Indirect commensalism promotes persistence of secondary consumer species

Abstract

Local species extinctions may lead to, often unexpected, secondary extinctions. To predict these, we need to understand how indirect effects, within a network of interacting species, affect the ability of species to persist. It has been hypothesized that the persistence of some predators depends on other predator species that suppress competitively dominant prey to low levels, allowing a greater diversity of prey species, and their predators, to coexist. We show that, in experimental insect communities, the absence of one parasitoid wasp species does indeed lead to the extinction of another that is separated by four trophic links. These results highlight the importance of a holistic systems perspective to biodiversity conservation and the necessity to include indirect population dynamic effects in models for predicting cascading extinctions in networks of interacting species.

Figure 1.

Population dynamics in the experimental insect communities. (a) Compositions of the three experimental community treatments (seven replicates of each). (b–e) Densities of the four insect species during the eight weeks of the experiment in treatments with both parasitoid species (purple diamonds), L. fabarum only (red circles) and Aphidius ervi only (blue triangles). Values are the means of the seven replicates in each treatment and error bars represent ± s.e.m.

Figure 2.

Persistence of experimental parasitoid populations. (a) L. fabarum in the presence (purple diamonds) and absence (red circles) of Aphidius ervi. (b) Aphidius ervi in the presence (purple diamonds) and absence (blue triangles) of L. fabarum.