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Density as a mechanism linking habitat disturbance to increased disease prevalence: evidence from a natural experiment

Barrile, Gabriel1; Chalfoun, Anna1; Walters, Annika1; Merkle, Jerod1

Published Oct 10, 2025 on Dryad. https://doi.org/10.5061/dryad.cfxpnvxgf

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Abstract

Sudden habitat loss associated with environmental disturbance can trigger animals to move from affected to undisturbed areas, where increases in local density may occur. Such increases in density can affect a number of ecological processes. Although pathogen transmission is strongly related to local density, how crowding after habitat loss affects infection dynamics in wild populations remains unclear. Here we conceptualize the Disturbance-Density-Disease (DDD) hypothesis, which posits that disturbance-induced habitat loss results in increased pathogen prevalence via increases in local density at adjacent, undisturbed patches. We then used empirical data from before, during, and after an extreme flooding event to test the DDD hypothesis in boreal toads Anaxyrus boreas boreas co-occurring with the pathogenic fungus Batrachochytrium dendrobatidis (Bd). We collected Bd samples from each captured individual during a 5-year (2015–2019) mark-recapture study of boreal toads (n = 1,295) that breed in beaver ponds in western Wyoming, USA. During spring of 2017, an extreme flooding event destroyed several beaver dams, resulting in the loss of breeding habitat. We compared host density and pathogen prevalence pre- and post-disturbance at sites affected versus unaffected by flooding. At affected sites, both population density and Bd prevalence increased at adjacent, undisturbed ponds following the sudden loss of habitat. Moreover, neither host density nor Bd prevalence increased at control sites that were in areas unaffected by flooding. Taken together, our results provide evidence that supports hypothesized links between disturbance, adjacent increases in density, and subsequent increases in pathogen prevalence. We thus demonstrate an important consequence of disturbance beyond proximate habitat loss, and introduce a clear conceptual approach (the DDD hypothesis) to understanding how pathogen transmission can be affected by disturbance via alterations to local density.