APPLICATION OF MARK-RECAPTURE MODELS TO ESTIMATION OF THE POPULATION-SIZE OF PLANTS

Mark-recapture models have been widely used in ecology to estimate pop ulation 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 prair ie plant Mead's milkweed (Asclepias meadii), which does not produce ab oveground parts every year and lives in dense vegetation where nonflow ering 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 ad equately estimate the total number of animals in an area. Using a fami ly of closed population models (CAPTURE), we applied mark-recapture me thodology to estimate population size of A. meadii. Over a 4-yr period , a total of 129 patches (aggregated collections of stems) was observe d, 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 M-t h of CAPTURE with these data, the estimated number of patches capable of flowering was 219. Although the confidence interval is broad (95% c onfidence interval of 175-302), these results emphasize that the obser ved number of patches in any one year, or even over a 4-yr period, und erestimates the actual population size.