Estimating Causal Impacts of Human Recreation on Wildlife in the Absence of Experimental Controls

Much recent research has focused on the impact of human recreation on wildlife, but relatively few studies have used causal inference approaches; doing so would strengthen recreation management and decision-making. Here, we use tools from the causal analysis literature and a multi-year observational dataset to assess how human road- and trail-use affects an apex predator, the grizzly bear (Ursus arctos horribilis). Our study leverages a natural experiment that reduced—via access restrictions and changes to tourism operator conditions—peak-season human recreation by ∼ 85% in 2023 compared to levels in 2018–2022.We used structural time series forecasting to quantify how weekly detection rates of grizzly bears changed in 2023 versus previous years, and “placebo tests” to strengthen causal inference and rule out competing hypotheses. We show that grizzly bear detections and temporal trends were 185% higher in late summer 2023 due to reduced human trail-use, providing robust evidence that human recreation can cause reduced wildlife activity in protected areas.