On the insularity of islands

Abstract

We propose that islands arc “less insular” than is generally perceived. This belief results, in part, from the paucity of studies on vagrant species that exploit islands but are not permanent residents with continual breeding populations. We show, via aneedotal evidence extracted from the literature and data acquired ON Gulf of California islands, that visitors to insular systems are fairly common. We delineate three types of events beyond the bounds of current biogeographical analysis that make islands less insular: I) migrants and “accidental” visitors. 2) individuals of a species whose foraging areas encompass many islands or the mainland and islands, and 3) species who “colonize” islands during opportune periods or years but become extinct during difficult times (source‐sink situations). Such events potentially significantly affect the ecology and evolution of island inhabitants by such means as increased predation and/or competition, transport of parasites and pathogens, dispersal of seeds and eggs, and genetic introgression and hybridization. Discussion of other “insular” habitats such as freshwater lakes and wildlife refuges illustrate that vagrancy events may be nearly ubiquitous. Studies addressing the frequency and ecological and evolutionary significance of vagrants are required, especially in light of recent and rapid extinctions on islands and the increasing fragmentation of habitats.