Temporal and spatial changes in a Western Rattlesnake (Crotalus oreganus) population in British Columbia

Effective conservation strategies and management plans for wildlife species, especially species-at-risk, require demographic information on populations and an understanding of how local landscapes and management regimes impact them. Historic comparisons are especially important because gradual declines across large landscapes can go unnoticed as wildlife managers in each generation and region start with a lower initial population size estimation. This phenomenon is called the Shifting Baselines syndrome. Replicating a historic study from 1981-1983, I conducted a mark-recapture study in 2018-2020 on Western Rattlesnakes in the North Okanagan region of British Columbia. The study area spanned two habitats with contrasting levels of human influence: Kalamalka Lake Provincial Park and Coldstream Ranch.The former receives an average of 32,500 visitors per month during the active season for snakes, while Coldstream Ranch allows cattle grazing but is off-limits to humans. In the 35 years since the original study, the Park population was estimated to have declined by 50% and the Ranch population by 31%, for an overall decline of 40%. A separate analysis using only adult snakes showed less severe declines, suggesting a lack of recruitment of snakes into the population may be contributing to this decline. Qualitative observations of behavioural differences between sites also prompted a standardized test of defensive behaviour in animals across the two contrasting habitats. Snakes in the area of high human visitation were 9.5× less likely to display defensive rattling behaviour, and allowed approaching investigators 2.5 m closer on average before initiating rattling behaviour, compared to snakes in habitats with negligible human presence.This study highlights the pitfalls of using point estimates of populations to assess the status of a species over a broad geographic area.These results also suggest that protected areas may not necessarily serve as ‘anchors’ for conservation. Further research spanning multiple landscapes and management types is necessary to differentiate between natural and anthropogenic pressures on populations. Revealing the specific pressures that may be at play in varying locations should be addressed for continued conservation.