Defensive insect symbiont leads to cascading extinctions and community collapse.

Animals often engage in mutualistic associations with microorganisms that protect them from predation, parasitism or pathogen infection. Studies of these interactions in insects have mostly focussed on the direct effects of symbiont infection on natural enemies without studying community-wide effects. Here, we explore the effect of a defensive symbiont on population dynamics and species extinctions in an experimental community composed of three aphid species and their associated specialist parasitoids. We found that introducing a bacterial symbiont with a protective (but not a non-protective) phenotype into one aphid species led to it being able to escape from its natural enemy and increase in density. This changed the relative density of the three aphid species which resulted in the extinction of the two other parasitoid species. Our results show that defensive symbionts can cause extinction cascades in experimental communities and so may play a significant role in the stability of consumer-herbivore communities in the field.

Plant-modified trophic interactions.

Plants can modify the interactions between herbivorous insects and their natural enemies in various ways. Chemical defences from the plants against herbivores may in fact harm the latter's natural enemies, thereby weakening the trophic interaction. On the other hand, volatile chemicals produced by the plant in response to herbivory may attract natural enemies, thereby strengthening the interaction. Recent research shows that effects of plants on insect interactions are not curious phenomena confined to a few specialist species but rather that they are ubiquitous in terrestrial ecosystems and often involve complex interactions among many species. The major challenge now is to study how the commonly reported short-term effects of plants affect long term dynamics of insect interactions in the context of complex natural communities.