PARENT SESSION
Oral Session #3: Plant Population Ecology. Presiding: N. Fetcher.
Monday, August 6, 2001. 8:00 AM to 11:30 AM. Hall of Ideas L&M.
Estimation of population size and probabilities of survival and detection in Mead's milkweed, a rare prairie plant.
SLADE, NORMAN1,2, ALEXANDER, HELEN1, KETTLE, DEAN3, 1 2 3
ABSTRACT- Recently ecologists have recognized the statistical similarities between estimating numbers of cryptic animals and of inconspicuous plants. Without flowers, Mead's milkweed (Asclepias meadii) is an unobtrusive plant in tallgrass prairies, hence the probability of locating plants increases with flowering, but most plants flower irregularly. We used maximum likelihood methods to estimate survival and probability of detecting plants for annual censuses from 1992-1999. Our statistical models were selected based on knowledge of life history, then tested for goodness of fit, rather than relying on model selection procedures. Models were fit to data from all the plants and to data from flowering plants only. Plants were marked with flags when located, so probabilities of initial discovery were much lower than probabilities of resighting. On our site, plants were mostly likely to flower and be detected in years in which the prairie was burned. Annual survival was estimated as ≥0.95 in 5 of 7 years using the flowering data and in 2 of 7 years using all data. In either case, aggregate survival over 8 years was so low as to predict rapid extinction of the population. We counted 177 plants in the field, but estimated the population as between 190 and 337. Thus, population size likely exceeds the number of plants seen, and more reproduction than we have observed is necessary to sustain the population.
KEY WORDS: rare plant, capture-mark-recapture, survival estimates, population estimates