Insect herbivory and propagule pressure influence Cirsium vulgare invasiveness across the landscape

Abstract

Abstract. A current challenge in ecology is to better understand the magnitude, variation, and interaction in the factors that limit the invasiveness of exotic species. We conducted a factorial experiment involving herbivore manipulation (insecticide-in-water vs. water-only control) and seven densities of introduced nonnative Cirsium vulgare (bull thistle) seed. The experiment was repeated with two seed cohorts at eight grassland sites uninvaded by C. vulgare in the central Great Plains, USA. Herbivory by native insects significantly reduced thistle seedling density, causing the largest reductions in density at the highest propagule inputs. The magnitude of this herbivore effect varied widely among sites and between cohort years. The combination of herbivory and lower propagule pressure increased the rate at which new C. vulgare populations failed to establish during the initial stages of invasion. This experiment demonstrates that the interaction between biotic resistance by native insects, propagule pressure, and spatiotemporal variation in their effects were crucial to the initial invasion by this Eurasian plant in the western tallgrass prairie.

Native insect herbivory limits population growth rate of a non-native thistle

“…Those model results suggested a growing C. vulgare population that was inconsistent with quantitative field observations (Andersen and Louda 2008). Our first experiment showed that greater propagule pressure and experimental reduction of herbivory led to the highest initial C. vulgare seedling density (Eckberg et al 2012); however, in that study we did not evaluate the effects of herbivory on plant population growth. In this study, we experimentally introduced C. vulgare into multiple currently uninvaded grassland sites in Lancaster and Saunders Counties, Nebraska, USA.…”

“…This approach precludes a rigorous assessment of the effects of native insect herbivory (biotic resistance) on the critical initial phase of invasion: plant population establishment. Our previous study showed that herbivory significantly reduced the rate at which the Eurasian C. vulgare seedlings invaded new grasslands, an effect that intensified with lower propagule pressure (Eckberg et al 2012). However, the effect of herbivory on population growth rate for newly established C. vulgare invasions remains unknown.…”

“…The influence of native fauna as a factor contributing to cases of failed plant invasions remains underevaluated experimentally (Eckberg et al 2012). The two dominant hypotheses have focused on how either escape from herbivores (enemy release) or accumulation of new herbivores (biotic resistance) affect the outcome of plant invasions (Maron and Vila 2001;Levine et al 2004;Williams et al 2010).…”

“…The two dominant hypotheses have focused on how either escape from herbivores (enemy release) or accumulation of new herbivores (biotic resistance) affect the outcome of plant invasions (Maron and Vila 2001;Levine et al 2004;Williams et al 2010). We know that biotic factors, such as native herbivores, can reduce survival, growth, and fecundity of nonnative plants (Maron and Vila 2001;Levine et al 2004;Eckberg et al 2012;. However, we still need quantitative, demographic assessment of the consequences of such effects by native biota on the population dynamics and density of non-native plants in their new environments.…”

“…Herbivory effects on specific plant life stages, even when severe, may have limited influence on population growth, density, or distribution (Parker 2000;Maron and Vila 2001;Levine et al 2004). Furthermore, there is often large spatial variation in the effect of herbivory on plant population dynamics (Louda 1982(Louda , 1983Eckberg et al 2012). Thus, population-level experiments at multiple sites or habitats are required, as has been done for native plant populations ).…”