A Multigenerational Field Experiment on Eco-evolutionary Dynamics of the Influential Lizard Anolis sagrei: A Mid-term Report

Only a handful of multi-generational experiments in natural systems of eco-evolutionary dynamics currently exist, despite Fussmann et al.'s call for more such studies nearly a decade ago. To perform such a study, in 2008 we introduced the lizard Leiocephalus carinatus, a predator (and possible food competitor) of the lizard Anolis sagrei, to seven islands having A. sagrei, with seven unmanipulated islands having A. sagrei as controls. Almost immediately, L. carinatus, which is larger and more terrestrial than A. sagrei, caused a major habitat shift in A. sagrei away from the ground and toward higher and thinner perches; focal behavioral surveys showed that on islands where its predator was introduced, A. sagrei had less conspicuous visual displays. The expected pattern for density of A. sagrei is that it would decrease markedly at first via predation from the larger lizard, but then it would increase as the habitat shift selected for individuals better able to live in higher vegetation. Data through 2015 show this pattern, but a return to previous densities (time-by-treatment interaction) was not yet significant. A previous within-generation selection study and comparative data suggest that short legs should evolve as the lizards adapt to better maneuver on the thin perches of higher vegetation. However, no indication of the expected morphological change in limb length was present through 2015. Previous studies showed A. sagrei producing many effects on lower levels of the food web, some quite large. In this study through 2012, we found significant differences only in spiders (web and ground). A possible complication is that the study site was hit by two major hurricanes in the last five years, decreasing population sizes of both lizard species and reducing the experimental perturbations. A benefit of the hurricanes, however, is that they eliminated lizards from some islands, providing the opportunity to monitor natural recolonization, the frequency of which has eco-evolutionary implications. Surveys of the 44 islands that lost lizards showed that recolonization is rather slow. To explore long-term patterns of morphological variation, we monitored morphology of 31 island populations for up to 19 years. Mean limb length oscillated across the 19-year period, both increasing and decreasing substantially, yet the net effect over that period is almost no change. In years following hurricanes, limb length increases significantly more than expected by chance.