Crossing boundaries: Introduced trout alter the bird community in a naturally fishless headwaters ecosystem

Abstract Alpine areas such as the headwaters of California's Sierra Nevada are increasingly imperiled by climate change and other human‐induced stressors. For one, the introduction of non‐native fishes into over 60% of its historically fishless watersheds has profoundly restructured the aquatic food web, depressed the abundance, biomass, and diversity of native invertebrates, and endangered endemic vertebrate taxa. Lakes and their surrounding land are reciprocally connected through flows of organic matter, such as emerging aquatic insects; however, the extent to which trout introductions disrupt such flows to the terrestrial environment is poorly understood. Emerging aquatic insects are a potentially important prey source for birds; thus, we hypothesized that the community composition, diversity, and abundance of birds differ between fishless lakes and those stocked with trout due to reduced emerging aquatic insect biomass and diversity. We conducted lakeside bird surveys in headwater lake basins in Sequoia and Kings Canyon National Parks for three summers (2014–2015, 2020) at a total of 39 lakes and quantified the emergence of mayflies between lake types using sticky traps. In addition to major reductions in mayfly densities, we documented lower overall bird abundance at stocked lakes, as well as turnover in avian community composition between stocked and fishless lakes. A fish eradication project at one lake between 2016 and 2019 allowed us to conduct a multiple‐control before‐after‐control‐impact (M‐BACI) case study to assess changes in the avian community following fish removal. After fish were removed, bird abundance was two times higher than at the control lake. Given the global decline of insect populations and their importance in bird diets, this study reinforces the importance of studying aquatic and terrestrial habitats as interdependent systems and motivates the restoration of naturally fishless habitats impacted by introduced fishes.