A trophic cascade is an ecological process in which a change at one trophic level of a food web triggers a chain reaction of changes on other levels, often affecting the entire ecosystem. A trophic level is the position that an organism occupies in a food chain, based on what it eats and what eats it.
Many trophic cascades are top-down, meaning that they begin with a change in a high trophic level such as by the loss of an apex predator or the addition of an invasive species. Such changes "cascade" down to the next lower trophic level and then to subsequent, still lower trophic levels.
Changes in predator species can affect prey not only by reducing their numbers but also by altering their behavior (for example, causing them to avoid certain areas or hide more). These changes in prey populations then affect how much they consume of the next level down, often plants or algae. As effects cascade through multiple trophic levels (i.e., levels in a food chain), they can lead to changes in species composition, nutrient cycling and biomass.
Trophic cascades can also be bottom-up, which are caused by changes in resources. For example, an increase in nutrients can boost plant or algae growth, which in turn supports more herbivores and, eventually, more predators.
A well-known example of a top-down cascade is the 1995 reintroduction of wolves into Yellowstone National Park in the Western United States. Wolves had largely been eliminated by the 1920s, leading to a major expansion in the elk population and a large increase in grazing on young trees and shrubs such as willow and aspen. This reduced the habitat and food for species such as beavers and some songbirds.
The reintroduction of wolves reduced elk numbers and changed elk behavior, causing them to avoid heavily grazed areas such as stream banks. This allowed shrubs and trees to recover. The resulting increase in vegetation provided resources for beavers, whose dam-building created ponds and wetlands, and improved conditions for fish, amphibians, aquatic insects and many bird species.