COMPLEX INTERACTIONS BETWEEN BIOTIC AND ABIOTIC FACTORS - EFFECTS ON MYCOPHAGOUS FLY COMMUNITIES

We measured the direct and interactive effects of larval density, ant predation, and soil moisture on larval survival and community structur e of mycophagous flies. First instar larvae of a brown-eyed Drosophila tripunctata mutant were added to Agaricus bisporus mushrooms (10, 20 or 40 larvae/mushroom). The mushrooms were placed on dry. moist, or we t soil in ant-access and ant-exclusion treatment cups. The cups were p laced in a woodland for 5 days where they were available to native fli es for oviposition. We used ANOVA and MANOVA to measure the effects of our independent variables on the percentage of mutant larvae completi ng development and the number and diversity of native metamorphs. Ants visited wet soils (30% visited) more frequently than moist (20% visit ed) or dry soils (10% visited). and had stronger negative effects on l arval survivorship and native metamorph abundance in wet and moist mic roenvironments. For example. ant predation reduced metamorph abundance by 60% on wet soil. 30% on moist soil, and only 8% on dry soil. Commu nity composition also changed with soil moisture: a phorid (Dohrniphor a sp.) dominated the dry soil mushrooms (49% of metamorphs). whereas d rosophilids were more abundant in mushrooms on moist (63%) and wet soi ls (74%). The relatively low larval densities used in this experiment had no effect on either survivorship or diversity. Larval survivorship and community structure were influenced more by the direct and intera ctive effects between predation and microclimate than by competitive i nteractions among flies.