For the past eight years, my lab has conducted intensive research on green anoles (Anolis carolinensis) in Palmetto State Park in Luling, Texas, about an hour east of San Antonio. This park is beautiful – it’s centered around a swampy area dominated by dwarf palmettos (Sabal minor), and the San Marcos River flows through it. We’ve marked lizards and mapped their home ranges, watched their behavior, measured their morphology and parasite loads, and so much more. In past years, we’ve calculated that the density of green anoles in the park is approximately 0.04 lizards/m2, or about four adult lizards in every 10m x 10m area. We could regularly get sample sizes of around 150 lizards for behavioral studies in the park, but we very rarely collected animals from the park – we left them where we found them!
But this year is different. On three field trips to the park this summer, we have found very few green anoles. On our first visit this year in May, we spent 16 person-hours searching for lizards and found four green anoles. On our second visit in early June, we spent 14 person-hours searching and found eight. Last week, we spent another 12 person-hours and found only two. We see green anoles all over the city of San Antonio, and the students in my team are all skilled lizard spotters and catchers, so this isn’t due to inexperience. Also, we see other species of lizards all over the park – most commonly, Texas spiny lizards, little brown skinks, and house geckos– as well as garter snakes, copperheads, and cottonmouths. We also see tons of frogs.
So what happened to the anoles? We’ve considered a number of possibilities. The first thing we thought of was the possibility of feral cats – but we haven’t seen any cats in the park, and we think cats should have the same effect on the other herp species. What if the insect population had crashed? But again, that would affect the other lizards, snakes, and frogs too. This isn’t a year of particular drought or excess rain (and in previous wet and dry years, we’ve still seen lots of anoles), and the vegetation throughout the park largely looks the same as it has in the past. Perhaps an anole-specific disease has spread through this population?
In any case, the paucity of anoles in the park this year suggests that there won’t be many next year either, as there’s almost no one around laying eggs. It’s a bummer, because we’ve had such success here in the past.
Any ideas to explain this, AA readers?
Work we’ve published from our previous research in Palmetto State Park:
- Dill, A.K., T.J. Sanger, A.C. Battles and M.A. Johnson. 2013. Sexual dimorphisms in habitat-specific morphology and behavior in the green anole lizard. Journal of Zoology 290: 135-142.
- Battles, A.C., T.K. Whittle, C.M. Stehle, and M.A. Johnson. 2013. Effects of human land use on prey availability and body condition in the green anole lizard, Anolis carolinensis. Herpetological Conservation and Biology 8: 16-26.
- Bush, J.M., M.M. Quinn, E.C. Balreira, and M.A. Johnson. 2016. How do lizards determine dominance? Applying ranking algorithms to animal social behavior. Animal Behaviour 118: 65-74.
- Stehle, C.M., A.C. Battles, M.N. Sparks, and M.A. Johnson. In revision. Prey availability affects territory size, but not territorial display behavior, in green anole lizards. Acta Oecologica.