cases “the big fish eats the little fish” to indicate that the will of the strongest normally prevails in life. But an ecologist would argue that this is essential in order to regulate the population of the small fish. His argument of course strictly refers to ecosystem interactions.
Ecosystems are carefully structured so that balance can be reached through a multitude of interactions between species that are part of different trophic levels, or in other words that serve different functions in the food chain. A predator for instance has an impact on their prey, both in terms of their population and behaviour. But is also appears that in many cases the impact of that predator cascades down the food chain or other tropic levels. This is what scientists are now calling the phenomenon of “trophic cascade”, a term coined by ecologist Robert T. Paine in the 1980’s¹,². Originally thought to be rare, trophic cascades are now understood to occur across diverse terrestrial, freshwater and marine ecosystems as well as in both a bottom-up and a top-down way. Trophic cascade phenomena do not only control species composition of an ecosystem but also biomass, and production of herbivores and plants. For example, overfishing of cod and other commercially exploited fish in the North Atlantic Ocean has led to an increase in small pelagic fish which would have otherwise been consumed by cod as well as snow crab and shrimp. This in turn led to decreased populations of large-bodied herbivorous zooplankton, which are consumed by small pelagic fishes, which then led to an increase in the phytoplankton.
While research is ongoing to fully understand these functions there are a number of well-established examples that illustrate the concept of a trophic cascade and reveal the various links that exist between species and plants; many of those links are far from evident and quite surprising!
Trophic cascade and wolves from Yellowstone
One of the most well-documented cases of trophic cascade is that of the Yellowstone National Park where wolves were re-introduced after having been absent for 70 years. Scientists have been closely monitoring their re-introduction and have identified a number of positive effects wolves have had to the whole ecosystem.
The presence of wolves in Yellowstone has had a direct impact on reducing prey numbers and density as well as altering prey behaviour. More specifically, elk have become wary of wolves and move faster and graze for shorter periods of time in the same area – this means that they browse on different species of vegetation and at different intensity. This has meant that local vegetation is under much less grazing pressure from elk. For the first time, streambank vegetation such as willow and aspen is regenerating after decades. The restored vegetation has meant that habitat for native birds, fish, beaver and other species is now available once more. Wolves have also had a positive impact by reducing the numbers of coyotes and the impact they have on other species such as pronghorn. In some areas, wolves have contributed to reductions in Yellowstone’s coyote population by as much as 50% which has radically increased pronghorn rates from 20% to 70%³.
The trophic cascade phenomenon serves to illustrate the detrimental impact that a lot of human activities can have on ecosystems. For example, hunting, overfishing and changes in land use can radically change the balance of an ecosystem in ways that are not easy to predict and therefore not possible to account for in advance.