STRATEGIES FOR COEXISTENCE IN 2 SPECIES OF NEW-ZEALAND HYDROPSYCHIDAE(TRICHOPTERA)

Strategies for the coexistence of two caddisflies, Aoteapsyche rarurar u and A. colonica (Trichoptera: Hydropsychidae) were considered in a S outh Island lake outlet, New Zealand. Three main strategies were asses sed, firstly that competition between species may be reduced by the pr esence of selective predators, secondly, that coexistence is maintaine d by periodic disturbance which reduces competition effects, and third ly that interspecific competition is negated by segregation of either life histories or feeding strategies, and diet or microhabitat prefere nces. The first strategy was rejected, as gut analyses of common fish and macroinvertebrate predators showed that both species of Aoteapsych e were taken in approximately equal proportions to their benthic densi ties (i.e. 10:1 A. raruraru to A. colonica), indicating that predation was unlikely to influence coexistence. Similarly the second strategy was not supported by observations of flow conditions during the study which were insufficient to move the substrata extensively colonised by both species of Aoteapsyche. Finally, temporal segregation of life hi stories was not observed, but analyses of larval guts indicated that d iet was affected by shelter location on the substrate. Sampling of sub strate microhabitats showed that A. raruraru larvae occurred on the up per surfaces, sides and under surfaces of large cobbles, although sign ificantly higher densities were collected from the upper surfaces and sides. In contrast, A. colonica aggregated on the sides and undersurfa ces of cobbles. In other streams A. colonica shows a similar microdist ribution but in a silted stream lacking seston, and in the absence of A. colonica, A. raruraru larvae occurred mainly on the under surfaces of stones. Co-existence of these two congeners would seem to be possib le by microhabitat segregation where food availability is high, howeve r in the absence of a plentiful food supply and A. colonica the microd istribution of A. raruraru may differ.