Research

My research program synthesizes topics from three different research fields – ecology, evolution, and wildlife management – first, to develop and test theory, and second, to aid in sustainable conservation of both organisms and ecosystems. In this page, I will describe relevant areas of research, and projects included therein.

Population ecology in Neotropical rain forests

For my dissertation I tested and integrated three hypotheses describing bottom-up, top-down, and parallel population regulation of a model assemblage at La Selva Biological Station, Costa Rica. First, I tested the litter-mosaic hypothesis that leaf-drop phenology of trees regulates population cycles and patch dynamics of terrestrial arthropods, frogs (Craugastor bransfordii, Oophaga pumilio), and lizards (Norops humilis). I used a comparative experimental approach, where I (1) compared seasonal abundance cycles of terrestrial litter and fauna beneath two tree species differing in phenology of leaf abscission (Castilla elastica, Dipteryx panamensis), and (2) experimentally manipulated leaf abscission beneath C. elastica with litter supplementation. Second, to better understand how predators influence prey frog and lizard ecology, I tested the mainland-island model prediction that predation exerts stronger effects than food resources for anoles and, by extension, frogs in Central America. I modeled the relative contribution of food, microhabitat, and predatory spiders (Ctenidae) to frog and anole occupancy. Third, I used eighteen months of mark-recapture observations of O. pumilio in a homogenous, fallow cacao plantation to test predictions of the conspecific attraction hypothesis.

Together, my results contribute to our understanding of patch dynamics for terrestrial frogs and lizards in Neotropical wet forests. My experimental results suggest that Norops humilis populations respond to pulses of terrestrial litter in ways consistent with the litter-mosaic hypothesis, but the O. pumilio and C. bransfordii do not. Dispersal of O. pumilio and other frogs through the landscape may be driven in part by conspecific attraction. Because predatory spiders exert stronger effects on the occupancy of terrestrial frogs at lower abundance of terrestrial litter, seasonal declines in frog abundance during the wet season may be driven in part by elevated predation pressure when leaf litter is scarce. Predator-prey models reject a large literature invoking predation as the dominant force shaping ecology of diverse mainland anoles, and suggest similar mechanisms may regulate the ecology and evolution of mainland and island anoles.

Population ecology and conservation of reptiles

The world is changing rapidly in the Anthropocene, and ecosystems risk perturbation and extinction by human-driven climate change, habitat loss, exploitation, invasive species, and pollution. Consequently, I feel an ethical duty as a wildlife biologist to conduct studies which not only test theory, but also have practical applications for sustainable conservation of wildlife and ecosystems.

Turtle are the most endangered vertebrate group on Earth, a problem I became acutely aware of while living in Alabama. The Mobile Bay drainage of Alabama has the highest diversity of turtles among any comparable hydrological unit on Earth, and this diverse assemblage is threatened by numerous conservation concerns. To this end, I became involved in conservation projects with two imperiled turtles in coastal Alabama – the Alligator Snapping Turtle (Macrochelys temminckii), the largest turtle species in North America which has been petitioned for federal listing, and the federally-endangered Alabama Red-bellied Turtle (Pseudemys alabamensis). Relevant projects on M. temminckii used long-term mark-recapture data to model reference population demography for conservation, built a mathematical population model for the species, and used population projection models to estimate population viability for three endangered populations across the species range. Additionally, we also evaluated the species’ distribution and abundance in a conservation context in Alabama. Our project on Alabama Red-bellied Turtles synthesized 30 years of data to describe population trends and extinction risk, which aided in a revised management plan for this endangered species.

