Alpine birds in a sky island: Resource subsidies from foothill areas

Material fluxes between ecosystems subsidize consumers in recipient ecosystems. While alpine zones are generally regarded as isolated, arthropods from lower elevations may be deposited on the snow surface in alpine zones by flight or wind. This arthropod fallout should be essential food resources for alpine consumers that are forced to deal with scarce food in environments. In this study, the source location of arthropods that fell onto the alpine snowpack was examined using data of their host plants. Furthermore, seasonal changes in the arthropod diets of two insectivorous and one primarily herbivorous alpine bird species were evaluated by fecal DNA metabarcoding with correction of PCR amplification bias among arthropod taxa using DNA mock assemblages of arthropods. We present quantitative evidence that winged aphids originating from the subalpine and montane zones are abundant on the snowpack. These subsidized aphids accounted for approximately 40% of the arthropod portion of the diet, and 6%–40% of the overall diet, of these birds during their early breeding seasons. Our findings indicate that material fluxes from foothill areas contribute to the maintenance of biotic communities in alpine ecosystems during less productive seasons.