Native insect herbivory and density influence the establishment and invasion trajectories of a non-native thistle

Background/Question/Methods

Identifying factors that affect whether or not a non-native species become invasive is an important goal for ecology and conservation management. Two factors shown to limit the growth, fecundity, and density of non-native species are low propagule density and native predators. However, we know little about their population level effects, and it is therefore challenging to evaluate the consequences of such effects on plant invasions. We tested the effects of seed density and native insect herbivory on the population dynamics of newly established Cirsium vulgare. We established plots (3 m²) at eight sites in one region of the central Great Plains, USA. At each site, we seeded 14 plots, using seven densities ( 150 – 1050 seeds/plot); half of the plots (one at each density) were treated with insecticide to exclude herbivores. We collected demographic data from 2006-2008 and used it to construct a density-dependent integral projection model for populations with and without insects. We parameterized the model using Bayesian analysis to quantify hierarchical levels of spatial variance. We used the model to determine how native insect herbivores and density dependence interact to influence population dynamics.

Results/Conclusions

Herbivory had strong negative effects on all vital rates. We found negative density dependence in growth and seed production but positive density dependence in survival. Overall, herbivory had a strong negative effect on population dynamics, driving the growth rate below replacement (λ = 1) at all densities and predicting population extinction at all seed densities in the presence of herbivores; weak positive density dependence occurred at low densities and weak negative density dependence occurred at high seedling densities. With experimental reduction of herbivores, population growth rate was strongly increasing (λ ≈ 6) but dropped below replacement at high densities, leading to a non-zero equilibrium population size. Thus, our results show that native insect herbivores can limit the invasiveness of C. vulgare and even cause local extinctions, regardless of the initial seed density. Native consumers can therefore play an important role in the establishment and invasion trajectories of non-native species.