oceanic food web essay assignment

The Food Chain of the Ocean

Oceanic food chains contain some of the largest organisms in the world, such as whales, feeding on some of the smallest organisms, such as phytoplankton. We know this thanks to the great work of many marine biologists, but the difficulties in exploring the depths of the ocean mean our understanding is still very limited. Also known as community ecology, synecology is the study of interactions between plant and animal species in ecological communities. Within this field of study, we learn about the relationships between living beings, including food relationships in the form of oceanic food chains and food webs.

At AnimalWised, we discover the food chain of the ocean , learning how energy and matter pass between species to create some of the most diverse ecosystems on the planet. We also learn about how specific aquatic food chains exist within the context of a large marine ecosystem food web.

Difference between food chains and food webs

The oceanic food chain, examples of food chains of the ocean.

Although the way in which aquatic plants and animals feed can be difficult to determine, we know that all living organisms are either autotrophs or heterotrophs. This means they can sustain themselves in one of two ways:

• Autotrophs : produce their own food without the need for another organism.
• Heterotrophs : consume other organisms to survive.

With this basic understanding, we can see that some organisms produce, while others consume. There are even some rare occasions where an organism is both. This can help us to understand the difference between a food chain and a food web.

A food chain shows how matter and energy move within an ecosystem through different organisms in a linear and unidirectional way. Food chains always begin with an autotrophic being that is the primary producer of matter and energy. These organisms are capable of transforming inorganic matter into organic matter and non-assimilable energy sources into assimilable energy.

A good example of this transformation is the conversion of sunlight into adenosine triphosphate (ATP), a source of energy for living beings. This occurs in autotrophic organisms with the ability to photosynthesize. The matter and energy created by the autotrophs will pass to the rest of the heterotrophic beings or consumers. These consumers are usually animal that can be primary, secondary and tertiary consumers.

On the other hand, a food web is a set of food chains that are interconnected. It is based on the same principles, but it provides a much more complex explanation of the movement of energy and matter. The marine ecosystem food web is one of the most complex and fascinating in nature.

Although the types of organisms vary greatly between aquatic and terrestrial ecosystems , the fundamental principles apply to both. They are both created by charting the interactions between producers and consumers. The main differences can be found in the types of species involved in the food chain of the ocean, as well as the amount. The biomass of terrestrial ecosystems is significantly greater than those of the ocean.

We can understand the food chain of the ocean by looking at the trophic levels . These are the positions an organism occupies within a food chain or food web. It is thanks to trophic levels that we think of different species being at the top or bottom of their respective food chains. Here we look at some of the trophic levels within oceanic food chains.

Primary producers

In the aquatic food chain, we can find primary producers which are mainly phytoplankton . Plankton is the term used for the many organisms which float through the ocean without an ability to propel themselves through the water. Phytoplankton are the autotrophic plankton which use photosynthesis to create their own energy.

One of the most common types of phytoplankton are the various types of algae which can be found in the ocean. They can be unicellular, such as those belonging to the phyla Glaucophyta , Rhodophyta and Chlorophyta or multicellular, such as those of the superphylum Heterokonta . The latter are the algae that we can see with the naked eye washed up on beaches. In addition, we can find bacteria at this level of the food chain, such as cyanobacteria which also carry out photosynthesis.

Learn more about this type of phytoplankton in the food chain of the ocean with our sister site's guide to what are cyanobacteria?

Primary consumers

The primary consumers in the aquatic food chain are usually herbivorous animals that feed on microscopic or macroscopic algae and even bacteria. This level is usually made up of zooplankton and other herbivorous organisms. Specific empales of primary consumers in the food chain of the ocean are small crustaceans, the larvae of larger animals, various fish species and even types of coral.

Discover more about certain types of marine primary consumers with our guide to types of aquatic insects .

Secondary consumers

Secondary consumers are notable for being carnivorous animals , feeding on herbivores in the lower level. They can be fish, arthropods, waterfowl or even mammals. For example, certain species of mackerel are considered secondary consumers as they will eat other smaller fish, but can themselves be eaten by larger carnivorous fish .

Tertiary consumers

Tertiary consumers are generally considered to be supercarnivores . These are carnivorous animals that feed on other carnivores, which form the trophic level of secondary consumers. In this group we can find animals of the marine ecosystem food web such as orcas, sharks and even certain species of tuna.

Although relevant, it is important to know that size is not the only factor in determining the trophic level of an animal in a marine ecosystem food web. For example, some of the largest aquatic animals are whales which are often considered secondary consumers since they do not hunt like certain shark or orca species.

Learn more about the orcas place in the food chain of the ocean by learning what do killer whales eat?

Although we have provided the basic trophic levels of oceanic food chains , they are generally more complicated to understand. One reason for this is that difference synecologists use different systems to understand marine ecosystem food webs. Below we look at some examples of different types of food chains in the ocean:

• The first example of an aquatic food chain consists of two links . This is the case of phytoplankton and whales. Phytoplankton is the primary producer and whales the only consumer.
• These same whales can form a three-link chain if they feed on zooplankton instead of phytoplankton. Then the chain would look like this: phytoplankton > zooplankton > whale. The direction of the arrows indicates where energy and matter are moving.
• In an semi-aquatic/terrestrial system such as a river, we could find a chain of four links : phytoplankton > mollusks of the genus Lymnaea > common barbel ( Barbus barbus ) > gray heron ( Ardea cinerea ).
• An example of a five-link chain where we can see a supercarnivore is the following: Phytoplankton > krill > emperor penguin ( Aptenodytes forsteri ) > leopard seal ( Hydrurga leptonyx ) > orca ( Orcinus orca ).

In a natural ecosystem the relationships between different species are never as simple as the ocean food chain suggests. Food chains are used to simplify trophic relationships and help us better understand the principles of marine life. It is important to remember food chains interact with each other in a complex network of food webs.

We must also consider the fact that aquatic food webs are also influenced by terrestrial animals. This is the case with birds which dive into the water to feed, as well as large mammals that eat fish such as polar bears. There are also marine animals that will venture partway onto land to eat terrestrial animals.

If you want to read similar articles to The Food Chain of the Ocean , we recommend you visit our Facts about the animal kingdom category.

• Hansson, LA, Nicolle, A., Granéli, W., Hallgren, P., Kritzberg, E., Persson, A., & Brönmark, C. (2013). Food-chain length alters community responses to global change in aquatic systems. Nature Climate Change , 3(3) , 228.
• Jake Vander Zanden, M., & Fetzer, W. W. (2007). Global patterns of aquatic food chain length. Oikos , 116(8) , 1378-1388.

Estimated Class Time for the Engagement: 20-30 minutes

EXPLORATION

This student-centered station lab is set up so students can begin to explore food webs. Four of the stations are considered input stations where students are learning new information about food webs, and four of the stations are output stations where students will be demonstrating their mastery of the input stations.  Each of the stations is differentiated to challenge students using a different learning style.  You can read more about how I set up the station labs here .

EXPLORE IT!

Students will be working in pairs to recreate the engagement activity that they went over at the beginning of the food webs lesson. Students will have 2 images that they will be looking at. The first image will require the students to list 7 organisms that live within a desert ecosystem. Students will explain how those organisms are related. The second image is the same as the first, however, includes a desert ecosystem food web. Students will explain how plants get their energy, the direction of the arrows, and what would the impact be if the hawk was removed from the web.

WATCH IT!

At this station, students will be watching a 4-minute video describing how wolves change rivers. Students will then answer questions related to the video and record their answers on their lab station sheet. For example, name 2 impacts the wolves had on the deer population at Yellowstone, how did the re-introduction impact tree populations, and how wolves impacted the flow of rivers in Yellowstone.

RESEARCH IT!

The research station will allow students to get online and participate in an interactive website about food webs. Students will read about food webs and what a trophic level means. Students interact with a food web game where they will drag and drop organisms within an Antarctic food web template. Once students have placed cards, they can check their answers and will be given opportunities to fix mistakes. With each concept, students will answer a few questions to help make the research more concrete.

READ IT!

This station will provide students with a one page reading about food webs. In the reading students will discover what the term ecology means and methods of ecological interdependence. Students will also learn from the reading that the many relationships that occur in an ecosystem that allows organisms to thrive an survive. There are 4 follow-up questions that the students will answer to show reading comprehension of the subject.

ASSESS IT!

WRITE IT!

Students who can answer open-ended questions about the lab truly understand the concepts that are being taught.  At this station, the students will be answering three questions like describing the impact of removing an organism from a food web, describe the flow of energy in a marine food web, and explain the reason why humans are dependent on a healthy ecosystem.

ILLUSTRATE IT!

