Recovery of Goat-Damaged Vegetation in an Insular Tropical Montane Forest

Abstract

The feral goat (Capra hircus) is an alien herbivore that has wreaked havoc in island ecosystems, including the dry, rugged, and relatively inaccessible montane koa parkland on the islands of Maui and Hawai'i. The objective of the present work was to evaluate the ability of koa parkland on Maui to recover naturally from browsing damage if goats are eliminated. We studied recovery over 7 years by periodically determining percent ground cover, species composition, and woody plant abundance inside and outside a goat exclosure. Initially, plant cover did not differ inside and outside the exclosure; but, after 3 years, plant cover inside was almost twice that outside. Molassesgrass (Melinis minutijora), an introduced mat-forming species, spread rapidly inside the exclosure and showed no sign of dying out or retreating. Goats exert some control over this species. After 7 years, tree regeneration has occurred only inside the exclosure except for koa (Acacia koa) regeneration, which also occurred outside the exclosure. Heights of koa seedlings indicated that those outside represented recent germinants only, while those inside represented older individuals as well as recent germinants. The results indicated that the aging koa overstory could be replaced by koa regeneration if goats were eliminated and that some native species (Dodonaea, Styphelia, Coprosma) could at least partially recover. Once established, some introduced species (e.g., Melinis minutiflora), are favored by elimination of goats and may severely inhibit recovery of native species. FERAL ANIMALS INTRODUCED into insular environments become ecologically dominant and have extensive adverse effects on native biota (Coblentz 1978). And tropical oceanic islands have not been immune to the assaults of alien herbivores. Examples of wholesale destruction of vegetation by ungulates include islands in the Marquesas (Adamson 1936, Holyoak 1973, Thibault 1973), the Galapagos (Hamann 1975, 1979), and the Hawaiian Archipelago (Mueller-Dombois 1979, Katahira 1980, Scowcroft and Giffin 1983). Alien ungulates, goats among them, were introduced into Hawai'i2 about 1780 A.D. Goats (Capra hircus) became established in the wild, and by the 1850's feral populations were large (Tomich 1969). The adverse impact of feral goats on Hawaiian forests has concerned managers and ecologists (Tomich 1969, Yocom 1967, Spatz and Mueller-Dombois 1973, Reeser 1976). Kramer (1971) considered the feral goat the most destructive ungulate introduced to the islands. Control efforts, which began about 19 1 0 (Tomich 1969), proved effective where rigorously applied, a situation illustrated by the fencing and goat eradication program conducted in Hawai'i Volcanoes National Park. Feral goats are still a problem in some areas on Hawai'i, Kaua'i, Maui, Moloka'i, and Received 10 May 1985, revision accepted 24 March 1986. 2 Diacritical apostrophes indicate glottal stops in Hawaiian words. Kaho'olawe. We studied one of the problem areas on Maui, where the native forest of koa (Acacia koa) appears to be decadent due to many years with little or no tree recruitment. This paper reports a study of changes in ground cover, understory species composition, and regeneration of tree species in an area where goat browsing was eliminated for 7 years. It also compares that area with an adjacent area in which goat browsing continued.