Population dynamics of the invasive weed Lupinus arboreus in Tasmania, and interactions with two nonnative pollinators

The factors that determine which plant species become invasive weeds are not well understood and there have been few studies of population dynamics in the early stages of invasion. Here, we examine changes in population size, pollinator visitation and seed set of the tree lupin, Lupinus arboreus, in Tasmania between 1999 and 2010. Lupinus arboreus is a native of California that has become a major environmental weed in New Zealand and Chile, but has not yet become a serious weed in Tasmania. Our data suggest that the main pollinators of L. arboreus are honeybees and the bumblebee, Bombus terrestris, which invaded in 1992. There was no clear evidence for an impact of the arrival of bumblebees. Lupinus arboreus population size increased by 76% between 1999 and 2010, despite weed control programmes. Populations appeared to be unstable; 43% of populations detected in 1999 were extinct by 2010, but this was more than offset by establishment of new populations. Inland populations tended to be smaller and were more likely to go extinct, compared with coastal populations, and some coastal populations had increased fourfold in 11 years. Large populations in 2010 tended to have higher seed set than smaller populations. The overall rate of increase suggests that L. arboreus may become a major environmental weed in Tasmania. Control of expanding populations is likely to become more difficult if, as we observed, seed set increases with population size.