Trophic cascades are powerful indirect interactions that can control entire ecosystems, occurring when predators in a food web suppress the abundance or alter the behavior of their prey, thereby releasing the next lower trophic level from predation (or herbivory if the intermediate trophic level is a herbivore). For example, if the abundance of large piscivorous fish is increased in a lake, the abundance of their prey, smaller fish that eat zooplankton, should decrease. The resulting increase in zooplankton should, in turn, cause the biomass of its prey, phytoplankton, to decrease.
The trophic cascade is an ecological concept which has stimulated new research in many areas of ecology. For example, it can be important for understanding the knock-on effects of removing top predators from food webs, as humans have done in many places through hunting and fishing.
A top-down cascade is a trophic cascade where the food chain or food web is disrupted by the removal of a top predator, or a third or fourth level consumer. On the other hand, a bottom-up cascade occurs when a primary producer, or primary consumer is removed, and there is a reduction of population size through the community. Predator removal can affect the ecosystem at either the population level (keeping the ecosystem functioning despite a shift in species composition) or the community level (in which case there is a risk of loss of ecosystem functioning and thus ecosystem service delivery).
Published on May 3, 2016
The short film opens with two questions: “So what determines how many species live in a given place? Or how many individuals of the species can live somewhere?”
The research that provided answers to these questions was set in motion by key experiments by ecologists Robert Paine and James Estes. Robert Paine’s starfish exclusion experiments on the coast of Washington state showed that removing starfish from this marine ecosystem has a big impact on the population sizes of other species, establishing the starfish as a keystone species. James Estes and colleague John Palmisano discovered that the kelp forest ecosystems of the North Pacific are regulated by the presence or absence of sea otters, which feed on sea urchins that consume kelp. These direct and indirect effects of sea otters on other species describe a trophic cascade. These early studies were the inspiration for hundreds of investigations on other keystone species and trophic cascades, as well as ongoing studies into the regulation of population sizes and species numbers.
The above four videos have been produced by Sustainable Human.