Experimental evidence that host species composition alters host-pathogen dynamics in a ranavirus-amphibian assemblage

Losses in biodiversity can alter disease risk through changes in host
species composition. Host species vary in pathogen susceptibility and
competence. Yet how changes in diversity alter host-pathogen dynamics
remains unclear in many systems, particularly with respect to generalist
pathogens. Amphibians are experiencing worldwide population declines
linked to generalist pathogens, such as ranavirus, and thus represent an
ideal group to investigate how host species composition affects disease
risk. We conducted experiments where individuals in the laboratory and
assemblages of three amphibian species (Pacific tree frogs, Pseudacris
regilla; Cascades frogs, Rana cascadae; and Western toads, Anaxyrus
boreas) or just A. boreas alone in outdoor mesocosms were exposed to
ranavirus as larvae. In laboratory experiment, we observed low survival
and high viral loads in P. regilla compared to the other species
suggesting that this species was highly susceptible to the pathogen. In
the mesocosm experiment, we observed 41% A. boreas mortality when alone
and 98% mortality when maintained with P. regilla and R. cascadae. Our
results suggest that the presence of highly susceptible species can alter
disease dynamics across multiple species, potentially increasing infection
risk and mortality in co-occurring species.