Robert Powell, Ph.D.
Research Interests
Critically endangered Cyclura ricordii from Parque Nacional Isla Cabritos, República Dominicana
In recent years my research interests have been focused primarily on the Hispaniolan herpetofauna (and, to a much lesser extent, that of the West Indies in general). Since 1986 I have made nearly 40 trips to the West Indies, usually accompanied by groups of undergraduates. The emphasis of most studies has been on life histories and community composition and structure, especially of amphibians and reptiles — the latter predicated largely by my enthusiasm for those animals but also because of their suitability for addressing questions amenable to work involving undergraduates. The results of these collaborative investigations have often been published in short notes in order to acknowledge more formally the active contributions of individual undergraduates in short-term studies. Also, students in NSF-funded Research Experiences for Undergraduates (REU) programs, which have as a major goal the promotion of graduate studies and careers in science by participants, have been successfully encouraged to take active roles in preparing the manuscripts pertaining to their own studies. As a result, many have earned senior authorship of published papers.
Lizard communities, particularly those on tropical and sub-tropical islands, are ideal models for studying life history traits and ecological relationships. West Indian lizards in particular are visible, often phenomenally abundant, and never cease to provide opportunities to ask new questions of what, how, and why. Also, very few studies have addressed the life histories of most species and only anoline communities have received any detailed attention by ecologists.
Many aspects of life history can be ascertained through museum-based studies. Although I will not place myself in the same class as Henry Fitch or Rick Shine, I share with them an interest in what animals eat and when and how they reproduce. These basic bits of knowledge are best acquired through the examination of museum specimens. Because these types of studies are also capable of being completed in relatively short periods of time, I have guided many of the undergraduates with whom I have worked toward such investigations. Still, diets are only of passing interest until one can relate food habits to habitats and availability of prey. Similarly, reproductive information is most significant when cause-and-effect relationships can be established between patterns and/or modes of reproduction and climate and habitat. These latter connections obviously require a field component, which I attempt as frequently as possible (studying fascinating animals in a tropical paradise is a tough road to travel, but someone has to do it, and I'm willing to make the necessary sacrifices whenever possible).
To begin exploring the nature of the relationships between populations, niche dimensions other than food, namely those of space and time, also must be defined — and to date, most of my work has addressed these rather fundamental stages. However, I have been able, with the willing assistance of several colleagues and many students, to examine aspects of niche structure in a variety of lizards (plus a couple of frogs and a snake or two), in particular species comprising communities in four very different settings. In the extremely xeric Llanos de Azua in the south-central Dominican Republic I have studied since 1986 the food habits, use of space, and times of activity of six co-existing species (three families and genera are represented). Also, since 1986, I have been examining aspects of the same niche dimensions in thirteen species (five families and six genera) occupying an extensively altered site on the moist coastal plain near the city of Barahona. More recently, since 1989, I also have investigated niche structure in eight species (four families and genera) found in the moist upland forests of the eastern slopes of the Sierra de Baoruco, just south of Barahona. Finally, in 1990, I began the process of evaluating niches of five species (four families and genera) that have colonized successfully the Cayos Siete Hermanos, an archipelago of seven small satellite islands off the northwestern coast of the Dominican Republic. Supplemented by occasional work with populations in other parts of the country, I have accumulated the considerable baseline data necessary to begin addressing the more complex questions of how resources are partitioned within these communities.
When seeking answers to complex questions, new questions inevitably arise (that is the beauty of biology). Although I have begun to address aspects of niche partitioning in each of the communities mentioned above, specific unresolved questions awaiting opportunities for more field work have precluded publication, except for preliminary analyses of the sites in the Llanos de Azua and on the eastern slopes of the Sierra de Baoruco. Data indicate that horizontal spatial partitioning is the most significant factor in the Llanos de Azua, although spatial use patterns appear to break down under unusually moderate (cool, moist) conditions. The community near Barahona exhibits both horizontal and vertical stratification, as well as temporal partitioning of some resources. The montane populations exhibit very stringent horizontal partitioning along habitat gradients, but vertical elements dominate the stratified forest itself. The populations occupying the Siete Hermanos demonstrate geographic partitioning, with no more than three, and often only one species found on a particular key. Compounding the difficulty of drawing definitive conclusions are differences in sizes, thermal regimes, and foraging modes among the species at all sites. Interestingly, at no site have I found significant differences in the exploitation of prey resources.
Describing the interactions, however, is only the second step after defining niche structure of the species examined. The most interesting questions address issues of why these constraints on resource use exist and how they developed. Although current data are too sparse for conclusive analysis, they suggest that interspecific competition is operative in only some instances — in other cases constraints involving physical features of the habitat appear to be far more important. In the moist forests of the Sierra de Baoruco, I also began to explore comparisons in geographically proximate and historically similar areas subjected by human activities to varying degrees of alteration. These studies have begun to shed light on which specific habitat components are necessary before an area can and will be used by a given species. A distinct reduction in diversity among sites is obvious and apparently correlated with the removal of particular “signals” used by different species to identify appropriate habitat. The reduction in diversity is accompanied by density compensation, as a result of which some remaining species increase in abundance and freely occupy microhabitats which are available but not used in less altered sites. The Dominican wildlife and national park services are extremely interested in these data, and have requested that I initiate similar studies in ecologically interesting areas threatened by squatters engaged in subsistence agriculture and charcoal production.
