Demographic rescue falters when pathogens are present

As natural populations continue to decline globally, direct forms of
intervention are increasingly necessary to prevent extinction. One type of
intervention, known as demographic rescue, occurs when individuals are
added directly to a population to increase abundance and ultimately
prevent population extinction. However, the role of infectious disease in
demographic rescue remains unknown. To examine the effects of pathogens on
demographic rescue, we used a host-pathogen system with the aquatic
crustacean Daphnia dentifera as the host and the fungus Metschnikowia
bicuspidata as the pathogen. We constructed a randomized 3 x 2 factorial
experiment with three rescue treatments (none, low, high) and two pathogen
treatments (unexposed, exposed), where the pathogen was introduced via
infected individuals during rescue events. We found that adding more
individuals to demographically depressed populations increased abundance
over the short term; highly supplemented populations initially had 62%
more individuals than populations that had no introduced individuals.
However, by the end of the experiment, populations that did not have any
individuals introduced averaged 640% higher abundance than populations
where infected individuals had been added. Thus, the introduction of
infected individuals can result in worse demographic outcomes for
populations than if no rescue is attempted.