コガタシマトビケラ幼虫個体群の分布に対する種間競争の影響

Effects of interspecific competition among three species of net-spinning caddisflies on the distribution of larval population of Cheumatopsyche brevilineata (the smallest species) were examined for 4 years and the mechanism of their coexistence was considered. At the current velocity lower than 139.2 cm/s, densities of third, fourth and fifth instar larvae of C. brevilineata and Hydropsyche orientalis (the intermediate-sized species) had very similar relationships with current velocity to each other. Therefore, the relationship between densities of both species in the same quadrat (50 X 50 cm) was positive in scarce years of fifth instar larvae of Stenopsyche marmorata (the largest species), but dome shaped in their most abundant year. This fact seems to indicate the exclusion of C. brevilineata by H. orientalis, which is intensified by fifth instar larvae of S. mamorata which narrow nesting sites of both species. Third instar larvae of H. orientalis are smaller than fifth instar larvae of C. brevilineata, whereas the exclusion of the former could not be detected. In all other combinations of third, fourth and fifth instar of both species, H. orientalis are larger than C. brevilineata, though only in combinations of the three larval instars of C. brevilineata and fifth instar of H. orientalis, density relationships were dome shaped. Critical densities over which the exclusion of C. brevilineata occurred were estimated from dome shaped relationships. In the most abundant year of S. marmorata larvae, 50.1% of C. brevilineata larvae over second instar were estimated to be under strong pressure of competition with H. orientalis. However, the larval population of C. brevilineata did not significantly decrease in the next year. C. brevilineata larvae seem to avoid severe interspecific competition by growing later than H. orientalis.