Amphibians are also another highly imperiled vertebrate group, with up to 40% of populations in decline worldwide. In particular, my study assemblage as La Selva Biological Station has experienced chronic declines over the last 40 years, likely a result of climate-change driven differences in leaf-litter and disease dynamics. Thus, during my dissertation, I also initiated long-term monitoring of a poorly-known group of frogs, the glassfrogs (Centrolenidae). Using mark-recapture methods at three replicate populations, I have been monitoring population demography of the Spined Glassfrog, Teratohyla spinosa, and also describing hitherto unknown natural history (e.g., male combat and the use of a sexually dimorphic weapon). Using frog-call surveys at 30 sites across the landscape at La Selva, I also collaborated with an undergraduate student, Nelson Rivera, who found that, similar to aquatic organisms, the riparian glassfrog community structure varies across the landscape in a pattern consistent with the stream continuum concept. These studies will provide benchmark data for population monitoring, while also publishing fundamental descriptive and hypothesis-driven studies of anuran population and community ecology.

Historical biogeography and systematics

While working in the Auburn University herpetology collections, my adviser was revising Bob Mount’s seminal guide The Reptiles and Amphibians of Alabama [9], and a general lab project was to construct a taxonomic key to amphibian and reptiles of the southeastern United States, the most biodiverse region in the country. To this end, we frequently used weekly “museum nights” to develop this key, and, while pawing through specimens, I became familiar with the numerous wide-ranging, geographically variable species that occur in the Southeast. During these activities, I delved into systematic research, and began reading about biogeography in the southeastern United States, an area with well-known biogeographic breaks for numerous diverse taxa [10]. These experiences began my interests in biogeographic and systematic hypotheses, and I became involved in projects to resolve taxonomic quandaries in the Southeast.

In 2014, a research group from the University of Florida published a new taxonomy for Alligator Snapping Turtle (Macrochelys temminckii) which split a single, widely distributed species into three different species [12]. This paper had immediate implications for me, because I was involved in a long-term demographic study of those turtles in Georgia [13] and my population was described as one of the new species. However, upon reading their paper, my co-author and I became skeptical of the data provided. We wrote a follow-up article which argued that the morphological analyses were only sufficient at diagnosing two, rather than three, species, and that they ignored critical published genetic data which question the validity of their third species. We revised the Macrochelys taxonomy to be diagnosable until more data are provided, and we provided a modified taxonomic key of functional use to museum and field biologists.

In 2016, another research group from the University of Florida published a new taxonomy for Eastern Indigo Snakes (Drymarchon couperi). The species is federally listed as Threatened by the US Government, because it has become rare throughout it’s range Coastal Plain of the southeastern United States. In this new taxonomy, the species was split into two: the Eastern Indigo Snake (Drymarchon couperi) and the Gulf Coast Indigo Snake (Drymarchon kolpobasileus). However, my colleagues and I have been actively writing a manuscript to reverse this taxonomic revision, where we described a series of datasets (gene sequence data, microsatellite data, morphology, philosophy) which suggest that there is a single species of Eastern Indigo Snakes.

The Red Salamander (Pseudotriton ruber) is a wide-ranging species in eastern North America, for which four subspecies have been used to describe morphological geographic variation. One of my first systematic projects collected genetic sequence data for individuals from 45 populations to test whether these allopatric and morphologically diagnosable subspecies actually represent undiagnosed lineages. Our phylogenies described significant structure between Coastal Plain and Appalachian populations, but were insufficient for diagnosing these populations as distinct taxa. Rather, we found data consistent with the hypotheses that P. ruber evolved in the Coastal Plain before dispersing into Appalachia, and that a Müllerian mimicry complex with toxic and aposematic newts (Notopthalmus) is driving geographic variation in coloration of P. ruber.

Colleagues and I documented and described a hybrid zone for Map Turtles (Graptemys sps.) in the Choctawhatchee River. Like fishes, Graptemys are highly aquatic turtles which are limited to large Gulf Coast riverine habitats and provide a text-book example of drainage-specific endemism driven by sea-level fluctuation and allopatric speciation in isolated river drainages. We first found that a hitherto unknown population in the Choctawhatchee River drainage actually represents two co-occurring and hybridizing populations, the first example of co-occurrence and hybridization of Graptemys to date and a puzzling exception to the pattern of drainage-specific endemism. In a separate paper, we also described geographic variation of the turtle Graptemys ernsti in a conservation context.