PARENT SESSION
Contributed Oral Session 93: Modeling: Movement, Populations, and Communities
Wednesday, August 10, 1:30 PM - 5:00 PM, Meeting Room 513 E, Level 5, Palais des congrès de Montréal

Impact of insect herbivory on Eurasian bull thistle in eastern Nebraska.

Tenhumberg, Brigitte ^*,, Louda, Svata,

ABSTRACT- Biological invasions represent a major threat to natural and agricultural ecosystems. However, not all exotic species are invasive and not all ecosystems are equally likely to be invaded. A hypothetically important factor in ecosystem resistance to invasion is pre-adapted natural enemies. Using a matrix model approach, we examined the hypothesis that insect herbivores limit the growth and spread of the introduced bull thistle, Cirsium vulgare, in the tallgrass prairie region of the upper Great Plains of the USA. Even though this species is highly invasive worldwide, it has remained at low densities for more than 40-50 years in eastern Nebraska. Pre-adapted, specialized insect herbivores from a similar, synchronously flowering native relative (Cirsium altissimum, tall thistle), have included the exotic thistle in their food range and reduce seed production over 90%. Our models quantify the impact of insect herbivory on population growth rate of bull thistle. To date, our models suggest that, while insect herbivory reduces population growth rate by an order of magnitude, thistle populations should still double each year. We quantified the impact of parameter uncertainty on this prediction using Monte Carlo sampling of the parameter space. Elasticity analysis reveals the importance of early life stages, such as seed production and transitions from seedlings to larger plants, to population growth rate, suggesting early competition and/or microhabitat availability are also important. Our research on what other factors contribute to the further restriction of bull thistle population growth in the region continues.

Key words: Cirsium vulgare, natural enemies hypothesis, invasion ecology