Vicuña antipredator diel migration drives spatial nutrient subsidies in a high Andean ecosystem

Spatial subsidies of nutrients within and among ecosystems have profound effects on ecosystem structure and functioning. Large animals can be important drivers of nutrient transport as they ingest resources in some habitats and release them in others, even moving nutrients against elevational gradients. In high Andean deserts, vicuñas (Vicugna vicugna) navigate a landscape of fear by migrating daily between productive wet meadows, where there is abundant water and forage but high risk of predation by pumas (Puma concolor), and open plains, where soils are nutrient-poor and forage is less abundant but the risk of predation is low. As they move, vicuñas also defecate and urinate in communal latrines to maintain the cohesion of their family groups. We investigated whether these latrines impacted soil and plant nutrient concentrations across three habitats in the Andean ecosystem (meadows, plains, and high-risk rugged canyons), and used stable isotope analysis to determine the source of fecal nutrients in latrines. We found that latrines increased the concentrations of nitrogen, carbon, phosphorus, and other nutrients in soils across all habitats. These inputs also corresponded with an increase in plant quality (lower C:N) at latrine sites in plains and canyons, but not in meadows. As we predicted, stable isotope mixing models revealed that a substantial proportion (30%) of nutrients in latrines in plains originated from vegetation in meadows, even though meadows accounted for only 2.6% of the landscape. Thus, vicuña diel migrations, motivated by predator avoidance, drive nutrient subsidies from low-lying, nutrient-rich meadows to more elevated, nutrient-poor plains. Scaling these results up to the landscape scale, we found that the amount of additional nitrogen and phosphorus in plains latrines were of the same order of magnitude as estimates of annual atmospheric nitrogen and phosphorus deposition for this region. These results suggest that vicuña-mediated nutrient cycling and deposition is an important process impacting ecosystem functioning in arid Andean environments, on par with other major inputs of nutrients to the system.