Natural-enemy release facilitates habitat expansion of the invasive tropical shrub Clidemia hirta

Normative, invasive plant species often increase in growth, abundance, or habitat distribution in their introduced ranges. The enemy-release hypothesis, proposed to account for these changes, posits that herbivores and pathogens (natural enemies) limit growth or survival of plants in native areas, that natural enemies have less impact in the introduced than in the native range, and that the release from natural-enemy regulation in areas of introduction accounts in part for observed changes in plant abundance. We tested experimentally the enemy-release hypothesis with the invasive neotropical shrub Clidemia hirta (L.) D. Don (Melastomataceae). Clidentia hirta does not occur in forest in its native range but is a vigorous invader of tropical forest in its introduced range. Therefore, we tested the specific prediction that release from natural enemies has contributed to its expanded habitat distribution. We planted C. hirta into understory and open habitats where it is native (Costa Rica) and where it has been introduced (Hawaii) and applied pesticides to examine the effects of fungal pathogen and insect herbivore exclusion. In understory sites in Costa Rica, C. hirta survival increased by 12% if sprayed with insecticide, 19% with fungicide, and 41% with both insecticide and fungicide compared to control plants sprayed only with water. Exclusion of natural enemies had no effect on survival in open sites in Costa Rica or in either habitat in Hawaii. Fungicide application promoted relative growth rates of plants that survived to the end of the experiment in both habitats of Costa Rica but not in Hawaii, suggesting that fungal pathogens only limit growth of C. hirta where it is native. Galls, stem borers, weevils, and leaf rollers were prevalent in Costa Rica but absent in Hawaii. In addition, the standing percentage of leaf area missing on plants in the control (water only) treatment was five times greater on plants in Costa Rica than in Hawaii and did not differ between habitats. The results from this study suggest that significant effects of herbivores and fungal pathogens may be limited to particular habitats. For Clidemia hirta, its absence from forest understory in its native range likely results in part from the strong pressures of natural enemies. Its invasion into Hawaiian forests is apparently aided by a release from these herbivores and pathogens.