Defensive symbionts mediate species coexistence in phytophagous insects

1. Competition of two species for the same resource is expected to result in competitive exclusion of the inferior competitor. In natural communities, however, other antagonists and symbionts moderate competition. Thus we have to go beyond studying pairwise interactions. 2. Natural enemies may facilitate coexistence if they affect the superior competitor more strongly, or they can hinder coexistence via apparent competition. Less well studied is the role of symbionts, which may influence species coexistence in conjunction with enemies. 3. Eukaryotes commonly harbor microbial endosymbionts that provide protection against natural enemies, but are costly in their absence. Such defensive symbionts could thus mediate coexistence of species competing for the same resource, both in the presence and in the absence of enemies, but as yet there is little evidence for this claim. 4. We addressed this proposed role of defensive symbionts in replicated simple communities consisting of two aphid ...