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Food Chains and Food Webs

Energy Pathways

So far, this module has mentioned several times that solar energy is the foundation of Earth's ecosystems. The previous page described how solar energy becomes part of the ecosystem through photosynthesis, which transforms solar energy into plant matter. But how does this energy continue to power the biosphere and cycle through ecosystems?

The answer is by eating! Organisms eat plants, other organisms eat those organisms, and so on, thereby allowing the movement of energy throughout the biosphere.

"When we die, our bodies become the grass, and the antelope eat the grass. And so we are all connected in the great circle of life." - Quote from Mufasa, Simba's father, in the Disney film The Lion King. Who knew that Mufasa was talking about energy pathways and trophic relationships!
"When we die, our bodies become the grass, and the antelope eat the grass. And so we are all connected in the great circle of life." - Quote from Mufasa, Simba's father, in the Disney film The Lion King. Who knew that Mufasa was talking about energy pathways and trophic relationships!

Food Webs

The movement of energy and nutrients throughout an ecosystem is typically portrayed through food chains and food webs. Food chains and food webs represent trophic relationships, or the feeding levels among organisms which make up energy pathways within an ecosystem.

  • Food Chains: an idealized, linear and uni-directional (one way) pathway depicting how energy flows in an ecosystem from producers (who make their own food) through to consumers (which eat producers), eventually ending in detritivores and decomposers (which break down organic matter)
  • Food Webs: a complex network of interconnected food chains with multidirectional branches

It is important to note that food chains and food webs are typically portrayed in an idealized, simplified way to make them more understandable. While some food webs are fairly simple, trophic relationships within ecosystems are generally highly complex. For instance, food chains show an idealized linear relationship between organisms, but consumers are often part of many different food chains. An organism that is a top consumer in some food chains may be prey to other consumers in others.

A simplified Antarctic Ocean food web. Phytoplankton (at the bottom) are the producers and use solar energy for photosynthesis. Krill and other herbivorous zooplankton are primary consumers which eat the phytoplankton, and are then consumed by secondary consumers like seals, squid, fish, penguins, Baleen whales, and other seabirds at the next trophic level. Tertiary consumers like sperm whales, orca, leopard seals, and humans consume the secondary consumers.
A simplified Antarctic Ocean food web. Phytoplankton (at the bottom) are the producers and use solar energy for photosynthesis. Krill and other herbivorous zooplankton are primary consumers which eat the phytoplankton, and are then consumed by secondary consumers like seals, squid, fish, penguins, Baleen whales, and other seabirds at the next trophic level. Tertiary consumers like sperm whales, orca, leopard seals, and humans consume the secondary consumers.

Trophic Levels

Organisms that share the same feeding level in a food chain have the same trophic level. Food chains generally have 3-6 levels, beginning with primary producers (plants) and ending with detritivores and decomposers, organisms that break down organic matter and the final link in idealized food chains.

Primary Producers

Primary producers, or autotrophs (self feeders) are the first level in a food chain. They make their own food from solar energy via photosynthesis. Primary producers are mainly plants in terrestrial ecosystems, and phytoplankton/algae in aquatic ecosystems.

Consumers

Consumers, or heterotrophs, are organisms that feed on organic matter (biomass) for energy. There are several trophic levels within the broader category of consumers.

  • Primary consumers: organisms that feed on producers (plants); generally considered herbivores (plant eaters)
  • Secondary consumers: organisms that mainly eat primary consumers; generally considered carnivores (meat eaters)
  • Tertiary consumers: organisms that eat both primary and secondary consumers; often called the 'top carnivore' in a food chain. For example, polar bears in the Arctic region eat bearded seals. Bearded seals are secondary consumers which eat fish and clams, which are in turn primary consumers eating phytoplankton and algae.
  • Omnivores: consumers that eat both producers (plants) and consumers (meat)

Detritivores and Decomposers

Detritivores and decomposers have similar functions within ecosystems, although they operate differently. Both of them play important roles for breaking down organic materials to release compounds and nutrients back into the ecosystem. They are thus both essential for the cycle of energy throughout the biosphere.

