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I am broadly interested
in the evolutionary ecology of amphibians and reptiles. My current research
interests are focused on the evolutionary and ecological factors that
promote the divergence of populations and diversification of species.
This line of inquiry relies on intensive fieldwork and molecular lab
techniques, and integrates analytical tools from a variety of disciplines
such as phylogenetics, population genetics, ecology, historical biogeography,
comparative biology, and GIS.
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Current Projects
Phylogeography and Diversification of the Ouachita Mountain Endemic Salamanders of the Plethodon ouachitae complex
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Salamanders of the Plethodon ouachitae complex are endemic to the Ouachita Mountains of west-central Arkansas and southeast Oklahoma, where they are largely restricted to high-elevation, mesic forest. Because environmental conditions in the intervening valleys are warmer and drier, the salamanders appear to be isolated on "sky islands" where gene flow among populations on different mountains may be restricted. Previous work using allozymes (Duncan and Highton, 1979) found significant genetic variation among populations on different mountains, and it would not be surprising if the number of distinct taxa within this complex is considerably higher than the three currently recognized species (P. caddoensis, P. fourchensis, and P. ouachitae). Dr. Frank Burbrink (College of Staten Island-CUNY) and Kelly Irwin (AR Game & Fish), and I are sampling the region more intensively and using DNA sequence data to analyze patterns of genetic diversity and assess species boundaries. This work will not only tell us more about species diversity in the region, which is important for conservation, but it will provide information about the processes involved in lineage diversification and ultimately the origin of new species. We have collected and sequenced the cyt b and ND4 mitochondrial genes (1816 bp) for 281 P. ouachitae from 55 localities and 130 P. fourchensis from 38 localities throughout the Ouachita Mountains. Results showed that P. ouachitae is composed of seven lineages structured across six major mountains and P. fourchensis is composed of four lineages structured across five montane isolates. The results of our work on P. ouachitae and P. fourchensis were recently published in two separate papers in Molecular Ecology (see links below). During the Spring 2009 field season, we concentrated on P. caddoensis and now have about 330 individuals from 62 localities. We have sequenced cytb and ND4 for all of these samples and our results are indicating this species is also composed of several very divergent, geographically structured lineages. We are in the process analyzing data and hope to have a manuscript on P. caddoensis submitted late-2009. In addition to our phylogeographic work employing mitochondrial genes, we are examing morphological variation and looking at several nuclear loci to better understand diversity and evolutionary history of this group. This work involves using unlinked gene trees from multiple nuclear loci to reconstruct the species tree that is the most likely representation of the evolutionary history of species. Related publications: Shepard and Burbrink. 2008. Molecular Ecology. (pdf); Shepard and Burbrink. 2009. Molecular Ecology. (pdf) |
Species Diversification and Delimitation in the Slimy Salamanders (Plethodon glutinosus complex)
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Delimiting species can be extremely difficult when morphological variation is lacking, hybridization is frequent, and gene lineages have not had sufficent time to sort. The Plethodon glutinosus complex is plagued by all of these problems and consequently has had a contentious taxonomic history. Historically, only a single widespread species (P. glutinosus) was recognized, but the advent of allozyme gel electrophoresis revealed a high level of cryptic genetic variation. Based on these allozyme studies, species were defined as populations exhibiting a genetic distance over a specific threshold value. Although this method has received much criticism, 12 species are currently recognized by most authorities. However, whether these taxa represent truly independent evolutionary lineages and whether additional species diversity remains undescribed have not been studied in a statistical phylogenetic framework using DNA sequence data from multiple unlinked genes with detailed population sampling. As a starting point for tackling the whole group, Frank Burbrink and I have begun examining phylogeography of P. albagula. We are finding that this species is not monophyletic and that some populations of