Life histories, secondary production and microdistribution of hydropsychid caddisflies (Trichophtera) in a tropical forest stream

Abstract

The ecology of seven species of hydropsychid caddisflies was investigated in Tai Po Kau Forest Stream (TPKFS), New Territories, Hong Kong, over a two-year period (1977-79). Quantitative benthic samples were taken 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 hydropsychid densities) and C. ventricosa (54.7 individuals 0.5 m-2; 24%) were the most abundant hydropsychids in TPKFS, followed by Macrostemum fastosum (15.2 individuals 0.5 m-2; 7%). Hydatopsyche melli, Herbertorossia quadrata, Polymorphanisus astictus and Hydropsyche chekiangana were present at relatively low densities (each < 2% of total hydropsychid abundance). Macrostemum fastosum made up 42% (mean value 1977-79; 40.0 mg AFDW 0.5 m-2) of total hydropsychid biomass. Hydatopsyche melli was ranked second (13.3 mg AFDW 0.5 m-2; 14%), followed by C. spinosa and C. ventriosa. There was little inter-year variation in total hydropsychid standing stocks: densities (mean of all species combined) 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, while Herbertorossia quadrata, Hydatopsyche melli and Hydropsyche cheklangana exhibited the converse pattern. Standing stocks of M. fastosum and C. spinosa were relatively stable between years. Total hydropsychid abundance in TPKPFS 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 numerous following recruitment during the wet season. Cheumatopsyche spp. and Herbertorossia quadrata had bivoltine life histories while P. astictus and M. fastosum were univoltine; the latter grew rapidly to the final instar and then spent several months increasing in weight before emergence. Hydatopsyche melli and Hydropsyche chekiangana were probably 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 fluctuations in annual mean biomass: thus production of P. astictus and C. ventricosa was higher in 1977-78, and production of Herbertorossia quadrata, Hydatopsyche melli and Hydropsyche chekiangana was greater in 1978-79. Production of M. fastosum, which constituted 35% of the total, was relatively stable between years. Species-specific annual production during 1977-79 was in the order M. fastosum > C. spinosa > C. ventricosa > Hydatopsyche melli > Herbertorossia quadrata > P. astictus > Hydropsyche chekiangana. Production:biomass (P:B) ratios varied from 3.59 (Hydatopsyche melli, 1977-78) to 25.46 (Herbertorossia quadrata, 1977-78), and P:B ratios (1977-79) decreased in the order C. ventricosa > C. spinosa > Herbertorossia quadrata > M. fastosum > Hydropsyche chekiangana > P. astictus > Hydatopsyche melli (Table II). Bivoltine species had the highest P:B ratios. All TPKFS hydropsychids (except P. astictus) were significantly more abundant in midstream microhabitats than close to the stream banks. This pattern was most marked in Hydatopsyche melli and Hydropsyche chekiangana. Multivariate statistics revealed that hydropsychid microdistribution was influenced significantly by sediment grain-size characteristics, although algae or detritus also had an influence on all species except Herbertorossia quadrata. Despite the statistical significance of best-fit regression models, the proportion of variation in the abundance of each species explained by the independent variables did not exceed 23% (in C. spinosa), and was typically much less.