Your visual students will love this station.  Students will be creating a sample food web from an ecosystem they would find at a nearby park. Students will include the Sun, at least 7 organisms and arrows depicting the flow of energy.

ORGANIZE IT!

The organize it station allows your students to place organisms on a food web template. The marine food web contains 9 cards that students will place in the correct order showing the correct flow of energy.

Estimated Class Time for the Exploration: 1-2, 45 minute class periods

EXPLANATION

The explanation activities will become much more engaging for the class once they have completed the exploration station lab.  During the explanation piece, the teacher will be clearing up any misconceptions about food webs with an interactive PowerPoint, anchor charts, and interactive notebook activities. The food webs lesson includes a PowerPoint with activities scattered throughout to keep the students engaged.

The students will also be interacting with their journals using INB templates for food webs.  Each INB activity is designed to help students compartmentalize information for a greater understanding of the concept.  The food webs INB template will diagram the flow of energy that takes place in a terrestrial, freshwater, and marine ecosystem. Estimated Class Time for the Exploration: 2-3, 45 minute class periods

ELABORATION

The elaboration section of the 5E method of instruction is intended to give students choice on how they can prove mastery of the concept.  When students are given choice the ‘buy-in’ is much greater than when the teacher tells them the project they will have to create.  Each of the food web projects will allow students to show their understanding of the flow of energy transfer from organism to organism.

Estimated Class Time for the Elaboration: 2-3, 45 minute class periods (can also be used as an at-home project)

The final piece of the 5E model is to evaluate student comprehension.  Included in every 5E lesson is a homework assignment, assessment, and modified assessment.  Research has shown that homework needs to be meaningful and applicable to real-world activities in order to be effective.  When possible, I like to give open-ended assessments to truly gauge the student’s comprehension.

Estimated Class Time for the Elaboration: 1, 45 minute class period

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Marine food webs.

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Feeding relationships are often shown as simple food chains – in reality, these relationships are much more complex, and the term ‘food web’ more accurately shows the links between producers, consumers and decomposers

Understanding food webs

In this video, Associate Professor Stephen Wing talks about how our understanding of food webs has changed over the years. We now have a much better understanding about the complex networks of interactions between organisms and the place of humans in the food web.

A food web diagram illustrates ‘what eats what’ in a particular habitat. Pictures represent the organisms that make up the food web, and their feeding relationships are typically shown with arrows. The arrows represent the transfer of energy and always point from the organism being eaten to the one that is doing the eating.

Trophic levels

Organisms in food webs are commonly divided into trophic levels. These levels can be illustrated in a trophic pyramid where organisms are grouped by the role they play in the food web. For example, the 1st level forms the base of the pyramid and is made up of producers. The 2nd level is made up of herbivorous consumers and so on. On average, only 10% of the energy from an organism is transferred to its consumer. The rest is lost as waste, movement energy, heat energy and so on. As a result, each trophic level supports a smaller number of organisms – in other words, it has less biomass. This means that a top-level consumer, such as a shark , is supported by millions of primary producers from the base of the food web or trophic pyramid.

Marine trophic pyramid

Food webs throughout the world all have the same basic trophic levels. However, the number and type of species that make up each level varies greatly between different areas and different ecosystems.

Producers are described as autotrophic, which means they are able to make their own food. Just like producers on land, producers in the marine environment convert energy from the sun into food energy through photosynthesis. Phytoplankton are the most abundant and widespread producers in the marine environment. Other producers include seaweeds (a type of macroalgae) and seagrasses (the only flowering plant found in marine environments). New Zealand has only 1 species of seagrass but many species of seaweed.

Consumers are described as heterotrophic, which means they are unable to make their own food and rely on consuming other organisms or absorbing dissolved organic material in the water column.

Consumers are divided into herbivores and carnivores and are typically further divided into 1st, 2nd or 3rd level consumers. For example, many zooplankton in the marine environment are herbivorous consumers. They form the 2nd level of the trophic pyramid and consume phytoplankton. Zooplankton are eaten by the 1st level carnivorous consumers, which includes juvenile stages of larger animals like fish and jellyfish as well as small fish and crustaceans. 2nd and 3rd level carnivorous consumers include larger fish and some species of squid and octopus. Predators at the top level of the trophic pyramid include animals like sharks and dolphins . However, not all top marine predators live in the sea. The albatross is an important predator at the top of the marine food web in Otago. Humans are also top-level consumers in the marine food web.

Tuna sandwich

On average, only 10% of the energy from an organism is transferred to its consumer. This means that a top-level consumer, such as a tuna, is supported by millions of primary producers from the base of the food web or trophic pyramid.

Decomposers

Decomposers exist on every trophic level. They are mainly bacteria that break down dead organisms. This process releases nutrients to support the producers as well as the consumers that feed through absorbing organic material in the water column. This process is very important and means that even top-level consumers are contributing to the food web as the decomposers break down their waste or dead tissue.

Changes to food webs

The effect of removing or reducing a species in a food web varies considerably depending on the particular species and the particular food web. In general, food webs with low biodiversity are more vulnerable to changes than food webs with high biodiversity. In some food webs, the removal of a plant species can negatively affect the entire food web, but the loss of one plant species that makes up only part of the diet of a herbivorous consumer may have little or no effect.

Some species in a food web are described as ‘keystone’ species. A keystone species is one that has a greater impact on a food web than you would expect in relation to their abundance. The removal of a keystone species characteristically results in a major change, in the same way that removing a keystone from an arch or bridge could cause the structure to collapse.

In Fiordland, the New Zealand sea star is a keystone species that controls the numbers of the species it feeds on, for example, mussels . If the sea star is removed, this can cause a large increase in the numbers of mussels, and this has flow-on effects throughout the food web.

Many scientists investigate food webs in order to better understand how they may be affected by human impacts such as fishing, pollution and tourism.

Marine ecosystem

Explore this interactive diagram to learn more about life in the sea. Click on the different labels to view short video clips or images about different parts of the marine ecosystem.

Useful links

Listen to this Radio New Zealand programme Sea Lions As Food Web Ambassadors . Lucy Jack is hoping that her research will give insights into marine food webs and how they’ve changed over time.

Learn about Trophic level: definition, categories, structure, examples and importance on Biology Online .

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What eats what?

aquatic Of or relating to water, such as an aquatic plant or animal.', FONTSIZE, '12px')" onmouseout="UnTip()">Aquatic food webs are complex groups of organisms that perform different functions in the ecosystem. Phytoplankton are small primary producers Organisms that make their own food from a primary energy source and are food for primary consumers.', FONTSIZE, '12px')" onmouseout="UnTip()">primary producers suspended in water. They use nutrient The minerals that every living organism needs to survive. Nutrients can become a problem if there are too much of them.', FONTSIZE, '12px')" onmouseout="UnTip()">nutrients along with carbon dioxide to harness sunlight energy and create biomass The total mass of all living material in a specific area, habitat, or region.', FONTSIZE, '12px')" onmouseout="UnTip()">biomass through the process of photosynthesis The process of changing carbon dioxide, water, and sunlight into energy and oxygen; used by plants, algae, and zooxanthellae.', FONTSIZE, '12px')" onmouseout="UnTip()">photosynthesis . Phytoplankton biomass is usually the primary food for other aquatic organisms, including zooplankton. Zooplankton are small, heterotrophic organisms that feed on phytoplankton and other zooplankton, and are themselves food for larger planktivore an animal with a diet that consists mainly of plankton', FONTSIZE, '12px')" onmouseout="UnTip()">planktivores . In this way, the sun’s energy is transferred up aquatic food webs, eventually feeding apex predators such as sharks and other large fish.

What factors shape food webs?

Aquatic food webs can be characterized by the number of trophic level A position in a food chain or Ecological Pyramid occupied by a group of organisms sharing the same function in the food chain and the same nutritional relationship to the primary sources of energy.', FONTSIZE, '12px')" onmouseout="UnTip()">trophic levels and the amount of biomass in each level. Nutrient availability is central in shaping food webs. Plants require nutrients in specific quantities in order to photosynthesize and convert nutrients into useable food. Low quantities of certain nutrients can limit the food energy available in the web. Another factor shaping food webs is the amount of biomass in each trophic level. For example, a food web with many predators may have little prey biomass than one with fewer predators, because the predators eat more of the prey.

Where do humans fit in?

