Climate impacts on bird and plant communities from altered animal--plant interactions

The contribution of climate change to declining populations of organisms remains a question of outstanding concern1–3. Much attention to declining populations has focused on how changing climate drives phenological mismatches between animals and their food4–6. Effects of climate on plant communities may provide an alternative, but particularly powerful, influence on animal populations because plants provide their habitats. Here, we show that abundances of deciduous trees and associated songbirds have declined with decreasing snowfall over 22 years of study in montane Arizona, USA. We experimentally tested the hypothesis that declining snowfall indirectly influences plants and associated birds by allowing greater over-winter herbivory by elk (Cervus canadensis). We excluded elk from one of two paired snowmelt drainages (10 ha per drainage), and replicated this paired experiment across three distant canyons. Over six years, we reversed multi-decade declines in plant and bird populations by experimentally inhibiting heavy winter herbivory associated with declining snowfall. Moreover, predation rates on songbird nests decreased in exclosures, despite higher abundances of nest predators, demonstrating the over-riding importance of habitat quality to avian recruitment. Thus, our results suggest that climate impacts on plant–animal interactions can have forceful ramifying effects on plants, birds, and ecological interactions. One of the clearest signals of anthropogenically driven climate change is the declining snowfall observed across mountains of the world1,7,8. Reductions in winter snowpack can influence the abundance and growth of individual tree species9,10, and changes in the plant community may then impact populations of animals that rely on these plants for habitat. Asmost animals, including breeding songbirds, have specific habitat requirements11, changes in habitat through plant responses to climate change have great potential to influence entire bird communities. Climate-driven change in plant communities can also influence the abundance and search efficiency of avian nest predators11. Thus, climate changemay indirectly affect songbird abundance via trophic (predation) as well as non-trophic (habitat) routes. These varied impacts of climate change on bird assemblages remain unstudied. Declining snowfall can influence plant communities by either negatively influencing plant demographics12,13, or by changing plant–herbivore interactions. Large ungulates are dominant herbivores in many high-elevation and high-latitude systems and can strongly influence plant productivity and demography14,15. Declining snowfall has great potential to influence plant communities indirectly if it alters interactions between ungulates and plants. Here, we demonstrate that reduced snowfall mediates