What is a trophic cascade?

A trophic cascade is ecological change that happens when a predator is removed from the food pyramid. The removal of a carnivore at the top of the pyramid affects prey animals and vegetation at the bottom. Many of these effects are unexpected, and biologists track and study them to learn how parts of an ecosystem connect. One example of a trophic cascade is the sea otter population in Alaska affecting kelp forests. Humans had hunted sea otters nearly to extinction by the early 1900s. When there were only a few left, scientists realized that the otters had been helping to keep kelp forests healthy. The otters ate the sea urchins that ate the kelp. With no otters, there were too many sea urchins, and the urchins ate too much kelp and depleted an important habitat for fish and other marine organisms. Another example involves gray wolves in Yellowstone National Park. The wolves were killed off in the 1930s. This indirectly led to a huge decline of beavers in the park over the next decades: the wolves preyed on elk, and the elk ate the same plants that beavers ate. When there were no wolves, the elk ate too many willow trees, and the beavers couldn’t find enough food in the winters. When wolves were reintroduced in 1995, everything changed again, and now there are about 10 healthy beaver colonies in the park. As these examples show, trophic cascades are often caused by humans. Overharvesting or overfishing is one way this happens: humans hunt or fish too much and substantially reduce the population of a predator in an area. Other causes include natural disasters like forest fires and extreme weather. Disease outbreaks can also cause cascades in an ecosystem.
By studying trophic cascades, scientists learn how to better manage ecosystems and work with conservationists to create ecological resilience.