Defensive Symbionts Mediate Host–Parasite Interactions at Multiple Scales

In protection mutualisms, defensive symbionts protect their hosts from natural enemies, including parasites. Protection mutualisms were historically considered rare ecological relationships, but recent examples demonstrate that defensive symbionts are both quite common and diverse. Defensive symbionts can have surprisingly large effects on host and parasite ecology at the individual, population, guild, and community scales. However, the highly context dependent nature of protection mutualisms makes it difficult to identify and quantify the roles that defensive symbionts play in host-parasite systems. The mutualism-parasitism continuum framework can be used to understand and predict the outcomes of these interactions under variable environmental and ecological contexts. Embracing and expanding this theory will improve future research, and may better prepare us to use defensive symbionts as biocontrol agents.