Trophic interactions (the consumption of prey by predators) define the flow of energy and nutrients in food webs. Many organisms involved in trophic interactions are also able to move (disperse) among areas of suitable habitat. For example, after eating a prey item in one patch of habitat, a predator may disperse to another habitat patch before consuming additional prey. Such dispersal of organisms may affect the outcome of trophic interactions by altering the size of populations within patches. At the same time, trophic interactions may influence population sizes, and thus determine the number of organisms available to disperse among patches. As a result, trophic interactions and dispersal likely combine to dictate the structure and dynamics of food webs at the local (within-patch) and regional (among-patch) scales. This study uses a system of freshwater rock pools on Appledore Island, Maine to determine how dispersal and trophic interactions combine to influence local and regional food-web dynamics. The simple rock-pool food web consists of phytoplankton, herbivorous zooplankton, and a predatory water bug (Trichocorixa). Previous research shows that predation by Trichocorixa strongly affects local food webs and that Trichocorixa is capable of dispersing among rock pools. A field experiment will be used to quantify the local and regional effects of dispersal by Trichocorixa on the rock-pool food web. A specific mathematical model has been developed to predict the outcomes of this experiment, and this model will be extended to other ecosystems in order to determine the general effects of dispersal on local and regional food-web dynamics.
This study combines theoretical and empirical approaches to increase understanding of the role of spatial structure in food-web dynamics. As such, results of this research will inform the conservation and management of ecosystems threatened by habitat fragmentation. This project supports the dissertation research of one graduate student and provides the opportunity for an undergraduate to be involved in field research as part of a multi-university internship program. Results will be disseminated broadly through scientific publications and presentations, lectures to undergraduate courses, and outreach to local high school students, teachers, and the general public.
Overview This research focused on understanding how populations and food webs persist in fragmented landscapes. In particular, we studied how dispersal among habitat fragments and predator-prey interactions occurring within fragments combine to promote persistence or extinction. We conducted experimental, observational, and modeling studies and focused our work on a system of freshwater rock pools on Appledore Island (Isles of Shoals, Maine, USA). There are approximately 1,500 pools on Appledore, each housing a relatively simple food web with a highly mobile and voracious top predator (water boatmen), and thus are a fantastic field system for our research. We conducted a mesocosm experiment to determine the proximate causes of top-predator dispersal and collected water boatmen from 100 pools on Appledore for a population genetics project to quantify how spatially structured their populations are as a consequence of this dispersal. As part of this project, we developed a genetic marker library (microsatellite and mitochondrial DNA) for the water boatmen. We also completed field studies examining the passive dispersal of organisms among pools via two mechanisms: overflowing water (caused by intense rain events) and gulls (which frequently visit the pools). Finally, we conducted modeling exercises examining the effects of prey density on predator dispersal rate and predator dispersal on their population genetic structure. The findings from these studies have been prepared into three scientific peer-reviewed manuscripts (one published already), one Ph.D. dissertation, one undergraduate honors thesis, and three conference presentations. Findings The results from our research program indicate that dispersal is a crucial factor contributing to population and food-web persistence in fragmented ecosystems like the Appledore rock pools. In the mesocosm experiment, predators were shown to emigrate far more frequently when prey (zooplankton) densities were lower, which a mathematical model suggests prevents extinction of the prey populations within habitat fragments by relieving the prey of predation pressure when they are at low densities (near extinction). Our population genetics survey and modeling shows that dispersal by the top predators among pools is indeed very frequent and promotes persistence of their populations as well. The markers we developed amplified highly variable genetic sequences, but populations were only found to be genetically distinct from each other at far distances. Our other studies indicate that dispersal occurring via overflows and gulls (Larus spp.) are also important in this system, in particular for organisms at lower trophic levels (e.g., phytoplankton, zooplankton) who lack the ability to disperse among pools on their own (via, e.g., flight). Dispersal via overflow was negatively related to trophic level, such that phytoplankton dispersed more than zooplankton, who dispersed more than water boatmen. Overflows, however, are relatively infrequent in the Appledore rock pools, and dispersal via overflow is much less frequent than via gulls. Gull-mediated dispersal, on the other hand, was shown to occur regularly and have a significant homogenizing effect on species diversity (pools in areas of higher gull density were more similar to each other). Training and Development This project provided research and teaching skills and experiences to the co-PI Joseph Simonis (now a post-doctoral researcher in applied population biology) and the undergraduate student Kara Pellowe (entering a Ph.D. program in marine conservation science in the fall). Through this project, I have developed my abilities at mentoring and advising undergraduates, project management, genetics lab techniques and analyses, and modeling ecological populations and food webs. Pellowe has received training in experimental design, field data collection, data management and analyses, and scientific writing. Outreach Activities The outreach for this project focused primarily on three activities: engaging the public regarding our research, education, and mentoring. Shoals Marine Lab (SML) provides a fantastic opportunity to engage a wide range of people about research. Visitors to Appledore Island include high school and undergraduate students and teachers, fishers, gardening/history enthusiasts, and the general public. In addition, SML is staffed by engineers, boat captains, chefs, laboratory personnel, and interns that all are interested and engaged in the science that goes on there. During summer 2011, we gave presentations regarding our experiments (placed prominently near the SML campus) to visitors from a variety of different groups (garden/island tours, courses, day trippers, staff). Additionally, Simonis gave guest lectures regarding our ongoing research to three undergraduate courses and one high school course conducted at SML. During a return trip in 2012, Simonis installed a research poster in the SML commons explaining the research done on the rock pools. Simonis (with Hairston) also advised and mentored Pellowe on her undergraduate thesis project. We worked together to develop and execute her project and are currently preparing a manuscript for submission.