P. albagula in the Ouachita Mountains of Arkansas are actually more closely related to species in the southeastern US. Further, P. kiamichi, P. sequoyah, and P. kisatchie appear to be distributed more broadly than previously recognized. As of September 2009, we are working on acquiring more samples of P. albagula from Missouri and Arkansas with the help of Jeff Briggler (MDOC) and Kelly Irwin (AGFC). We are also sequencing our existing samples for the mitochondrial genes ND4 and cytb, and screening several nuclear loci to determine if they will be useful for delimiting species. Any legally acquired tissues (liver, muscle, tail clips, toe clips, etc) from salamanders of the Plethodon glutinosus complex would be greatly appreciated. Please email me if you have some to donate: dshepard@ou.edu |
Survey and Inventory of Amphibians and Reptiles of the Wildlife Management Areas of Oklahoma
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The wildlife management areas of Oklahoma offer some of the best maintained and most undisturbed natural habitats in the state. With the increasing threat of development and urbanization, these areas will play a critical role in conserving the state's diverse flora and fauna. However, no detailed information currently exists on the species that occur in these areas and this baseline data is needed for effective management and in order to gauge the impact of future habitat modifications. From 2006-09, Dr. Laurie Vitt, Dr. Jan Caldwell, and all of the herpetology students at the Oklahoma Museum of Natural History are intensively surveying several wildlife management areas for amphibians and reptiles to generate species lists, assess relative abundances, and collect data on species' ecology. Check out the project webpage. |
Past Projects
Survey and Inventory of the Brazilian Cerrado Herpetofauna
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The Cerrado is Brazil's second largest biome and is currently one of South America's most threatened ecosystems due to rapid agricultural expansion. Little is known about the Cerrado herpetofauna, but recent work has shown that diversity is much higher than previously thought. I assisted Dr. Laurie Vitt and Dr. Jan Caldwell in their NSF funded project in collaboration with Dr. Guarino Colli and his graduate students at the University of Brasilia (UnB) to survey the amphibians and reptiles of the Cerrado and study their ecology and biogeography. Click here for more about this project. Related publications: Caldwell and Shepard. 2007. Journal of Herpetology. (pdf); Vitt et al. 2008. Copeia. (pdf); Vitt et al. 2007. Journal of Zoology. (pdf) |
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Diet Composition and Overlap between Recently Metamorphosed Southern Leopard Frogs (Rana sphenocephala) and Crawfish Frogs (Rana areolata): Implications for a Frog of Conservation Concern
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Local anuran density can be high immediately following metamorphosis, which could lead to intense competition between species that breed in the same habitats and metamorphose simultaneously. The range of R. areolata is overlapped entirely by the range of R. sphenocephala, and these two species often breed concurrently in the same habitats, metamorphose during the same time period, and are morphologically similar. John Crawford, Chris Conner, and I collected recently metamorphosed frogs of these two species and compared morphology and diet to assess the potential that competitive interactions at this life stage could be negatively affecting populations of R. areolata. We found that both species were similar in body size, although head morphological variables increased at a faster rate relative to body size in R. areolata. Both species ate primarily hemipterans, beetles, and spiders, and exhibited high dietary overlap. Stomach content volume was significantly lower in R. areolata, which could be due to competitive interactions with R. sphenocephala. Related publication: Crawford, Shepard, and Conner. In press. Copeia. (pdf) |
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Lizard Predation: Effects of Habitat and Body Shape
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Predation is a major selective force in the evolution of morphological characteristics of organisms. Visually oriented predators (e.g. birds) develop search images for prey using features such as pattern and the body contour and outline. Lizards that rely on crypsis as a defense have body shapes that enhance their ability to avoid detection whereas lizards that depend on flight possess body shapes that promote rapid locomotion. In December 2004, I used clay models to test whether predator attack rates varied among lizards of four different body shapes. I conducted this experiment in three Cerrado habitats in Brazil with different vegetative characteristics to examine potential differences in attack rates among habitats. Predator attacks did not differ among the model shapes, but did differ among habitats. Predator attacks were most frequent in the most structurally complex habitat and were higher in more exposed microhabitats (Can you tell which lizards on the left are real and which ones are models?). Related publication: Shepard. 2007. Herpetologica. (pdf) |
