Three years ago I received a message from Ray Huey asking me if I’d be interested in collaborating on an NSF grant to return to Puerto Rico and replicate the studies on Anolis thermal biology and ecology that we had conducted during the 1970s. The idea was to have the original investigators, including Ray, myself, and Paul Hertz, work in the same study areas, utilize the same techniques, and document changes that had occurred over the past 35-40 years. Our ultimate goal was to understand the impact of climate warming on Anolis populations over a range of habitats, from the Luquillo rainforest in northeastern Puerto Rico to the Guanica dry forest in the southwestern corner of the island.
We eventually received NSF funding for the project, and to date I have carried out field work in Luquillo and Guanica during July 2011 and January 2012. Andres Garcia, my long term colleague from the National Autonomous University of Mexico, joined me in Puerto Rico as a co-investigator on the grant. At the Luquillo study site, we successfully repeated all phases of the research I conducted in July 1976 and January 1977 (Lister, B. C. 1981. Seasonal niche relationships of rainforest anoles. Ecology 62(6):1548-1560). The major components of this study were (1) a July and January census of forest dwelling anoles (A. gundlachi, A. evermanni, and A. stratulus) (2) measurements of perch heights, perch diameters, and perch site insolation (3) recording of body temperatures for all forest species (4) sweep samples of the forest understory during both the winter and summer seasons (5) collection of male and female A. gundlachi for subsequent scoring of reproductive condition and stomach content analysis.
Comparison of our results with those of my previous study indicate significant changes in all of the above areas. (1) The total density of anoles within the 15mx15m census quadrat is 50% lower, and biomass over 25% lower. The density of A. evermanni in the study area, formerly an abundant species along the edges and interior of the forest, has dropped 10-20 fold. A. gundlachi abundances are down by about 25%. We did not see any A. stratulus within the study area, or in other areas of the interior forest at Luquillo. Commonly observed along edges and more open areas in the 1970s, A. stratulus is now very rare in these habitats. (2) Within the study area, both male and female A. gundlachi have greatly expanded the range of perch heights and diameters they utilize. (3) Compared to 1976 and 1977, body temperatures of male and female A. gundlachi were somewhat higher (~.50 C) during July and January. Interestingly, perch site insolation of males and females was substantially lower during July-January 2011-2012 than in July-January 1976-1977. (4) Our sweep samples indicate that arthropod abundance is now 10-15 fold less, and arthropod biomass 5-10 fold lower, than in July-January 1976-1977. (5) In July 2012, only 58% of female A. gundlachi were gravid (one or two oviducal eggs) vs. 92% in July 1976.
In conclusion, I am pleased to report that we have decisively disproved the null hypothesis of no change in the ecology of Luquillo anoles over the past three and a half decades! But what about climate warming? Our analyses of weather data from the El Verde station (1972-2011) indicate that mean rainfall has remained virtually unchanged, while average air temperature has increased by 20 C. This is substantial, particularly for a tropical location. So there’s a smoking gun right smack dab in the middle of our crime scene investigation. Let’s book the suspect! But wait. We’re faced with the usual problem in historical sciences like ecology and evolution that deal with complex, adaptive systems: a myriad number of alternative hypotheses present themselves. Perhaps, for currently obscure reasons, a sudden, critical transition occurred in the Luquillo ecosystem over the past 35 years, shifting the forest and it’s resident anoles into an alternative stable state. If this scenario actually occurred, we’re faced with the even more difficult task of cracking the proverbial “cold case”. Or, maybe 2011-2012 was just an anomaly and the lizards will return to the same basin of attraction they occupied in the 1970s. Then again, it’s possible that we really are hot on the trail of the historical and ongoing effects of climate change.
Especially for all of us who love anoles, the stakes are high. The prospect of documenting the extinction of these most beautiful and wonderful species over the coming decades is a dismal one to say the least. We’ll be back in Puerto Rico next July to gather more data and begin transplant experiments that may give us deeper insight into the effects of climate warming. I’ll keep you updated on our results via the Anole Annals blog.