Factors affecting the occurrence of facilitative effects in interspecific interactions: an experiment using two species of Drosophila and Aspergillusniger

The designs of many previous experiments investigating interspecific intera ctions have been such that the frequency of competition has been overestima ted. We report here the results of a laboratory experiment which investigat ed the interactions between two species of Drosophila, D. melanogaster and D, hydei, and a mould, Aspergillus niger, The experiment was designed to re veal both positive (facilitative) and negative (inhibitory) effects, and so give a better indication of the relative Frequency of competition, amensal ism and other interaction types. Our experimental arrangement included thre e important features: (a) three species rather than two, so that indirect e ffects. including 'enemy's enemy', could be manifested: (b) disparate taxon omic distances. with two species bring very closely related, the third quit s distinct; and (c) non-synchronous 'colonization' of the resource. permitt ing investigation of temporal priority effects. The results showed that the tr;pe of interaction which occurred between any two species in the system was highly variable, with all six theoretically possible types of interacti on being found. In general, there appeared a contest for resources between the three species. There was no prevailing incidence of Facilitation produc ed by incorporating species from divergent ta,va. The effect of temporal pr iority was to give an advantage to the early species, causing severs inhibi tion of the later species. However, facilitation did occur through indirect effects; D. melanogaster facilitating D. hydei by reducing the surface cov er of the mould. This indirect facilitation can b; likened to the occurrenc e of positive effects in other systems where the mechanism is based on the amelioration of environmental conditions which are harsh for one of the spe cies involved. The consequences of variation in interactions between specie s forming metacommunities are discussed.