Application of mark-recapture models to estimation of the population size of plants

Abstract

Mark–recapture models have been widely used in ecology to estimate population sizes of animals. In contrast, estimation of plant population size has usually been assumed to be much easier. However, detection of individuals is difficult for perennial plants, such as the rare prairie plant Mead’s milkweed (Asclepias meadii), which does not produce aboveground parts every year and lives in dense vegetation where nonflowering stems are hard to observe. In these cases, a count of the number of plants observed in a particular year may greatly underestimate the true population size, just as a count of animals in traps does not adequately estimate the total number of animals in an area. Using a family of closed population models (CAPTURE), we applied mark–recapture methodology to estimate population size of A. meadii. Over a 4-yr period, a total of 129 patches (aggregated collections of stems) was observed, with 124 flowering in at least one year. In any one year, however, the number of flowering patches ranged from 15 to 105. Using model Mth of CAPTURE with these data, the estimated number of patches capable of flowering was 219. Although the confidence interval is broad (95% confidence interval of 175–302), these results emphasize that the observed number of patches in any one year, or even over a 4-yr period, underestimates the actual population size.