Human activities change aquatic environments and food webs in many ways.  We dam rivers to the sea and then reclaim land and re-populate it. Aquatic organisms may be transported around the world through, for example, bilge water in ships, potentially establishing populations of invasive species non-native (or alien) to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm.', FONTSIZE, '12px')" onmouseout="UnTip()">invasive species . Most human populations reside near water, and human activities introduce excess nutrients to aquatic systems through sewage, detergents, fertilizer and animal waste. Humans rely on the oceans for food, and thus play a role as apex predators, influencing aquatic food webs through overfishing Removing fish from a reef faster than the population can grow.', FONTSIZE, '12px')" onmouseout="UnTip()">overfishing . Human activities may decrease the biomass of fished animals, decrease biodiversity The variety of living organisms in an ecosystem. This term can refer to different ecosystems, species, genes, and their relative abundance.', FONTSIZE, '12px')" onmouseout="UnTip()">biodiversity , and alter the species present in an ecosystem.

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Energy and food webs.

• All living things require energy in order to survive and carry out their life processes, such as growth, reproduction and for their metabolism. For example, when thinking about our Ocean Tracks species, a large amount of energy is required to migrate the thousands of miles they may travel.  
• This energy comes from the organism’s ecosystem and in many cases from the food that organism eats. But where did the energy in those food sources come from?  
• For much of the life on Earth, the primary source of energy is from the sun. Through photosynthesis , plants are able to capture energy from sunlight and use that energy to power reactions that transform carbon dioxide and water into oxygen and sugar molecules. This process removes carbon dioxide from the atmosphere and provides the oxygen that we breathe.
• Life is possible, however, even in the absence of sunlight. For example, microbes living in hydrothermal vent communities are able to use inorganic chemical compounds through a process known as chemosynthesis to create energy. These chemosynthetic microbes are the foundation of the food web in hydrothermal vent communities. Photosynthesis and chemosynthesis make it possible for life to exist on Earth!  
• Organisms that are able to capture energy either from sunlight or chemicals and convert it into a form that other organisms can use are called autotrophs.  
• Autotrophs are also known as primary producers, and are are a highly important food source for other organisms. On land, primary producers are mostly plants such as grass in trees. In the ocean, 95% of the primary production is done by microscopic phytoplankton . Phytoplankton contribute 50% of the oxygen in our atmosphere. Some phytoplankton are bacteria and others are protists. Two important types of phytoplankton that are diatoms and dinoflagellates.  
• In coastal regions of the ocean, algae, such as kelps and rockweeds, and plants, such as sea grasses, are important primary producers.  
• For organisms that cannot make their own food, they must ingest other organisms to fulfill their energy requirements. These organisms are called heterotrophs. Heterotrophs are also called consumers because they must consume other organisms for energy and nutrients.  
• There are herbivores that feed on plant material. In the ocean, an example of an herbivore would be a periwinkle grazing on some algae.
• There are carnivores that kill and eat other animals. In the ocean, of course one of the greatest carnivores is the great white shark.
• There are scavengers and detritivores that feed on dead plants and animals, such as a hagfish feeding on a dead whale in the deep ocean.
• O mnivores feed on both plants and animals.  The hawksbill sea turtle is an omnivore, feeding on sea urchins, mollusks, crustaceans and algae.
• Decomposers are bacteria that chemically break down organic matter.  
• A food chain is a set of linkages that show who eats who in an ecosystem and the transfer of energy that takes place.  
• Food chains start with a primary producer . Energy is then transferred to a primary consumer , then secondary , tertiary , and quaternary consumers in sequence.  
• The primary consumer is an organism that eats a primary producer, which can include a zooplankton or snail in the ocean.  
• The secondary consumer is an organism that eats a primary consumer, and includes fish species that feed on the zooplankton.  
• Tertiary consumers feed on secondary consumers, and quaternary consumers feed on tertiary consumers. These groups include higher level predators such as sharks.  
• In reality most ecosystems are more complicated than a simple chain of feeding interactions. Many species consume more than one type of species, creating a complex web of interactions known as a food web.  
• In the visual below, you can see an example of a food web in the open ocean ecosystem and also one food chain that is a part of that food web. You may notice, however, that even the picture of the food web is incomplete since only a small number of ocean species are represented.

• Each step of the food web or chain is called a trophic level . Primary producers are always the first trophic level and are represented at the bottom of an ecological pyramid. The diagram below shows an example of an ecological pyramid for the ocean.

• These pyramids can also show how much energy is available at each trophic level of a food web. The average amount of energy transferred from one trophic level to the next is 10%. For example, 10% of the solar energy that is captured by phytoplankton gets passed on to zooplankton (primary consumers). Ten percent of that energy (10% of 10%, which is 1%) gets passed on to the organisms (secondary consumers) that eat the zooplankton.
• With more trophic levels that exist between the primary producer and a consumer, the smaller the amount of energy that gets passed on to the consumer. The shape of the pyramid reflects the idea that they amount of energy gets smaller as you move up the food chain. The visual below shows how little energy gets passed along as you get higher in the food chain.

Marine Food Webs

Watch this brief, video picture of practice that captures everyday classroom life and provides real-life examples of how students learn and think about ocean topics.

Earth Science, Oceanography

Food chains are relatively easy concepts for students to understand. They show linear, one-way relationships between organisms within a given ecosystem. Students learn to tell stories of food chains as connections between living things that need food. However, students may not know that arrows in food chains represent the flow of matter and energy to the next trophic level. Another potential challenge is that students are taught about food chains with land-based plants and animals as examples. The plant grows, a deer eats the plant, and a wolf eats the deer. But what does the food chain look like in the ocean? Additionally, feeding relationships between organisms are much more complicated than simple food chains, especially as shown in food webs. Watch this video of 5th grade students in Laguna Niguel, California—a coastal community. The purpose of this classroom video is to see examples of ways students describe differences between food webs and food chains in the ocean. For additional classroom context, video analysis, and reflection opportunities, read the Picture of Practice page for "Marine Food Webs" in the One Ocean Environmental Literacy Teacher Guide, page 59.

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Food Web: Concept and Applications

Introduction

There are two types of food chains: the grazing food chain, beginning with autotrophs, and the detrital food chain, beginning with dead organic matter (Smith & Smith 2009). In a grazing food chain, energy and nutrients move from plants to the herbivores consuming them, and to the carnivores or omnivores preying upon the herbivores. In a detrital food chain, dead organic matter of plants and animals is broken down by decomposers, e.g., bacteria and fungi, and moves to detritivores and then carnivores.

Food web offers an important tool for investigating the ecological interactions that define energy flows and predator-prey relationship (Cain et al. 2008). Figure 1 shows a simplified food web in a desert ecosystem. In this food web, grasshoppers feed on plants; scorpions prey on grasshoppers; kit foxes prey on scorpions. While the food web showed here is a simple one, most feed webs are complex and involve many species with both strong and weak interactions among them (Pimm et al. 1991). For example, the predators of a scorpion in a desert ecosystem might be a golden eagle, an owl, a roadrunner, or a fox.

The idea to apply the food chains to ecology and to analyze its consequences was first proposed by Charles Elton (Krebs 2009). In 1927, he recognized that the length of these food chains was mostly limited to 4 or 5 links and the food chains were not isolated, but hooked together into food webs (which he called "food cycles"). The feeding interactions represented by the food web may have profound effects on species richness of community, and ecosystem productivity and stability (Ricklefs 2008).

Types of Food Webs

Applications of food webs, food webs are constructed to describe species interactions (direct relationships)..

The fundamental purpose of food webs is to describe feeding relationship among species in a community. Food webs can be constructed to describe the species interactions. All species in the food webs can be distinguished into basal species (autotrophs, such as plants), intermediate species (herbivores and intermediate level carnivores, such as grasshopper and scorpion) or top predators (high level carnivores such as fox) (Figure 1).

These feeding groups are referred as trophic levels. Basal species occupy the lowest trophic level as primary producer. They convert inorganic chemical and use solar energy to generate chemical energy. The second trophic level consists of herbivores. These are first consumers. The remaining trophic levels include carnivores that consume animals at trophic levels below them. The second consumers (trophic level 3) in the desert food web include birds and scorpions, and tertiary consumers making up the fourth trophic level include bird predators and foxes. Grouping all species into different functional groups or tropic levels helps us simplify and understand the relationships among these species.

Food webs can be used to illustrate indirect interactions among species.

Indirect interaction occurs when two species do not interact with each other directly, but influenced by a third species. Species can influence one another in many different ways. One example is the keystone predation are demonstrated by Robert Paine in an experiment conducted in the rocky intertidal zone (Cain et al. 2008; Smith & Smith 2009; Molles 2010). This study showed that predation can influence the competition among species in a food web. The intertidal zone is home to a variety of mussels, barnacles, limpets, and chitons (Paine 1969). All these invertebrate herbivores are preyed upon by the predator starfish Pisaster (Figure 3). Starfish was relatively uncommon in the intertidal zone, and considered less important in the community. When Paine manually removed the starfish from experimental plots while leaving other areas undisturbed as control plots, he found that the number of prey species in the experimental plots dropped from 15 at the beginning of the experiment to 8 (a loss of 7 species) two years after the starfish removal while the total of prey species remained the same in the control plots. He reasoned that in the absence of the predator starfish, several of the mussel and barnacle species (that were superior competitors) excluded the other species and reduced overall diversity in the community (Smith & Smith 2009). Predation by starfish reduced the abundance of mussel and opened up space for other species to colonize and persist. This type of indirect interaction is called keystone predation.

