Ecological effects on estimates of effective population size in an annual plant

Effective population size (Ne) is a critical indicator of the vulnerability of a population to allele loss via genetic drift, and it can also be used to assess the evolutionary potential of a population. While some plant conservation plans have focused on outcrossing through cross-pollination as a way to increase estimated Ne, variance in reproductive output determined by ecological factors such as competition can also strongly affect estimated Ne. We examined the effects of intraspecific and interspecific competition, stressful soils, and local adaptation on estimates of Ne in an annual plant species. While ecological influences on plant growth rate variance have been predicted to influence estimates of Ne/N, we found a significant effect on the estimate of Ne/N, but no significant ecological effects on growth rate variance. Lower survivorship on stressful soil was the most important effect reducing estimates of Ne/N. If stochastic mortality is greater in environments that are abiotically stressful, then populations in these stressful environments may be slower to adapt because of lower census sizes and reduction of Ne/N. In populations of conservation concern, increasing survivorship may be of greater benefit for maximizing Ne than the reduction of variability in reproductive output among surviving adults.