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

Defensive symbionts (i.e., symbionts that protect hosts against natural enemies) are common in nature, infecting many arthropod species. Defensive symbioses likely arose from host-symbiont interactions that are costly for hosts and evolved over time to a facultative mutualism. Hence, the role symbionts play in protecting hosts against natural enemies might be one of the reasons for their successful spread in nature. Here, we conducted a meta-analysis to investigate the costs and benefits of hosting symbionts in arthropods. We gathered approximately 3,000 effect sizes from 226 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 their hosts. In the absence of natural enemies, symbionts caused little reduction in host fitness. Overall, protection was six times higher than the cost to hosts, indicating that symbionts provide high protection at low cost to hosts. We also found that natural enemies attacking hosts infected with symbionts have a significant reduction in their fitness. Moreover, the level of protection and cost for both hosts and natural enemies varies between host families, fitness components, and between natural and introduced symbionts (i.e., if the symbiont is natural of that species or if it was transinfected into a new species). Our results reveal a broad generality of protection at little cost for hosts, which may explain the successful spread of symbionts in wild arthropod populations.