Food webs can be used to study bottom-up or top-down control of community structure.

Top-down control occurs when the population density of a consumer can control that of its resource, for example, predator populations can control the abundance of prey species (Power 1992). Under top-down control, the abundance or biomass of lower trophic levels depends on effects from consumers at higher trophic levels. A trophic cascade is a type of top-down interaction that describes the indirect effects of predators. In a trophic cascade, predators induce effects that cascade down the food chain and affect biomass of organisms at least two links away (Ricklefs 2008). Nelson Hairston, Frederick Smith and Larry Slobodkin first introduced the concept of top-down control with the frequently quoted "the world is green" proposition (Power 1992; Smith & Smith 2009). They proposed that the world is green because carnivores depress herbivores and keep herbivore populations in check. Otherwise, herbivores would consume most of the vegetation. Indeed, a bird exclusion study demonstrated that there were significantly more insects and leaf damage in plots without birds compared to the control (Marquis & Whelan 1994).

Food webs can be used to reveal different patterns of energy transfer in terrestrial and aquatic ecosystems.

As a diagram tool, food web has been approved to be effective in illustrating species interactions and testing research hypotheses. It will continue to be very helpful for us to understand the associations of species richness/diversity with food web complexity, ecosystem productivity, and stability.

References and Recommended Reading

Cain, M. L., Bowman, W. D. & Hacker, S. D. Ecology . Sunderland MA: Sinauer Associate Inc. 2008.

Cebrian, J. Patterns in the fate of production in plant communities. American Naturalist 154 , 449-468 (1999)

Cebrian, J. Role of first-order consumers in ecosystem carbon flow. Ecology Letters 7 , 232-240 (2004)

Elton, C. S. Animal Ecology . Chicago, MI: University of Chicago Press, 1927, Republished 2001.

Knight, T. M., et al. Trophic cascades across ecosystems. Nature 437 , 880-883 (2005)

Krebs, C. J. Ecology 6 th ed. San Francisco CA: Pearson Benjamin Cummings, 2009.

Marquis, R. J. & Whelan, C. Insectivorous birds increase growth of white oak through consumption of leaf-chewing insects. Ecology 75 , 2007-2017 (1994)

Molles, M. C. Jr. Ecology: Concepts and Applications 5 th ed. New York, NY: McGraw-Hill Higher Education, 2010.

Paine, R. T. The Pisaster-Tegula interaction: Prey parches, predator food preferences and intertide community structure. Ecology 60 , 950-961 (1969)

Paine, R. T. Food web complexity and species diversity. The American Naturalist 100 , 65-75 (1966)

Paine, R. T. Food webs: Linkage, interaction strength and community infrastructure. Journal of Animal Ecology 49 , 667-685 (1980)

Pimm, S. L., Lawton, J. H. & Cohen, J. E. Food web patterns and their consequences. Nature 350 , 669-674 (1991)

Power, M. E. Top-down and bottom-up forces in food webs: do plants have primacy? Ecology 73 , 733-746 (1992)

Schoender, T. W. Food webs from the small to the large. Ecology 70 , 1559-1589 (1989)

Shurin, J. B., Gruner, D. S. & Hillebrand, H. All wet dried up? Real differences between aquatic and terrestrial food webs. Proc. R. Soc. B 273 , 1-9 (2006) doi:10.1098/rspb.2005.3377

Smith, T. M. & Smith, R. L. Elements of Ecology 7 th ed. San Francisco CA: Pearson Benjamin Cummings, 2009.

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Essay On Oceanography: Marine Food Web

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Topic: Energy , Vitamins , Internet , Food , Customers , Life , Ocean , Water

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Introduction

Our knowledge of ocean life is limited to big fishes and salty water. However, with new discoveries it is now evident that ocean life is mainly kept in balance by microorganisms. Billions of micro, nano and singular celled organisms called planktons are the primary producers of the marine fish web. Any living being in order to survive needs energy in the form of food and on earth the sole supplier of energy is the Sun. Be it ocean food chain or terrestrial food chain it always start with the sun. In case of terrestrial chains plants and weeds are the primary producers while for ocean life phytoplankton are the primary producers of food chain. Marine life is very different than the life on the land. The chemistry is different, the animals are different and the basic environment is different. This essay will first touch upon the marine environment to discuss upon the important characteristics for animals to survive in that environment and then will move on to the discussion about the marine food chain and food web comparing it with the terrestrial food, simultaneously highlighting the energy efficiency of the food chain.

Marine Environment

Marine environment is divided into many zones based on five primary factors called depth, light, temperature, substrate and salinity. The basic zonation is based on substrate. Pelagic zone is the exclusive water environment and on the other hand benthic zones are zones near to the bottom surface of the ocean. Pelagic zones are mainly divided into two zones; neritic zone which includes the areas around continental shelf, and oceanic zone. Oceanic zone is further divided into two main categories. Photic zone is the upper surface where sunlight can penetrate whereas aphotic zones are dark part of the ocean. Fig 1: (State College of Florida, 2013) There are several factors affecting the marine life. Light is one of the most important among them. Light is required for photosynthesis which is the primary way to convert solar energy inside a living cell. The whole food chain survives because of light which makes photosynthesis happen. Light generally can penetrate up to about 150 meter but it can vary depending on wavelength and turbidity (State College of Florida, 2013). Most of the ocean life actually lives in this zone because food is abundant in this layer of the ocean. Apart from light from the sun, in deep water some marine organism produces light by biochemical reactions known as bioluminescence. Another important factor for marine life is temperature. Metabolic rates of living bodies depend on temperature. Higher temperature means higher metabolism rates. Higher metabolism rate means more energy production. As most of the living being lives within the first 150 meter of the marine surface where temperature variation is not much so marine bodies are not fit to survive huge temperature variations. Nutrients in the sea water are extremely important for the marine life to grow and survive. The main nutrients required by the marine life are nitrogen and phosphorous and to a lesser extent calcium and phosphorous (State College of Florida, 2013). Marine plants recycle these nutrients to keep balance in the water. In fact there are certain oceanic areas in the world where because of the abundance of nutrients marine life is seen in large numbers. Another very important nutrient for the marine bodies is iron. Iron directly may not have much impact on the marine life but it indirectly helps nitrogen to assimilate in the phytoplankton which essentially helps in the photosynthesis process. Salinity is another main factor important for the sustainability of life in the oceanic environment. Salinity varies from 6-40 ppt (State College of Florida, 2013). This huge variation can be primarily contributed to factors like evaporation rates, fresh water supply rates and sea ice formation. Salinity mainly varies at the surface level of the ocean. However, deep ocean level salinity varies far less. Salinity is important because of the way different species process water. Most of the marine species are isotonic. This means that the salinity of water inside the body of species is exactly same as that of the ocean. This is ideal to survive in any level of salinity. However, some fishes like bony fish are hypotonic. Water inside the body of bony fish is almost pure containing no salinity. Hence they continuously lose water and are threatened by dehydration. To prevent that bony fish continuously drinks and processes water in their gills to take out the salt. Salinity also means greater density of the water which helps in flotation. Most of the marine organisms are lighter than salt water density so they naturally float. However, most of the fishes are slightly heavier but they use gas filled swim bladders to maintain buoyancy.

Primary Producers

Fig 2: (Harris, 2013) In nature everyone needs energy in some form or other to survive. The sole source of energy in earth is the sun but most of the organisms cannot directly convert solar energy into a form which can be used by other animals. These organisms are known as the primary producers in the food chain (Stewert, 2005). On the land trees, weeds, mosses and grasses are the primary producers. In the ocean phytoplankton and seaweeds are the only ones who can do photosynthesis from the solar energy. Planktons are a community of organisms that are mainly identified and categorized by their locomotion. Planktons live in the photic zone as they require sunlight to do the photosynthesis (Corey and Beutel). Phytoplanktons are plants and zooplanktons are animals. They can be of many types starting from soft bodies to hard cells to a single cell organism. Phytoplanktons are responsible for almost all types of primary productivity of ocean life. Phytoplanktons along with seaweeds are the only organisms which can create their own food and get energy from that and that is why they are called autotrophs. All other types of marine lives are heterotrophs meaning they cannot generate their own food and are dependent on others for nutrient and energy.

