Increased competitive ability of an invasive tree may be limited by an invasive beetle

Invasive plants are often more vigorous in novel habitats than in their native ranges. Sapium sebiferum (Chinese tallow tree) is a major invader of habitats in the southern United States. Long-term common garden experiments in Texas and Hawaii (USA) with S. sebiferant genotypes from its native range (Asia) and from areas in North America where it is invasive suggest that post-introduction evolutionary changes may contribute to its invasiveness in Texas. In Texas, where there was uniformly low herbivory, fast-growing, poorly defended invasive North American genotypes outperformed slower growing, better defended native Asian genotypes in common garden plots. In Hawaii, in contrast, an Asian herbivore, Adoretus sinicus (Chinese rose beetle) is abundant, and S. sebiferum is not invasive. In Hawaiian common garden plots, A. sinicus caused greater damage to North American genotypes, and Asian genotypes were competitively superior. Our results suggest that exotic plants freed from herbivory can evolve greater competitive ability, allowing them to become much more abundant than in their native ranges. This did not seem to occur, though, if herbivores from the native range were abundant.