Blackwell Science, LtdOxford, UKEREEcological Research1440-17032004 Ecological Society of Japan196633643Original ArticleStream subsidies to riparian spidersC. Kato et al.
Ecological Research (2004) 19: 633–643
Prey use by web-building spiders: stable isotope analyses of trophic flow at a forest-stream ecotone Chika KATO,1 Tomoya IWATA1* and Eitaro WADA2 1
Center for Ecological Research, Kyoto University, Hirano-cho, Kamitanakami, Otsu, Shiga 520–2113, Japan, 2Research Institute for Humanity and Nature, 335 Takashima-cho, Kamigyo-ku, Kyoto 602–0878, Japan
A forest-stream trophic link was examined by stable carbon isotope analyses which evaluated the relationship of aquatic insects emerging from a stream to the diets of web-building spiders. Spiders, aquatic and terrestrial prey, and basal resources of forest and stream food webs were collected in a deciduous forest along a Japanese headwater stream during May and July 2001. The d13C analyses suggested that riparian tetragnathid spiders relied on aquatic insects and that the monthly variation of such dependence is partly associated with the seasonal dynamics of aquatic insect abundance in the riparian forest. Similarly, linyphiid spiders in the riparian forest exhibited d13C values similar to aquatic prey in May. However, their d13C values were close to terrestrial prey in both riparian and upland (150 m away from the stream) forests during June to July, suggesting the seasonal incorporation of stream-derived carbon into their tissue. In contrast, araneid spiders relied on terrestrial prey in both riparian and upland forests throughout the study period. These isotopic results were consistent with a previous study that reported seasonal variation in the aquatic prey contribution to total web contents for each spider group in this forest, implying that spiders assimilate trapped prey and that aquatic insect flux indeed contributes to the energetics of riparian tetragnathid and linyphiid spiders. Key words: adult aquatic insects; forest-stream linkages; riparian web-building spiders; stable carbon isotope ratios.
Introduction A strong linkage between food webs of forests and streams has been well documented in many regions (Likens & Bormann 1974; Wallace et al. 1997; Nakano et al. 1999). In particular, inputs of particulate organic carbon from riparian forests (i.e. leaf-litter and terrestrial arthropods) have long been emphasized as a critical energy base of consumer communities in most headwater streams (Mason & MacDonald 1982; Nakano et al. 1999; Hall et al. 2000). Moreover, recent studies have recognized increasingly that carbon outputs from streams via a biological vector (e.g. emerging aquatic insects) often fuel the production of riparian generalist predators (Power & Rainey 2000; Nakano & Murakami 2001; Iwata et al. 2003). Thus, forest and stream communities are interdependent through the exchange of organic materials across their common boundary. Previous studies have reported that adult aquatic insects emerging from streams are an important prey item for riparian web-building spiders. In many regions, web-building spiders often concentrate in
riparian zones and trap emerging aquatic insects to a substantial degree (Williams et al. 1995; Henschel et al. 2001; Kato et al. 2003). Henschel et al. (2001) further demonstrated the essential role of emerging aquatic insects and spiders in riparian food webs by showing that aquatic insect flux promotes aggregation of spiders along stream edges, which in turn depresses terrestrial herbivore populations (e.g. leafhoppers). Moreover, riparian spiders have the potential to facilitate energy transfer from stream to forest ecosystems by consuming small aquatic insects while serving as prey for large riparian insectivores such as forest birds and bats (Jackson & Fisher 1986; Iwata et al. 2003). Therefore, identifying the pathways and magnitude of energy flow from streams to spiders is a key to understanding food web dynamics at forest-stream ecotones. Notwithstanding, previous studies have not demonstrated that energy *Author to whom correspondence should be addressed. Present address: Department of Ecosocial System Engineering, University of Yamanashi, 4-3-11 Takeda, Kofu, Yamanashi 400–8511, Japan. Email:
[email protected] Received 4 August 2003, Accepted 2 June 2004.
