Fluctuations in Drosophila Populations in a Tropical Area

Fluctuations in the populations of D. ananassae, D. hypocausta, D. anuda, and D. melanogaster were studied on Moen Island, Truk, Eastern Caroline Islands. Almost daily collections were made from fruit-baited traps for a period extending over approximately thirteen months. In this wet tropical area, with very little variation of temperature and humidity, the major population expansions of D. ananassae coincided with the autumn and spring fruiting of a large breadfruit tree near the fly traps. The population of D. hypocausta underwent a large expansion during the longer period of autumnal fruiting of the breadfruit tree but failed to expand during the spring fruiting. The population of this species had dwindled to very low numbers following the autumnal expansion. The highest population expansion of D. anuda occurred more than a generation's time subsequent to the autumn breadfruit harvest. The population of this species was probably dependent upon a source of food other than the breadfruit. D. melanogaster did not undergo any expansions during the period of collection. Small month to month fluctuations were observed in the population of D. ananassae during the period between the autumnal and spring fruiting of the big breadfruit tree near the fly collecting station. Cyclic large population expansions of a species in a uniformly tropical wet climate are believed to be dependent upon the fruiting of certain types of fruit trees. Small irregular month to month fluctuations of a species in such an area are probably non-cyclic but depend upon variations in the local food supply during the time between the fruiting of major food supply trees.