Wildlife Provisioning Selects for Higher Pathogen Virulence in Hosts with Incomplete Immunity

AbstractHumans provide massive inputs of food to wildlife, with profound ecological and evolutionary consequences. By potentially altering wildlife host immunity, density, and behavior, provisioning can influence transmission of wildlife pathogens and thus may impose strong selection pressure on pathogens. But surprisingly we lack theory on the eco-evolutionary consequences of provisioning for host-pathogen dynamics. Here we develop a mathematical model of the eco-evolutionary dynamics of a wildlife pathogen under provisioning, motivated by Mycoplasma gallisepticum, a bacterial pathogen that emerged, spread, and evolved higher virulence in provisioned house finches. We model how provisioning influences the evolution of pathogen virulence, defined here as the mortality increase associated with infection in identical background hosts. In our model, house finches recover from infection and acquire incomplete immunity; this incomplete immunity is stronger if their initial infection was with a more virulent pathogen strain (as previously found empirically). We find that even when provisioning improves individual host fitness (via survival, fecundity, or immune defenses), it should still select for higher pathogen virulence and thus may actually lead to declines in host populations. These negative effects arise because provisioning magnifies the impact of incomplete immunity, selecting for higher virulence and driving host populations down. Our results highlight that food provisioning can select for more virulent pathogens, with potentially far-reaching implications for conservation.