Empirical Evidence for the Rescue Effect from a Natural Microcosm

SIMPLE SUMMARY: A widely assumed ecological process called the “rescue effect” holds that populations of organisms that receive immigrants from other populations should be less vulnerable to local extinction than populations that do not receive such immigrants. While sensible, the rescue effect has had little strong support from real-world populations because of the difficulty of testing this idea. Using a study system of plant-specialist frogs from the rainforests of Madagascar, this study provides evidence for the rescue effect as well as weaker evidence for a related (but opposite) process referred to as the “abandon-ship effect”. ABSTRACT: Ecological theory predicts that populations which receive immigrants are less vulnerable to extinction than those that do not receive immigrants (the “rescue effect”). A parallel but opposite process may also exist, where emigration increases the risk of local extinction (the “abandon-ship effect”). Using a natural microcosm of plant-specialist frogs from Madagascar, empirical evidence for both processes is provided. Populations receiving immigrants were less extinction-prone than those without immigration, and those populations losing individuals through emigration were more extinction-prone than those in which no emigration occurred. The number of immigrants and emigrants was also elevated and depressed (respectively) in patches that did not go extinct. These data provide some of the first definitive empirical evidence for the rescue effect and provide suggestive initial data on the abandon-ship effect. Both of these processes may be important to understanding the dynamics of populations.