The hotter the better: high mean annual temperature, not seed predation, hastens the decline of invasive Bromus tectorum

Bromus tectorum invasion degrades biodiversity and ecosystem functioning in the Great Basin Desert. To explain, predict, and manage B. tectorum invasion, we must understand the biotic and abiotic factors that influence its establishment and persistence. We explored interactions between two key constraints for invaders in general and B. tectorum in particular: post-dispersal seed predation and climate, including mean annual temperature, rainfall, snowfall, and aridity. At six study sites in undisturbed, climax Artemisia tridentata communities across northern Nevada and Utah, we performed one-time additions of 100 B. tectorum seeds to microsites outfitted with either a dummy (“open”) cage that exposed seeds to rodent foraging, or a functional (“closed”) cage that protected them from rodent foraging. Living Bromus tectorum plants within experimental cages were censused one and 5 years after seed additions. At both 1- and 5-years censuses, and regardless of climate, B. tectorum densities were similar in open and closed microsites, suggesting that rodent foraging did not affect B. tectorum establishment or persistence. Compared to 1-year censuses, B. tectorum counts declined by 47% across all sites after 5 years, but declines were sharpest at sites with the highest mean annual temperatures. Taken together, our findings suggest that undisturbed, climax A. tridentata communities can resist B. tectorum expansion, and this resistance increases with mean annual temperature, not rodent foraging. We suggest that controlling nascent populations of B. tectorum may be particularly urgent in intact climax A. tridentata communities in relatively cool climates, where community-level resistance against B. tectorum persistence is lowest.