Strategies for the coexistence of two caddisflies, Aoteapsyche raruraru and A. colonica (Trichoptera: Hydropsychidae) were considered in a South Island lake outlet, New Zealand. Three main strategies were assessed, firstly that competition between species may be reduced by the presence of selective predators, secondly, that coexistence is maintained by periodic disturbance which reduces competition effects, and thirdly that interspecific competition is negated by segregation of either life histories or feeding strategies, and diet or microhabitat preferences. The first strategy was rejected, as gut analyses of common fish and macroinvertebrate predators showed that both species of Aoteapsyche were taken in approximately equal proportions to their benthic densities (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 histories was not observed, but analyses of larval guts indicated that diet was affected by shelter location on the substrate. Sampling of substrate microhabitats showed that A. raruraru larvae occurred on the upper surfaces, sides and under surfaces of large cobbles, although significantly higher densities were collected from the upper surfaces and sides. In contrast, A. colonica aggregated on the sides and undersurfaces of cobbles. In other streams A. colonica shows a similar microdistribution 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 possible by microhabitat segregation where food availability is high, however in the absence of a plentiful food supply and A. colonica the microdistribution of A. raruraru may differ.
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