BIOTROPICA 41(2): 196–201 2009 10.1111/j.1744-7429.2008.00463.x Substrate Availability May Be More Important than Aquatic Insect Abundance in the Distribution of Riparian Orb-web Spiders in the Tropics Eric K. W. Chan 1 , Yixin Zhang 2 , and David Dudgeon 1,3 1 Division of Ecology & Biodiversity, School of Biological Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China 2 Department of Biology, Texas State University, 601 University Drive, San Marcos, Texas, U.S.A. ABSTRACT Spiders that are abundant along streams may depend on energy subsidies across land–water ecotones, but the effects of season and habitat structure on this trophic linkage remain poorly understood in the tropics. We carried out surveys and a manipulative experiment to investigate the effects of season and substrate availability on the distribution of riparian orb-web spiders in Hong Kong, southern China. In the surveys, spider abundance, prey, substrate use, and web orientation were recorded. The experiment involved installation of in-stream artiﬁcial substrates (ropes and bamboo poles) to increase substrate availability for web attachment. We found no seasonal difference in web abundance, but seasonal differences were observed for the prey on webs: aquatic insects (mostly Ephemeroptera and chironomid midges) contributed 69 percent of total prey collected during the wet season, but only 38 percent during the dry season. Most webs (50–80%) were < 0.5 m above the water and 45–51 percent of them tended to be orientated horizontally to the water surface and supported by overhanging vegetation and boulders. The addition of artiﬁcial substrates resulted in a 23–34 percent increase in the number of webs at the four treatment sites compared to controls, indicating that availability of web-building substrates is a critical determinant of the spider distribution. Our results suggest that riparian zones are potential ‘hotspots’ of food availability for spiders, and that the aquatic insect subsidy allows this habitat to support increased densities of spiders when the constraint of substrate availability is relaxed. Key words: BACI; China; ecotone; habitat structure; Hong Kong; riparian vegetation; subsidy. HABITATS ARE OFTEN LINKED BY THE TRANSFERS OF NUTRIENTS AND ENERGY (hereafter referred to as ‘subsidy’), and stream riparian zones are land–water ecotones where there may be reciprocal exchanges of nutrients and energy of importance to consumers in recipient habitats (Polis et al. 1997, Baxter et al. 2005, Naiman et al. 2005, Ballinger & Lake 2006). For example, the diet of drift-feeding ﬁshes may be subsidized by terrestrial insects falling into streams (Nakano & Murakami 2001). Conversely, terrestrial predators (e.g., lizards, birds, bats, spiders) consume emergent aquatic insects, which show strong seasonal ﬂuctuations in magnitude of emergence and mostly aggregate within 10–20 m of streams (Iwata et al. 2003, Sanzone et al. 2003, Power et al. 2004, Uesugi & Murakami 2007). A recent study in New Zealand has also demonstrated a positive correla- tion between the biomass of benthic stream insects and riparian arachnids (spiders and harvestmen), while spider web density also declined strongly with increasing distances from stream margins (Burdon & Harding 2008). As a result, stream riparia may be ‘hotspots’ of food availability for terrestrial predators during some parts of the year. Manipulative experiments have shown that the reductions in the subsidy of adult aquatic insects can reduce the densities or growth of riparian insectivores and may indirectly affect the community structure in these habitats (e.g., Sabo & Power 2002a, b; Kato et al. 2003; Marczak & Richardson 2007). Given the potential importance of land–water interactions for streams and their riparian zones, a better understanding of their dynamics is needed, but such data are generally lacking from the seasonal tropics (but see Lynch et al. 2002, Chan et al. 2007). In addition, very few studies (including those carried out in temperate Received 22 March 2008; revision accepted 19 July 2008. 3 Corresponding author; e-mail: ddudgeon@hkucc.hku.hk regions) have examined the inﬂuence of riparian habitat structure on the distribution and abundance of insectivores that feed on adult aquatic insects. For instance, riparian conditions could limit spider foraging along streams (Laeser et al. 2005). In monsoonal Hong Kong, tetragnathid spiders are a conspicuous component of streamside habitats and orb webs are attached to overhanging vegetation, lianas, logs, and boulders along forest streams. The webs are situated close to the water surface, suggesting that emerging adult aquatic insects may be a major food source for these spiders. We carried out ﬁeld surveys of webs along ten reaches of four different streams during the wet and dry seasons to record seasonal changes in the abundance of spiders and their prey, and the substrates used for web attachment. We hypothesized that riparian spiders and their catches of aquatic prey would be more abundant during the wet season, when more adult aquatic insect prey are available (Chan et al. 2007). Anecdotal observation indicates that tetragnathid spiders in Hong Kong usually aggregate in stream reaches with dense over- hanging vegetation or boulders, suggesting that the availability of suitable substrates is a crucial factor in affecting their abundance in food-rich ‘hotspots’ along streams. The distribution and abun- dance of riparian spiders may be particularly sensitive to the dis- tribution and availability of robust substrates (e.g., tree branches), which are usually more limiting above streams than in other habi- tats such as forest and shrubland (cf. Buskirk 1975, Laeser et al. 2005). To investigate this, we conducted a manipulative ﬁeld ex- periment to test the hypothesis that spider density is a function of substrate availability for web-building along streams. We expected that increasing the substrate availability for web-building would re- sult in higher spider densities in sites where aquatic insect prey are abundant. 196 C  2009 The Author(s) Journal compilation C  2009 by The Association for Tropical Biology and Conservation THE฀JOURNAL฀OF฀TROPICAL฀BIOLOGY฀AND฀CONSERVATION