The influence of feral goats on koa (Acacia koa Gray) reproduction in Hawaii Volcanoes National Park

Abstract

Goats were introduced to the Hawaiian Islands nearly 200 years ago. They have become wild and today roam in many Hawaiian ecosystems with dry-season climates from the lowlands to the mountains. A quantitative analysis was made to evaluate the influence of feral goats on tree reproduction of Acacia koa, in the mountain parkland ecosystem on the east flank of Mauna Loa. In this ecosystem, the endemic koa is the only important tree species. Here it reproduces vegetatively from root suckers. Suckering has resulted in the formation of small, dense tree colonies. Ten transects were established through a goat exclosure that was constructed three years earlier (in 1968). In addition, six transects were run across several typical nonfenced koa colonies. All suckers were counted, measured for height and mapped. It was shown that koa reproduction below 10 cm height is abundant outside the exclosure and at the unfenced colonies. Almost totally missing are suckers between 0.5 m and 2 m height. However, hundreds of this height grow inside the goat exclosure. Most of the few trees of this height found outside the fenced area were dying or dead showing that the current goat pressure is so high that the reproduction cycle of koa is nearly disrupted. The dense and vigorous sucker growth inside the exclosure, which is the result of current release from goat browsing pressure, was found to be an artifact. The artifact has resulted from increased suckering density caused by goat feeding and probably trampling on shallow roots. Thus, the entire forest stand structure in the mountain parkland is directly related to herbivore feeding and departs definitely in spacing and probably in height growth from the original forest structure as evolved during island ecosystem evolution.