Humans change environments in many different ways, including accidentally or purposefully provisioning wildlife with novel food resources. For example, bird feeders, salt licks, ecotourism feeding stations, and dumpsters all provide concentrated food resources for wildlife. Does this food provisioning influence disease dynamics?
First, let’s consider how food provisioning might influence disease dynamics for parasites with any given transmission mode. The big rates that we care about here are the transmission rate (a function of contact rate and transmission success), birth and death rates, and immigration and emigration rates. If resource provisioning increases host population density via increased aggregation of individuals, increased birth rates or decreased death rates, and/or increased immigration or decreased emigration rates, then transmission rates for pathogens with direct contact density-dependent transmission should increase. In the same scenarios, pathogens with frequency-dependent transmission may not be affected by provisioning, or transmission rates may even decline if high birth rates dilute the prevalence of infection in the population. For pathogens that are transmitted via environmental stages, environmental stages may build up at resource provisioning sites when high densities of animals hang out there for long periods, increasing transmission. For pathogens that are transmitted via intermediate hosts, transmission may be reduced if hosts switch from foraging on intermediate hosts to foraging on human-provided resources.
Factors besides transmission mode might also be important. For instance, regardless of transmission mode, if resource provisioning increases host resistance – for instance, by increasing body condition – then transmission rates should decrease. Conversely, if resource provisioning decreases host resistance – for instance, if the provisioned food is nutritionally poor or there is high competition at sites with provisioning and body condition is reduced – then transmission rates should increase. Resource provisioning might also alter host tolerance to infection, so that hosts aren’t as sick but continue shedding infectious agents longer than they would if they didn’t have supplemental resources.
Ok, I think that covers most of the possibilities. Back to our question: does food provisioning influence disease dynamics? Yes, sometimes. Most of the mechanisms listed above were supported by at least one study in a recent review by Becker et al. (2015). Based on the long, messy list above, you can probably guess that sometimes food provisioning increases transmission, sometimes it decreases transmission, and sometimes nothing notable happens. The big take-home message is that there isn’t just one universal outcome when we provision wildlife with supplemental resources, and it’s important that we conduct more and better studies aimed at elucidating the epidemiological mechanisms underlying the observed relationships. Cool stuff!
Have you read this post about house finch conjunctivitis yet?
Becker, D.J., D.G. Streicker, and S. Altizer. 2015. Linking anthropogenic resources to wildlife–pathogen dynamics: a review and meta-analysis. Ecology Letters 18: 483–495.