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Test of the release from natural enemies hypothesis using the invasive tropical shrub Clidemia hirta.
DEWALT, SAARA1, ICKES, KALAN1, DENSLOW, JULIE2, 1 2
ABSTRACT- The primary hypothesis proposed to account for changed habitat distribution and increased abundance of plant invaders between their native and introduced ranges is the release from natural enemies (RFNE) hypothesis. This hypothesis states that herbivores and pathogens (natural enemies) limit population size and distribution in native areas; their absence in areas of introduction accounts for the observed increase in abundance or size. We tested the RFNE hypothesis by conducting parallel field experiments with a tropical woody species, Clidemia hirta (Melastomataceae), in adjacent open and shaded environments within native (Costa Rica) and introduced (Hawaii) areas. We evaluated the growth and survival of C. hirta plants exposed to or protected from aboveground insect herbivores or fungal pathogens. Overall mortality and herbivory was much higher for Costa Rican than Hawaiian plants, particularly in the shade. Insecticide and fungicide application had a significantly positive effect on survival of Costa Rican plants in the shade, and had no effect on survival of Costa Rican sun plants or Hawaiian plants in either light environment. Fungicide application increased total aboveground biomass of Costa Rican plants in both shade and sun but only of Hawaiian plants grown in the shade. These results support the predictions of the RFNE hypothesis and suggest that the scarcity of C. hirta in forest understory in its native range is at least in part due to the activity of fungal pathogens and insect herbivores.
KEY WORDS: Clidemia hirta, Hawaii, invasive species, natural enemies