A new paper in the journal Nature Climate Change finds evidence that declining snowfall in the southwestern US indirectly influences plants and associated birds by allowing greater over-winter herbivory by elk. Abundances of deciduous trees and associated songbirds have declined with decreasing snowfall over 22 years of study in montane Arizona. The researchers experimentally tested the hypothesis that declining snowfall indirectly influences plants and associated birds by allowing greater over-winter herbivory by elk, by excluding elk from one of two paired snowmelt drainages and replicating this paired experiment across three distant canyons. Over six years, the exclosures reversed multi-decade declines in plant and bird populations by experimentally inhibiting heavy winter herbivory associated with declining snowfall. 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.
These findings imply that climate change and increasing abundance of herbivores may interact to impact other trophic levels. Although not specifically addressed in the study, in some cases increasing ungulate abundance may be due to loss of top predators. Although evidence from various studies do not yet clearly indicate how ubiquitous such top-down regulation is, the new study supports conclusions from other areas that restoration of wolves and other top carnivores may interact with climate change to trigger complex indirect effects on other species.
The full paper is available here.