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GROUP 8 PRESENTATION

• Costa, Carolina Lemes Nascimento; University of Campinas, Biology Institute, Brazil
• Leles, Suzana Gonçalves; Federal University of Rio de Janeiro, Graduate Program in Ecology, Brazil
• Maia, Kate Pereira; University of São Paulo, Department of Ecology, Brazil
• Vannucchi, Fabio Stucchi; São Paulo State University, São Vicente Campus, Brazil
• Mesquita Filho, Walter; University of São Paulo, Graduate Program on Entomology, Brazil
• Zamani, Maryam; Shahid Beheshti University, Department of Physics, Iran

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• Ayana Martins
• Juliana Berbert

Taken from http://www.nature.com/scitable/knowledge/library/trophic-cascades-across-diverse-plant-ecosystems-80060347

One example of trophic cascades occurs when predators limit the density and/or behavior of their prey and thereby enhance survival of the next lower trophic level. A simple example is depicted in Figure (1). Understanding trophic cascades is important to assess vulnerability of ecological communities and extinction risks.

Example of Figure (1) involves two trophic links. The present project will be based on an experiment where more species are present.

The main reference for you project is the recent paper by D. Sanders, L. Sutter and F. J. F. van Veen, The loss of indirect interactions leads to cascading extinctions of carnivores, which appeared in Ecology Letters, 16, pp. 664-669 (2013), available at http://onlinelibrary.wiley.com/doi/10.1111/ele.12096/abstract .

The authors present results from an experiment with insect communities consisting of a food plant, three aphid species, and three parasitoid wasp species. Each parasitoid is a specialist on one of the aphid species. Aphid species compete for the same plant species. The experiment consists in harvesting one of the parisitoid species. The main result is that harvesting of single carnivorous parasitoid species led to a significant increase in extinction rate of other parasitoid species, separated by four trophic links.

The baseline explanation for this is that harvesting one of the parisitoids released its prey, leading to stronger competition between aphid species, lowering the population of the other aphids. This resulted in increased extinction rates of non-harvested parasitoid species when their prey had become rare relative to other aphids. Harvesting one parasitoid lowered the abundance of other parasitoids.

But, wait! If you carefully read the Discussion section of the paper you will note that, although the broad view outlined above provides a rationale for the observed results, some details show that life is not so simple! Indeed, competitive exclusion of hosts, due to release from top-down control, did not always occur. As they state, in no case, any aphid population got extinct. Only in one case, one of the aphid population clearly declined. The paper advances some speculative reasons for this.

The main starting point is to model the system in order to undercover which mechanisms are compatible with the results. Furthermore, if you have time, you could study what happens when some of the assumptions are relaxed. For instance, what if one of the predators is a generalist? can this possibly alter the outcome of the experiments?

This work has been discussed in http://pegejournalclub.wordpress.com/2013/12/16/the-loss-of-indirect-interactions-leads-to-cascading-extinctions-of-carnivores/ and at http://www.sciencedaily.com/releases/2013/02/130228124144.htm.