Competitive displacement between ecological homologues

Abstract

Competitive displacement among three species of parasitic Hymenoptera which are ecological homologues has occurred in the field in southern California. Aphytis chrysomphali (Mercet) [Hymenoptera: Aphelinidae] was eliminated in 10 years (1948-1958) from nearly all of its former range—an area of some 4,000 square miles—by Aphytis lingnanensis Compere, which was imported from the Orient in 1948. Subsequently, Aphytis melinus DeBach, another exotic from India-Pakistan, virtually displaced A. lingnanensis from about 500 square miles of interior climatic areas in 4 years (1957-1961), but during the same time A. lingnanensis precluded the establishment of A. melinus in the milder intermediate climatic areas of San Diego County. Competitive displacement occurred in places in spite of food [the host scale insect, Aonidiella aurantii (Maskell)] being abundant. Thus food scarcity is not necessary for competitive displacement to occur. Two other species of parasites, Comperiella bifasciata Howard and Prospaltella perniciosi Tower, which attack the same host but are not ecological homologues of the Aphytis spp., continued to coexist with Aphytis spp. Experimental studies in laboratory population cages supported and helped to explain the field results. Single species cultures served as checks and could be maintained indefinitely. Success in single species cultures, as determined by the average population density attained, was not correlated with inherent fecundity. When any two of the three species—Aphytis fisheri DeBach, A. melinus, and A. lingnanensis—were cultured together, one species always eventually eliminated the other. Differences in F1 progeny production explained the results. Inherent differences in fecundity were largely responsible for the differences in F1 progeny production, but aggressive competition between larvae tended to modify inherent fecundity. Competition between adults was not a factor in competitive displacement, but relative ability to find hosts in the field when hosts are scarce is important. Physical environmental conditions also affect F1 progeny production; hence, in different environments the winner in competition may differ. When A. lingnanensis and A. fisheri were cultured together with a constant surplus of hosts, A. fisheri was eliminated after nine generations. This corroborates the field observations that food (host) scarcity is not necessary for competitive displacement to occur. Aphytis lingnanensis and Comperiella bifasciata, which have slightly different ecological niches, were shown to coexist in laboratory cultures as they do in the field. However, Prospaltella perniciosi was displaced. Reasons for this are discussed. Mechanisms of competitive displacement and the importance of competitive displacement in population ecology are also discussed.