Context-dependent biological control of an invasive thistle

Carduus nutans (nodding or musk thistle) is an important invasive plant of Eurasian origin. Biological control of this species, using insects that attack rosettes or developing seed heads, has met with varied success in different parts of its invaded range. Here we develop and compare simple demographic matrix models for populations of this species in Australia and New Zealand, to explore reasons for these differences. In a New Zealand population, rapid population growth of C. nutans is driven by early life history transitions. In an Australian population, fecundity of C. nutans is of reduced importance, and survivorship of rosettes plays an increased role. These differences suggest how biocontrol agents that are successful at providing control in one situation may fail in another. Theoretical explorations of the models show which life history transitions drive the differences in matrix elasticities. We suggest that characteristics of the invaded community also play a role in invasion success of this species, and develop theoretical and empirical approaches to assess what factors may drive population growth, and hence what control methods are most likely to work, under different circumstances.