Detritivores are organisms that feed on detritus, or dead organic debris (e.g. fallen leaves, waste products, dead bodies) produced by living organisms. Examples of terrestrial detritivores include worms, mites, centipedes, snails, slugs, crabs, and termites. Bottom feeders are examples of aquatic detritivores.

Decomposers are organisms that digest organic debris outside of their bodies (as opposed to eating debris, as detritivores do). Through this process, they absorb and release nutrients back to the ecosystem. Bacteria and fungi are examples of decomposers. Any time you smell something rotting, you are observing decomposers at work - the "rotting" is the metabolic work of microbial decomposers in action!

Sunlight (1) shines on the microscopic algae (2) in the sea ice. The sea ice algae convert carbon dioxide (CO2) to sugars through a process called photosynthesis (3). The algae in the ice are eaten by small animals like copepods (4). Some of the algae in the ice are not eaten and melt out of the ice and sink into the water (5) where they can be eaten by the small animals (4) or whales (6). Some of the algae are heavy and sink to the sea floor (7) where they can be eaten by bottom-feeding animals such as isopods (8; which are crustaceans, like the copepods). The small animals in the water (4) and on the seafloor (8) might in turn be eaten by fish (9). The fish are the food choice of seals (10). Seals are a preferred food of polar bears (11), which inhabit the area and roam on top of the ice in search of food. Inuit hunters (12) are also part of this food web. While only one Inuit hunter is pictured here, Inuit hunters often work together. They share their catch among many families, and they are careful to not waste any part of the animal. If they take a larger animal, just one may give them enough food to feed their whole family through the winter. (Image and text sourced from "Ask A Biologist," Arizona State University: https://askabiologist.asu.edu/arctic-food-webs

Sunlight (1) shines on the microscopic algae (2) in the sea ice. The sea ice algae convert carbon dioxide (CO2) to sugars through a process called photosynthesis (3). The algae in the ice are eaten by small animals like copepods (4). Some of the algae in the ice are not eaten and melt out of the ice and sink into the water (5) where they can be eaten by the small animals (4) or whales (6). Some of the algae are heavy and sink to the sea floor (7) where they can be eaten by bottom-feeding animals such as isopods (8; which are crustaceans, like the copepods). The small animals in the water (4) and on the seafloor (8) might in turn be eaten by fish (9). The fish are the food choice of seals (10). Seals are a preferred food of polar bears (11), which inhabit the area and roam on top of the ice in search of food. Inuit hunters (12) are also part of this food web. While only one Inuit hunter is pictured here, Inuit hunters often work together. They share their catch among many families, and they are careful to not waste any part of the animal. If they take a larger animal, just one may give them enough food to feed their whole family through the winter. (Image and text sourced from "Ask A Biologist," Arizona State University Links to an external site.)

Biological Amplification

The relationships between trophic levels can concentrate the levels of certain chemical compounds as they cycle throughout the ecosystem, a process known as biological amplification or biomagnification. This is of particular concern for certain chemicals used in chemical pesticides. While some chemicals disperse fairly quickly in air or water, others are long-lived and stable. Some of these chemicals are soluble in the fatty tissues of consumers, with the result that they become increasingly concentrated at each higher trophic level.

A famous example of this process is DDT, a pesticide that was widely used in the United States after World War II. DDT was biomagnifying, particularly in birds. Its increasing concentration in fat tissues caused the eggshells of birds at higher trophic levels to thin, increasing hatchling mortality. In the 1970s, DDT was banned for agricultural use in the U.S. This ban is credited for saving certain bird species, such as the Brown Pelican and Peregrine Falcon, from extinction.

As a result of biological amplification, certain pollutants can actually pose greater threats to organisms higher up in the food web. In the Puget Sound region, orcas or 'killer whales' are threatened by high levels of contaminates including PCBs (polychlorinated biophenyls). These pollutants are stored in the whales' tissues and are believed to be a primary cause of declining resident orca popuations in the pacific Northwest.