Any observant individual has noticed and possibly even been astonished by the incredible densities that some insular anole populations (i.e. A. sagrei) can achieve. Islands necessarily create a unique combination of environmental factors, several of which have traditionally been suggested as reasons that insular species are capable of attaining such densities. Species richness tends to be quite low on islands and so the diversity of predators remains low and there are fewer other species with which to compete for resources. A lack of predation pressure and competition can allow a species to more broadly utilize a traditionally occupied niche or even evolve to fill new regions of adaptive space, further utilizing resources in ways that increase population growth. A newly published meta-analysis of lizard densities across the globe confirms some of what we already knew about island biogeography, but also challenges some traditional thinking on the subject.
Novosolov et al. combined data for 346 lizard species around the world. They split these species into four groups based on their distribution: species found only on the mainland (mainland endemics), species found only on islands (insular endemics), species from the mainland but introduced to an island (insular non-endemics), and species from an island but introduced to the mainland (mainland non-endemics). For each species included in the analysis, they collected data from past studies and large databases (i.e. GARD) on site-specific population densities as well as local lizard richness, climatic factors, number and richness of predators (as measured by mammal, snake, and bird diversity), and net primary productivity (NPP); all of which were utilized to determine which factors had the greatest effects on population density. They used both phylogenetic and non-phylogenetic models and accounted for species specific qualities (i.e. body size) that affect population density.
As expected, insular endemics, particularly those inhabiting snake-free islands, have the highest population densities and mainland endemics have the lowest, but there were a few unexpected trends. Insular lizard densities were highest on islands with low NPP possibly suggesting that a reduction in endotherms (associated with low NPP) may correlate to a reduction in predation pressure as endotherms typically are more negatively affected by low NPP than ectotherms. They also found no correlation between insular population density and island area, lizard species richness, or predator richness. This may suggest that all islands impose similar constraints on population density, regardless of their land area, and that competitor and predator densities may be more important than richness (these were not accounted for in this study).
The most interesting trend was that mainland non-endemics (insular species that colonized the mainland) had higher population densities than mainland endemics. This suggests that even when insular species invade the mainland, they retain their ability to manifest high population densities. Traditionally, such high densities are thought to arise from the unique qualities of island ecosystems; however, it might be that some species inherently are capable of achieving higher densities and these species are simply more likely to invade islands and survive once they arrive. As demonstrated in this study, even when they return to the mainland, their uniquely high population densities persist. This is known as ecological sorting and the authors suggest that high population densities are not strictly the result of island environments but rather result from this phenomenon. In further support of this assumption, their analyses suggest that population density is strongly related to phylogeny, so some groups have simply evolved the ability to maintain high densities while others have not. This might mean that the ability to achieve high densities on the mainland increases the likelihood of island colonization and reduces the probability of extinction once colonization has occurred. The authors end by hypothesizing that island lizard communities appear to be composed of species with the ability to attain high densities and suggest that their study is the first quantitative evidence that “high population density on islands arises, at least partially, independently of environmental factors.”