The strength of the Yellowstone trophic cascade after wolf reintroduction

Abstract

Trophic cascades, the indirect effects of predators propagating downward through food webs, play a critical role in shaping ecosystems. We evaluated the strength of a large carnivore-induced trophic cascade in northern Yellowstone National Park, focusing on riparian willows (Salix spp.) as primary producers. Using the log₁₀ response ratio, a standardized indicator of trophic cascade strength, we quantified changes in willow crown volume following the 1995–96 reintroduction of gray wolves (Canis lupus), which completed the large carnivore guild. Reduced herbivory pressure from Rocky Mountain elk (Cervus canadensis) followed their reintroduction, leading to increased growth in willows. Crown volume, a proxy for above-ground biomass, was calculated using a predictive model based on willow height and was used to index primary producer response. Data from a 20-year study (2001–2020) revealed a relatively strong trophic cascade, with a ∼1500 % increase in average willow crown volume and a log₁₀ ratio of 1.21. This ratio surpassed 82 % of those reported in a global meta-analysis of trophic cascades. These results emphasize the importance of long-term monitoring to capture gradual and nonlinear ecosystem responses following predator reintroductions. They also underscore the substantial effect restored large carnivores can have on riparian vegetation and highlight the utility of crown volume as a metric for assessing trophic cascade strength.