Ecology and Conservation of the Eastern Massasauga (Sistrurus catenatus)
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Started in 1999, this project is a long-term study on all aspects of the life history, ecology, and conservation of the eastern massasauga at Carlyle Lake, Illinois (with Dr. Chris Phillips, Mike Dreslik, and Ben Jellen of the Illinois Natural History Survey). Areas of research include spatial ecology, habitat use and preference, population ecology, thermal ecology, the effects of roads, foraging ecology, population genetics, reproductive ecology, etc. We are currently working on several manuscripts on various topics related to this project as well as continuing our mark-recapture study. Related publications: Shepard et al. 2004. American Midland Naturalist (pdf); Jellen et al. 2007. Journal of Herpetology. (pdf); Shepard et al. 2008. Copeia. (pdf); Shepard et al. 2008. Animal Conservation. (pdf). |
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Evolutionary Ecology of Foam-nesting Frogs of the Genus Leptodactylus
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Frogs of the genus Leptodactylus deposit their eggs in foam nests; however, nest deposition site and the extent to which larvae develop in the foam nest vary among species. Foam nests may be deposited on the water surface, on land at the margins of water, in burrows below ground adjacent to water, or in burrows below ground away from water. Larvae may enter water shortly after hatching or after an extended period in the foam nest, or they may complete development within the foam nest independent of water. Reproductive modes are hypothesized to have a phylogenetic basis, but data to evaluate this are lacking for most species of Leptodactylus. In 2003, Dr. Jan Caldwell and I studied the reproductive ecology of Leptodactylus labyrinthicus at a locality in the Cerrado region of Brazil. We found that foam nests were deposited on land about 2 m from the water's edge. Less than 20% of the eggs in nests developed and tadpoles remained in the nest after hatching, feeding on the undeveloped eggs. Heavy rains washed tadpoles out of foam nests and into adjacent ponds where they then fed on the eggs of other frogs, primarily small hylids. Related publication: Shepard and Caldwell. 2005. Copeia. (pdf) |
Body Size, Age, Growth, and Alternative Mating Tactics in Toads: Satellite Males are Smaller but not Younger than Calling Males
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Alternative mating tactics occur in many species. In toads, non-calling males parasitize calling males and attempt to intercept females. These satellite males are usually smaller in size than the calling males they parasitize and this has led to the assumption that because they are smaller, they are also younger. Chris Leary and I analyzed size and age data on caller and satellite Bufo cognatus and Bufo woodhousii and found that although callers were larger in size than satellites, they were not different in age. The size difference between callers and satellites of the same age was the result of different growth trajectories, with callers growing faster. Related publication: Leary et al. 2005. Animal Behaviour. (pdf) |
Aggressive Behavior and Territoriality in the Green Frog (Rana clamitans)
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This project was my Master's work at Illinois State University. I compared aggession, site tenacity and spatial relationships of male Green Frogs (Rana clamitans) during the breeding and non-breeding seasons to test if their behavior was consistent with territoriality. I found that although males are aggressive during the breeding season, their site tenacity and spatial relationships were not entirely consistent with territoriality. Related publications: Shepard. 2002. American Midland Naturalist. (pdf); Shepard. 2004. Copeia. (pdf) |
Amphibians and Reptiles of Camp Gruber, Muskogee Co., Oklahoma
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During spring and summer of 2003, Drs. Jan Caldwell, Laurie Vitt, and I surveyed for amphibians and reptiles at Camp Gruber, a U.S. Army Training Grounds in eastern Oklahoma. While avoiding a hail of gunfire, we documented 59 species (19 amphibians and 40 reptiles). For a species list click here. |
last updated September 2009