As urban areas expand, wildlife show adaptations to urban ecosystems. We tested two hypotheses for urban populations of red fox (Vulpes vulpes (Linnaeus, 1758)) in urban areas: the population pressure hypothesis, which posits that urban foxes make do with suboptimal habitat, and the urban island hypothesis, which presumes that urban areas provide high-quality habitat. We investigated habitat quality by investigating anthropogenic food in fox diets across a rural–urban gradient in Lancaster, Pennsylvania (USA). We used stable carbon isotopes because human food can have a distinct stable carbon isotope signature. We collected fox hair and stomach samples from 21 locations and extracted land use and land cover characteristics within a 100 ha buffer area. We found that higher δ13C values in fox hair were positively correlated with impervious surface cover and developed open spaces, key metrics of urbanization, and negatively associated with agricultural land cover, an indicator of rural habitats. Overall, fox hair δ13C was less related to urbanization and more related to the availability of developed open spaces that provide habitat with vegetation cover and access to nearby food sources. Our results suggest that urban habitats are high quality and support the growing literature revealing that certain species may thrive in urban areas.

Nitrogen is an essential, often limiting, element for biological growth that can act as a pollutant if present at high concentrations. Excessive nitrogen in highly biologically available forms, especially nitrate (NO 3 − ), is common in urban ecosystems. Urban ecosystems tend to have higher NO 3 − burdens due to higher inputs from fertilizer application (Baker et al., 2001;Law et al., 2004), deposition from fossil fuel combustion (Bettez & Groffman, 2013;Hale et al., 2014), and treatment of sewage (Lauver & Baker, 2000). In addition, urban stormwater infrastructure quickly transports runoff to storm drains that can discharge into water bodies (Baker et al., 2001;Kaye et al., 2006). Whether this high NO 3 − burden causes eutrophication partially depends on the capacity of the aquatic ecosystem to attenuate NO 3 − inputs. Wetland ecosystems can be hotpots for NO 3 − attenuation due to the availability of water, organic matter, and variable oxygen zones (Ehrenfeld, 2000;Mitsch & Gosselink, 2015). These environmental conditions are conducive to NO 3 − attenuation through pathways such as denitrification and dissimilatory NO 3 − reduction to ammonium (NH 4 + ) (DNRA). Indeed, urban wetland ecosystems are thought to

Cessation of cattle grazing has resulted in the reestablishment of wetlands in some streams of the U.S. Southwest. Decades of cattle grazing prevented vascular plant growth in Sycamore Creek (Arizona, U.S.A.), resulting in stream reaches dominated by diatoms and filamentous green algae. Establishment of vascular plants can profoundly modify ecosystem processes; yet, the effects on nitrogen (N) cycling remain unexplored. We examined the consequences of this ecosystem state shift on N cycling in this N‐limited desert stream. We compared results from whole‐reach ammonium‐N stable isotope (15NH4+) tracer additions conducted before (pre‐wetland state) and 13 yr after (wetland state) free‐range cattle removal from the watershed. Water column estimations showed that in‐stream N uptake and storage were higher in the pre‐wetland than in the wetland state. N turnover was also higher in the pre‐wetland state, indicating that assimilated N was retained for shorter time in stream biomass. In addition, N uptake was mostly driven by assimilatory uptake regardless of the ecosystem state considered. Water column trends were mechanistically explained by the fact that the dominant primary uptake compartments in the pre‐wetland state (i.e., algae and diatoms) had higher assimilatory uptake and turnover rates than those in the wetland state (i.e., vascular plants). Overall, results show that the shift in the composition and dominance of primary producers induced by the cessation of cattle grazing within the stream‐riparian corridor changes in‐stream N processing from a dominance of intense and fast N recycling to a prevalence of N retention in biomass of primary producers.

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