In the1970s and 1980s, observations of “ecological release” were interpreted as evidence that interspecific competition is weak in island ecosystems relative to continental mainlands. However, Antillean lizards have been shown to live at higher densities on fewer resources, to grow more slowly, to reproduce later, and to live longer than mainland counterparts. Living at high densities should select for the ability to subsist on minimum resource levels, the definition of a superior resource competitor. Hutchinsonian size ratios of feeding guild members and low alpha vs. gamma diversity further support the conclusion that intra- and interspecific competition is strong in island communities and relatively weak in mainland communities, a circumstance I deem to be the consequence of reduced predation pressure on islands. Accordingly, I conclude that continental predator regimes prevent island species from invading mainlands, whereas mainland species are prevented from invading island communities by intense competition and associated low resource levels.