Conservation Implications of Individual Scale Spatial Pattern in the Threatened Dune Thistle, Cirsium pitcheri

Abstract

For plants and other sessile organisms, the dispersion of individuals in a population can influence the strength of ecological interactions, and can have important implications for the conservation of these species. We investigated the spatial pattern in a population of the monocarpic perennial Cirsium pitcheri (Pitcher's thistle), a dune species endemic to the western Great Lakes, U.S.A., by mapping individuals with a GPS. Using a refined nearest neighbor analysis of the mapped point data combined with Monte Carlo randomization tests, we found that individual plants were clustered on the scale of about a meter, which was smaller than expected if aggregations were caused by major habitat features such as dune height or aspect. The size of clusters was consistent with reports of relatively short-distance dispersal of seeds. We found no evidence of self-thinning via nonrandom mortality, and regression analyses indicated no density dependent effects on reproductive effort at a range of ecologically reasonable scales. However, we did find a suggestion of density dependent effects on juvenile size at several scales. The neighborhood radius that maximized the variance explained was 25 cm, roughly the radius spanned by the largest juvenile individuals in our study. Incidence of herbivory was not concentrated in denser patches of C. pitcheri; in fact, we found a trend in the opposite direction: isolated individuals were more likely to have been damaged by herbivory than those with a crowded local neighborhood. Our results show that explicit attention to such individual-scale spatial patterns can lead to increased understanding and thus more effective management of local plant populations. We call for more systematic studies examining local spatial patterns in this and other threatened plant species.