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incorporated into the spider tissue is actually derived from streams (but see Collier et al. 2002). Stable carbon isotope ratios are used increasingly to examine the trophic flow across terrestrial and freshwater ecosystems (France 1995; Finlay 2001). Terrestrial and freshwater plants often exhibit different ratios of stable carbon isotope as a result of variation in plant physiology or inorganic carbon sources (Rounick & Winterbourn 1986; Peterson & Fry 1987). Moreover, there is little shift in the carbon isotopic ratio associated with trophic transfer of organic carbon (DeNiro & Epstein 1978; Fry & Sherr 1984; Wada et al. 1993). Organisms that consume both forest- and stream-based resources should fall along a continuum in carbon isotopic signatures, depending on the proportion of each category in their diet. Thus, stable carbon isotope analyses are applicable to assessing the contribution of stream-derived energy to web-building spiders. The present study is intended to evaluate the importance of adult aquatic insects in the diets of web-building spiders using stable carbon isotope analyses. In this analysis, we measured stable isotope ratios of spiders in both riparian and upland forests to deduce the spatial extent of such stream subsidy, because adult aquatic insects in riparian forests often concentrate around stream channels and their abundance declines exponentially with distance from streams (Petersen et al. 1999; Iwata et al. 2003). Furthermore, temporal variations in the contribution of stream-derived energy were also assessed because web-building spiders can exhibit dynamic dependence on aquatic insect flux, which varies seasonally (Williams et al. 1995; Kato et al. 2003).
Methods Study area A field study was conducted during May and July 2001 in a deciduous forest plot established along a headwater reach (1.2-km stretch) of the Horonai Stream in the Tomakomai Experimental Forest (TOEF; 42∞43¢N, 141∞36¢E) in Hokkaido, Japan. The study area is situated in the cool-temperate region, having a mean annual temperature of approximately 7∞C. The Horonai Stream is a low gradient, spring-fed stream (15.4 km2 in drainage area, 14 km in total length, 2–5 m width, gradient 0.05, n = 20). Prior to the scraping, we gently rubbed and repeatedly rinsed the tile surface to cleanse other organic matter from the epilithic material. In addition, 12 samples of coarse particulate organic matter (hereafter CPOM: ≥1 mm) were collected from the stream bed by hand as an alternative basal resource in the stream. In each of the riparian and upland transects, we picked 18 leaves from live C3 plants (chosen haphazardly) and collected six samples of detritus from the A0-layer of forest soils as terrestrial basal resources (C3 plants and forest soils, respectively). Web-building spiders were also collected every month in each of the riparian and upland transects (26– 30 May, 23–29 June and 25–31 July). Spiders were collected ≥2 weeks after monthly sampling of aquatic and terrestrial prey because terrestrial arthropods require 1–3 weeks to turn over C and N (Ostrom et al. 1997; Oelbermann & Scheu 2002). Three species of Tetragnathidae (Metleucauge yunohamensis, Tetragnatha maxillosa and Tetragnatha praedonia), two species of Araneidae (Cyclosa kumadai and Zilla sachalinensis) and three species of Linyphiidae (Linyphia angulifera, Linyphia emphana and Linyphia longipedella) were sampled as most abundant spiders (see Appendix 1 for sample sizes). However, it was difficult to ensure a sufficient sample size of tetragnathid spiders in the upland transects because of the very low abundance.
Thus, this spider group was collected only in the riparian transects. No spiderlings were collected because of the difficulty of species identification. All specimens of spiders, prey and basal resources for stable isotope analyses were stored in glass vials or plastic bags on ice in the field, and later refrigerated in the laboratory until processing. Stable isotope measurement In the laboratory, samples of thawed whole animal organisms (spiders, and aquatic and terrestrial prey), CPOM, C3 plants and forest soils were dried at 60∞C for 48 h and were ground to a fine powder. The CPOM (mean dry mass per sample = 2.78 ± 1.27 SD mg) and forest soil (2.54 ± 0.70 SD mg, n = 36 for both) samples were then rinsed with 5% hydrochloric acid (HCl) to dissolve any calcium carbonate (CaCO3), followed by a distilled water rinse, and dried again at 60∞C for 48 h. Periphyton collected from a ceramic tile was concentrated on a glass fiber filter, rinsed with 5% HCl under gentle vacuum filtration followed by a distilled water rinse, and then dried at 60∞C for 48 h. Carbon isotope ratios of prepared samples were measured using a CNH analyzer combined with a Finnigan MAT Delta-S (Thermo Electron Corporation, Waltham, MA, USA) mass-spectrometer through a Conflo–II interface. In addition, nitrogen isotope ratios of those samples were measured simultaneously. Stable isotope ratios are expressed in d notation as parts per thousand (‰) differences from the standard:
