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Insular species are predicted to broaden their niches, in response to having fewer competitors. They can thus exploit a greater proportion of the resource spectrum. In turn, broader niches are hypothesized to facilitate (or be a consequence of) increased population densities., We tested whether insular lizards have broader dietary niches than mainland species, how it relates to competitor and predator richness, and the nature of the relationship between population density and dietary niche breadth., We collected population density and dietary niche breadth data for 36 insular and 59 mainland lizard species, and estimated competitor and predator richness at the localities where diet data were collected. We estimated dietary niche shift by comparing island species to their mainland relatives. We controlled for phylogenetic relatedness, body mass and the size of the plots over which densities were estimated., We found that island and mainland species had similar niche breadths. Dietary niche breadth was unrelated to competitor and predator richness, on both islands and the mainland. Population density was unrelated to dietary niche breadth across island and mainland populations., Our results indicate that dietary generalism is not an effective way of increasing population density nor is it result of lower competitive pressure. A lower variety of resources on islands may prevent insular animals from increasing their niche breadths even in the face of few competitors. [ABSTRACT FROM AUTHOR]

Aim Islands organisms usually have fewer predator and competitor species than mainland ones. This is thought to result in high population densities on islands. We hypothesize that insular lizards have denser populations than mainland species and that density, in general, is negatively correlated with competitor and predator richness. Location Global. Methods We compared densities of 346 lizard species on islands and the mainland while examining the relationship between density and, predator and competitor richness, primary productivity, seasonality and island area. We controlled for phylogenetic non-independence, body mass and study area, which are known to strongly affect population density. Results Insular populations (especially on snake-free islands) are denser than mainland ones. Mainland populations of lizard species that also inhabit islands were denser than those of species that do not inhabit islands. Population density was the highest on islands with low net primary productivity and was not significantly affected by competitor or predator richness. Moreover, insular populations show high density regardless of island area. Main conclusions We conclude that the ability of mainland species to reach high population densities may increase their chances in reaching and successfully colonizing islands. We postulate that population density may be affected by predator and competitor density rather than by their richness. Density increase on islands may result not from the environmental simplicity of island faunas but through propagule sorting or pressure. [ABSTRACT FROM AUTHOR]

Aim 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. Location World-wide. Methods 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. Results 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. Main conclusions 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. [ABSTRACT FROM AUTHOR]