A dilution effect in the emerging amphibian pathogen Batrachochytrium dendrobatidis

Abstract

Global declines in biodiversity are altering disease dynamics in complex and multifaceted ways. Changes in biodiversity can have several outcomes on disease risk, including dilution and amplification effects, both of which can have a profound influence on the effects of disease in a community. The dilution effect occurs when biodiversity and disease risk are inversely related, whereas the amplification effect is a positive relationship between biodiversity and disease risk. We tested these effects with an emerging fungal pathogen of amphibians, Batrachochytrium dendrobatidis (Bd), which is responsible for catastrophic amphibian population declines and extinctions worldwide. Despite the rapid and continued spread of Bd, the influence of host diversity on Bd dynamics remains unknown. We experimentally manipulated host diversity and density in the presence of Bd and found a dilution effect where increased species richness reduced disease risk, even when accounting for changes in density. These results demonstrate the general importance of incorporating community structure into studies of disease dynamics and have implications for the effects of Bd in ecosystems that differ in biodiversity.

Fig. 1.

Average infection severity and prevalence of Bd with varying host density and species richness. There were three A. boreas-only treatments with two, three, or six A. boreas individuals and three mixed-species combinations with six individuals and either two or three species. Treatments are labeled on the x axis representing the number of individuals of each species indicated as “A” for A. boreas, “P” for P. regilla, and “R” for R. cascadae. (A and C) Points represent the average Bd infection severity (log + 1-transformed; ±SE) for A. boreas (A) and for all species present (C). (B and D) Bars represent (+SE) the percentage of A. boreas testing positive for infection (B) and the percentage infection for all species (D). For the 2A treatment, all replicates showed 100% infection. Averages for all figures were calculated with all Bd-exposed aquaria, and each treatment was replicated nine times.

Fig. 2.

Average total Bd infection by treatment. There were three A. boreas-only treatments with two, three, or six A. boreas individuals and three mixed-species combinations with six individuals and either two or three species. Treatments are labeled on the x axis representing the number of individuals of each species indicated as “A” for A. boreas, “P” for P. regilla, and “R” for R. cascadae. Points represent average total infection for Bd-exposed treatments (±SE). Only Bd-exposed treatments were used in calculating averages, and each treatment was replicated nine times.