COMMUNITY STRUCTURE AND ENVIRONMENTAL-STRESS - DESICCATION PROMOTES NESTEDNESS IN MYCOPHAGOUS FLY COMMUNITIES

In a previous field experiment, communities of mycophagous flies that emerged from Agaricus bisporus mushrooms exhibited a nested-subset pat tern related to mushroom size. A phorid species and Leucophenga varia (Drosophilidae) emerged from small, medium, and large mushrooms, but o ther drosophilid species were usually limited to large mushrooms. Here , we conducted two experiments and one field survey to determine wheth er this nestedness pattern is caused by nested desiccation tolerances among species, coupled with the mediating effects of mushroom size. In the first experiment, 20 first instar larvae of either Drosophila tri punctata or D. putrida were added to A. bisporus mushrooms that weighe d 5 g, 10 g, or 20 g. Mushrooms were placed in environmental chambers at different temperatures for 5 d (maxima of 25 degrees C, 30 degrees C, or 35 degrees C). Both mushroom size and temperature had significan t direct effects on the proportion of larvae completing development; l arvae in small mushrooms are more susceptible to desiccation stress th an larvae in large mushrooms. In a second experiment, we examined the effect of desiccation stress on community structure. Woodland mushroom s collected in the field were cut in half; the halves were placed at d ifferent incubation temperatures (maxima of either 25 degrees C or 35 degrees C) for 5 d, and emerging flies were counted and sorted by spec ies. The halves incubated at 35 degrees C produced significantly neste d communities, the halves incubated at 25 degrees C did not. Correlati ons between fly abundance and mushroom mass were also more significant at higher temperatures. These results were consistent with the hypoth esis that desiccation stress, differentially mediated by mushroom size , drives nested subset structure in these communities. In a four-week field survey, however, nestedness was not related to mean temperature or rainfall. We contend that small habitats are less resistant to chan ges in environmental conditions than larger habitats, so the intensity of environmental stress will correlate with habitat size. So, some in itially variable and non-nested communities might decay to a nested-su bset pattern of species composition as a result of differential habita t buffering under stressful environmental conditions.