Thursday, August 6, 2009: 3:40 PM
Picuris, Albuquerque Convention Center
Background/Question/Methods Species establishment rates are expected to increase with propagule pressure as immigration rates or the number of introductions increase, but it is unclear whether this relationship will be linear or follow some other form. Over realistic ranges, the propagule pressure-establishment relationship may be asymptotic or unimodal if competition among seedlings leads to increased mortality at high density. Alternatively, allee effects—driven by factors such as seed predation or allelopathy—may lead to a sigmoidal propagule pressure-establishment relationship if high establishment only occurs beyond a critical threshold. Few studies have quantified such relationships or determined if they vary with environmental conditions. We tested for such differences by manipulating seed density of Lespedeza cuneata in a native Kansas grassland. Because plant community structure commonly varies with disturbance and fertility, we crossed our propagule pressure treatments with six combinations of fertility (ambient, fertilized) and disturbance (no disturbance, simulated haying, simulated grazing).
Results/Conclusions After two growing seasons, a substantial number of individuals had established with large differences among environmental treatments in mean density (80 versus 264 m-2) and cover (35 versus 85%) for the highest propagule pressure treatments. The propagule pressure-establishment relationships varied from linear to asymptotic depending upon disturbance. In contrast, increased fertility reduced the slope or asymptote under all disturbance conditions. These patterns resulted from differences in the early germination phase as mortality rates of individuals beyond the true-leaf stage were low. Nevertheless, there were environmentally dependent differences in survivorship that contributed to the propagule pressure-establishment relationships. Our results suggest that environmental conditions modify the influence of propagule pressure on plant colonization and invasion.
Results/Conclusions After two growing seasons, a substantial number of individuals had established with large differences among environmental treatments in mean density (80 versus 264 m-2) and cover (35 versus 85%) for the highest propagule pressure treatments. The propagule pressure-establishment relationships varied from linear to asymptotic depending upon disturbance. In contrast, increased fertility reduced the slope or asymptote under all disturbance conditions. These patterns resulted from differences in the early germination phase as mortality rates of individuals beyond the true-leaf stage were low. Nevertheless, there were environmentally dependent differences in survivorship that contributed to the propagule pressure-establishment relationships. Our results suggest that environmental conditions modify the influence of propagule pressure on plant colonization and invasion.
