The Influence of Feral Goats on Koa Tree 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 var, hawaiiensis Rock in the mountain park—land 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 constucted 3 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, but 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 replacement cycle of koa is nearly disrupted. The vigorous growth of suckers inside the exclosure was a direct response to their release from goat browsing. In contrast, the high stocking density of suckers was a response to goat browsing prior to fence construction, since it was found that goat feeding of small herbaceous suckers increased suckering density at the fringes of the existing koa colonies. Trampling on shallow roots may contribute to an increase in suckering density. When released from goat pressure these suckers grow into stands probably much denser than those that existed before goats were introduced to the islands. From the viewpoint of community evolution on islands, this distortion of stocking density is unnatural.