Ecosystem size and flooding drive trophic dynamics of riparian spiders in a fire-prone Sierra Nevada river system

Abstract

Disturbance can play an important role in structuring stream food webs. Although floods have received the greatest attention as a disturbance agent in rivers, wildfire — which can strongly influence fluvial ecosystem structure and function — may also drive consumer trophic dynamics. We measured the relative effects of wildfire, hydrologic disturbance, ecosystem size, and canopy openness (as a proxy for in-stream productivity) on trophic position and reliance on aquatically-derived nutritional subsidies of riparian spiders of the family Tetragnathidae along two rivers on the west slope of the Sierra Nevada in California, USA. Ecosystem size received strong support as an environmental determinant of both trophic measures, with variability in flood magnitude emerging as an important mechanism linking ecosystem size and trophic responses. Piecewise linear regression revealed significant breakpoints in spider trophic position and reliance on aquatically-derived nutritional subsidies that were related to thresholds in fire extent within the catchment. These nonlinear relationships with wildfire may lend additional insight into the potential interactions among ecosystem size, productivity, and disturbance that determine stream–riparian food-web architecture.

Local and landscape drivers of aquatic‐to‐terrestrial subsidies in riparian ecosystems: a worldwide meta‐analysis

“…This may be due to the fact that the importance of ecosystem size and the direction of its relationship to predator diets can be system‐specific, as conflicting relationships have been reported (Iwata , Stenroth et al. , Jackson and Sullivan ). In our study, agricultural land use and urbanization at the landscape scale, however, did have strong and consistent effects on terrestrial consumer diet, which might be driven by either direct or indirect effects.…”

“…This number was estimated as the difference between the consumer δ 15 N and mean basal resource δ 15 N divided by 3.4‰ (McHugh et al. , Jackson and Sullivan ). When raw data for stable isotope were available for consumers, we used the simmr package (Parnell et al.…”

“…, Schindler and Smits ), except for the recent studies of McHugh et al. (), Jackson and Sullivan (), Stenroth et al. (), and Carlson et al.…”

“…At the local scale (100-m buffer), landscape structure has an impact on predator diet by facilitating or preventing subsidies from entering recipient ecosystems (Greenwood 2014, Muehlbauer et al 2014. At the landscape (catchment) scale, ecosystem size and land use effects on riparian ecosystem food webs were poorly studied (Marczak et al 2007, Schindler andSmits 2017), except for the recent studies of McHugh et al (2010), Jackson and Sullivan (2017), Stenroth et al (2015), and Carlson et al (2016). Land use, at local and landscape scales, influences the composition and biomass of both aquatic insect communities (via water quality, terrestrial subsidies, and canopy cover: Dol edec et al 2006, Schindler andSmits 2017) and predator communities (Hendrickx 2007, Lafage et al 2015a).…”

Riparian spiders indicate the magnitude and sources of polychlorinated biphenyl contamination at a large contaminated sediment site

“…In contrast, araneid spiders are distributed from riparian to upland habitats and build stronger, vertical orb webs in which they catch aquatic and terrestrial insects. In near‐shore environments (e.g., within 5 m of the water's edge), both taxa demonstrate strongly aquatic diets, as indicated by a natural abundance of carbon and nitrogen isotopes and by the accumulation of aquatic contaminants in spider tissues (Walters et al , 2010; Raikow et al ; Speir et al ; Jackson and Sullivan ). Even though tetragnathid and araneid spiders are increasingly used as indicators of aquatic contaminants, it remains to be seen whether these 2 taxa tell a similar story about PCB contamination in the environment.…”