The effect of island type on lizard reproductive traits
The origins of islands influence island colonization and radiation dynamics, thus exerting differential selection pressures on the species that inhabit them. The occurrence of lower numbers of predator and competitor species on islands than the mainland selects for ‘slow’ life-history attributes (the ‘island syndrome’). Animals colonizing, and radiating on, oceanic islands probably face more novel environments than do those inhabiting continental fragment and land-bridge islands. We hypothesized that oceanic island endemics will show the slowest life histories, whereas land-bridge island species will resemble mainland species the most. We predicted that species on old, small and isolated islands will also have slow life histories.
We assembled life-history data for 540 mainland and 319 insular endemic lizard species. We tested whether clutch size, brood frequency, hatchling mass and productivity differed between islands of different origin and between islands and the mainland. We controlled for female size, for latitude and for phylogenetic relationship using the R package caper. In addition, we tested the influences of island age, area and isolation on species life histories.
Oceanic island endemics have the smallest clutches and the largest offspring, and, together with continental fragment island endemics, lay most frequently. Clutch size, brood frequency and productivity increase with increasing island age. Isolation and area have little effect on lizard life history.
Our findings support the proposition that selection pressure differs across island type. The predator-poor environments on oceanic islands select for few, large offspring, while the predator-rich environments of the mainland and land-bridge islands select for many, small offspring. Island geological origin creates the environment within which evolution takes place, and thus plays a major role in life-history evolution. As islands grow older, lizards adapt by increasing their yearly reproductive effort.