Complex dynamics in experimental populations of an annual plant, Cardamine pensylvanica

To study the numerical dynamics of plant populations, 12 experimental populations of an annual greenhouse weed, Cardamine pensylvanica, were maintained for 15 generations in controlled-environment growth chambers by growing plants in an array of pots and allowing seed for the next generation to disperse into an adjacent array of fresh pots. Discrete generations were enforced by harvesting mature plants after seed dispersal, but germination, recruitment, competition, and dispersal occurred naturally. The numerical dynamics of the experimental populations cycled from high to low density with a period of four to five generations, as indicated by negative autocorrelations in population size at lags of two and three generations. Demographic data collected during the experiment indicate that population density affected plant growth and seed set. Independent estimates of low-density recruitment were also high enough to predict complex population dynamics from simple models of direct density-dependent population regulation. However, simple population models fit to the time series data predicted stable dynamics. Similar models including time-lagged density dependence qualitatively reproduced the dynamics of the experimental populations. Delayed feedback through maternal effects or interacting herbivores or pathogens may be possible causes of the observed dynamics. This suggests that although plant population dynamics may be stabilized by direct density dependence, delayed density dependence could destabilize dynamics.