History, impact, and management options for invasive vines in the Mariana Archipelago
Outside their native range, invasive species have the potential to alter communities and ecosystems. Island plant communities are especially vulnerable because flora has established without competition from invasive species. The invasive vines chain of love (Antigonon leptopus) and scarlet ivy gourd (Coccinia grandis) were introduced for ornamental and agricultural purposes to the island of Saipan in 1970 and 1990, respectively. This resulted in the invasion of over 15,000 acres (52% of Saipan’s land mass). Although several studies have documented the negative effects of invasive species on biodiversity, few have attempted to determine which cover type is most affected by invasive species. We evaluated the effect of invasive vines on the abundance, richness, and diversity of flora in cover types across Saipan. We surveyed plots with and without invasive vines across the three main cover types (native limestone forest, secondary mixed forest, and introduced tangantangan).
Plots with invasive vines showed a reduction in plant abundance, richness, and diversity, and a negative relationship between increased vine cover and plant diversity was found. Vines displayed the largest negative effect on plant richness and diversity in native limestone forest, and the largest negative effect on vegetation abundance in tangantangan. From these results, we identified native limestone as the cover type most at-risk of degradation. As such, conservation efforts should prioritize native limestone forest.
Prior to our study, three biological controls including one moth (Melittia oedipus) and two weevils (Acythopeus cocciniae) and (Acythopeus burkhartorum) were released for scarlet ivy gourd. Current status of M. oedipus and A. burkhartorum is unknown, and the weevil A. cocciniae is experiencing predation from a hymenopteran parasite. Because the effectiveness of the A. cocciniae is reduced via predation, strategies to facilitate its population growth should be considered. Rerelease of A. cocciniae in native limestone forest could help boost their population levels and allow them to cope with predation.
Additional research into the ecological factors affecting the effectiveness of the biological controls coupled with other methods of control (manual removal and herbicide application) could further aid in control. As funds and management options are limited, prioritizing conservation efforts is essential. A targeted management approach coupled with consistent monitoring of affected areas will facilitate greater success in the conservation of Saipan’s endemic forests.