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Notes and Discussion Web-building Spider Response to a Logjam in a Northern Minnesota Stream ABSTRACT.—Logjams, or accumulations of wood in streams, can increase aquatic macroinvertebrate production through organic material retention and habitat diversification. Past studies showed a positive correlation between web-building spiders near streams and aquatic insect emergence. We hypothesized there would be an increase in terrestrial webbuilding spider density near logjams. To test this, we counted webs within 6 m of the stream bank along a 40 m reach in a northern Minnesota stream centered on a spanning logjam. We then estimated the availability of web-building substrate during late May and late Aug. of 2011. Webs in May heavily concentrated around the logjam, whereas webs in Aug. appeared dispersed throughout the reach. The web-building substrate did not show a significant correlation with web density in May, but it had a significant effect in Aug. These results cautiously suggest logjams have a positive effect on spider web density, but that effect varies through time. Further studies may explicitly link logjam-mediated prey to spider web distributions. INTRODUCTION Web-building spiders represent a notable beneficiary of aquatic prey subsidies and respond to variability in insect emergence (Henschel et al., 2001; Kato et al., 2003; Marczak et al., 2007). In fact, spiders in areas near rivers and streams can derive the majority of their diet from aquatic origins (Akamatsu et al., 2004; Kato et al., 2004). Web-builders often act as sit-and-wait predators but demonstrate enough mobility to respond to spatial variation in prey abundance. Consequently, these spiders may respond to resource gradients within the stream (Iwata, 2007; Burdon and Harding, 2008). Evidence exists to suggest the abundance of emerged aquatic prey may be the chief factor dictating spider distributions at these ecotones, whereas other variables, most notably riparian vegetation, exert relatively little influence (Iwata, 2007). However, Chan et al. (2009) showed that prey subsidies may be a major driver of spider density only when web-building structure is readily available. Therefore, studies that examine the relationship between prey availability and habitat structure address the processes underlying riparian predator distribution. Logjams comprise stream features that may have a positive influence on the distribution of emergent prey, and consequently, spider web distribution. Logjams, or woody accumulations, increase aquatic invertebrate production in several ways: First, logjams retain organic material, a major food source for many benthic macroinvertebrates (Wallace et al., 1995). Second, logjams create areas of varying current and depth, thereby increasing microhabitat diversity (Schneider and Winemiller, 2008). Third, accumulated logs and fallen branches can provide shelter from predators for invertebrates (Everett and Ruiz, 1993). Finally, emergent logjam surfaces may facilitate aquatic insect emergence by providing easy access to the riparian habitat (Petersen and Hildrew, 2003). These processes may result in a concentration of prey around logjams, which may elicit a response in the distribution of spiders. Based on these potential positive influences on invertebrate abundance, we conducted a small-scale study to test the hypothesis spider web density would increase near logjams. We also investigated whether web-building substrate (trees, shrubs, and other scaffolding) represented a limiting resource to spiders and if it could account for patterns in web distribution. METHODS STUDY SITE

Field work took place at Cabin Creek (47u34944.9970N, 91u8942.7220W), a low-order stream in the Superior National Forest of northern Minnesota during late May and late Aug. of 2011. The stream occurs in the Northern Lakes and Forest ecoregion of the Mixed Wood Shield (Omernik, 1987). Pine, fir, aspen and spruce dominate the forest overstory. Woody and herbaceous vegetation (e.g., willows and grasses) occur in riparian understory areas.

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Data collections occurred in late morning during two sampling periods: May 21–25 and Aug. 20–24, 2011. The sampling area ran along a 40 m stream reach and extended 6 m into the adjacent riparian habitat on either side of the stream. The study reach centered on a logjam that spanned the entire width of the stream. We designated this logjam as the ‘‘primary’’ logjam to differentiate it from other accumulations of wood within the reach. Notes on weather were recorded from the Isabella, Minnesota weather station (of the Western Regional Climate Center) and corroborated by on-site observations. WEB SURVEY

We counted and identified spider webs by systematically walking the sampling area and taking successive visual sweeps, starting at the downstream end and working upstream until the entire stream reach and the 6 m of bordering riparian vegetation had been surveyed. All webs up to 3 m from ground/stream bed level were counted. We measured the location of each web relative to fixed reference points – ten terrestrial posts and 50 structures anchored in the stream – which gave coordinates for each web in 2 dimensional space. The location of all reference points occurred relative to the primary logjam and the stream bank. We estimated the availability of web-building substrate by creating a Web Space Index (WSI): The WSI estimated the percentage of available 3 dimensional vegetative structure in the 2 m3 volume around each reference point. We multiplied nonwoody vegetation percentages by 0.5 because of the relative frailness of nonwoody vegetation compared to woody vegetation. The calculation involved summing these two percentages (woody and nonwoody vegetation), rounding, and then converting to a 1–10 scale. DATA ANALYSIS

