COS 243-5 - Toward resolution in diversity-disease theory: an empirical investigation of mechanism and scale, and a whole-ecosystem manipulation

Abstract: Biodiversity can have complex effects on infectious disease. For instance, changes in community diversity can lead to changes in parasite populations via several interrelated (and challenging to disentangle) mechanisms. Moreover, the direction and magnitude of these changes can be scale-dependent and can yield different diversity-disease relationships depending on how disease is quantified. Hence, efforts to resolve diversity-disease theory will benefit from mechanistic approaches that explore multiple transmission processes and perspectives.

We comprehensively tested diversity-disease theory in an amphibian-trematode system by analyzing a long-term dataset and conducting an in-situ manipulation of biodiversity. Through analyzing 11 years of infection data from over 900 multi-host, multi-parasite communities, we assessed three diversity-linked mechanisms that may alter transmission: species richness, host density, and the density of predators that consume parasites. When considering risk of infection for individual hosts (i.e., taking the host’s perspective), we found that diversity generally decreased disease for all five amphibian hosts as they encountered all four trematode parasites. This negative diversity-disease association arose through the effects of species richness, with no influence of either host density or predator density. Conversely, when we considered a parasite’s probability of successful infection (i.e., taking the parasite’s perspective) we found that greater diversity increased or maintained transmission. This positive diversity-disease association arose from additive assembly patterns, where more species rich communities had greater total host density (and thus afforded greater opportunities for parasite transmission). We built on these findings with a whole-ecosystem manipulation in which we experimentally altered species richness. During the amphibian breeding season (before trematode transmission starts), we relocated newt egg masses from removal ponds to addition ponds to isolate the effects of species losses on transmission. Our manipulation successfully relocated 20,000 newt egg masses (over 100 kg of biomass) resulting in substantial changes to newt densities, with consequences for transmission. Collectively, our results help reconcile diversity-disease controversy by empirically testing how diversity-associated mechanisms alter parasite transmission, and by illustrating how these effects vary across biological scales.