Marine Food Web

Fig 3: (Britannica, 2005) The food chain in marine life starts from the phytoplankton. The most common form of phytoplankton is the diatoms. Diatoms take energy from the sun and through photosynthesis convert that energy to nutrients and store it. Phytoplankton are the primary producers in the marine food chain. Zooplanktons then eat the phytoplankton. Zooplanktons, some kind of shrimps, are the primary consumers as they only survive on the primary producers. The most common forms of zooplanktons are pteropods and copepods (Corey and Beutel). Zooplanktons are the primary food for small fishes. Small fishes are the secondary consumers. In some cases big zooplanktons are also food for some marine mammals like whales. Because of the sheer volume and biomass in the whole food chain copepods are the most important link between primary producers and the secondary consumers. Copepods make the largest animal mass in the ocean. Among the secondary consumers amphipods and alewife fish and menhaden fishes are voracious eaters of zooplankton and if their population increases in the ocean then that will create a dent in the zooplankton population creating havoc in the whole food web (Stewert, 2005). However, that is prevented by the next layer of secondary consumers. Bigger fishes like bluefish eats menhaden and alewife fishes thus preventing unprecedented increase of menhaden population in the ocean. Further down the food chain bigger fishes hunt smaller fishes. The hunt at this level is cutthroat and there is no singular relationship of food chain. This means that at this level fishes even can eat each other. For example, Bluefin tuna is targeted by whales, swordfish, sharks and even other types of tuna. Apart from this type of predation, larval forms are often a target by many other types of fishes (Corey and Beutel). For example, squid feeds on bluefish larva but squid on the other hand is a prey for the adult bluefish. Apart from this standard food cycle energy circulates in other forms as well. For example, large whales and sea turtles are not targeted by any other animals in the ocean but they often generate waste as part of excretion or dead tissue. Bacteria work on them and release nutrients from the waste which then is used by the plants to start the food cycle again. Fig 4: (LGFL, 2005) Organisms higher up the food chain are larger in size but fewer in number than lower level. This is mainly because of the loss of energy while going from one trophic level to another. The energy efficiency is only 10% and 90% of the energy is lost with every trophic level (LGFL, 2005). That is why most of the food chains do not have more than a few levels. It is very unlikely to see food chains with more than 6 trophic levels. For example, if a phytoplankton generates 100 Calorie from photosynthesis then by eating one phytoplankton, zooplankton will get only 10 calorie. Subsequently, by eating a zooplankton an amphipod will only get 1 calorie of energy (LGFL, 2005). It can be seen that only within 3 levels the energy level has come down to only a single calorie. That is why the food chain lengths generally are not high, further the population of the animals as we go up the food chain are much less than that of primary producers and primary consumers.

Terrestrial vs. Marine Food Web

Terrestrial and Marina food chains and webs are similar in many aspects but also have many differences. Apart from the difference in the medium which is water in case of marine food chain and land in case of terrestrial food chain, there are significant differences between the two food webs. The main primary producers in case of terrestrial food chain are crops, trees and other green vegetation and forests. In almost all the cases these are well developed multi cellular complex organism. On the other hand the primary producer in the marine food web is the phytoplankton. Most of the primary producers are single celled, small and primitive organisms. Even the primary consumers in case of marine life are small primitive organism in many cases. For example, copepods are the largest primary consumers in the ocean life and they are primitive organism. In case of terrestrial food chain, most of the primary consumers are well developed animals. For example, in case of terrestrial food chains, deer is one of the primary consumers of primary producers. Land food chains have less trophic level than ocean food chain. Almost all ocean food chains have minimum 4 trophic levels. On the other hand, many of the terrestrial food chains and web have only three trophical levels.

Effects of Overfishing on Marine Food Web

Fig 5: (Scheffer, 2005) As we know that for any marine food chain due to loss of energy as we go up the food chain the number of organisms up the chain decreases. For example, number of zooplanktons should always be much more than the small fishes in ocean as small fishes require large quantities of zooplankton to survive and get the required energy. That’s why as we go up the chain the number should come down. However, marine food webs are not that simple. For example, as per the above theory phytoplankton should be the highest in ocean (Scheffer, 2005). However, that is not the case. The amount of zooplankton in the ocean is more than the amount of phytoplankton. Phytoplankton may be smaller in number than zooplanktons but they have quick replenishment time. That is when phytoplankton grows very fast. If some of the phytoplanktons are eaten by zooplanktons then phytoplanktons can grow back quickly to fulfill the void. This way the balance in food chain is maintained between primary producers and primary consumers in marine food web. Furthermore, the weight of big fishes compared to small fishes in the oceans is more. This is also contrary to our basic understanding of the food chain as primary food for the big fishes are the small fishes. This also is somewhat compensated by the fact that small fishes can grow back very quickly. In case of big fishes, the number is high because they hunt for each other as well. For example, squids hunt the larvae of blue fish; sharks eat tuna and so on. Fishing in large scale creates some really dangerous changes in the marine food cycle. For example, most of fishing happens for large fishes. Especially the population of large fishes like tuna, cod, and salmon has gone down drastically. This is causing a decrease in the number of large fishes in the marine food cycle, leading to a top-down problem in the marine food chain. As the number of big fishes decreases the number of small fishes gets on a rise (Scheffer, 2005). Fishes like shrimp, crab, sunfish and lantern fish are on a rise. This increase in the population of small fish means that they need more zooplankton than before to survive. This is causing the zooplankton population to decrease. Especially there are many evidences that large bodied zooplanktons have decreased substantially. During fishing due to the usage of heavy fishing nets and crawlers used by the fishermen lot of zooplanktons are getting affected. This also adds to the reduction of zooplankton population. A decrease in zooplankton population means that the phytoplankton population is on a rise as there is not enough zooplankton to eat them (Scheffer, 2005). If this goes on then the marine food chain will be disrupted. This in the long run can change the marine environment completely if not addressed immediately. In fact to preserve the current balance in the marine food chain a good many countries have banned Cod fishing to encourage the fishing of small organisms like prawn.

Food chains or more accurately food webs are linear and circular sequence of organisms, each of which feeds on the preceding or in some cases feeds on each other. Primary producers are the main link in any food chain. Phytoplanktons are the primary producers in the marine food chain. They convert the energy from the sun into nutrients and then that is consumed by the primary and secondary consumers in the marine food chain. Phytoplanktons are crucial to marine food chain and so are the zooplanktons which are most consumed organisms in the ocean. Marine food chain is different than terrestrial food chain in some aspects. The marine food chains are generally longer than the terrestrial food chain. Modern fishing has enabled the fishermen to fish at a very large quantity. This is changing the ecology of the marine food chain drastically. Some measures have been taken to restrict the fishing of big fishes but much more need to be done if we want the marine food web to remain the way it is today.

Marine Environment and Primary Productivity (2013). State College of Florida (SCF). Retrieved on 22nd November 2013 from <http://faculty.scf.edu/rizkf/OCE1001/OCEnotes/chap11.htm> Efficiency of Energy Flow between Trophic Levels. Boundless. Retrieved on 22nd November 2013 from < https://www.boundless.com/biology/ecosystem-function/movement-of-energy-between-trophic-levels/efficiency-of-energy-flow-between-trophic-levels/> What happens to energy and biomass at each stage in a food chain (2013). London Grid for Learning (LGFL). Retrieved on 22nd November 2013 from <http://lgfl.skoool.co.uk/content.aspx?id=757> Food Chains and Webs, Making a Food Chain (2006). Cornwall Wildlife Trust (CWT) UK. Retrieved on 22nd November 2013 from <http://www.cornwallwildlifetrust.org.uk/> Valenti, Christopher. Malinda Schaefer Zarske and Denise Carlson (2004). Got Energy? Spinning a Food Web. Teach Engineering. Retrieved on 22nd November 2013 from <http://www.teachengineering.org/view_activity.php?url=collection/cub_/activities/cub_bio/cub_bio_lesson03_activity1.xml> Gratton, Claudio (2012). Midges connect aquatic and terrestrial food webs. Retrieved on 22nd November 2013 from < http://gratton.entomology.wisc.edu/2012/05/02/midges-connect-aquatic-and-terrestrial-food-webs/> Ecosystem: food chain in the ocean (2012). Encyclopedia Britannica. Retrieved on 22nd November 2013 from < http://www.britannica.com/EBchecked/media/119100/A-food-chain-in-the-ocean-begins-with-tiny-one> Corey, Tony and Beutel, Dave. The Marine Food Web. Sea Grant, Rhode Island. Retrieved on 22nd November 2013 from < http://seagrant.gso.uri.edu/factsheets/foodweb.html> Stewert, Robert R (2005). Marine Fisheries Food Webs. Department of Oceanography, Texas A&M University. Retrieved on 22nd November 2013 from < http://oceanworld.tamu.edu/resources/oceanography-book/marinefoodwebs.htm> Scheffer, Marten (2005). Cascading effects of overfishing marine systems. Aquatic Ecology and Water Quality Management Group, Department of Environmental Sciences, Wageningen University. Retrieved on 22nd November 2013 from <http://www.sciencedirect.com/science/article/pii/S0169534705002752> Harris, David (2013). Seeking sailors to help measure phytoplankton populations. PNAS. . Retrieved on 22nd November 2013 from <http://firstlook.pnas.org/seeking-sailors-to-help-measure-phytoplankton-populations/>

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19.1: Introduction to and Components of Food Webs

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Introduction

All living things require energy in one form or another. Energy is required by most complex metabolic pathways (often in the form of adenosine triphosphate, ATP), especially those responsible for building large molecules from smaller compounds, and life itself is an energy-driven process. Living organisms would not be able to assemble macromolecules (proteins, lipids, nucleic acids, and complex carbohydrates) from their monomeric subunits without a constant energy input.

