Food Webs and Bioaccumulation

Chipmunk with acorn in its mouth
We all know that wildlife, including people, cannot survive without food.  Just think how tired and weak you become when you don’t eat enough during the day.  Our bodies convert the food we eat into usable energy allowing us to move around, and grow into healthy adults.  

The energy we receive from food can be traced back to the Sun.  As the Sun shines, it radiates light energy.  Plants absorb the light energy, convert it to sugars (photosynthesis) and make energy usable by plants as well as other wildlife.

Once plants convert light energy into food energy, things get interesting.  Animals, such as deer and rabbits, eat plants to absorb the energy, and then the deer and rabbits are eaten in turn by wolves and other predators.  The energy from the Sun moves its way through ecosystems by predators eating their prey. 

  • Producers: Plants are producers because they make usable energy from light.  They turn sunlight, carbon dioxide and water into sugar energy.  Producers are the foundation of the food web, because they provide the base energy needed by all wildlife.
  • Primary Consumers: Herbivores are primary consumers, because they receive their energy directly from plants. 
  • Secondary Consumers: Carnivores are secondary consumers, because they receive their energy by eating primary consumers.  Secondary consumers are predators.  An omnivore acts as both a primary and secondary consumer, because omnivores eat both animals and plants. 
  • Decomposer:  Decomposers get energy by breaking down dead plants and animals.  They are extremely important, because decomposers convert dead matter into energy and release nutrients that can be added back to soils and ecosystems.

What is a Food Web?
A food web is a diagram displaying how all the producers, decomposers and primary and secondary consumers interact in an ecosystem.  It shows how energy is transferred between species. 

A food web can be very simple - with one producer, consumer and decomposer- or a food web can be extremely complicated.  A food web of an entire woodland ecosystem becomes complex when you include every species from plants to insects and mammals. 

There Is More to a Food Web than Energy

When animals eat their prey, they consume more than just energy.  They also absorb all the chemicals and nutrients inside the prey.  For example, when you eat a banana you get energy from the banana, as well as the added benefits of potassium and vitamin A. 

Sometimes animals ingest pollutants that can become stored in their fat and tissues.  Human-caused pollution has added heavy metals, oil, and industrial and pharmaceutical chemicals to the environment.  Plants, fish and other species absorb these toxins, and as they are eaten by predators, the toxins are then absorbed into the predators’ tissues.  As the chain of predator and prey continues up the food web the toxins become more concentrated and move higher and higher up the food web.  The pollutants can have a disastrous effect on the food web and potentially kill species.

What happens when a Chemical is Added to the Food Web?
To explain the true impacts of chemicals on the food web, we’re going to use the real world example of mercury poisoning. 

Coal-fired power plants burn coal and release mercury into the atmosphere as a byproduct.  Over time, mercury falls to Earth through rain, snow and natural settling.  Rain carries the mercury to streams and rivers and it eventually settles in lakes and ponds. 

After mercury enters lakes and ponds, bacteria transform mercury into a more easily absorbed toxic substance called methylmercury.  Aquatic plants, bacteria and plankton absorb methylmercury from the surrounding water. 

It’s at this point that mercury becomes added to the food web.  Eventually, the contaminated plants, bacteria and plankton will be eaten by predators, such as fish.  The methylmercury toxins will move into the tissues of the fish and poison a new level of the food web. 

Magnifying Up the Food Web
Individual plants, plankton and bacteria only have a small amount of methylmercury.  The problem begins at the next level of the food web.  Fish don’t eat just one plankton or plant – they can eat hundreds or thousands of them!  All the mercury in each of the plankton or plants has now been eaten by a fish and absorbed into the fat and tissues.  After eating 100 plankton, the methylmercury in the fish is now 100 times what it was in the plankton! 

It doesn’t stop there.  The higher and higher up the food chain you go, the more food is necessary to maintain energy and activity. 

    • If a small fish eats 50 mercury contaminated plants.
    • And a large fish eats 100 small fish
    • And an eagle eats 100 large fish.

50X100X100 = 500,000  The concentration of mercury in the eagle is 500,000 times larger than it was in the plankton!!

The process that causes the concentration of a substance to increase as it moves up the food web is called bioaccumulation.  Methylmercury is a famous example of bioaccumulation, because mercury poisoning causes neurological disorders, reduced reproduction and even death in raptors and mammals.  People are susceptible to mercury poisoning by eating too much contaminated fish.  

Study the diagram to see how mercury bioaccumulates up this common food web. 

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