What controls the population dynamics of the invasive thistle Carduus nutans in its native range?
The invasive thistle Carduus nutans causes major economic losses in the Americas, Australia and New Zealand. For the first time, we have modelled its population dynamics in its native range, Eurasia, where it rarely reaches problematic densities, in order to identify ways to improve management strategies for this weed in the invaded range. In particular, we investigated whether specialist enemies in the thistle's native range suppress thistle populations, as predicted by the enemy-release hypothesis, and, if so, how this effect relates to other factors that may limit population growth. We constructed population transition matrix models with data from three French populations. A vital rate elasticity analysis revealed that reproduction determines between 33% and 61% of the projected growth rate of the populations, and thus is a key driver of the population dynamics of this monocarpic short-lived perennial. Decreases in population size were predicted by the models for all three populations (λ<1). Using limiting factor omission analyses, we showed that the suite of native insect herbivores causing seed losses had a larger impact than the joint effects of rosette damage by sheep and summer drought acting on seedling establishment. Removal of insect herbivores increased the native population growth rate by 166% on average; removal of sheep damage and summer drought from the model increased population growth by 51%. Specialist herbivores and drought interacted synergistically to affect reproduction. Synthesis and applications. We show that vital rate elasticity analysis provides more management information than elasticity analysis on the level of matrix elements, particularly when management options affect only certain vital rates. For example, it can be used to make predictions about the effectiveness of predispersal floral herbivores in control management. The model can also help identify ways in which biocontrol can be augmented using integrated weed management that reduces seedling establishment probabilities, for example by preventing overgrazing. This method illustrates how native range studies of invasive species can be used to generate insights into managing populations in the invaded range.