Coexistence mechanisms of mycophagous drosophilids on multispecies fungal hosts: aggregation and resource partitioning

1. In nature, various mechanisms that provide some refuges for inferior spe cies to escape from competition act together to promote the species coexist ence in a local community. We propose a method to evaluate simultaneously t he relative importance of different mechanisms for the species coexistence, focusing on a mycophagous drosophilid community depending on a multispecie s fungal host system, where two mechanisms - aggregation over patches of si ngle fungal species and resource partitioning at the fungal species level - can act together. The data are based on the numbers of adult insects emerg ing from naturally occurring fungi collected from a temperate forest in nor thern Japan. 2. We confirm the finding of Sevenster & van Alphen (1996) that large patch es are low-density refuges despite the large absolute numbers of competitor s on them, and therefore adopt new measures for intra- and interspecific ag gregations and the species-persistence criterion, which were developed by S evenster (1996) to take account of variable patch sizes in calculating the aggregation measures. 3. The calculated measures an compared among three analyses for fungal reso urces: AR-analysis (the original data-set from multispecies fungal patches) in which both aggregation and resource partitioning can act together; R-an alysis (combining the data for all patches of each fungal species) in which only resource partitioning at the fungal species level can act; and A-anal ysis (calculating the measures independently for each fungal species) in wh ich only aggregation process can act as a coexistence mechanism. 4. Intraspecific aggregation over multispecies fungal patches is strengthen ed by both aggregation and resource partitioning processes. 5. We reconfirm the finding of Sevenster & van Alphen (1996) that interspec ific associations are consistent over years, and support their claim that s uch consistent associations should be taken as real aspects of the communit y structure. 6. Measures for the persistence criterion (T), the 'relative effect of comp etitor aggregation', are less than unity, satisfying the necessary and suff icient condition for the species persistence, in most drosophilid species c oncerned; these tend to be smaller in AR-analysis than in R- or A-analysis, suggesting facilitation of the species coexistence by both aggregation and resource partitioning in the mycophagous drosophilid community studied. 7. Differences in T-value between AR- and R-analyses and between AR- and A- analyses may indicate relative contributions of aggregation and resource pa rtitioning, respectively, to the species coexistence. We propose a method t o compare various communities in a multi-dimensional space with regard to t he relative importance of different coexistence mechanisms.