Temporal Variation in Recruitment and Mortality for the Pasture Weed Hieracium floribundum: Implications for a Model of Population Dynamics

Abstract

(1) Threefold differences in annual recruitment were recorded for the pasture weed Hieracium floribundum* during an 8-year study. There were also eight-fold differences in the maximum value of annual mortality during the same period. (2) Annual variation in the effects of herbivores (e.g., Microtus sp.) apparently accounted for most of the change in mortality and recruitment. Recruitment may also have been influenced by the availability of soil moisture early in the growing season. (3) Population density increased more slowly than predicted by a mathematical model, because the model did not allow for this temporal variation in recruitment and mortality. After 6 years, predicted density was an average of 60% greater than actual density. Models to predict long-term changes in weed density must therefore allow for annual variation in recruitment and mortality.