We tallied web counts for each collection period (in 2 m increments that extended longitudinally away from the primary logjam), and we determined their distances from the primary logjam from reference point coordinates. Because we assumed webs responded to aquatic insect emergence from the stream, and the dispersal of emerging individuals to terrestrial habits typically shows an exponential decay (for review, see Gratton and Vander Zanden, 2009), we performed exponential regressions on these distributions in R (v. 2.11.1) by taking the natural log of the web count for each distance class and then conducting a linear regression on the log-transformed web count as a function of distance from the logjam. To examine the relationship between estimated available structure and web count, we tallied the number of webs nearest each terrestrial reference point and plotted them against the reference point’s WSI. Given our WSI was discretely numeric, and Chan et al. (2009) observed a direct relationship between available structure and web count, we performed linear regressions on these plots using R. To visualize web distribution patterns, we used ArcGIS 10 to map spider webs by overlaying web locations onto GPS data provided by Karen Gran and Grant Neitzel of the University of Minnesota Duluth from a total station survey. RESULTS We counted 106 webs counted in May and 422 in Aug. A peak in web density centered on the primary logjam occurred in May, with web count decreasing exponentially with increased distance from the jam (F1,8 5 19.98, P 5 0.002, r2 5 0.71; Figs. 1A, 2A). This trend disappeared in Aug. when web densities increased and became abundant throughout the reach (F1,8 5 0.09, P 5 0.769; Figs. 1B, 2B). The May sampling period was cool, moist and overcast with temperatures ranging 6–12 C and an average of 3.9 mm precipitation per day. The Aug. sampling period was warm, dry and sunny, with temperatures ranging 14–22 C. Stream depth was 45 (616 SE) cm in May, and dropped to 14 (610 SE) cm in Aug. The effect of available web-building structure also changed through time. In May no significant relationship occurred between WSI and web count (F1,8 5 2.36, P 5 0.16, r2 5 0.23). However, in Aug. WSI showed a positive relationship to web numbers (F1,8 5 6.47, P 5 0. 03, r2 5 0.45; Fig. 3).

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FIG. 1.—Web counts at different distances from the primary logjam in the Cabin Creek study reach. In May (A), web number decreased exponentially with increasing distance to the primary logjam, but this trend disappeared in Aug. (B)


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FIG. 2.—Maps of the Cabin Creek study reach made in May and Aug. 2010 with web locations overlaid. The greater web density near the primary logjam can be seen in the May map (A). An increase in total web count from May to Aug. can be observed (B)


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FIG. 3.—The number of spider webs in the Cabin Creek study reach plotted against their respective Web Space Index rating for May (dashed line) and Aug. (solid line) DISCUSSION Webs appeared clustered around the primary logjam in May, supporting our hypothesis, but this pattern disappeared in Aug. These results suggest that the logjam drives the distribution of spider webs in May but does not have as much influence in Aug. In contrast the estimated web-building substrate did not correlate with web counts in May but showed a positive relationship in Aug. This seasonal difference corroborates other work showing that available structure can be a constraint on riparian spider distribution (Rypstra et al., 1983; Chan et al., 2009). These data suggest a temporal shift in the factors that determine where riparian spiders build their webs. Specifically, the logjam may affect web distribution when available space does not (apparent in May), and vice versa (apparent in Aug.). Two primary explanations for this seasonal change exist: (1) the communities of spiders present during May and Aug. may differ substantially and respond to environmental features differently and/or (2) the increase in the number of web-building spiders from May to Aug. may result in higher competition, forcing some spiders to occupy sub-optimal habitat farther from the logjam. Because of its preliminary nature, our study had several limitations. We did not identify and count the prey species found in webs. While we assumed spiders responded to insects emerging from the stream, we did not verify this. Nor did we quantify densities of emergent prey in regions where we found webs clustered. Therefore, we could not confirm if indeed web-building spiders responded to local prey availability. Finally, the short duration of the study prevented us from establishing whether the patterns detected were ephemeral or long lasting. Consequently, the precise mechanism of the logjam’s positive influence on spider web density remains unclear. Nevertheless, other studies have shown wood in streams can increase macroinvertebrate production (Johnson et al., 2003; Coe et al., 2009) and insect emergence (Rolauffs et al., 2001), and local spikes in prey abundance can be reflected in the distribution of riparian webs (Iwata, 2007; Burdon and Harding, 2008). The connection among these elements, however, has yet to be unambiguously established. Future studies exploring the linkage between logjam-mediated aquatic emergence and riparian spiders should quantify aquatic emergence in the vicinity of logjams to assess prey availability and identify prey captured in webs to establish correlates with aquatic-derived emergence. A longer-term study would not only help to demonstrate


