Tropical dry forests are among the most threatened natural areas as a result of human activities, yet relatively little is known about the impacts of anthropogenic disturbance and species introductions in these diverse biomes. Island biotas are especially susceptible to anthropogenic disturbances and invasion because they are typically disharmonic, have high levels of endemism, and small populations. Hawaiʻi is an emblematic case which can serve as a model system for other tropical dry forests in island biomes. In this study we revisited remnant dry forest patches that were previously surveyed in 1950 and 1970 in the Mokulēʻia Forest Reserve on Oʻahu, Hawaiʻi in order to assess long-term native persistence. We tested the hypothesis proposed by Hatheway (1952), suggesting that where disturbances by free-roaming cattle or fire were absent and natives were in sufficient abundance, native Hawaiian forests can maintain themselves, suppress alien invasion, and even expand their ranges in a broadly invaded landscape.
Results/Conclusions
Our resurvey of the seven original remnants found that natives declined in basal area in every plot except one, and maintained dominance over aliens in only three of seven plots. Natives experienced substantial declines in abundances, and the greatest reductions in terms of native basal area and species richness occurred in formerly well-developed native forest plots, which previously had the highest native diversity. Only the most common native species were able to maintain their populations, and these were most dominant in the rockiest plots. At the same time, non-natives increased in richness in four out of seven plots, and increased their basal area in six out of seven plots. These results indicate that despite absence of grazing cattle and fire, native Hawaiian dry forest remnants are being invaded by nearby non-natives, and will likely continue to decline if not supported by active management.