Effects of long-term ungulate exclusion and recent alien species control on the preservation and restoration of a Hawaiian tropical dry forest

Summary

Study 1

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).

Study 2

Tropical dry forest, once widespread in Hawaii, has been severely reduced in extent and remaining areas are severely degraded, mainly through detrimental anthropogenic impacts, such as deforestation, grazing and introduction of non-native species. Hawaiian tropical dry forests are now confined to small, isolated remnant patches, much reduced in quality from their original high biodiversity.

The introduction of non-native species has occurred in Hawaii since the early 1900s. Alien species, such as perennial fountain grass Pennisetum setaceum, out-compete native vegetation, preventing growth by choking the understory. Fountain grass also has the added risk of increasing forest fires. Introduced mammals, such as rodents, have also been responsible for continual degradation, by consuming fruits and seeds, thus reducing regeneration. Control of invasive species is an ongoing process which requires intensive management, and few studies have quantified its effects.

Study site: The Kaupulehu preserve is located at around 600 m elevation, 17 km east of Kailua-Kona, Hawaii. The preserve is 2.3 ha in area and was fenced in 1956, excluding cattle and goats (i.e. 40 years since ungulate removal). Grazing continued outside the exclosure which acted as a control to asses the impacts of alien species removal inside of the preserve.

Rodent control: Initial rodent trapping showed there were three species present in the study area: Polynesian rat Rattus exulans, black rat R.rattus and house mouse Mus musculus. In late 1996, permanent bait stations were set up in the preserve at 50 m intervals, 25 stations in all. The rodenticide diphacinone was placed in the bait stations, and replenished when empty.

Fountain grass control: The perennial, non-native fountain grass Pennisetum setaceum was abundant both inside and outside of the preserve. In late 1995 the removal of fountain grass from the preserve began by weed-whacking, followed by the application of the grass-specific herbicide Fusillade, using back pack sprays. Due to the absence of native grass species, this did not affect the native flora. A year after treatment a 90% reduction of fountain grass cover was recorded, but due to the persistence of small patches, follow-up management was needed. The manual removal of clumps and spot-spraying was continued from 1997 onwards.

Monitoring: To assess the effects of fountain grass and rodent control, in April 1996 (before control attempts commenced), 5 × 5 m quadrats were established inside the preserve at 50 m intervals (56 in total) and, in the adjacent area two 200 × 20 m transects were set up, 50 and 100 m from and parallel to, the exclosure fence. Quadrats were placed at 20 m intervals along each transect (20 in total) with a further six placed at random within the 0.8 ha area of adjacent grazed forest. In each quadrat the number of individuals and/or the percentage cover of seedlings and adults of each plant species were recorded, together with the diameter at breast height of all woody stems. The same quadrats and parameters were recorded in 1997 an 1998 (after invasive control) in the preserve area. However, due to land-ownership, permanent quadrats could not be established in the adjacent area but the same sampling methods were used.

Seedling survival: To analyse the effects of control on emergent Hawaiian ebony Diospyros sandwicensis seedlings, 50 × 50 cm quadrats were set up inside and outside the preserve in July 1997. Diospyros is the main canopy tree in the forest. A total of 25 quadrats, each containing at least eight Diospyros saplings, were established at random inside the preserve. Due to the low abundance of mature Diospyros trees outside the preserve, 25 quadrats (10 covered with wire cages to exclude grazers) were placed beneath three adult trees to examine the effects of grazing on seedling growth and survival. The number of seedlings were recorded every three months, and in July 1998, stem height, sapling diameter and the number of leaves, were recorded for each plant.

Vegetation composition: In 1997, (a year with particularly high rainfall) there was a higher abundance of Diospyros seedlings inside the preserve, after the control of fountain grass and rats, compared to the initial 1996 survey (see Table 1, attached). There were also more Diospyros seedlings, in concentrated patches, in the adjacent grazed area, after the control. Herbaceous species had also increased in abundance. There were 17 new species recorded within the preserve, 16 of which were naturalized exotics. In 1998, (a particularly dry year) fewer Diospyros seedlings were noted inside the preserve than the previous year (see Table 1, attatched). The two native vines Cnavalia hawaiiensis and Cocculus orbicularis increased in cover from 0% to 0.3% and 0.9% respectively in 1997, and to 1.4% and 3.2% in 1998.

Seedling survival: Diospyros seedling survival was greatest in the caged quadrats outside the exclosure, lowest survival occurred in the adjacent unprotected area. Higher levels of survival were noted in 1997 compared to the particularly dry year of 1998. Caged seedlings adjacent to the preserve had survival levels of around 70% in September 1997, falling to around 60% in June 1998, compared to seedlings inside the preserve, which had survival levels of around 65% in September 1997, falling to around 45% in June 1998.

Seedlings in the uncaged quadrats had around 50% survival in September 1997, which dropped to around 40% in June 1998. Seedling size in the exclosure was greater than those in both the adjacent, caged and uncaged quadrats (see Fig. 1, attached). Seedlings in the exclosure also had higher numbers of leaves (mean ± SE: 1.72 ± 0.42) than those in both the caged and uncaged quadrats (combined mean ± SE: 0.27 ± 0.16).

Conclusions: The control of fountain grass and rodents had a positive effect on native plant regeneration in the preserve area, with greater seedling abundances noted for almost all species. This was more notable in the 1997 census, which may be attributed to the high level of precipitation that year, which was almost double that of the average.

The increase in Diospyros seedlings in the preserve suggests that fountain grass and rodents were inhibiting seedling growth and survival of this important canopy species. Increased regeneration of Diospyros was also noted outside of the preserve. This suggests that perhaps rodent control was effective beyond the confines of the exclosure (where the control had been undertaken) having a wider effect on rodent abundance in the surrounding areas also.

Differences in the survival and size of Diospyros seedlings, both inside the preserve, after the control of invasives, and in the adjacent area, where fountain grass was still widespread, were apparent. The high survival rates noted in the caged quadrats of the adjacent area, along with the low levels of survival in the uncaged quadrats, suggests that fountain grass is not the only factor affecting seedling survival, and that grazing also decreases survival. Increased vigor of Diospyros seedlings inside the preserve indicates that although seedlings can survive in the presence of fountain grass, their size and growth is enhanced by grass control.

This and other studies (see Case 344) suggests that intense management of tropical dry forests is needed if any reversal in habitat degradation is to be acheived. Due to the complexity of these ecosystems, it is difficult to separate out which methods are most successful, therefore, a combination of fountain grass and rodent control, plus removal of non-native grazing animals, is recommended for future restoration programmes.


Note: If using or referring to this published study, please read and quote the original paper. Please do not quote as a www.conservationevidence.com case as this is for previously unpublished work only.

 

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).


Note: If using or referring to this published study, please read and quote the original paper. Please do not quote as a www.conservationevidence.com case as this is for previously unpublished work only.