Food webs illustrate how energy flows directionally through ecosystems, including how efficiently organisms acquire it, use it, and how much remains for use by other organisms of the food web.

Food Chains and Food Webs

In ecology, a food chain is a linear sequence of organisms through which nutrients and energy pass: primary producers, primary consumers, and higher-level consumers are used to describe ecosystem structure and dynamics. There is a single path through the chain. 

Food chains do not accurately describe most ecosystems. Even when all organisms are grouped into appropriate trophic levels, some of these organisms can feed on species from more than one trophic level; likewise, some of these organisms can be eaten by species from multiple trophic levels. In other words, the linear model of ecosystems, the food chain, is not completely descriptive of ecosystem structure. A holistic model—which accounts for all the interactions between different species and their complex interconnected relationships with each other and with the environment—is a more accurate and descriptive model for ecosystems. A food web is a graphic representation of a holistic, nonlinear web of primary producers, primary consumers, and higher-level consumers used to describe ecosystem structure and dynamics (Figure 1). 

Figure 1. Example of simplified food chains (a) and food webs (b) of terrestrial and marine ecosystems. Developed by LadyofHats and licensed under CC0. 

Though more complex than a food chain, a food web remains a simplified illustration of the direct and indirect trophic interactions among species in an ecosystem. Food webs often aggregate many species into trophic groups, which are functional groups of species that have the same predators and prey in a food web. Software can be used to model more complex interactions (Figure 2), but no food web model can capture all of the complexity found within a natural ecosystem. 

Figure 2: An example of a more complex food web developed by Hoover et al. 2021 using a program called Ecopath. This food web depicts trophic relationships among species in the Canadian Beaufort Sea. Horizontal lines represent trophic level. Image licensed under CC-BY 4.0. 

Components of a Food Web

The three basic ways in which organisms get food are as producers, consumers, and decomposers.

Producers (autotrophs) are typically plants or algae. Plants and algae do not usually eat other organisms, but pull nutrients from the soil or the ocean and manufacture their own food using photosynthesis. For this reason, they are called primary producers. In this way, it is energy from the sun that usually powers the base of the food chain (Cengage Learning 2002). An exception occurs in deep-sea hydrothermal ecosystems, where there is no sunlight. Here primary producers manufacture food through a process called chemosynthesis (van Dover 2000).

Consumers (heterotrophs) are species that cannot manufacture their own food and need to consume other organisms. Animals that eat primary producers (like plants) are called herbivores. Animals that eat other animals are called carnivores, and animals that eat both plants and other animals are called omnivores.

Decomposers (detritivores) break down dead plant and animal material and wastes and release it again as energy and nutrients into the ecosystem for recycling. Decomposers, such as bacteria and fungi (mushrooms), feed on waste and dead matter, converting it into inorganic chemicals that can be recycled as mineral nutrients for plants to use again.

Energy is acquired by living things in three ways: photosynthesis, chemosynthesis, and the consumption and digestion of other living or previously living organisms by heterotrophs.

Photosynthetic and chemosynthetic organisms are both grouped into a category known as autotrophs : organisms capable of synthesizing their own food (more specifically, capable of using inorganic carbon as a carbon source). Photosynthetic autotrophs ( photoautotrophs ) use sunlight as an energy source, whereas chemosynthetic autotrophs ( chemoautotrophs ) use inorganic molecules as an energy source. Autotrophs are critical for all ecosystems. Without these organisms, energy would not be available to other living organisms and life itself would not be possible.

Photoautotrophs, such as plants, algae, and photosynthetic bacteria, serve as the energy source for a majority of the world’s ecosystems. Photoautotrophs harness the solar energy of the sun by converting it to chemical energy in the form of ATP (and NADP). The energy stored in ATP is used to synthesize complex organic molecules, such as glucose.

Chemoautotrophs are primarily bacteria that are found in rare ecosystems where sunlight is not available, such as in those associated with dark caves or hydrothermal vents at the bottom of the ocean (Figure 3). Many chemoautotrophs in hydrothermal vents use hydrogen sulfide (H2S), which is released from the vents as a source of chemical energy. This allows chemoautotrophs to synthesize complex organic molecules, such as glucose, for their own energy and in turn supplies energy to the rest of the ecosystem.

Figure 3 Swimming shrimp, a few squat lobsters, and hundreds of vent mussels are seen at a hydrothermal vent at the bottom of the ocean. As no sunlight penetrates to this depth, the ecosystem is supported by chemoautotrophic bacteria and organic material that sinks from the ocean’s surface. This picture was taken in 2006 at the submerged NW Eifuku volcano off the coast of Japan by the National Oceanic and Atmospheric Administration (NOAA). The summit of this highly active volcano lies 1535 m below the surface.

Not Your Average Food Web: Deep Sea \(\PageIndex{1}\)

Food webs in the deep sea vary depending on proximity to seamount, hydrothermal vents, and trenches. In areas near hydrothermal vents, chemosynthetic bacteria are the major primary producers. These chemoautotrophs are what provides energy for the rest of the trophic levels in this system.

A comparison of photosynthetic (left) vs. chemosynthetic (right) food webs. Diagram developed by GRID-Arendal and licensed under CC-SA-NC.

Species in deep-sea ecosystems have adapted to interact with each other in many ways. One key interaction is the symbiosis between many species and chemosynthetic bacteria in hydrothermal vent systems. These bacteria live within the body of species like tubeworms, which are dependent on the bacteria to survive, similar to the relationship between zooxanthellae and coral. Another important type of deep sea community develops when a dead whale (or other large marine organism) carcass sinks to the ocean floor and provides an influx of nutrients. The communities support scavengers like hagfish, opportunists like bristle worms, and eventually enter a sulfophilic stage that appears similar to a hydrothermal vent community.

Whale falls serve as an extremely important influx of nutrients to the sun-starved deep ocean. This photo shows a Whale skeleton submerged in Monterey Bay National Marine Sanctuary, covered in octopuses and several other species. Photo by National Marine Sanctuaries is licenced under CC 2.0. 

Heterotrophs

Unlike autotrophs, heterotrophs consume rather than produce biomass energy as they metabolize, grow, and add to levels of secondary production. A food web depicts a collection of polyphagous heterotrophic consumers that network and cycle the flow of energy and nutrients from a productive base of self-feeding autotrophs (Pimm et al. 1991; Odum and Barrett 2005; Benke 2010). Autotrophs and heterotrophs come in all sizes, from microscopic to many tonnes - from cyanobacteria to giant redwoods, and from viruses to blue whales.

A gradient exists between trophic levels running from complete autotrophs that obtain their sole source of carbon from the atmosphere, to mixotrophs (such as carnivorous plants) that are autotrophic organisms that partially obtain organic matter from sources other than the atmosphere, and complete heterotrophs that must feed to obtain organic matter. 

There are different kinds of feeding relations that can be roughly divided into herbivory, carnivory, scavenging and parasitism. Some of the organic matter eaten by heterotrophs, such as sugars, provides energy. An often overlooked but key component of food webs are the decomposers. 

Not Your Average Food Web: Wasp-Waist Ecosystems \(\PageIndex{2}\)

Food webs can be controlled by top-down mechanisms (predator abundance determines the abundance of lower trophic levels), bottom-up mechanisms (primary producer abundance determines the abundance of higher trophic levels), or a combination of both. In wasp-waist food webs, population dynamics are controlled by planktivorous lower trophic level species such as sardine, anchovy, and small squids rather than the bottom or the top (Cury et al. 2011). These lower trophic level species often have high abundance but low diversity. The term “wasp-waist” describes the shape of these food webs, with many species existing at lower trophic levels (i.e., the plankton) and at higher trophic level (i.e., the predators), but very few lower trophic level species linking the plankton and the predators. These lower trophic level species exert top-down control on zooplankton and bottom-up control on top predators, with environmental factors largely affecting their abundance (Cury et al. 2000; Cury et al. 2003). Wasp-waist ecosystems are highly vulnerable to collapse when forage fish decline due to the critical energetic links that they provide between highly available zooplankton and larger predators (Shannon 2000). 