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whether the patterns described here are robust but may uncover dynamics in the response of webbuilding spider to fluxes in aquatic subsidies. Acknowledgments.—We would like to thank Laura Gaffney, Kelly Murray, John Schoen, Zach Snobl, Ong Xiong, and Evan Weiher for their help in various aspects of the research. This was an undergraduate research project funded by the National Science Foundation (NSF DEB-06-42512 to T.W.), the U.S. Forest Service, and the Office of Research and Sponsored Programs at the University of Wisconsin – Eau Claire.

LITERATURE CITED AKAMATSU, F., H. TODA, AND T. OKINO. 2004. Food source of riparian spiders analyzed by using stable isotope ratios. Ecol. Res., 19:655–662. BURDON, F. J. AND J. S. HARDING. 2008. The linkage between riparian predators and aquatic insects across a stream-resource spectrum. Freshwater Biol., 53:330–346. COE, H. J., P. M. KIFFNEY, G. R. PESS, K. K. KLOEHN, AND M. L. MCHENRY. 2009. Periphyton and invertebrate response to wood placement in large Pacific coastal rivers. River Res. Appl., 25:1025–1035. CHAN, E. K. W., Y. ZHANG, AND D. DUDGEON. 2009. Substrate availability may be more important than aquatic insect abundance in the distribution of riparian orb web spiders in the tropics. Biotropica, 41:196–201. GRATTON, C. AND M. J. VANDER ZANDEN. 2009. Flux of aquatic insect productivity to land: comparison of lentic and lotic ecosystems. Ecology, 90:2689–2699. HENSCHEL, J. R., D. MAHSBERG, AND H. STUMPF. 2001. Allochthonous aquatic insects increase predation and decrease herbivory in river shore food webs. Oikos, 93:429–438. IWATA, T. 2007. Linking stream habitats and spider distribution: spatial variations in trophic transfer across a forest-stream boundary. Ecol. Res., 22:619–628. JOHNSON, L. B., D. H. BRENEMAN, AND C. RICHARDS. 2003. Macroinvertebrate community structure and function associated with large wood in low gradient streams. River Res. Appl., 19:199–218. KATO, C., T. IWATA, S. NAKANO, AND D. KISHI. 2003. Dynamics of aquatic insect flux affects distribution of riparian web-building spiders. Oikos, 103:113–120. ———, ———, AND E. WADA. 2004. Prey use by web-building spiders: stable isotope analyses of trophic flow at a forest-stream ecotone. Ecol. Res., 19:633–643. MARCZAK, L. B. AND J. S. RICHARDSON. 2007. Spiders and subsidies: results from the riparian zone of a coastal temperate rainforest. J. Anim. Ecol., 76:687–694. OMERNIK, J. M. 1987. Ecoregions of the conterminous United States. Map (scale 1:7,500,000). Ann. Assoc. Am. Geogr., 77(1):118–125. ROLAUFFS, P., D. HERING, AND S. LOHSE. 2001. Composition, invertebrate community and productivity of a beaver dam in comparison to other stream habitat types. Hydrobiologia, 459:201–212. RYPSTRA, A. L. 1983. The importance of food and space in limiting web-spider densities - a test using field enclosures. Oecologia, 59:312–316. SCHNEIDER, K. M. AND K. O. WINEMILLER. 2008. Structural complexity of woody debris patches influences fish and macroinvertebrate species richness in a temperate floodplain-river system. Hydrobiologia, 610:235–244. WALLACE, J. B., M. R. WHILES, S. EGGERT, T. F. CUFFNEY, G. J. LUGTHART, AND K. CHUNG. 1995. Long term dynamics of coarse particulate organic matter in three Appalachian mountain streams. J. N. Am. Benthol. Soc., 14:217–232. CHRISTOPHER WOJAN1, AARON DEVOE, ERIC MERTEN, AND TODD WELLNITZ, Biology Department, University of Wisconsin - Eau Claire, Eau Claire, 54701. Submitted 2 October 2012; Accepted 17 October 2013.

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Corresponding author: Telephone: (715)-574-4856; e-mail: [email protected]


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