A food chain is a useful simplification, but in nature almost every species eats, and is eaten by, several others. A food web maps all these feeding links. Food webs reveal something food chains cannot: that a change in one species can ripple through the whole ecosystem, sometimes in surprising directions.
What you will learn
- How food webs are structured and why they matter
- What keystone species, ecosystem engineers and foundation species are
- The difference between top-down and bottom-up control
- How trophic cascades work, with real examples
From chains to webs
In a food web, each species is a node and each feeding link is a connection. Some species, called generalists, eat many things; others, called specialists, depend on one or a few food sources. The giant panda, which eats almost only bamboo, is an extreme specialist; the brown bear, which eats berries, roots, insects, fish and mammals, is an extreme generalist.
Food webs with many connections tend to be more resilient, because if one food source declines, consumers can switch to another. Specialists are more vulnerable to the loss of their food.
Keystone species
A keystone species has an effect on its ecosystem far larger than its numbers would suggest. The term comes from architecture: the keystone at the top of an arch holds the other stones in place, and removing it makes the arch collapse.
The ecologist Robert Paine coined the term after his experiments on rocky shores in Washington State in the 1960s. When he removed the predatory starfish Pisaster ochraceus, mussels took over the rocks and crowded out other species. Within a few years, the number of species in his study plots fell from 15 to 8.
Other keystone species include:
- Sea otters, which control sea urchins and protect kelp forests.
- African elephants, which knock down trees and keep savannas open.
- Beavers, which build dams that create wetlands for many other species (beavers are also ecosystem engineers).
- Fig trees in tropical forests, which fruit year-round and feed many animals when other food is scarce.
Ecosystem engineers and foundation species
- Ecosystem engineers physically create or modify habitats. Beavers, elephants, termites, earthworms, corals and prairie dogs are examples.
- Foundation species are abundant species that form the structure of a habitat, such as reef-building corals, kelp, mangroves and dominant trees.
Top-down and bottom-up control
- Bottom-up control: the abundance of organisms is set by the resources at the bottom of the food web. More nutrients mean more plants, which mean more herbivores and more predators.
- Top-down control: predators at the top regulate the abundance of the levels below them.
Most ecosystems experience both, but the balance varies. Nutrient-poor ecosystems tend to be more bottom-up; ecosystems with effective top predators often show strong top-down effects.
Trophic cascades
A trophic cascade happens when a predator affects not only its prey but also the level below its prey. Because the effect skips a level, it alternates in direction: more predators mean fewer herbivores, which means more plants.
Sea otters and kelp forests
Along the North Pacific coast, sea otters eat sea urchins. Urchins graze kelp. Where fur hunters wiped out otters in the eighteenth and nineteenth centuries, urchin numbers exploded and turned kelp forests into bare “urchin barrens”. Where otters recovered, urchins declined and kelp forests regrew, bringing back fish, seals and seabirds. In the 1990s, killer whales in parts of Alaska began eating sea otters, apparently after their usual seal prey declined, and the cascade ran in reverse: fewer otters, more urchins, less kelp.
Wolves in Yellowstone
Wolves were eliminated from Yellowstone National Park by the 1920s. Without them, elk numbers rose and elk browsed heavily on willows and aspen along streams. Wolves were reintroduced in 1995–1996. Elk numbers fell and elk changed where they fed. In some areas willows and aspen have since recovered, providing food for beavers and shade for streams.
This story is often told as a dramatic transformation, but ecologists still debate how large and widespread the effect has been. Other factors, including drought, bears, cougars, human hunting outside the park and the loss of beavers, also shaped elk numbers and vegetation. Yellowstone is a good example of how a real trophic cascade is usually messier than the popular version.
Lions, leopards and baboons
In parts of sub-Saharan Africa where lions and leopards have declined, olive baboon numbers have increased. Baboons raid crops and eat smaller animals, and some studies have linked their rise to increased parasite loads in people living nearby. The loss of large predators can have effects that reach humans.
Why food webs matter for conservation
- Protecting top predators can protect whole ecosystems.
- Removing species can have effects that are hard to predict, including the release of smaller predators, known as mesopredator release. When coyotes declined in parts of California, for example, smaller predators such as foxes and domestic cats increased, and songbirds declined.
- Introducing species can disrupt food webs, as cane toads have in Australia and rats have on islands worldwide.
- Overfishing often removes top predators first, a process called fishing down the food web, leaving ecosystems dominated by smaller species such as jellyfish.
Try it yourself
Draw a food web for a habitat you know, with at least eight species. Choose one species and cross it out. Which other species would be affected directly? Which would be affected indirectly, two steps away?
Common misconceptions
- “Predators are bad for prey populations.” Predators often keep prey populations healthy by removing sick and weak individuals and preventing overgrazing.
- “Removing a predator always increases its prey.” Sometimes a smaller predator increases instead and has an even larger effect.
- “The Yellowstone wolves single-handedly restored the park.” Wolves were one important factor among several.
Key terms
- Food web: the network of feeding relationships in an ecosystem.
- Keystone species: a species with a disproportionately large effect on its community.
- Ecosystem engineer: a species that physically modifies its habitat.
- Trophic cascade: indirect effects of predators on levels below their prey.
- Mesopredator release: the increase of mid-sized predators when top predators decline.
Quick quiz
Test what you have learned. Scoring 60 percent or more marks this module as complete.
Further reading
- Estes, J. A. et al. (2011). “Trophic downgrading of planet Earth.” Science.
- Ripple, W. J. and Beschta, R. L. (2012). “Trophic cascades in Yellowstone: the first 15 years after wolf reintroduction.” Biological Conservation.