Effects of behaviour on trophic cascades

The presence or absence of top predators has the potential to trigger trophic cascades in two ways. The first trigger arises through direct prey on herbivores and the second trigger is the indirect effect a predator has on the behaviour of its prey. The second trigger is termed the anti-predator behaviour and has large impacts on ecosystems, and plant abundance (Schmitz, 1997). Anti-predator behaviour has evolved in many prey species in response to the presence of predators. Prey behaviour involve the adaptation of foraging strategies in order to reduce the risk of predation, whether it be reduced foraging time or even avoiding certain areas. While anti-predator behaviour reduces the risk of predation, there are some trade-offs which also effect the organism. Such trade-offs include reduced foraging and feeding times, resulting in a risk of starvation, life history alterations and influence of habitat selection. These behavioural traits in prey (often herbivores) have intense effects on primary producer populations (Schmitz 1997). Thus, changes in predator abundance influences prey/herbivore behaviour and thus primary producer populations. And hence we see a trophic cascade.

An experiment was conducted by Schmitz et al. (2004) to demonstrate the effect of prey anti-predator behaviour on trophic cascades. This experiment involved green crabs (Carcinus maenas) who feed on herbivorous Littorina snails. The crabs were placed in tubs with the snails. The tub also contained patches of algae. In presence of the crabs the snails minimised their feeding time. This study concluded that the effect of prey anti-predator behaviour are one of the two main drivers of trophic cascades (Schmitz 2004).

Sources

Schmitz, O.J, Beckerman, A.P & O’Brien, K.M 1997, ‘Behaviourally mediated trophic cascades: effects of predation risk on food web interactions’, Ecology, vol. 78, no. 5, pp. 1388-1399.

Schmitz, O.J, Krivan, V & Ovadiam, O 2004, ‘Trophic cascades: the primacy of trait-mediated indirect interactions’, Ecology Letters, vol. 7, pp. 153-163.