A commonly observed, but little studied, aspect of tropical herpetology is the seasonal shift in some species’ relative abundance in forested habitat and adjacent, nearby streams. The general pattern is that during the dry season, some species of forest frogs, lizards, and snakes seem easier to detect along streams than in the forest and vice versa during the wet season. Despite this intuitively unsurprising seasonal shift in macrohabitat use being noticed in the 1960s by researchers like Jay Savage and Norm Scott, there has been little work done to document it. In an upcoming issue of the Journal of Herpetology is a paper titled: Seasonal Shifts in Relative Density of the Lizard Anolis polylepis (Squamata, Dactyloidae) in Forest and Riparian Habitats.
The difficulty in documenting seasonal macrohabitat shifts is twofold. First, field sampling must encompass both seasons and be continuous. Second, simultaneous sampling needs to occur in both forest and streams across seasons. For many tropical herpetologists, the opportunity and time for such a study do not come about often. In December 1999, I had this opportunity when I spent three years studying the herpetofauna along the south-central Pacific coast of Costa Rica. I was a young, precocious and budding herpetologist and wanted to understand the ecological habits of all the local amphibians and reptiles. So, out of curiosity I set up transects in a 25-hectare forest patch and a stream that ran through the forest at the Tropical Forestry Initiative (TFI) research station. For 29-months, with the help of field assistants (Deborah Merritt and Yemaya Maurer St. Clair) we sampled the transects regularly, documenting and observing species diversity and habitat use in the forest and stream. While I was organizing the data, an interesting pattern emerged in regard to Anolis polylepis. Of all of the species in the local lizard fauna, A. polylepis showed the strongest seasonal shift in relative density between the two habitats!
Anolis polylepis is the most common anole along the Pacific coast of Costa Rica, reaching densities of up to 300 individuals per hectare (Andrews 1971; Scott 1976). The species can be found in a wide variety of forested habitats ranging from old growth forest to gardens with ample shade trees. In my experience, the only necessary habitat requirement for A. polylepis is shade from a closed canopy. The high density and generalist habits of A. polylepis make it a wonderful study species.
Like many forest anoles, A. polylepis is active in the understory during the day. However, obtaining accurate population counts can be difficult because individuals are wary and can be difficult to detect. For example, A. polylepis will jump to the ground or circle around a tree when observed. This avoidance behavior can be problematic when attempting to obtain reliable counts by increasing the likelihood of missing a lizard. To counter this difficulty, I surveyed for A. polylepis at night, which facilitated easier detection. Anolis polylepis, like many species of anoles, sleeps visibly on leaf tops, twigs, branches and vines from 0.5 to 4 meters above the ground. Thus, it is easier to obtain better counts of relative density for some anole species when lizards are sleeping and inactive. Nocturnal surveys can be very informative for addressing certain questions related to anole biology.
In total, 41 nocturnal surveys were conducted between January 2001 and February 2002, covering one wet and one dry season. We found significant seasonal differences in A. polylepis relative densities between the wet and dry season. During the dry season, A. polylepis density was 0.052 lizards per meter in the stream and 0.010 lizards per meter in the forest. This pattern reversed in the wet season when stream relative density was 0.002 lizards per meter and forest relative density was 0.036 lizards per meter. This seasonal change in relative abundance suggests that wet-dry seasonality influences macrohabitat use in A. polylepis in Costa Rica.
One major limitation of our study was that we did not use mark-recapture. Use of such an approach would give insight into the individual movements associated with our observed patterns. For example, we could test whether lizards are moving large distances to the stream during the dry season, or whether deep forest lizards are moving to moist microhabitats within the forest such as tree buttresses, to name two possibilities.
As with many pilot field projects, ours documents a novel pattern, but raises additional questions. Future work on this issue should extend to other species and regions and use mark-recapture or radio telemetry to elucidate the details of seasonal migrations. An understanding of seasonal movements in environments with distinct wet and dry seasons has implications for how anoles and other herps can tolerate the harsh dry season.
Andrews, R.M. 1971. Food resource utilization in some tropical lizards. Unpubl. PhD diss. University of Kansas, Lawrence.
Scott, N.J. 1976. The abundance and diversity of the herpetofauna of tropical forest litter. Biotropica 8:41-58.