Landscape Heterogeneity, Patch Connectivity, and Host-Parasitoid Population Dynamics

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Landscape heterogeneity, patch connectivity, and host-parasitoid population dynamicsNSF Ecological Studies Grant (DEB-0211359)September 1, 2002 - August 31, 2005 The fragmentation and loss of natural habitats is a dire world-wide problem. Ecologists have made significant in-roads toward understanding the impact that increased habitat fragmentation or spatial patchiness might have on the long-term persistence of predator-prey populations. However, most fragmentation studies have disregarded the vegetational cover that exists between patches (known as the matrix), generally assuming that the matrix is featureless and neutral to the movements and population fluctuations of both predator and prey species. In reality, the matrix is complex and ever-changing. Any alteration in matrix composition may significantly impact how predators interact with their prey. This proposal outlines a course of study designed to address how different matrix types influence the movement and population fluctuations of an herbivore and its predator. The planthopper, Prokelisia crocea, and the parasitoid wasp, Anagrus columbi, which are distributed among discrete patches of prairie cordgrass in the Great Plains of the United States form an ideal system for this type of study. The crux of this proposal is a relatively large-scale field experiment that is designed to alter the ability of planthoppers and wasps to move among patches (i.e., patch connectivity) by embedding cordgrass patches in either of two dominant matrix types (mudflat or the exotic grass species brome). Emigration, immigration, the pattern of spatial spread, and the within-patch distribution of insects will be assessed from the recapture of fluorescent-marked individuals. The experiment also will be carried out over 3 spatial scales and 6 generations to provide information on the effect of the matrix manipulation on parasitism and density fluctuations among patches and over time. Never before has the patch structure and matrix been manipulated in predator-prey population studies in the field. As a complement to this field experiment, trials will also be conducted to determine the behavioral basis for matrix-dependent movement patterns of the planthopper and wasp. Finally, based on information derived in the above studies, a simulation model will be developed with the goal of evaluating the impact of habitat fragmentation and degradation (e.g., changes in the landscape matrix) on the long-term persistence of predator-prey interactions. Results from this renewal of NSF grant DEB-9973789 should provide critical and much needed information on the effects of heterogeneous landscapes on predator-prey interactions, and hopefully catalyze a broader approach to studies of predator-prey interactions. This research also has broad ramifications for conservation programs that are interested in the connectivity of reserves: a matrix that facilitates dispersal (i.e., a high-quality matrix) may serve a function identical to that of stepping stones or corridors. Finally, this research may have a direct bearing on the success of pest management programs that employ strategies that purposely modify the matrix (e.g., through inter-cropping, terracing, or leaving some fields fallow). Knowledge of the effect of these changes on pest and natural enemy movement and interactions may be critical to developing a successful pest-control program.