Fifty Years of Observations on Anolis Lizards at Barro Colorado Island, Panama

Abstract: I present the results of 50 yr (1971–2020) of annual censuses of Anolis apletophallus on Barro Colorado Island, Panama. The main objectives were to assess why abundance in end-of-the-year censuses varied substantially from year to year and why it declined over time. Abundance was negatively correlated with annual rainfall, 90% of which occurs in the wet season when eggs are laid. Lizard abundance is indirectly linked to rainfall through the interaction between Anolis eggs and their major predator, Solenopsis ants. More eggs are killed by ants when rainfall is relatively high because ants are more active and encounter more eggs than when rainfall is relatively low. While rainfall accounts for variability in abundance, it has not changed over time and thus may not account for the overall decline in abundance. Model selection of AICc analyses identified two other factors correlated with abundance. Abundance was positively correlated with the Southern Oscillation Index (SOI) lagged by 1 yr. High SOI (and high abundance) is associated with cool and wet La Niña conditions and low values with dry and warm El Niño conditions. The prediction that low abundance is associated with dry and warm El Niño conditions (low SOI) conflicts with the negative correlation between abundance and rainfall where low abundance is associated with high rainfall. Moreover, abundance was negatively correlated with Tmin (minimum annual temperature). The mechanism by which increasing Tmin during the census period is linked to declining abundance is unknown. Three climatic factors are correlated with lizard abundance, but none of them explain why abundance has declined. A third objective was to examine the relationship between species richness and species dominance of Anolis communities with respect to rainfall patterns. Tropical forests typically have a maximal richness of 7–8 species. Our study sites in Panama have high species richness, but Anolis apletophallus individuals made up ≥96% of all records, an unexpected level of species dominance. Comparisons among sites suggest that the number of Anolis species in a community is related to annual rainfall, and dominance is related to seasonality of rainfall. Dry forests have few Anolis species and wet forests have as many as 7–8 species. Forests with short wet seasons (months with >100 mm rainfall) have a high likelihood that individuals of one species will dominate the community.