Factors Affecting the Occurrence of Facilitative Effects in Interspecific Interactions: An Experiment Using Two Species of Drosophila and Aspergillus niger

The designs of many previous experiments investigating interspecific interactions have been such that the frequency of competition has been over-estimated. We report here the results of a laboratory experiment which investigated the interactions between two species of Drosophila. D. melanogaster and D. hydei, and a mould. Aspergillus niger. The experiment was designed to reveal both positive (facilitative) and negative niger. The experiment was designed to reveal both positive (facilitative) and negative (inhibitory) effects, and so give a better indication of the relative frequency of competition, amensalism and other interaction types. Our experimental arrangement included three important features: (a) three species rather than two, so that indirect effects, including enemy's enemy. could be manifested; (b) disparate taxonomic distances, with two species being very closely related, the third quite distinct; and (c) non-synchronous colonization' of the resource, permitting investigation of temporal priority effects. The results showed that the type of interaction which occurred between any two species in the system was highly variable, with all six theoretically possible types of interaction being found. In general, there appeared a contest for resources between the three species. There was no prevailing incidence of facilitation produced by incorporating species from divergent taxa. The effect of temporal priority was to give an advantage to the early species, causing severe inhibition of the later species. However, facilitation did occur through indirect effects; D. melanogaster facilitating D. hydei by reducing the surface cover of the mould. This indirect facilitation can be likened to the occurrence of positive effects in other systems where the mechanism is based on the amelioration of environmental conditions which are harsh for one of the species involved. The consequences of variation in interactions between species forming metacommunities are discussed.