LIFE-HISTORIES, SECONDARY PRODUCTION AND MICRODISTRIBUTION OF HYDROPSYCHID CADDISFLIES (TRICHOPTERA) IN A TROPICAL FOREST STREAM

The ecology of seven species of hydropsychid caddisflies was investiga ted in Tai Po Kau Forest Stream (TPKFS), New Territories, Hong Kong, o ver a two-year period (1977-79). Quantitative benthic samples were tak en from different microhabitats in the stream on 47 occasions and, in combination with light-trap catches of adult caddisflies, were used to investigate life-history patterns and estimate secondary production. Cheumatopsyche spinosa (141.9 individuals 0.5 m(-2); 63% of total hydr opsychid densities) and C. ventricosa (54.7 individuals 0.5 m(-2); 24% ) were the most abundant hydropsychids in TPKFS, followed by Macrostem um fastosum (15.2 individuals 0.5 m(-2); 7%). Hydatopsyche melli, Herb ertorossia quadrata, Polymorphanisus astictus and Hydropsyche chekiang ana were present at relatively low densities (each < 2% of total hydro psychid abundance). Macrostemum fastosum made up 42% (mean value 1977- 79: 40.0 mg AFDW 0.5 m(-2)) of total hydropsychid biomass. Hydatopsych e melli was ranked second (13.3 mg AFDW 0.5 m(-2); 14%), followed by C . spinosa and C. ventricosa. There was little inter-year variation in total hydropsychid standing stocks: densities (mean of all species com bined) were 7% higher in 1978-79 than in 1977-78 (mean value 1977-79: 224.8 individuals 0.5 m(-2)), while total biomass was only 3% greater (mean value 1977-79: 96.1 mg AFDW 0.5 m(-2)). However, mean densities and biomass of C. ventricosa and P. astictus were higher in 1977-78, w hile Herbertorossia quadrata, Hydatopsyche melli and Hydropsyche cheki angana exhibited the converse pattern. Standing stocks of M. fastosum and C. spinosa were relatively stable between years. Total hydropsychi d abundance in TPKFS followed an annual pattern of wet-season decrease and dry-season increase, but this tendency was more apparent in some species (e.g. C. spinosa) than others. Macrostemum fastosum was most n umerous following recruitment during the wet season. Cheumatopsyche sp p. and Herbertorossia quadrata had bivoltine life histories while P. a stictus and M. fastosum were univoltine; the latter grew rapidly to th e final instar and then spent several months increasing in weight befo re emergence. Hydatopsyche melli and Hydropsyche chekiangana were prob ably univoltine. Total hydropsychid annual production over the period 1977-79 was 537.0 mg AFDW 0.5 m(-2), and was 11% higher during 1978-79 than in 1977-78. Inter-year variations in production reflected fluctu ations in annual mean biomass: thus production of P. astictus and C. v entricosa was higher in 1977-78, and production of Herbertorossia quad rata, Hydatopsyche melli and Hydropsyche chekiangana was greater in 19 78-79. Production of M. fastosum, which constituted 35% of the total, was relatively stable between years. Species-specific annual productio n during 1977-79 was in the order M. fastosum > C. spinosa > C. ventri cosa > Hydatopsyche melli > Herbertorossia quadrata > P. astictus > Hy dropsyche chekiangana. Production:biomass (P:B) ratios varied from 3.5 9 (Hydatopsyche melli, 1977-78) to 25.46 (Herbertorossia quadrata, 197 7-78), and P:B ratios (1977-79) decreased in the order C. ventricosa > C. spinosa > Herbertorossia quadrata > M. fastosum > Hydropsyche chek iangana > P. astictus > Hydatopsyche melli (Table II). Bivoltine speci es had the highest P:B ratios. All TPKFS hydropsychids (except P. asti ctus) were significantly more abundant in midstream microhabitats than close to the stream banks. This pattern was most marked in Hydatopsyc he melli and Hydropsyche chekiangana. Multivariate statistics revealed that hydropsychid microdistribution was influenced significantly by s ediment grain-size characteristics, although algae or detritus also ha d influence on all species except Herbertorossia quadrata. Despite the statistical significance of best-fit regression models, the proportio n of variation in the abundance of each species explained by the indep endent variables did not exceed 23% (in C. spinosa), and was typically much less.