Spatial and temporal dynamics of land-water interactions mediated by arthropods in tropical stream riparia

Abstract

Knowledge of land-water energy exchange across tropical stream riparia remains poor. I characterized the spatio-temporal dynamics of water-to-land energy transfer mediated by arthropods by quantifying and describing patterns of aquatic-insect emergence, as well as the spatio-temporal variability of predatory arthropods, within riparia of two forest streams in tropical Hong Kong. I also estimated the importance of the water-to-land subsidy of insects to these terrestrial predators.

Mean annual emergence of aquatic insects ranged from 9,399-17,127 individuals m^(-2) and were comparable to studies elsewhere but, in biomass terms (0.713-1.168g DW m-2), were at the lower end of reported values. Emergence and abundance of adult aquatic insects were high over seven months (April to October) including the entire wet season. Inter-stream differences in abundance were likely attributable to local spate events that appeared to cause short-term reductions in aquatic insect populations. Seasonal trends in abundance of volant aquatic and terrestrial insects were similar, although aquatic insects accounted for 74-87% of captures in light traps but only 13-14% in Malaise traps. The relative abundance of aquatic versus terrestrial insects was similar between seasons, suggesting that aquatic subsidy may remain important for riparian insectivores during the dry season when all insects are scarce.

Predator reliance on aquatic insects varied according to their hunting modes. The web-building tetragnathid spider (Orsinome diporusa) had the greatest reliance (~40-55%) on this water-to-land subsidy, followed by two species of damselfly (40-50%), three cursorial spiders (32-51%) and two neustic gerrids (17-36%). Such reliance also showed interspecific differences among cursorial spiders according to the extent of their distribution inland. Two gerrids and two cursorial spiders that were active year-round showed consistent reliance on aquatic insects between seasons, reflecting the lack of seasonal variability in the relative abundance of aquatic and terrestrial prey.

Spiders constituted the vast majority of ground-dwelling predators along forest stream riparia. Of the three most abundant spiders captured along stream margins, Heteropoda sp. (Sparassidae) and Draconarius spp. (Agelenidae) were more abundant inland and present throughout the year. Only Pardosa sp. (Lycosidae) showed some degree of aggregation at the stream banks, indicating a potential reliance on aquatic insect prey, and was virtually absent during the dry season when aquatic-insect emergence was low. The stable-isotope signature of Pardosa sp. confirmed its dependence on aquatic prey, whereas Heteropoda sp. assimilated less of this aquatic subsidy. In general, forest-stream riparia in Hong Kong did not appear to be feeding hotspots for ground-dwelling spiders, and may be attributable to the low biomass of aquatic insects emerging from the study streams, as well as the availability of terrestrial prey in the surrounding forest. The biomass of Heteropoda sp., the largest of the three spiders, exceeded total standing stocks of other spiders within riparia by 2 - 80 times. Although its dietary dependence on aquatic insects was lower than Pardosa sp., the high biomass and inland distribution of Heteropoda sp. could make it a potential conduit for the stream-to-land transfer of energy.