Beaver activity increases aquatic subsidies to terrestrial consumers MAGNUS MCCAFFERY † AND LISA EBY The Wildlife Biology Program, College of Forestry and Conservation, The University of Montana, Missoula, MT 59812, USA SUMMARY 1. The occurrence and importance of fluxes of nutrients and organic matter between aquatic and terrestrial habitats is well established, but how catchment characteristics influence these fluxes remains unclear. Beaver (Castor canadensis) alter freshwater ecosystems and increase aquatic production, but it is unknown how these changes influence the magnitude and lateral dispersal of aquatic nutrients into terrestrial ecosystems. 2. We examined differences in abundances of dominant aquatic invertebrates, wolf spiders (Lycosidae), and deer mice (Peromyscus maniculatus), at beaver and non-beaver sites. We used stable isotopes to track aquatic-derived carbon in terrestrial consumers and linear mixed-effects models to examine the importance of beaver presence and distance from stream channel on the percentage of aquatic-derived carbon in terrestrial consumers. 3. Sites with beaver activity had >200% higher aquatic invertebrate emergence rates as well as 60% and 75% higher abundances of spiders and deer mice, respectively, relative to non-beaver sites. 4. The tissues of both spiders and deer mice exhibited a greater percentage of aquatic-derived carbon at sites with beaver activity than at non-beaver sites. 5. Aquatic-derived carbon in deer mice declined linearly with distance from the stream edge at both beaver and non-beaver sites. The contribution of aquatic-derived carbon in mice extended farther from the stream edge in beaver-modified catchments. Aquatic-derived carbon in spiders also declined linearly with distance from the stream at beaver sites but not at non-beaver sites. 6. We documented a novel example of increased aquatic subsidy to riparian areas with beaver activity, leading to changes in the magnitude of the lateral dispersal of aquatic nutrient subsidies to the terrestrial environment in small stream systems. Understanding the effects of natural disturbance regimes, such as beaver modification, will be important for management and, where appropriate, restoration of natural catchment processes. Keywords: aquatic-derived carbon, deer mouse, resource subsidies, stable isotopes, wolf spider Aquatic and terrestrial ecosystems are inextricably linked, and the effects of habitat alteration in one habitat can propagate throughout their linked food webs (Rein- ers & Driese, 2001; Saunders & Fausch, 2007). The input of terrestrial detritus (e.g. leaves, woody debris), sedi- ment and nutrients into lotic systems was a crucial com- ponent of early ecological conceptual models describing longitudinal gradients from headwaters to river ecosys- tems (Vannote et al., 1980). More recently, studies have elucidated the importance of reciprocal fluxes that pro- vide habitat, nutrients and energy to subsidise food webs. For instance, subsidies can increase individual growth rates (e.g. Wipfli et al., 2003) and population sizes (e.g. Barrett et al., 2005) of organisms in receiving systems, as well as influence community structure and trophic dynamics (e.g. Baxter, Fausch & Carl Saunders, 2005; Burdon & Harding, 2008; Richardson, Zhang & Marczak, 2010). As an understanding of subsidies con- tinues to evolve, ecologists are investigating the influ- ence of factors such as spatial scale (e.g. Stapp & Polis, Correspondence: Magnus McCaffery, The Wildlife Biology Program, College of Forestry and Conservation, The University of Montana, Missoula, MT 59812, USA. E-mail: magnus.mccaffery@tedturner.com † Current address: Turner Endangered Species Fund, 901 Technology Blvd., Bozeman, MT 59718, USA. 518 © 2016 John Wiley & Sons Ltd Freshwater Biology (2016) 61, 518–532 doi:10.1111/fwb.12725