A 19-year study of the Mead's milkweed (Asclepias meadii): Insights on detection of individual plants and on population ecology

Relative to animals, plants are viewed as “easy to count” because of their sessile nature.  Our nineteen year study of the Mead’s milkweed, Asclepias meadii, however suggests that detection of individual plants is a major issue for this rare, long-lived herbaceous perennial.  Two features contribute to this result: the occurrence of a dormant stage and the fact the species lives in a matrix of dense prairie vegetation.  To study the population biology of this plant, we combined yearly monitoring of plants with analyses based on mark-recapture methodologies.  We conclude that plants have, on average, a 96% probability of survival and that recruitment of new plants occurs frequently enough to balance plant mortality.  Thus, population sizes are likely to be relatively constant over time despite large variation in the number of plants observed in any one year.  Analyses of our long-term data set also reveal that our method of defining plants is effective and that spring burning increases the probability of flowering.  Further, we discovered that multiple-year data sets are essential for accurate survival estimates.  By having a long- term data set, we also were able to study the plant in unusual years with large numbers of flowering plants.  These years were important both for the biology of the species (i.e. high seed production) but also because they made it possible to detect a greater proportion of plants in the population.  Our general monitoring and data analysis approaches should be applicable to other similar species.