In 1994 I revisited the Cayos Siete Hermanos for the purpose of compiling a complete floral survey of the islands, necessary in order to address questions of why particular species are found on some keys and not others and also useful in trying to determine patterns of colonization. One published hypothesis suggests that different floral communities have allowed different colonizing species to become established. To date, my data suggest otherwise, with chance alone capable of explaining the presence of any one species on any one of several islands.
As adjuncts to the studies described above, I also have, as a result of interests expressed by students over the years, had to learn a great deal about animal behavior and parasites infecting Dominican amphibians and reptiles. Behavioral studies have focused primarily on two questions, one pertaining to the apparent absence of both territorial and stereotypical behavior in a grass anole, the second to interspecific responses among closely related species in a recently established contact zone. Parasitological publications to date include eight descriptions and a redescription of coccidian parasites, a new species of nematode, and an examination of a pentastomid infection of a gecko. We also have identified several additional host records. Some published studies have suggested that parasites constrain population sizes of lizard hosts, apparently by diverting resources from reproduction. Also, some evidence exists to support the idea that parasitized lizards are more susceptible to infections by other types of parasites. As I continue studies of lizard populations, I hope simultaneously to examine the effects of parasites on their hosts. One particular system, composed of a pentastomid parasite and a lizard introduced to Hispaniola and apparently confined to an altered urban area, is of particular interest. In the absence of any substantial predation pressure, this lizard may prove to be an ideal tool for exploring the hypothesis that parasites are responsible for limiting population growth.
I have also worked closer to home. In addition to efforts attempting to more precisely define the distributions of amphibians and reptiles in Missouri, from 1987–1991 I studied the herpetological community composition on natural prairie areas subjected to different management methods. Although the acquisition of conclusive data was compromised by unplanned alterations in the management of the study sites (not under my control), preliminary results were sufficiently effective that the Missouri Department of Conservation subsequently initiated a revised management plan for aquatic habitats within the prairie. Another interesting observation of different microhabitat use by two species of box turtles led to a more detailed study by Dan Sammartano, a graduate student at Southwest Missouri State University, on whose graduate committee I served.
Realization of my research goals has been affected by the heavy teaching loads expected of faculty at Avila University and by constraints imposed as a result of working exclusively with undergraduates. However, my involvement over a number of years with undergraduate research has resulted in building gradually a foundation upon which I can continue to address the real and fascinating questions that drew me to biology in the first place. In summary, my interests have led to studies in a fascinating geographic region and to investigations that will never cease to provide opportunities to ask new questions and explore any number of alternative explanations.
CURRENT ACTIVITIES
Two projects, triggered by REU experiences in the Dominican Republic, are basically systematic in nature. One question regarding the status of a geographically isolated cybotoid anole arose as a result of discussions with Al Schwartz after my first trip to the Cayos Siete Hermanos. Because of the exceedingly small sample available at the time, Al had not been able to determine whether these lizards represented a mere isolate, a subspecies, or even a separate species. When preliminary meristic studies failed to resolve the issue, even after a substantial series was collected during our trips, it became obvious that DNA sequences would be necessary. Because I have no dedicated research space at Avila (student research must be conducted in teaching labs), I could take the project no further. Fortunately, Rich Glor, a former REU student, became interested in the question and has expanded the scope of the study to include other cybotoid anoles, including satellite island populations of similarly uncertain status. Interestingly, the population in the Cayos and on the adjacent main island are most closely related to nearby populations assigned to Anolis cybotes— instead of those they resemble most closely and which also occupy exceedingly xerix habitats. Another investigation, whether two currently recognized species of Ameiva are, in fact, merely color variants, grew out of a REU study seeking to determine the conditions that permitted apparent syntopy of three congeners of similar size and habits. The two were merely pattern variants (not entirely unexpected as similarly unpatterned variants are known to occur in other populations within the same species complex).
Additional projects in various stages, with either Avila students or REU participants, include comparisons of diets in sympatric lizard species of similar size but with different foraging modes, the diet of a widely distributed blind snake, an autecology of an anoline lizard apparently more inclined than most to be terrestrial, efforts to determine the nature of niche partitioning operative in montane forest communities, habitat utilization and activity of dwarf geckos, possible effects of melanism on thermal biology in closely related species of Ameiva from the Anguilla Bank, foraging behavior in an arboreal boid, niche partitioning (or lack thereof) in two apparently syntopic anoles of the Grenada Bank, and effects of human habitat alterations on amphibian and reptilian distributions and population densities.