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

Abstract

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 ...