Parental environmental effects and cyclical dynamics in plant populations

Parental environmental effects have been widely reported in plants, but these effects are often weak relative to direct effects of current environmental conditions. Few studies have asked when consideration of such effects is necessary to understand long-term plant population dynamics. In this article, I show that inclusion of effects of parental density on offspring mass fundamentally changes population dynamics models by making recruitment a function of population size in two previous generations (Nt+1 = f(Nt, Nt-1)), rather than one (Nt+1 = f(Nt,)). Models without parental density effects predict either stable population dynamics or sharp crashes from high to low population size (flip bifurcations). When parental effects are at least one-third the size of direct density effects, gradual cycles from high to low population size (Hopf bifurcations) are possible. In this study, I measured effects of parental and offspring density on offspring quality in an annual plant, Cardamine pensylvanica, by manipulating plant density independently in parent and offspring generations and by comparing the effects of parent and offspring density on offspring performance. Parental density effects were detectable but were noticeably weaker than offspring density effects. Nonetheless, the parental effect was large enough to change population dynamics predictions. Thus, parental effects may be an important component of the numerical dynamics of plant populations.