Trophic Cascades - Final

Trophic cascades occur when a change in number of top predators affects the density/behaviour of prey, thus shaping the survival of the next trophic level.

I hope that my blogs have been informative and given a better understanding of the connectivity between, well, everything. It just shows how coevolution has such an important role on what we see today. And as everything is connected, we cannot dismiss individual species – for their size, lack of beauty or even because we are afraid of them, such as many top predators. Indeed, our fear is threatening many shark species, and our greed the wolves. We must remember that our actions are not only affecting the abundance of these populations, but every biotic and abiotic factor linked to them as well.

 

Howling for Justice (2014)

Keystone species like Otters and ecosystem engineers like the ingenious beaver and humans also hold the balance of an ecosystem in their own abundance.

I still get excited about the extent of the impact one organism can have on entire ecosystem – they can change rivers and even influence the carbon cycle.

There are many aspects of trophic cascades that are still yet to be fully understood, but it is a popular topic in the literature. As an aspiring conservationist, I was really interested in how we can use current knowledge of trophic cascades to predict the outcomes of certain actions, and thus develop ways to prevent extinctions, as well as ‘rewilding’ already altered ecosystems.

I hope you enjoyed this blog as much as I have.

How to predict the magnitude of trophic cascades

Based on a study conducted by Borer et al. (2005) called What determines the strength of a trophic cascade?

Certain biological characteristics can be used to predict the strength of a trophic cascade. Predator taxonomy, thermal regulation and herbivore mass-specific metabolic rates describe 31% of the variation in cascade strength. There have been many studies exploring the best indicators for trophic cascade strength. The conclusions in this blog were based on a compilation of 114 studies, and seven different ecosystems. It seems the prime indicator of trophic strength is the biological/behavioral characteristics of the herbivores – grazing efficiency and high predation vulnerability. The efficiency of herbivores and even predators also has a significant effect on trophic cascade strength. The studies show that invertebrate herbivores create stronger cascades than vertebrate herbivores and invertebrate predators. While all of these factors are generally good indicators, plant and herbivore defenses can reduce herbivore efficiency and thus reduce the strength of cascades. It is uncertain whether diversity influences cascade strength. Diversity within trophic levels may limit trophic cascade strength due to compensatory dynamics, but diversity at each of the three levels was not related to cascade intensity. It is believed that the studies could not accurately determine diversity effects because there were not enough subsets of the community to replicate dynamic relationships. Species richness and system productivity appear to have no effect on cascade strength. Theoretically, factors relating to plant palatability (e.g. plant productivity, nutritional quality, turn over rates of plants, etc.) should contribute to cascade strength. These studies give a very good idea of some of the factors that can be used to predict trophic cascade strength, but there are still some unanswered questions. Therefore, of course, this is a topic that needs further studies to clarify some ideas.

E. T. Borer, E. W. Seabloom, J. B. Shurin, K. E. Anderson, C. A. Blanchette, B. Broitman, S. D. Cooper, and B. S. Halpern 2005. ‘What determines the strength of a trophic cascade?’, Ecology, vol.86, pp. 528–537.

Trophic cascade alters ecosystem carbon exchange

The scientific report ‘Trophic cascade alters ecosystem carbon exchange’ by Strickland et al. (2013) explores the possible effects of trophic cascades on the carbon cycle. 

The effects of cascades on biogeochemical cycles are largely unexplored. “We’re discovering that predators are having important effects on shaping the make-up of ecosystems,” says Dr. Oswald Schmitz (2013), “but we’ve not really spent a lot of time measuring how that translates into other functions like nutrient cycling and recycling.” 

The experiment tested whether predator and herbivore abundance and their effect on the abundance and physiology of plants, influenced carbon dynamics in plants – carbon storage, fixation and allocation. In a meadow ecosystem a spider (Pisaurina mira) and a grasshopper (Melanoplus femurrubrum) were the carnivore and herbivore species. 

Predators have a huge effect on carbon cycle (Schmitz 2013)

Strickland et al. (2013) hypothesised that 'carnivores should increase plant community carbon fixation and reduce respiration, thereby increasing carbon retention by causing herbivores to reduce their foraging impacts on plants’. The experiment showed that carnivores indeed increased carbon fixation and reduced carbon loss via respiration by plants. Thus the report accepted the hypothesis, concluding that trophic cascades do influence carbon dynamics in terrestrial ecosystems; the presence of carnivores result in the increase of carbon retention in plants. “It’s going to force some thinking about the vital roles of animals in regulating carbon,” concludes Dr. Schmitz (2013), pointing to the fact the UN’s body of scientific experts who study climate change don’t consider these multiplier effects in their models. “People are arguing for a paradigm change.”

Trophic cascades affect so many different aspects of ecosystems. I constantly wonder at the delicate balance in nature but am also impressed by the flexibility of ecosystems to adapt when the equilibrium is disturbed - though this flexibility is not limitless. 

References

Schmitz, O.J 2013, ‘Study shows dramatic effects of predators on carbon cycle’, RedOrbit, <http://www.redorbit.com/news/science/1112876639/effects-of-predators-on-carbon-cycle-061813/ >

Strickland, M.S, Hawlena, D, Reese, A, Bradford, M.A & Schmitz, O.J 2013, ‘Trophic cascade alters ecosystem carbon exchange’, pnas, vol.110, no.27, pp. 11035-11038.

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.