The ‘island syndrome’ is an alternative state

AbstractAimIn the half‐century since publication of the Theory of Island Biology, ecologists have come to recognize the importance of predation as a decisive determinant of alternate states in many ecosystems. Island species are notorious for their vulnerability to introduced predators, yet the strength of island predator regimes has not been fully incorporated into our understanding of the forces that structure island consumer communities.LocationThe Greater and Lesser Antilles.TaxonBirds andAnolislizards.MethodsField surveys of sclerophyll and rainforest sites on islands ranging in size from 3.5 km2Terre‐de‐Haut to 76,000 km2Hispaniola.ResultsEvidence gathered in the 1970s and 1980s shows that Antillean anoles live at higher densities on fewer resources, grow more slowly, reproduce later and live longer than mainland counterparts in conformity with the ‘island syndrome’. Data from this period show that Antillean bird communities display density overcompensation, community saturation, size‐structured foraging guilds, low species diversity and low species packing, all traits consistent with the island syndrome and a regime of low predation and intense competition. Mainland species and communities display none of these features.Main conclusionsI propose that the island syndrome is an alternative state that distinguishes low‐predation island communities from high‐predation mainland counterparts. It follows that strong mainland predation regimes tend to prevent island species from colonizing. Conversely, invasion‐resistant, size‐structured island communities, despite low species diversity, prevent mainland species from colonizing islands. These predictions are experimentally testable withAnolislizards and, if confirmed, could set island biogeography on a new course.