Dispersal patterns of two phytophagous prairie beetles: Implications for the conservation of insects in fragmented habitats.

摘要

For many species dispersal to and colonization of empty habitat patches in spatially structured landscapes is critical to long-term persistence because local populations are susceptible to extinction. For such species that function as metapopulations, however, it remains unclear what characteristics of a species or its habitat predispose it to metapopulation function. In this thesis I investigate, using mark-recapture techniques, the factors contributing to the dispersal patterns and hence, landscape-level population structure of two phytophagous beetles in the agricultural landscape of east-central Iowa.; I examined movement patterns of Chrysochus auratus and Rhyssomatus lineaticollis, monophagous herbivores of Apocynum cannabinum and Asclepias syriaca, respectively. For C. auratus, the frequency of dispersal between host patches and the average movement distance within a patch were positively related to beetle density in the patch but not with the host ramet count. Recapture probabilities within a patch were affected by the distance to neighboring patches regardless of their size. For R. lineaticollis, movement patterns and recapture probabilities were not related to beetle densities or isolation distances. For both species, frequent dispersal between host patches and the absence of vacant patches in the study area indicate that they are functioning as a patchy population, not as a metapopulations. Furthermore, I showed that small, experimenter-created, clusters of host plant may not be suitable for colonization by C. auratus but may facilitate movement between larger host patches by functioning as stepping-stones for dispersers.; Taken together, I showed that these two beetle species do not function as metapopulations although they have the spatial framework to do so. The spatial and temporal instability of their host plants may have promoted strong dispersal capabilities and patchy population structure in C. auratus and R. lineaticollis even before the fragmentation of their habitat. Monophagous herbivores of stable host plants may be more likely to function as metapopulations than herbivores dependent on weedy species.