Plant-scale spatial pattern affects density dependence and incidence of herbivory in the threatened dune thistle, Cirsium pitcheri.
Girdler, E. Binney1, Radtke, Theresia1, 2, 1 Kalamazoo College, Kalamazoo, MI2 Teton Science School, Kelly, WY
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 Pitcher's Thistle (Cirsium pitcheri), a Great Lakes dune endemic, by mapping individuals with a GPS (n = 408). 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 non-random mortality, and preliminary regression analyses indicate 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 above-ground area 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. Since herbivory has been implicated as a conservation threat for local occurrences of this threatened species, explicit attention to plant-scale spatial pattern will be important to restoration efforts. We call for more systematic studies examining local spatial patterns in this and other threatened plant species and suggest that GPS mapping may be a useful tool for field studies of spatial pattern in plant communities.
Key words: Cirsium pitcheri, spatial, density, aggregations
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