Positive Density Dependence Promotes Host Persistence in the Face of Infectious Disease.

Sociality offers benefits to species that can enhance their fitness. However, pathogen transmission can be higher in larger groups, potentially negating the advantages of group living. Despite the important paradoxical effects of population density on disease impacts and recovery, the competing effects of density remain unexplored. Here, we examine the response of a social bat species to pathogen invasion by comparing the effect of colony size on disease impacts during the summer (disease-free period) and winter (disease period). During pathogen invasion, larger winter colonies initially experienced relatively higher declines than smaller colonies. Conversely, summer colony size positively influenced colony growth immediately following pathogen invasion and during recovery, suggesting that Allee effects may be important in population resilience. Our results show that hosts faced with a novel pathogen may experience both benefits and costs of group living, and balancing these competing effects could impede evolutionary selection pressure toward asociality.