Biodiversity inhibits natural enemies: Broad evidence for the dilution effect
Human activities are dramatically reducing biodiversity, and the frequency and severity of outbreaks of harmful natural enemies (e.g., herbivores and parasites) in wildlife populations are increasing. These concurrent patterns have prompted suggestions that biodiversity and the success of natural enemies may be linked. For example, the dilution effect hypothesis posits that diverse host communities inhibit the spread of parasites through several mechanisms including regulation of susceptible host populations and interfering with parasite transmission, i.e., “encounter reduction”. Despite support in some systems, this hypothesis remains controversial, especially when applied to zoonotic diseases of humans. Still, a critical question remains: what is the evidence for the dilution effect across ecological systems? We addressed this question with a meta-analysis of 202 effect sizes on 61 parasite species and accounted for non-independence within parasite species and experiments.
Overall, we found strong evidence that host diversity inhibits parasite abundance. The strength of dilution effects did not differ between (a) parasites that infect humans vs. wildlife, (b) macro- vs. microparasites, (c) parasites with complex vs. direct lifecycles, (d) observational vs. manipulative studies, and (e) studies with fully resistant diluter species vs. (at least partially) susceptible ones. A second analysis incorporating 136 effect sizes on 39 herbivore species revealed similarly strong evidence for dilution effects in plant-herbivore systems. Although there can be exceptions, our results indicate that biodiversity generally decreases both parasitism and herbivory. These broad patterns suggest that further human-mediated reductions in biodiversity could exacerbate pest outbreaks and impair human health, species conservation, and the stability and function of ecosystems. However, these results also suggest that biodiversity conservation may yield a promising strategy to minimize pest outbreaks and mitigate these consequences.