Viability of multiple populations across the range of a species at risk: The case of Pitcher's thistle, Cirsium pitcheri, in Canada

Abstract

A quantitative, probabilistic approach to species and population assessment can be used to inform a species' legal at-risk designation and help determine priorities and targets for population recovery and protection measures. To that aim, we analysed the viability of a large number of individual populations of a species across its range in Canada, we compared that viability among populations, and we assess the viability of all those populations as a whole. We used Pitcher's thistle, Cirsium pitcheri, as a case because it is listed at risk in both Canada and the United States but its actual risk of extinction remains unclear. Also, for this monocarpic plant endemic to Great Lakes dunes and beaches, there are 35 years of monitoring data available for the populations in Pukaskwa National Park on Lake Superior, and nearly 17 years of data for populations on Lake Huron. Means and variances of the annual population growth rates were computed from those time series, to estimate the cumulative distribution function for time-to-extinction, using the diffusion approximation as well as simulations. We also ran multi-population stochastic projections to estimate the risk of decline of the whole Canadian population. We found that many Pitcher's thistle populations have a high probability of either extinction or undergoing a 50% decline at some point in their future. This is due mainly to steady declines experienced by small populations. However, multi-population stochastic projections and the projected growth of many large individual populations indicate that Pitcher's thistle is likely to persist in Canada over the next 100 years, assuming past conditions continue in the future. Keywords: Viability analysis, Species at risk, Species status assessment, Pitcher thistle, Plant population dynamics, Environmental variation, Endemic species