dX = (Rsample/Rstandard - 1) ¥ 1000 where X is 13C or 15N, R is 13C/12C or 15N/14N, and appropriate standards were PeeDee Belemnite (PDB) and atmospheric nitrogen for carbon and nitrogen, respectively. Analytical precision was better than ±0.1‰ and ±0.2‰ for carbon and nitrogen, respectively. Data treatment For data analyses of stable isotope ratios, aquatic prey samples were grouped into three trophic guilds, herbivores, detritivores or predators (Appendix 1), although their feeding habits become occasionally flexible depending on the kind of food available (e.g. Mihuc 1997). Trichopteran grazers (Glossosoma spp., Neophylax ussuriensis and Apatania abberans) and two mayfly grazers (Ameletus spp. and Baetis thermicus) were classified as aquatic herbivores. An Ephemeroptera (Ephemera japonica), Plecopteran shredders (Nemouridae spp. and Leuctridae spp.) and a Dipteran cranefly (Antocha spp.) were assigned to aquatic detritivores. Predators that consume aquatic macroinvertebrates were Plecoptera (Perlodidae spp. and Chloroperlidae
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spp.), a Trichoptera (Rhyacophila arefini), a Dipteran midge (Tanypodinae spp.) and Dipteran craneflies (Dicranota spp. and Limnophyla spp.). Other aquatic insects (mainly gatherers such as Drunella spp., Paraleptophlebia chocorata, Chironominae spp. and Orthocladiinae spp.) were classified as a polyphagous group. Terrestrial prey samples were also grouped (Appendix 1), but typical terrestrial detritivores were not collected. Instead, many dipteran flies (a major prey item of web-building spiders) were captured. Because terrestrial dipteran flies, except predatory diptera, usually exhibit omnivorous feeding habits, these insects were assigned to a group (hereafter terrestrial diptera) for convenience. Terrestrial herbivores were rarely collected in May because of their low abundance; they were excluded from analyses in that month as an unimportant prey item of spiders. Web-building spiders were analyzed by family, including Tetragnathidae (horizontal orb weavers), Araneidae (vertical orb weavers) and Linyphiidae (sheet weavers) because spider species belonging to the same family exhibit similar foraging habits (Wise 1993). Tetragnathids and araneids usually spin horizontal and vertical sticky orb webs, respectively, while linyphiids build sheet webs of entirely non-sticky silk, which basically comprise a horizontal sheet with scaffolding above and below (Wise 1993).
Results Aquatic and terrestrial insect abundance The riparian transects had 12 times greater abundance of adult aquatic insects than the upland transects when all months were combined (Fig. 1). Two-way ANOVA on adult aquatic insect abundance revealed that transect, month and interaction effects were all significant (transect, F1,12 = 44.8, P < 0.001; month, F2,12 = 6.6, P = 0.012; interaction, F2,12 = 7.0, P = 0.010). Because the interaction term was significant, we made a one-way ANOVA and found a significant difference in their abundance among the six groups (two transects ¥ 3 months; F5,12 = 14.4, P = 0.0001). Subsequent multiple comparisons (Fisher’s protected least significant difference [PLSD] test) showed a significant trend that adult aquatic insects increased from May to June and then decreased drastically in July, but this monthly trend occurred only in riparian transects (Fig. 1). Two-way ANOVA also revealed a significant effect of month on terrestrial insect abundance (F2,12 = 5.43, P = 0.021), but neither transect (F1,12 = 0.66, P = 0.431) nor interaction effects (F2,12 = 0.95, P = 0.413) were significant (Fig. 1). Multiple comparisons (Fisher’s PLSD test) showed a significant and a marginally significant difference in terrestrial insect abundance between May and
Fig. 1. Abundance of aquatic and terrestrial insects collected by Malaise trap sampling in riparian () and upland () transects in the deciduous forest plot during May and July 2001. There is no significant difference (P ≥ 0.05) between circles with the same letter (assessed by Fisher’s PLSD tests). Mean and SE are presented (n = 3 for each). (a), aquatic insects; (b), terrestrial insects.
