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.
Mean (±SE) perch heights of A. sagrei shift higher (F1,7 = 17.2, P = 0.004 repeated measures MANOVA; Effect size [log ratio] = 0.79) on islands after the experimental introduction of the predatory lizard L. carinatus. Islands on which lizards were extirpated during the course of the experiment (see text) were not included in this or subsequent analyses.
Mean (±SE) perch diameters of A. sagrei decrease (F1,7 = 5.3, P = 0.05 repeated measures MANOVA; Effect size [log ratio] = 1.27) on islands after the experimental introduction of the predatory lizard L. carinatus.
Mean (±SE) densities of A. sagrei on islands with and without the introduced predatory lizard L. carinatus. Also included are mean (±SE) densities of L. carinatus after introduction. See text for statistical analysis.
Changes in habitat use and pattern of natural selection from Losos et al. (2006). For use of the ground (top) and perch diameter (middle), data from May 2003 represent habitat use before the initiation of the experiment. All data are for individuals initially measured and marked in May 2003. Lizards grew throughout the experiment, probably explaining the increase in perch diameter on control islands (an intraspecific relationship between body size and perch diameter is well established in Anolis lizards). (Bottom) Selection gradients were calculated for two time periods, May 2003 to November 2003 and November 2003 to May 2004. Selection gradients in the figure were adjusted for log-transformed island area (included in the repeated-measures analysis as a covariate) by using least squares means from the ANCOVA. Open symbols indicate control islands; filled symbols, introduction islands. Error bars ± SE.
Mean (±SE) tibia length for treatment islands with introduced L. carinatus and control islands with only A. sagrei (P = 0.53 repeated measures MANOVA). Note that this measure was taken with x-rays, and Figure 3's measure was done by hand, as well as being hindlimb not tibia. The correlations between the hindlimb measures taken by hand and the tibia by x-ray are very high: males r = 0.98 (n = 15), females r = 0.92 (n = 12) using data from mainland Great Abaco.
Mean (±SE) relative limb length (residuals from the tibia length vs. snout–vent length regression, separate by sex) across all islands has fluctuated over the 19-year study period with little net change (year-to-year change results from within-island evolution, population extinction, and the inclusion of different islands at different points in the study; trends are similar when only the nine islands sampled across the 19-year period are considered [results not shown]). Limb values increase after hurricanes (P = 0.005, see text).
Contributor Notes
Associate Editor: D. M. Green.
From “Eco-Evolutionary Dynamics in Cold Blood,” an ASIH-sponsored symposium at the 2016 Joint Meeting of Ichthyologists and Herpetologists in New Orleans, Louisiana.