Defensive symbionts provide high protection against natural enemies at low cost to hosts: a meta-analysis

Defensive endosymbionts (i.e., endosymbionts that protect hosts against natural enemies) are widespread in terrestrial arthropods, especially insects. Protection against natural enemies may contribute to their successful spread in nature. However, the outcome of host-symbiont interactions (mutualism or parasitism) depend on the balance between the costs and benefits of the interaction and on the ecological conditions experienced by the host, such as the presence or absence of natural enemies. Here, we conducted a meta-analysis to quantitatively assess the costs and benefits of symbiont-mediated protection in terrestrial arthropods. We gathered approximately 3,425 effect sizes from 246 studies. Our results show that in the presence of natural enemies (i.e., hosts infected with pathogens or parasites), symbionts provide protection, positively affecting the fitness of hosts. In the absence of natural enemies, symbionts caused little reduction in host fitness. Overall, symbiont-mediated protection was 7.1 times stronger than the cost observed in the absence of natural enemies, indicating strong protection and little evidence of substantial fitness costs to hosts. Natural enemies attacking symbiont-infected hosts had substantial reduction in their fitness. Moreover, the level of protection and cost for both hosts and natural enemies varies between the fitness components, host families, symbiont species, and natural enemy group. Our results reveal a broad symbiont-mediated protection with little evidence of substantial costs associated, which may help explain the successful spread of symbionts in wild arthropod populations.