Individual study: The effects of long-term grazing removal on recovery of dry tropical forest in Hawaii, USA
Cabin R.J., Weller S.G., Lorence D.H., Flynn T.W., Sakai A.K., Sandquist D. & Hadway L.J. (2000) Effects of long-term ungulate exclusion and recent alien species control on the preservation and restoration of a Hawaiian tropical dry forest. Conservation Biology, 14, 439-453
Tropical dry forests are among the world's most diverse ecosystems, containing high levels of biodiversity. However, they are also one of the most threatened environments with massive declines in cover and quality due to deforestation through deliberate cutting, burning, and in some cases, detrimental effects caused by introduction of exotic species.
Dry tropical forests, once widespread in Hawaii, now cover less than 10% of their previous extent due primarily to deforestation. Remaining forest is patchily distributed and only a few isolated remnants remain. These forest patches are largely degraded, with invasive non-native species choking the native understory. Regeneration by native species has been effectively halted, largely attributable to the effects of grazing by cattle and goats, as well as the competition from exotic plants. The removal of grazing pressures, was considered the most feasible step towards initiating long-term restoration of these forests. The effects of cattle removal on other degraded systems has had mixed responses. This study documents the effects of long-term grazing removal on the habitat composition of a remnant tropical dry forest in Hawaii.
Study site: The Kaupulehu preserve is located (around 600 m elevation) 17 km east of Kailua-Kona, Hawaii. The preserve is 2.3 ha in extent and domestic/feral grazing animals were excluded by fencing the entire area in 1956 (i.e. over 40 years since ungulates removal when this study took place). Grazing continued outside of the exclosure.
Vegetation surveys: Vegetation surveys of the ungrazed and an adjacent grazed area, 0.8 ha in size, were carried out in 1995. All native trees and shrubs, (excluding the shrub Kulu'i Nototrichium sandwicense, which was abundant in both sites and too time consuming to measure individually) were recorded both inside and outside of the preserve. The diameter at breast height was measured for all woody stems ≥ 30 cm high, in order to detect the impacts of ungulate removal.
Vegetation responses: There were clear differences in vegetation structure between the grazed and ungrazed sites. All native species present within the exclosure were also present in the grazed area. However, all of the native tree and shrub species found inside the preserve occurred at higher densities than in the adjacent grazed area (see Fig. 1, attached). The canopy tree Hawaiian ebony Diospyros sandwicensis was over three times more abundant in the ungrazed preserve than the grazed plot. However, there was little sign of recruitment in either areas, with over 95% (n = 920) of all native trees in the preserve consisting of mature individuals, and all trees (n = 68) recorded in the grazed area were mature.
Conclusions: Differences in species abundance between the ungrazed and grazed areas of tropical dry forest were apparent, with higher densities of native species occurring inside the preserve. This seems to indicate that grazing exclusion for 40 years has had a significant positive impact on the habitat composition of the protected area. However, due to the lack of recruitment, it is possible that other factors, aside from grazing, are having a detrimental impact on the species composition of the ungrazed area.
Invasive plants, such as fountain grass Penisetum setaceum, choke-out and compete with the native vegetation, while rats Rattus spp. eat their fruit, thus inhibiting recruitment. Both grass and rats were highly abundant at both areas studied. Therefore, exclusion of non-native ungulate grazers on its own as a technique for restoring Hawaian dry tropical forests, even over several decades, appears not to result in reinstatement of the desired plant community structure. In order to achieve this goal, a combination of other measures, such as the control of invasive plant species and reintroduction of natives, may produce a better result. The long-term control of fountain grass and removal of rodents, which predate on fruit and seeds, has commenced following these initial findings (see Case 345).
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