Competition in Natural Populations of Mycophagous Drosophila

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Abstract
In the northeastern United State, individual mushrooms commonly harbor the larvae of up to four species of Drosophila (D. Falleni, D. Recens, d. putrida, and D. Testacea), as well as larvae of crane flies (Tipulidae), wood gnats (Anisopodidae), and other, small flies. An experiment showed that larvae of these species commonly exhaust the food in single mushrooms under natural conditions. Supplemental mushroom increased survival to adulthood in three drosophila species and resulted in larger adult flies. There was substantial variation among individual mushrooms in the degree of food depletion by larvae; while some mushrooms were completely devoured, others appeared to provide more than enough food for the larvae. Mean body size of Drosophila that matured in nature were similar to those of flies reared in our experiments without supplemental food, which suggests that resource depletion and larval competition are common in natural populations of these species. (While the evidence for resource limitation is compelling, our methods do not allow us to distinguish between intra— and interspecific competition.) As a result, fitness of flies in nature should vary greatly as a function of the amount of food available to larvae. We speculate that low rates of parasitism allow mycophagous drosophila populations to deplete food resources more commonly than do phytophagous insects.