June (P = 0.007) and between May and July (P = 0.054), respectively. No significant difference in their abundance was, however, detected between June and July (P = 0.292). These results indicate that monthly trends in riparian and upland transects follow a similar pattern, in which terrestrial insects increased from May to June and remained at about the same level in July (Fig. 1). Stable isotope ratios of basal resources, prey and spiders Stable isotope analyses showed that aquatic prey were distinct from terrestrial prey in d13C, but not in d15N,
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throughout the study period (Figs 2 & 3). Periphyton exhibited unexplained high temporal variability in both d13C and d15N during the study period (Fig. 2), as has been reported by previous studies of headwater streams (France 1995; Finlay et al. 1999). d13C values of CPOM were similar to those values for forest soils in riparian (Fig. 2) and upland (mean ± SE: May, -29.0 ± 0.3‰; June, -28.7 ± 0.2‰; July, -28.7 ± 0.1‰, n = 6 for all) forests. However, C3 plant d13C values in riparian (Fig. 2) and upland (May, -30.5 ± 0.3‰; June, -30.9 ± 0.2‰; July, -30.9 ± 0.2‰, n = 18 for all) forests were depleted relative to CPOM d13C values (Fig. 2). As a result, basal resource d13C values overlapped between stream and terrestrial food webs. Despite these facts, aquatic herbivores, detritivores and predators and others aquatic insects all tended to show depleted d13C relative to terrestrial herbivores, predators and diptera in riparian and upland forests (Figs 2 & 3), indicating that d13C signatures were available for assessing variations in aquatic prey contribution to spider diets among groups, transects and months. In the riparian transects, d13C signatures of webbuilding spiders differed considerably among groups, with tetragnathid d13C being most depleted (Fig. 3). In May, tetragnathid and linyphiid spiders had significantly more depleted d13C than araneid spiders in the riparian transects (P < 0.05 by Sheffe tests after oneway ANOVA; F2,34 = 6.28, P = 0.005). In both June and July, only tetragnathid d13C was significantly depleted relative to both riparian araneid and linyphiid d13C values (P < 0.05 by Sheffe tests after one-way ANOVA; June, F2,36 = 33.66, P < 0.001; July, F2,35 = 15.02, P < 0.001). Although largely qualitative, riparian tetragnathid spiders had d13C values intermediate between aquatic and terrestrial prey in May, similar to aquatic prey in June, and again intermediate between aquatic and terrestrial prey in July (Fig. 3). In upland transects, linyphiid spiders exhibited d13C signatures similar to terrestrial prey throughout the study period (Fig. 3). However, linyphiid spiders in the riparian transects had significantly more depleted d13C than those in the upland transects in May, exhibiting intermediate d13C values between aquatic and terrestrial prey. Different from tetragnathid and linyphiid spiders, araneid spiders had d13C values similar to terrestrial prey in both the riparian and upland transects throughout the study period (Fig. 3).
Fig. 2. Carbon and nitrogen stable isotope signatures (‰, mean ± SE) of riparian web-building spiders () and aquatic () and terrestrial () resources during May and July 2001. Data for the upland transects are not shown. ar, araneids; de, detritivores; di, diptera; he, herbivores; li, linyphiids; ot, other aquatic insects; pr, predators; te, tetragnathids. (a), May; (b), June; (c), July.
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Fig. 3. Carbon stable isotope signatures (‰, mean ± SE) of aquatic and terrestrial prey (left panels: aquatic prey, open symbols; terrestrial prey, solid symbols) and web-building spiders (right panels), collected in riparian and upland transects during May and July 2001. Circles, herbivores; triangles, detritivores; squares, predators; inverse triangles, other aquatic insects; diamonds, terrestrial diptera. ar, araneids; li, linyphiids; te, tetragnathids. *, significant differences in d13C values between riparian and upland transects by t-tests (P < 0.05). Ellipses circled d13C values of each of aquatic and terrestrial prey. (a), May; (b), June; (c), July.
