AGGREGATION AND THE COEXISTENCE OF MYCOPHAGOUS DROSOPHILA

(1) Intraspecific aggregation has been postulated to facilitate the coexistence of competing species that share patchy resources. Data on adult emergence numbers of four mycophagous species of Drosophila from field-collected mushrooms show that these species exhibit highly aggregated distributions, both in collections comprising several naturally occurring species of mushrooms and in those of a single species for which mushroom size, condition, exposure time, and location in the field were controlled. (2) The relation between the mean and the variance in the numbers of flies emerging per mushroom reveals that the mean level of larval crowding increases with overall population density. Aggregation of larvae of each species is due to (i) females laying eggs in clutches of more than one egg, and (ii) non-random distributions of ovipositing females across breeding sites. (3) The emergence number of different species of Drosophila tend to be positively correlated across mushrooms, especially between species belonging to the same species group. In fact, within species groups, intraspecific and interspecific aggregation of ovipositing females contribute about equally to the proportional increase in the number of competitors an individual larva shares a mushroom with. Thus, aggregation does not appear to play an important.role in the coexistence of species belonging to the same species group. (4) The probability of parasitism of emergent Drosophila by the nematode Howardula aoronymphium Welch is generally independent of emergence numbers. Thus, although density-dependent parasitism could promote coexistence of species that have independent aggregated distributions, we find little evidence of such density dependence in these species.