A diagram showing the structure of a wasp-waist model for the California Current Large Marine Ecosystem. Arrows indicate inputs of a trophic group to another. Figure modified from Madigan et al. 2012.

Decomposers, Detritivores, and Scavangers

Decomposers are organisms that break down dead or decaying organisms; they carry out decomposition, a process possible by only certain kingdoms, such as fungi (NOAA 2014). Like herbivores and predators, decomposers are heterotrophic, meaning that they use organic substrates to get their energy, carbon and nutrients for growth and development. 

Figure 4: Fungi are the primary decomposers in most environments, illustrated here Mycena interrupta . Only fungi produce the enzymes necessary to decompose lignin, a chemically complex substance found in wood.

Detritivores (also known as detrivores , detritophages , detritus feeders , or detritus eaters ) are heterotrophs that obtain nutrients by consuming detritus (decomposing plant and animal parts as well as feces) (Wetzel 2001). There are many kinds of invertebrates, vertebrates and plants that carry out coprophagy. By doing so, all these detritivores contribute to decomposition and the nutrient cycles. Detritivores are usually arthropods and help in the process of remineralization. 

Plant tissues are made up of resilient molecules (cellulose, chitin, lignin and xylan) that decay at a much lower rate than other organic molecules. Detritivores perform the first stage of remineralization, by fragmenting the dead plant matter, allowing decomposers to perform the second stage of remineralization (Keddy 2017). The activity of detritivores are the reason why we do not see an accumulation of plant litter in nature (Keddy 2017; Sagi et al. 2019). 

While the terms decomposer and detritivore are often interchangeably used, detritivores ingest and digest dead matter internally, while decomposers directly absorb nutrients through external chemical and biological processes (Keddy 2017). Thus, invertebrates such as earthworms, woodlice, and sea cucumbers are technically detritivores, not decomposers, since they must ingest nutrients - they are unable to absorb them externally (Sagi et al. 2019). 

Detritivores are an important aspect of many ecosystems. They can live on any type of soil with an organic component, including marine ecosystems, where they are termed interchangeably with bottom feeders. Typical detritivorous animals include millipedes, springtails, woodlice, dung flies, slugs, many terrestrial worms, sea stars, sea cucumbers, fiddler crabs, and some sedentary polychaetes such as worms of the family Terebellidae.

Scavengers are animals that consume dead organisms that have died from causes other than predation or have been killed by other predators (Tan and Corlett 2011). While scavenging generally refers to carnivores feeding on carrion, it is also a herbivorous feeding behavior (Getz 2011). Scavengers play a fundamental role in the environment through the removal of decaying organisms, serving as a natural sanitation service (Ogada et al. 2011). While microscopic and invertebrate decomposers break down dead organisms into simple organic matter which are used by nearby autotrophs, scavengers help conserve energy and nutrients obtained from carrion within the upper trophic levels, and are able to disperse the energy and nutrients farther away from the site of the carrion than decomposers (Olson et a. 2016). Decomposers and detritivores complete this process, by consuming the remains left by scavengers. Scavengers are not typically thought to be detritivores, as they generally eat large quantities of organic matter. 

Decomposers are often left off food webs, but if included, they mark the end of a food chain (Hutchinson 2013). Thus food chains start with primary producers and end with decay and decomposers. Since decomposers recycle nutrients, leaving them so they can be reused by primary producers, they are sometimes regarded as occupying their own trophic level (Kane et al. 2016; Pahl and Ruedas 2021). 

Not Your Average Food Web: Detrital Web \(\PageIndex{3}\)

Detritus is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts communities of microorganisms that colonize and decompose (i.e. remineralize) it. In terrestrial ecosystems it is present as leaf litter and other organic matter that is intermixed with soil, which is denominated "soil organic matter". The detritus of aquatic ecosystems is organic material that is suspended in the water and accumulates in depositions on the floor of the body of water; when this floor is a seabed, such a deposition is denominated "marine snow".

Earthworms are soil-dwelling detritivores.

Two Adonis blue butterflies lap at a small lump of feces lying on a rock.

In a detrital web, plant and animal matter is broken down by decomposers, e.g., bacteria and fungi, and moves to detritivores and then carnivores (Gönenç et al. 2007). There are often relationships between the detrital web and the grazing web. Mushrooms produced by decomposers in the detrital web become a food source for deer, squirrels, and mice in the grazing web. Earthworms are detritivores that consume decaying leaves and are then consumed by a variety of wildlife, especially birds.

Trophic Levels 

Figure 7: Food web diagram showing the various ways in which organism roles can be differentiated. Developed by N. Gownaris. 

The trophic level of an organism is the position it occupies in a food web. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. The trophic level of an organism is the number of steps it is from the start of the chain. A food web starts at trophic level 1 with primary producers such as plants, can move to herbivores at level 2, carnivores at level 3 or higher, and typically finish with apex predators at level 4 or 5. The path along the chain can form either a one-way flow or a food "web". Ecological communities with higher biodiversity form more complex trophic paths. The word trophic derives from the Greek τροφή (trophē) referring to food or nourishment (merriam-webster.com, 2017). 

Trophic levels can be represented by numbers, starting at level 1 with plants. Further trophic levels are numbered subsequently according to how far the organism is along the food chain.

• Level 1: Plants and algae make their own food and are called producers.
• Level 2: Herbivores eat plants and are called primary consumers.
• Level 3: Carnivores that eat herbivores are called secondary consumers.
• Level 4: Carnivores that eat other carnivores are called tertiary consumers.
• Apex predators by definition have no predators and are at the top of their food web.

Figure 8: Examples of species found at each trophic level of a terrestrial ecosystem. 

The trophic level concept was introduced in a historical landmark paper on trophic dynamics in 1942 by Raymond L. Lindeman. The basis of trophic dynamics is the transfer of energy from one part of the ecosystem to another (Odum and Heald 1975; Cortés 1999). The trophic dynamic concept has served as a useful quantitative heuristic, but it has several major limitations including the precision by which an organism can be allocated to a specific trophic level. Omnivores, for example, are not restricted to any single level. Nonetheless, recent research has found that discrete trophic levels do exist, but "above the herbivore trophic level, food webs are better characterized as a tangled web of omnivores.” (Pauly et al. 1998). 

Figure 9: Killer whales (orca) are apex predators but they are divided into separate populations that hunt specific prey, such as tuna, small sharks, and seals.

The fisheries scientist Daniel Pauly sets the values of trophic levels to one in plants and detritus, two in herbivores and detritivores (primary consumers), three in secondary consumers, and so on. The definition of the trophic level, TL, for any consumer species is (Pauly and Palomares 2005)

\[ T L_{i}=1+\sum_{j}\left(T L_{j} \cdot D C_{i j}\right) \nonumber\]

where 

\[ T L_{j} \nonumber\]

is the fractional trophic level of the prey j, and 

\[ D C_{i j}  \nonumber\]

represents the fraction of j in the diet of i. That is, the consumer trophic level is one plus the weighted average of how much different trophic levels contribute to its food.

In the case of marine ecosystems, the trophic level of most fish and other marine consumers takes a value between 2.0 and 5.0. The upper value, 5.0, is unusual, even for large fish (Cortés 1999), though it occurs in apex predators of marine mammals, such as polar bears and orcas (Pauly et al. 1998). 

Not Your Average Food Web: Microbial Loop \(\PageIndex{4}\)

Simplified microbial food web in the sunlit ocean by Anders et al. is licensed under CC-BY-SA 4.0. Left side: classic description of the carbon flow from photosynthetic algae to grazers and higher trophic levels in the food chain. Right side: microbial loop, with bacteria using dissolved organic carbon to gain biomass, which then re-enters the classic carbon flow through protists. Based on DeLong & Karl (2005).

The microbial food web refers to the combined trophic interactions among microbes in aquatic environments. These microbes include viruses, bacteria, algae, heterotrophic protists (such as ciliates and flagellates) (Mostajir et al. 2015). Scientists have relatively recently begun to appreciate the importance of this microscopic food web to the functioning of higher trophic levels. 

