Meredith Swartwout is a graduate student in the Department of Biological Sciences. She studies population ecology, community ecology and conservation biology in reptiles and amphibians, and spent the summer in Costa Rica working on a project to learn more about ant predation on lizard eggs. Her report is below. 

Swartwout. Photo by Russell Cothren.

If you flip through a field guide to tropical reptiles, you will likely find the reproduction section to be seriously lacking, with a lot of words like “assume” and “presume.” This is because not many people are crazy enough to go poking through leaf litter for hours, trying to find lizard eggs that are roughly the size and shape of a Tic Tac. However, for some small anole lizard species, such as my study organism, the litter anole (Anolis humilis), egg survival may be very important for populations because adult survival is low. For that reason, I spent this summer at La Selva Biological Station in Costa Rica, searching the rainforest floor for lizard eggs, and collecting eggs from gravid females for a field experiment.

Tropical fire ants (Solenopsis geminata) swarming around a predated lizard egg. Photo by Meredith Swartwout.

My project, titled “Scrambled eggs: Disentangling mechanisms influencing ant predation on lizard eggs,” seeks to “unscramble” complex effects of habitat and food web changes driven by climate on lizard populations. This summer, I combined searches for anole lizard eggs with a field experiment looking at the effect of litter depth on lizard egg survival and invertebrate communities. I manipulated leaf litter depth in 30 small field plots by translocating litter from 10 random plots to another 10, with the final third serving as control plots. Over three months, I placed more than 50 lizard eggs in the plots to monitor hatching success and predation rates. To assess treatment effects on invertebrate communities and ant densities, I used a combination of pitfall traps, sticky traps, and Berlese samples from the plots.

Ants are the most abundant invertebrates at La Selva, and some species, such as fire ants from the genus Solenopsis, are known to predate on lizard eggs. A pilot study that I conducted last year suggested that ants occur at higher densities in plots with deeper litter. Consequently, there is higher potential ant predation pressure on lizard eggs. However, deeper litter also contains more prey invertebrates for the lizards, suggesting that there is still a benefit of litter depth. I expected to find that more eggs would survive in plots with less litter because of the reduction in numbers of ants.

Sometimes snakes like to use litter plots, too. At center, just above the blue flagging tape, is a neonate Terciopelo, or Fer-de-lance (Bothrops asper). On the upper left is a pink flag denoting where a lizard egg has been set. In the lower left of the photo there is a recently set pitfall trap that was not open at the time of the photo. Photo by Meredith Swartwout.

Although final results are pending, I did observe some interesting trends. Lizard eggs were much more frequently encountered in fallen logs or buttresses (large root structures found on many tropical trees) than in plots with just leaf litter. Egg mortality due to predation was high, likely between 20 percent and 50 percent, suggesting that there may be some pressure for females to lay eggs in more protected microhabitats. More research on leaf litter dynamics and lizard egg survival will likely be needed to finish “unscrambling” these eggs.