Abstract
Terrestrial predators have been shown to aggregate along stream margins during periods when the emergence of adult aquatic insects is high. Such aggregation may be especially evident when terrestrial surroundings are relatively unproductive, and there are steep productivity gradients across riparia. In tropical forests, however, the productivity of inland terrestrial habitats may decrease the resource gradient across riparia, thus lessening any tendency of terrestrial predators to aggregate along stream margins. We elucidated the spatio-temporal variability in the distribution of ground-dwelling spiders and terrestrial arthropod prey within the riparia of two forest streams in tropical Hong Kong by sampling arthropods along transects at different distances from the streams during the wet and dry seasons. Environmental variables that may have influenced spider distributions were also measured. The vast majority of ground-dwelling predators along all transects at both sites were spiders. Of the three most abundant spiders captured along stream margins, Heteropoda venatoria (Sparassidae) and Draconarius spp. (Agelenidae) were terrestrially inclined and abundant during both seasons. Only Pardosa sumatrana (Lycosidae) showed some degree of aggregation at the stream banks, indicating a potential reliance on aquatic insect prey. Circumstantial evidence supports this notion, as P. sumatrana was virtually absent during the dry season when aquatic insect emergence was low. In general, forest-stream riparia in Hong Kong did not appear to be feeding hotspots for ground-dwelling predators. The lack of aggregation in ground-dwelling spiders in general may be attributed to the low rates of emergence of aquatic insects from the study streams compared to counterpart systems, as well as the potentially high availability of terrestrial insect prey in the surrounding forest. Heteropoda venatoria, the largest of the three spiders maintained a high biomass (up to 28 mg dry weight/m2) in stream riparia, exceeding the total standing stock of all other spiders by 2–80 times. The biomass and inland distribution of H. venatoria could make it a likely conduit for the stream-to-land transfer of energy.
Highlights
Fluxes of aquatic insects from streams to the terrestrial landscape provide an important energy source for riparian insectivores, which may exhibit shifts in their spatio-temporal distribution in response to the availability of this water-to-land subsidy (Baxter, Fausch & Carl Saunders, 2005; Ballinger & Lake, 2006; Richardson & Sato, 2015)
The emergence of aquatic insects, and consequential water-to-land trophic subsidy, is likely to result in a steep productivity gradient across the riparian zones that leads to the aggregation of consumers, such as ground-dwelling insectivores, along stream margins; the aggregation can be especially evident in instances where rivers or streams are bordered by wide, relatively unproductive habitats such as gravel beds or sand bars (e.g., Sabo & Power, 2002; Sanzone et al, 2003; Paetzold, Schubert & Tockner, 2005)
One of the few studies of volant insects that has been undertaken in tropical stream riparia showed that aquatic insects were largely confined to the margins of forest streams in Hong Kong (50–85% of total abundance within 10 m of the banks), whereas the distribution of volant terrestrial insects was unaffected by proximity to the stream (Chan, Zhang & Dudgeon, 2007)
Summary
Fluxes of aquatic insects from streams to the terrestrial landscape provide an important energy source for riparian insectivores, which may exhibit shifts in their spatio-temporal distribution in response to the availability of this water-to-land subsidy (Baxter, Fausch & Carl Saunders, 2005; Ballinger & Lake, 2006; Richardson & Sato, 2015). Riparian web-building tetragnathid spiders aggregated in and along the stream channel (Chan, Zhang & Dudgeon, 2009), and the same phenomenon has been reported for spiders along small streams draining temperate forests (Marczak & Richardson, 2007) Due to their relatively limited mobility, ground-based predators are likely to be less sensitive to changes in the availability of aquatic insects than are flying insectivores (Power et al, 2004). We expected that spiders would show some degree of aggregation along the stream banks, but that the extent of aggregation might depend on the relative availability of ground-dwelling prey and the species-specific characteristics of the spiders The results of this investigation, in combination with data on spider dietary reliance on the aquatic subsidy, have implications for the roles of spiders in water-to-land energy transfer (e.g., Paetzold, Schubert & Tockner, 2005)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
