population stability of a tropical lizard
Populations of the lizard Anolis limifrons were censused for 19 yr at the Lutz site on Barro Colorado Island (BCI) and for 4 yr at 20 additional sites in central Panama. Census data provided estimates of population density at the end of the wet season (December) and indices of per-capita food intake and annual recruitment. Year-to-year changes in density of as much as 5- to 8-fold were observed at Lutz over 19 yr and of 2- to 3-fold were observed over 4 yr at other sites. Changes in population density at 11 BCI sites were synchronous over 1983-1986, while population density fluctuated independently at non-BCI sites. Populations of A. limifrons exhibited significantly greater generation-to-generation variability than populations of West Indian Anolis and of lizards in temperate-zone habitats (mostly arid lands in North America). An index of per-capita food intake was negatively related to density both for temporal comparisons at Lutz and for spatial comparisons among sites. The relative number of young individuals at the end of the wet season was negatively related to population density and positively related to an estimate of food intake. Partial correlations indicated that the positive association between the relative number of young individuals in the population and food intake was independent of the negative association between food intake and population density. Recruitment was thus depressed when density was high and enhanced when food was relatively abundant. Despite the potential for density-dependent regulation, Anolis limifrons populations are highly unstable, presumably because of random perturbations of the environment. Rainfall is implicated causally although population density was only weakly associated with rainfall - density at BCI was positively associated with rainfall during the dry season and negatively associated with rainfall during the wet season and with total annual rainfall. Population attributes that contribute to instability are rapid population turnover and a middle-level trophic position in a complex food web.