The comparative ecology of two closely related, sympatric species of Dacus (Diptera) in Queensland

Two sibling species of tephritid fruit fly, Dacus neohumeralis and D. tryoni, are common along the coastal region of Queensland. Natural populations of these flies were sampled at Rockhampton and Yeppoon at 2-monthly intervals between May 1962 and March 1964 to determine the extent to which the two species utilize the same resources. Estimates of their relative abundance were obtained by catching adult flies on fruit trees and by collecting fruit containing immature stages. Both species used the same kinds of fruit for oviposition and larval development. At Rockhampton, where D. tryoni constituted 90% of the population, this species utilized five favourable kinds of fruit equally, whereas D. neohumeralis utilized some to a greater extent than others. At Yeppoon, where D. neohumeralis was the more abundant species, it utilized four different kinds of fruit equally whereas D. tryoni utilized some more than others. The same kinds of fruit were available at the two places, suggesting that "preferences" were associated with the locality and the relative abundance of the two species rather than with the kinds of fruit. At Rockhampton, D. neohumeralis remained rare relative to D. tryoni at all times of year and no distinct seasonal changes in their relative abundance were detected. When fruit trees were classified according to the shelter they provided, D. neohumeralis was more abundant in trees with dense foliage than in open, exposed trees. D. tryoni was equally abundant in the fruit from all types of trees. A third species of fruit fly, Afrodacus jarvisi, shared the larval food resources of D. neohumeralis and D. tryoni in late summer. Samples with a high density of A. jarvisi contained fewer D. neohumeralis and D. tryoni than samples with a low density of A. jarvisi, but their relative proportions were unchanged, indicating that the deleterious effect was not differential. The results of this study are examined in the light of the competitive displacement theory. Flies that emerged from fruit were large and not at all like the small flies which can be reared in the laboratory when food is in short supply. Competition between the two species for larval food thus appears to be unimportant. Moreover, there was no evidence that one species might be displacing the other in the Rockhampton area. Both species preferred the same kinds of fruit and fruit trees when a choice was available, suggesting that both species can live together without exerting any deleterious effects on one another.