The significance of ratios of detritus types and micro‐organism productivity to competitive interactions between aquatic insect detritivores

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Abstract
Summary: Investigations of competitive interactions emphasize non‐detrital resources, even though detritus is a major component of most food webs. Studies of competing species focus usually on single resource types, although consumers in nature are likely to encounter mixtures of resource types that may affect whether competition results in exclusion or coexistence. The invasive mosquitoAedes albopictusis capable of excluding the native mosquitoOchlerotatus triseriatusin competition for single detritus types in laboratory and field microcosms. In this study, we used nine ratios of two detritus types (animal and leaf) common in natural containers to test whether detritus ratios affect the outcome of competition. Under intraspecific and interspecific competition,A. albopictusattained higher survival and estimated population growth rate than didO. triseriatus. Unlike past studies, both species had positive growth and high adult survival, with little evidence of competitive effects, under one resource ratio (10 : 1 ratio of leaf : animal detritus) regardless of mosquito densities, suggesting potential coexistence. Path analysis showed that densities of larvae had negative effects on population growth forO. triseriatusbut not forA. albopictus, indicating competitive superiority ofA. albopictus. Population growth of both species was affected strongly by the direct paths from animal (positive) and leaf (negative) detritus, and the indirect effect of leaf detritus via bacterial production (positive). Field sampling established that detritus entered real tree holes in ratios similar to those in our experiment, suggesting that natural variation in detritus ratios may influence local coexistence of these species. Seasonal variation in ratios of plant and animal detritus indicated that temporal as well as spatial variation in inputs may be important for potential coexistence.