Chronic, low-level nitrogen deposition enhances abundances of ant-protected herbivores inhabiting an imperiled foundation species

Abstract

Atmospheric nitrogen (N) deposition is driving changes in plant assemblages worldwide, but animal responses are not well understood, especially in relation to current widespread rates of low-level N deposition. Big sagebrush (Artemisia tridentata) is a foundation species in steppe ecosystems of western North America, where elevated, but low-level N deposition is prevalent. Sagebrush hosts numerous herbivorous and predatory arthropods, some of which form mutualistic (i.e. mutually beneficial) herbivore-predator associations that may be harmful for sagebrush. Here, I evaluated the response of a sagebrush arthropod assemblage to four and nine years of low-level N enrichment in Colorado, USA. Though sagebrush cover and arthropod abundances were not different in the fourth year, by the ninth year, N additions increased arthropod abundances due to greater numbers of sap-feeding insects that are mutualist partners of predaceous ants. Ant and non-mutualist herbivore abundances did not significantly respond to N treatments in the ninth year, but non-mutualist predators were only found in ambient N plots, suggesting ants provided greater anti-predator protection for mutualist herbivores under N enrichment. These responses occurred during drought conditions, which may have increased sagebrush nutritional quality for sap-feeding insects. Sagebrush is experiencing multiple stressors that are transforming steppe ecosystems; these results suggest chronic exposure to common rates of low-level N deposition may create further stress as herbivorous insects proliferate. Additional work is needed to understand and manage the repercussions of N deposition for plant-animal assemblages, particularly in contexts including species of conservation concern and both mutualistic and antagonistic species interactions.