In aquatic environments, microbes constitute the base of the food web. Single celled photosynthetic organisms such as diatoms and cyanobacteria are generally the most important primary producers in the open ocean. Many of these cells, especially cyanobacteria, are too small to be captured and consumed by small crustaceans and planktonic larvae. Instead, these cells are consumed by phagotrophic protists which are readily consumed by larger organisms. Viruses can infect and break open bacterial cells and (to a lesser extent), planktonic algae (a.k.a. phytoplankton). Therefore, viruses in the microbial food web act to reduce the population of bacteria and, by lysing bacterial cells, release particulate and dissolved organic carbon (DOC). DOC may also be released into the environment by algal cells. The microbial loop describes a pathway in the microbial food web where DOC is returned to higher trophic levels via the incorporation into bacterial biomass.

Ecological Pyramids

Figure 10: Illustration of a range of ecological pyramids, including top pyramid of numbers, middle pyramid of biomass, and bottom pyramid of energy. The terrestrial forest (summer) and the English Channel ecosystems exhibit inverted pyramids. Note: trophic levels are not drawn to scale and the pyramid of numbers excludes microorganisms and soil animals. Abbreviations: P=Producers, C1=Primary consumers, C2=Secondary consumers, C3=Tertiary consumers, S=Saprotrophs (Odum and Barrett 2005).  

Ecological trophic pyramids are typically one of three kinds: 1) pyramid of numbers, 2) pyramid of biomass, or 3) pyramid of energy (Odum and Barrett 2005). In a pyramid of numbers, the number of consumers at each level decreases significantly, so that a single top consumer, (e.g., a polar bear or a human), will be supported by a much larger number of separate producers. There is usually a maximum of four or five links in a food chain, although food chains in aquatic ecosystems are more often longer than those on land. Eventually, all the energy in a food chain is dispersed as heat (Odum and Barrett 2005).

Ecological pyramids place the primary producers at the base. They can depict different numerical properties of ecosystems, including numbers of individuals per unit of area, biomass (g/m2), and energy (k cal m−2 yr−1). The emergent pyramidal arrangement of trophic levels with amounts of energy transfer decreasing as species become further removed from the source of production is one of several patterns that is repeated amongst the planet’s ecosystems (Pimm et al. 1991; Raffaelli 2002; Proulx et al. 2005). The size of each level in the pyramid generally represents biomass, which can be measured as the dry weight of an organism (Rickleffs 1996). Autotrophs may have the highest global proportion of biomass, but they are closely rivaled or surpassed by microbes (Whitman et al. 1998; Grommbridge and Jenkins 2002). 

Pyramid structure can vary across ecosystems and across time. In some instances biomass pyramids can be inverted. This pattern is often identified in aquatic and coral reef ecosystems. The pattern of biomass inversion is attributed to different sizes of producers. Aquatic communities are often dominated by producers that are smaller than the consumers that have high growth rates. Aquatic producers, such as planktonic algae or aquatic plants, lack the large accumulation of secondary growth as exists in the woody trees of terrestrial ecosystems. However, they are able to reproduce quickly enough to support a larger biomass of grazers. This inverts the pyramid. Primary consumers have longer lifespans and slower growth rates that accumulate more biomass than the producers they consume. Phytoplankton live just a few days, whereas the zooplankton eating the phytoplankton live for several weeks and the fish eating the zooplankton live for several consecutive years (Spellman 2008). Aquatic predators also tend to have a lower death rate than the smaller consumers, which contributes to the inverted pyramidal pattern. Population structure, migration rates, and environmental refuge for prey are other possible causes for pyramids with biomass inverted. Energy pyramids, however, will always have an upright pyramid shape if all sources of food energy are included and this is dictated by the second law of thermodynamics (Odum and Barrett 2005; Wang et al. 2009). 

Figure 11: A pyramid of biomass shows the total biomass of the organisms involved at each trophic level of an ecosystem. These pyramids are not necessarily upright. There can be lower amounts of biomass at the bottom of the pyramid if the rate of primary production per unit biomass is high.

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Ocean Food Chain

The ocean or marine food chain shows the relationship among the organisms living in the ocean. Since organisms live underwater, they differ from those dwelling in terrestrial environments. Food chains are different from one oceanic environment to another.

The marine biome is the largest worldwide, covering three-quarters of the Earth’s surface. About 15% of all the species living on Earth, containing almost 300,000 species, are marine dwellers. The marine ecosystem consists of a series of interconnected producers and consumers.

A typical example of the ocean food chain is sharks eating tunas, which eat small fish. The small fishes consume plankton and crustacean, which feed on the microscopic, single-celled organism.

Like terrestrial food chains, the primary ocean food chain also has different levels.

Level 1: Primary Producers (Photoautotrophs)

They form the foundation of the ocean food chain. Producers in the ocean food chain are mostly invisible, although they are great in numbers. They are one-celled organisms called phytoplanktons that cover the ocean’s upper layer. Some photoautotrophic bacteria capture the sun’s energy to produce food by photosynthesis . In the coastal areas, seaweeds and grasses also perform the same function.

Together these compounds play a significant role in producing food that sustains the entire ocean’s food chain. They also contribute more than half of the oxygen we breathe.

Level 2: Primary Consumers (Herbivores)

The second food chain level consists of groups that feed on photoautotrophs for food. In their larval stages, microscopic animals called zooplankton include jellyfish, crustaceans, and mollusks. Larger herbivores include larger fishes like surgeonfish, parrotfish, and green turtles.

Although small, herbivores are significant eaters in the food chain, only to be eaten by the succeeding food chain elements, the carnivores.

Level 3: Secondary Consumers (Carnivores)

The zooplankton of level two sustains a diverse group of small carnivores such as sardines, herring, and menhaden. Secondary consumers include larger carnivores such as octopuses, feeding on crabs and lobsters, and fishes feeding on invertebrates.

Although they successfully catch prey, they also fall prey to the animals in the next level of the food chain – the tertiary consumers also called the predators.

Level 4: Tertiary Consumers (Top Predators)

They reside at the top of the food chain of the ocean. Tertiary consumers are enormous and fast-moving animals well-adapted to catch their prey. Top predators generally have an extensive lifespan, higher generation time, and lower reproduction rates. Such organisms include sharks, tunas, dolphins, penguins, seals, and walruses.

Thus, protecting these groups of animals is extremely important as their numbers are often slow to rebound and can affect the balance of the entire food chain.

Although predators reside at the topmost level of the ocean food chain, they are not safe from the ultimate predators – humans.

Alternative Ocean Food Chains

The primary ocean food chain based on plant productivity constitutes the majority of organisms in such ecosystems. However, other marine ecosystems also exist entirely independent of sunlight. The primary producers of such ecosystems are chemoautotrophs that use chemical energy to prepare food. Chemoautotrophic bacteria in the seafloor of hydrothermal vent ecosystems are a classic example.

Food chains also vary from one oceanic environment to another. The weather and climate differ from one geographic location to another. Accordingly, there are five leading ocean food chains in the marine biome:

• Coral Reef Food Chain in the coral reefs is located mainly near the equator, where the water is warm, and the environment is tropical.
• Arctic Ocean Food Chain is found in the Arctic Ocean in the polar circle of the northern hemisphere. The weather there is cold enough even to reach the sub-zero level.
• Atlantic Ocean Food Chain is located in the Atlantic Ocean. It is the second-largest of all oceans in the world and is home to billions of marine organisms.
• Pacific Ocean Food Chain in the pacific is the largest ocean ecosystem in the world. Some of the areas of the ecosystem are the most productive in the world and home to thousands of species not found elsewhere.
• The Southern Ocean , popularly known as the Antarctic Ocean, encircles Antarctica. It is home to marine life, like whales, penguins, and seals.
• Marine Food Chain – Education.nationalgeographic.org
• Energy and Food Webs – Oceantracks.org
• What is the Ocean Food Chain? – Study.com
• Marine Food Chains – Flexbooks.ck12.org

Article was last reviewed on Friday, February 17, 2023

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Effects of climate change on oceanic food webs

Climate change has significant effects on oceanic food webs , which are complex networks of interactions between different species in the marine ecosystem. Rising temperatures, changes in ocean currents, altered nutrient availability, and ocean acidification are among the factors impacting oceanic food webs. These changes can have cascading effects throughout the trophic levels of the food web. For example, warmer ocean temperatures can cause shifts in the distribution and abundance of plankton, which are the foundation of the food web. This, in turn, can affect the availability of food for higher trophic levels , such as fish, marine mammals, and seabirds. Changes in ocean currents can influence the transport of nutrients, affecting primary productivity and the availability of prey for higher-level consumers . Ocean acidification can impact the survival and growth of shell-forming organisms , which are important food sources for many species. Overall, climate change can disrupt the balance and structure of oceanic food webs, leading to changes in species composition, distribution, and abundance, and potentially affecting the productivity and functioning of marine ecosystems.

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