There has been long-standing concern that increasing connectivity between habitat patches could have the negative effect of allowing diseases to more easily spread. Most of this concern has been proven unfounded, although the complexity of disease transmission means there are still many unknowns about how connectivity may help or hinder pathogens.
Throwing more evidence into the debate is a new paper by Heard et al. in Ecology Letters, which examines the spread of chytridiomycosis (caused by the fungus Batrachochytrium dendrobatidis) in endangered growling grass frogs (Litoria raniformis) of Australia. Chytrid is a facultative pathogen, meaning that infection can come from other individuals that act as reservoir hosts, or from the environment, where spores can hang out for months. Is it possible that managing for connectivity in these frogs also allows chytrid to spread more easily?
In looking at how chytrid prevalence and wetland connectivity affect the probabilities of colonization and population persistence across 214 wetland sites, the answer appears to be no. The authors found that that preserving connectivity was beneficial, both in terms of population persistence and recolonization. In essence, the authors did not find a tradeoff between the spread of chytrid by migrants and metapopulation viability. The reason is that chytrid travels on many other frog species, through connecting waters and on other animals. Hence, growling grass frogs aren’t a major source of infections for each other, and migration rates between populations don’t appear to be related to the risk of epidemics.
Importantly, Heard et al. found that the major risk factors for chytrid were wetland attributes that define environmental suitability for chytrid. In the focal system, wetlands were found to varying substantially in water temperature regime and salinity, both of which are known to affect the pathogenicity of chytrid. The authors found that growling grass frogs were much more likely to persist in warm and slightly saline wetlands which are unfavorable for chytrid. Warm and slightly saline wetlands therefore provide natural refuges from disease.
The main message is that providing environmental refugia for frogs may significantly increase their likelihood of persisting in the face of chytrid, and enabling migration and connectivity should only be beneficial for metapopulation persistence when the pathogen has other pathways for transmission. Maintaining connectivity and environmental heterogeneity between patches therefore remain key management strategies for ensuring population persistence.
Heard, G. W., C. D. Thomas, J. A. Hodgson, M. P. Scroggie, D. S. L. Ramsey, and N. Clemann. 2015. Refugia and connectivity sustain amphibian metapopulations afflicted by disease. Ecology Letters 18(8): 853-863.
Hess, G. 1996. Disease in metapopulation models: implications for conservation. Ecology 77(5): 1617-1632.