Have you seen the YouTube video below, “How wolves change rivers”? This is a pretty amazing example of a trophic cascade: the affect that the top, apex predators can have on lower trophic levels, or vice versa. This example was mentioned by Professor Robert Warner from the University of California in his seminar at Macquarie University. Although Prof. Warner studies marine environments and the affects fishing can have on entire marine ecosystems, this terrestrial example emphasises the complexity of trophic cascades in both types of systems.
The human impact on marine environments is typically seen in the higher trophic levels, or the predator level3 (see diagram of trophic levels below). As Prof. Warner suggested, a trophic cascade can result from the removal of a predator from a system, which may lead to a change in the abundance of the lower trophic species1,3. However, predator loss can also influence the function of the lower trophic species, such as their behaviour3. If the behaviour of lower trophic species is changing, then what is happening to resources below this level?
Predators in all types of systems – freshwater, marine and terrestrial – can cause various prey responses1. For example, prey can alter their activity level and habitat use depending on whether predators are present or absent1. This is a kind of trade off1 – do they collect food or shelter from predators?
In his seminar, Prof. Warner focussed on prey feeding and movement when predators are increased or decreased in a marine ecosystem. In the presence of predators, prey use behavioural mechanisms such as avoidance and increased vigilance, leading to less feeding and changes in their diets4. Prof. Warner has found that prey weigh less in the presence of predators.
Furthermore, Madin, Gaines & Warner (2010) mention that the prey excursion distance (the distance to which the prey leave their shelter for food) can be affected by predator presence/absence. When a predator declines in a system, the prey have a kind of free range over an area, increasing their excursion rates4. The burning question is, would the system just ‘revert’ back to its pristine state when a predator is introduced back into the system?
Image retrieved from http://qetza.deviantart.com/art/School-of-Fish-Bus-252847804
The study by Madin et al. (2012), which was also discussed by Prof Warner, found this to be the case. Madin et al. (2012) compared various reefs on Line Islands and the Great Barrier Reef, with results indicating prey foraging behaviour returned to the pre-fishing state once predators recovered in the system3. This recovery actually occurred quite quickly3. Although the species studied were not representative of an entire fish ecosystem, the authors believe these results could be widespread – location wise and taxonomically3. These results could have significant implications for conservationists looking to restore marine systems to their pristine or near pristine state3.
The halo effect
It has been discussed that predators can affect the behavioural response of prey, but as Prof Warner discussed, does risk avoidance lead to changes in prey food distributions? According to Madin et al. (2012), the responses of prey to a loss of predators can cascade through the system, eventually affecting the distribution of macro-algae (primary producers)3.
In the presence of predators, herbivores will only graze in their immediate surroundings, leading to changes in distribution of algae and seagrasses – viewed as the halo effect2 (see image below). On the other hand, the absence of predators leads to a homogenous (more even) use of resources4. Hence, it may be common to assume that predator loss leads to an increase in herbivore densities, leading to a decrease in primary producers2. But is this always the case?
Image retrieved from http://www.1770reefcruises.com/
According to Prof. Warner, the predicted flow on effects can change once mesopredators are taken into account. Mesopredators are the prey that prey on smaller prey, particularly new recruits4. An increase in mesopredators will therefore see a decrease in recruits at the lower trophic levels, as well as change their behaviour4. Smaller species will avoid areas where mesopredators are present4. This then leads to an increase in the heterogeneity and abundance of macroalgae4, which can change the entire dynamics of a marine ecosystem.
Putting this into perspective
The complexity of marine ecosystems and the significance of behaviour responses is enormous. Prey behaviour is influenced by many factors – structure of habitat, food resources available, and composition of predators and grazers3. Understanding this type of complexity should have implications for current management of marine areas. This includes fishing practices as well as politically short-sighted government policies such as shark culls.
1. Madin, E.M., Gaines, S.D., & Warner, R.R. (2010). Field evidence for pervasive indirect effects of fishing on prey foraging behavior. Ecology, 91(12), 3563-3571. doi: 10.1890/09-2174.1
2. Madin, E.M., Gaines, S.D., Madin, J.S., & Warner, R.R. (2010). Fishing indirectly structures macroalgal assemblages by altering herbivore behavior. The American Naturalist, 176(6), 785-801. doi: 10.1086/657039
3. Madin, E.M., Gaines, S.D., Madin, J.S., Link, A.K., Lubchenco, P.J., Selden, R.L., & Warner, R.R. (2012). Do behavioral foraging responses of prey to predators function similarly in restored and pristine foodwebs?. PloS one, 7(3), 1-9. doi: 10.1371/journal.pone.0032390
4. Warner, R. (2014, March 19). Fear and longing: Predator change and the role of behaviour in marine conservation. BioSeminar. Conducted from Macquarie University, North Ryde, NSW.