Environmental pathogen reservoirs and habitat heterogeneity in a metapopulation

Background/Question/Methods

Many wildlife diseases are caused by pathogens that are able to persist – and remain infectious – for long periods of time in the environment. This persistence allows for the formation of environmental pathogen reservoirs that can play an important role in pathogen persistence and spread. In a metapopulation context, the importance of these reservoirs depends on patterns of host occupancy and movement, which are determined at least in part by patch quality. In particular, high-quality habitat patches might become contaminated with the pathogen, yet still attract colonists, thereby creating an infectious trap for the host. In this situation, low quality patches might serve as refuges that allow susceptible hosts to escape infection. The goal of this study is to investigate these possibilities using a theoretical stochastic patch occupancy model where individual patches vary in quality. This model was explored across a range of pathogen environmental longevities and patch quality variances to investigate how environmental pathogen reservoirs interact with habitat heterogeneity.

Results/Conclusions

The model assumes that high quality patches have lower extinction rates and higher colonization rates than low quality patches.  As a result, high quality patches have a greater capacity to support occupancy. Though some of that capacity does support susceptible occupants, high quality patches experience a greater number of infection events than low quality patches and spend a greater proportion of their occupied time infected. This suggests that high quality patches may serve as traps that attract and infect susceptible colonists, through either direct contact with infected colonists or through the environmental reservoir. In this context, low quality patches, which have high extinction and low colonization rates, are unable to support regular occupancy or serve as consistent susceptible refuges. At the metapopulation level, increasing the variance in the patch quality distribution increases the probability of a pandemic (the pathogen spreads through the entire susceptible host population) when environmental longevity is sufficient for the pathogen to persist. This suggests that, in addition to forming infectious traps that maintain the pathogen, the high quality patches in these high-variance metapopulations also facilitate its spread.