From food chains to food webs

In the made-up meadow below, the mouse eats three things and is eaten by three, so taking it out leaves every other species with something to eat.

Why one chain is not enough

A food chain is easy to draw and easy to reason about, and OpenStax points out that this is exactly why scientists still use chains in mathematical models. The trouble is that real animals rarely eat just one thing. The opossum shrimp in Lake Ontario eats both algae and the small animals that eat algae, so it sits on two levels at once. Some animals are also eaten by several different predators. Draw all these links together and the chain turns into a web: a map of every feeding link in a community.

A food web gives a truer picture, but at a price. It needs far more data, and a big web, such as the one for the Amazon rainforest, cannot realistically be measured completely. Ecologists therefore use both: chains for clear reasoning about energy, webs for asking what happens when the community changes.

Taking a species out

The model above uses two simple rules. First, if a consumer loses every one of its foods, it is lost too; ecologists call this a secondary extinction. The rule is applied again and again, because one loss can cause another. Second, if a species loses every one of its predators, it is marked as released, meaning its numbers could grow quickly. The panel also lists species that lost only part of their food.

Try it. Take out the grass and only the grasshopper is lost, because it is the only animal that eats nothing else. The rabbit, mouse, deer and songbird lose part of their diet but survive on other food. Take out the hawk and the snake has no predators left. Now take out grass and shrubs together: the grasshopper, the rabbit and the songbird all go, and 7 of the 12 species remain. Animals with many food sources are much harder to knock out than specialists that depend on one.

Webs and stability

OpenStax uses two words for how an ecosystem copes with change. Resistance is the ability to stay at equilibrium despite a disturbance. Resilience is the speed with which it recovers afterwards. A web with many links can resist the loss of one species better, because other links carry the energy that used to flow through it. A forest fire is a natural disturbance; the ground is first taken over by grasses, then shrubs, and later trees, until the forest returns.

People push webs in the same way, but often faster: overfishing removes predators, fertiliser runoff feeds algae, and new species arrive on ships. In the last unit of this course you will see what an invader did to the rivers of the Mississippi basin.