My research interests broadly include the trophic, spatial, and growth dynamics of marine vertebrates, with particular interest in characterizing life history variation, and its causes and consequences, in protected species. Understanding life history complexities can be critical to the development of effective conservation and management strategies, particularly in highly mobile marine species. My research relies on the integration of two primary tools: (1) sclerochronology, the study of concentric growth layers in calcified animal tissues (e.g., turtle bone, marine mammal teeth), and (2) ecogeochemistry, the use of stable isotopes and trace elements to infer past diet and habitat use of consumers. Together these techniques can provide invaluable insight into the ecology of otherwise difficult to study species and life stages. Much of my research focuses on the development and application of novel geochemical tools for the study of marine vertebrate ecology. A final aim of my research is to maximize the use of dead stranded marine organisms—both modern and historical—in ecological research. Information gleaned from their tissues can enable us to investigate previously intractable questions in marine ecology.
Marine Top Predator Trophic Dynamics
Despite decades of food web research, we still have a poor understanding of how environmental change alters marine top predator trophic interactions through time. For my NSF Postdoctoral Fellowship, and in collaboration with Drs. Kelton McMahon (URI), Aleta Hohn (NOAA), and Larisa Avens (NOAA), I will use a retrospective approach to investigate how marine mammals and sea turtles have adjusted their foraging strategies in response to shifts in climate and prey, predator, and competitor abundance over the past century. To this end, I will sample specimens from historical collections (e.g., Smithsonian, NOAA) for nitrogen isotopes of amino acids to examine how common bottlenose dolphin, harbour porpoise, and loggerhead sea turtle trophic position and niche breath have changed over time. Together, these data will provide a mechanistic understanding of the trophic response of key marine top predators to recent environmental change. Such information is paramount to predicting the consequences of future ecosystem change, and these topics were highlighted as high priority research areas in the recent decadal review of ocean science by the National Academy of Sciences.
Sea Turtle Growth Dynamics
Sea turtle growth rates are highly variable within and among species, populations, and life stages. Yet, little is known about the specific factors underpinning this variability given the difficulty of studying growth rates in such highly mobile species. This limitation can be overcome through the study of their humerus bones (pictured right), which have been collected from dead, stranded turtles for decades and contain annual growth layers that record information on body size, age, growth, diet, and habitat use. Using skeletochronology I have characterized the ontogenetic and spatiotemporal growth patterns of loggerhead and Kemp's ridley sea turtles. I have also examined the influence of both natural (climate change, changing population density, diet composition) and anthropogenic stressors (Deepwater Horizon oil spill) on sea turtle growth rates, in some cases through complementary isotopic analyses. As sea turtle population dynamics are highly sensitive to small changes in demographic rates, characterizing the proximate drivers of somatic growth variation and subsequent influences on population dynamics is of high importance to sea turtle conservation and management.
Movement Ecology & Resource Shifts
When isotopic or elemental differences exist among resources (habitat and/or diet) utilized by animals, this natural variation can be used to study animal migrations. Through complementary skeletal and geochemical analyses, I have harnessed this variation to provide novel insights to the oceanic stage duration and timing of oceanic-to-neritic ontogenetic habitat shifts in loggerhead, Kemp's ridley, and hawksbill sea turtles. When paired with complementary somatic growth rate information contained within their bones, such information has also allowed me to characterize growth dynamics surrounding these habitat shifts and, in some cases, to test ontogenetic niche theory. More recently, I have worked in collaboration with Drs. Alyssa Shiel (OSU) and Jessica Miller (OSU) to develop lead (Pb) isotope and trace element tools to study the connectivity of population subgroups within nearshore habitats. I am actively developing these and other geochemical techniques to enhance the tools available to study the movement of marine megafauna.
Life History Variation & Population Dynamics
Despite having population-level distributions spanning whole continental shelves or ocean basins, individual sea turtles often display remarkable inter-annual fidelity to specific foraging grounds, resources, and migratory routes. Such life history variation has been linked to differences in a suite of demographic rates in multiple sea turtles species and thereby holds the potential to fundamentally alter a species population dynamics. However, this information has yet to be robustly integrated into demographic models, largely due to insufficient data. I am actively working in collaboration with Dr. Selina Heppell to develop spatially-explicit, age-structured matrix population models for the critically endangered Kemp's ridley sea turtle, a species whose population is divided into distinct subgroups (Atlantic vs. Gulf of Mexico) that have unique demographic rates (growth, maturation schedules) and oceanic stage duration. With these models we are evaluating the relative contribution of Atlantic Kemp's Ridleys to overall species population growth.