Higher survival at low density counteracts lower fecundity to obviate Allee effects in a perennial plant

1. At low densities, plants may produce fewer seeds than at high densities, due to reduced pollinator visits or reduced receipt of compatible pollen. In principle, lower seed production could lead to an Allee effect (a decline in the population growth rate with declining density), which, if severe, could cause the population to decline to extinction when recruitment is insufficient to replace dying individuals. 2. However, plants at low density may also grow or survive better than those at high densities, due to release from intraspecific competition. Few studies have examined the combined effects on plant population dynamics of positive density dependence in fecundity and negative density dependence in survival or growth. 3. To do so, we parameterized a density-dependent population projection matrix model for Piriqueta caroliniana, a widespread sandhill plant that often occurs at low density, using data from a demographic study of natural populations and from field experiments testing for effects of density on fecundity, growth and survival. We then tested for an Allee effect when various combinations of positive and negative density dependence are included in the model. 4. The model exhibits an Allee effect when only positive density dependence in fecundity is included, but when negative density dependence in survival is added, the Allee effect disappears. Thus the increase in survival as density declines overwhelms the effect of declining fecundity on the population growth rate. This occurs because survival has a greater influence on the population growth rate than fecundity (as measured by vital rate elasticities), increasingly so as density declines. 5. Synthesis. Our results caution against considering only declining fecundity at low density. We show that survival may increase at low densities, and will often have stronger effects on population growth than does fecundity, especially for perennial plants. Of course, reproductive failure and population collapse must eventually occur as the density of any non-selfing species approaches zero, or if pollinator abundance falls to low levels.