Discussion The riparian forest supported more abundant adult aquatic insects than the upland forest (Fig. 1). This was most likely caused by the lateral dispersal pattern of emerging aquatic insects that many previous studies have reported. Those studies showed that aquatic insect abundance decreased exponentially with distance from streams (e.g. Petersen et al., Iwata et al. 2003). Moreover, aquatic prey flux into the riparian forest varied
monthly; the abundance of emerging aquatic insects increased from May to June and then dramatically decreased to July (Fig. 1). This pattern accords with the general seasonal variability of aquatic insect emergence from the Horonai Stream, where the emergence peaks before canopy closure in early to mid summer (Nakano & Murakami 2001). However, even in both May and June in the riparian transects, the abundance of terrestrial insects was far greater than that of aquatic insects (see also Kato et al. 2003), indicating that aquatic prey
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was less available (in terms of abundance) to webbuilding spiders in the riparian forest. Despite the small amounts, aquatic prey dynamics appeared to have affected the prey use of riparian tetragnathid spiders. In the present study, carbon isotope values provided a tool for distinguishing between aquatic and terrestrial trophic pathways to spiders, although nitrogen isotope values overlapped among aquatic and terrestrial prey and spider groups. We cannot discern why aquatic and terrestrial preys were distinct in d13C because the isotopic discrimination between aquatic and terrestrial basal resources was less clear. However, this might be caused by the contribution of unsampled resources with enriched d13C to terrestrial food webs, such as terrestrial fungi associated with decomposing wood (Kohzu et al. 1999). Tetragnathid spider d13C values similar to aquatic prey suggest the incorporation of stream-derived carbon into their tissue. Moreover, seasonal variation in their d13C appears to be associated, in part, with aquatic prey dynamics. The d13C values were most depleted when aquatic insect abundance was at its peak (in June), and those were enriched (became closer to terrestrial prey) when aquatic insects dramatically decreased in July (Fig. 3). Although the reason why tetragnathid spiders prefer aquatic prey remains unclear, a previous study conducted in this forest reported that 50–90% (dry mass) of prey caught in riparian tetragnathid orb-webs was of aquatic origin during May and July and that the proportion varied with the seasonal dynamics of aquatic insect flux even in the condition in which terrestrial prey abundance was far greater than adult aquatic insects (Kato et al. 2003; see also Williams et al. 1995). The isotopic pattern observed in this study affirms such a linkage between forest and stream food webs on a nutritional basis: tetragnathid spider predation on emerging aquatic insects. Linyphiid spiders’ d13C values in the riparian forest were depleted (similar to aquatic prey), relative to those in the upland forest (similar to terrestrial prey) in May (Fig. 3). However, during June to July, their d13C values were similar to terrestrial prey in both riparian and upland forests. The d13C depletion of riparian linyphiid spiders in May may be caused by their predation on emerging aquatic insects because terrestrial prey d13C did not differ between the riparian and upland forests throughout the study period (Fig. 3). In fact, aquatic prey was observed to be a main prey item trapped in riparian linyphiid sheet webs in May, when terrestrial prey abundance was limited in this forest (Kato et al. 2003). In addition, our results provide useful information on the spatial scale of such forest-stream linkage; stream-derived carbon seemed to scarcely enter linyphiid spiders in upland forests 150 m away from the stream. This fact contradicts the result provided by
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Power & Rainey (2000) from stable carbon isotope analysis which noted that aquatic prey constitutes at least half of the carbon of linyphiid spiders found hundreds of meters away from the South Fork Eel River in north-western California. Because the South Fork Eel River (ª30 m active channel width) is a larger and more open stream than the shaded headwater reach of the Horonai Stream (2–5 m), we expect that higher algal production and its associated greater biomass of aquatic insects in the former stream can extend the spatial extent of stream subsidy compared to the Horonai Stream. Araneid spiders’ d13C values were similar to terrestrial prey throughout the study period, with the pattern also being consistent with the previous result that prey items caught in their webs were primarily terrestrial insects from May through to July (Kato et al. 2003). Araneids build two-dimensional, vertical orb webs, which are thought to adapt for the capture of large, fast flying insects (Olive 1981; Craig 1987; Uetz & Hartsock 1987). This characteristic foraging tactic can account, at least in part, for the dependence of araneids on terrestrial prey even in the riparian forest, because adult aquatic insects are light, weakly flying prey (see Brodsky 1994). These findings suggest that webbuilding spiders assimilate prey caught in their webs, and that in fact, aquatic insect flux from the stream contributes to the energetics of tetragnathid and linyphiid spiders in the riparian forest. In particular, considering that these two spider groups gained energy from the relatively lower abundance of adult aquatic insects compared to terrestrial prey, we infer that the carbon pathway from the stream plays a disproportionately large role in sustaining tetragnathid and linyphiid spiders at the forest-stream ecotone. Terrestrially derived organic materials supplied from riparian forests or from upstream areas have long been considered a critical resource base for consumer communities in headwater stream food webs (Vannote et al. 1980; Wallace et al. 1997; Hall et al. 2000). Finlay (2001) isotopically confirmed such prediction in many temperate and Arctic streams in watersheds
