COS 234-1 - Infectious disease can decrease the success of demographic rescue

Abstract: Demographic rescue is the process by which adding individuals to a small population can reduce the population’s chance of extinction. These supplementation programs can reduce extinction because increasing the number of individuals in a population can help buffer it from stochastic fluctuations. However, the likelihood that supplementation is successful in rescuing a population may decrease if individuals added to the population are infected with a pathogen that reduces survival or fecundity. It is important to understand how supplementation impacts population size in the presence of infected individuals. Using a model system in ecology and evolution (Daphnia dentifera infected with the fungus, Metschnikowia bicusipdata), we performed a microcosm experiment introducing different levels of supplementation with or without infected individuals. We focused on answering the following questions: 1) Does supplementation reduce extinction and increase population size? 2) What is the overall demographic effect of supplementation when individuals added to populations are infected with a deadly pathogen? We found that when populations were initiated at very low densities, supplementation increased short-term population size even when a pathogen was introduced with supplemented individuals. However, once the pathogen established, supplemented populations with the pathogen present had lower abundance than populations that never received supplementation. This pattern persisted and there was no detectable difference in final abundance among uninfected populations (i.e., no difference across levels of supplementation), but supplemented populations with the pathogen had dramatically smaller population sizes than those without the pathogen. These results suggest that programs focusing on demographic rescue via supplementation can be successful, but the presence of pathogens can cause supplementation to have a negative overall effect on small populations.