Hawaiian Biodiversity Loss Driven by Feral Ungulates

By Violet K. Dixon
2011, Vol. 3 No. 02 | pg. 1/1

The activity of feral ungulates such as pigs, goats, and deer has resulted in extensive biodiversity loss in Hawaii. These animals were introduced by the Polynesians as domesticated livestock, and now play a destructive role in the local ecosystem. Ungulate populations have played a destructive role in the fragile ecosystems of Hawaiian Islands in a variety of ways. Grazing and rooting result in damage of ground cover and consumption of native plants. The soil disturbance from these activities results in erosion and sediment run-off which damages coral reefs. Disturbed earth is also vulnerable to rooting of invasive plant species and breeding grounds for mosquito larvae. Native Hawaiian species evolved in the absence of large predators such as these ungulates and are thus especially vulnerable to predation and habitat loss. In addition, there are no predators besides humans of these wild ungulates which allows them to reproduce rapidly. These wild populations must be managed and controlled at a rate faster than they can reproduce. In addition, restoration projects are in place to preserve the vegetation in Hawaii’s dry forests which are degraded at a more accelerated rate than the rainforests. Management and restoration plans are in place already; however, opposition from certain groups and more data must be collected and established to convince these parties that in order to preserve Hawaii’s biodiversity, ungulates must be controlled and removed.

There are no land mammals native to Hawaiian Islands; instead, mammals were introduced in two main ways. Dogs, pigs and rats were brought by Polynesians. Others were introduced by Europeans as gifts or trades with Hawaiian kings. “Goats and European hogs were brought in 1778, sheep in 1791 and cattle in 1793.” (Hobdy 1993) More recently, axis deer, California black tail deer, mule deer, antelope and mouflon sheep have been introduced to the islands for hunting game (Yocom 1967).

The Hawaii Conservation Alliance (HCA) has conducted research on effective methods of controlling ungulate populations and has developed four strategies. The first is to develop ungulate barriers such as fences. This process can be expensive, but it is extremely important for isolating the animals from any pristine habitat. The second strategy is to remove or kill animals faster than they can reproduce. The third is inspecting and maintaining barriers. The fourth is vigilance in monitoring the animals and the barriers. The HCA recommends professional trapping and shooting of animals as well as snares and live capture and sterilization of animals. They recognize that opposition exists to these techniques, but remain adamant that “To sacrifice watershed health and potentially dozens or hundreds of rare and endangered species to extinction in order to spare the lives of a few individuals of a populous and widespread species runs counter to sound ethical logic (Controlling Ungulate Pops 3, 2005).”

According to Maguire et al. 1997, unattended neck snares are a particularly controversial control method that involves snaring pigs and other ungulates by the neck in remote areas only accessible by a helicopter. These snares sometimes cause slow and painful deaths for the animals which has resulted in opposition by animal rights groups and humane societies as well as local hunters who feel the meat should not be wasted. Also, native Hawaiians were not all supportive of this method which has local implications. “Some Hawaiians viewed the use of unattended snares by some land managers as violating both humaneness and respectful use of the land’s resources, principles important in native Hawaiian culture.”(Maguire et al. 1997) Maguire et al. 1997 was a collaborative workshop with the intent of discussing existing control strategies and brainstorming new ones. They developed criteria for control alternatives to unattended neck snares. The alternative method must be effective, cost-effective, legal, safe, humane, not wasteful, have a low impact locally, and must have community support.

The high cost of conservation is a prevalent issue for preservation and ungulate control. According to Goldstein et al. 2006 there are three main financial barriers to conservation of ecosystems such as the Koa forest in Hawaii. The first is high up-front costs of research and conservation. The second is long periods of time with no revenue derived from the conserved area. The third is that there is a high project risk associated with conservation projects that entails a great deal of uncertainty and a long time period.  These barriers are from the economic standpoint and are essential for understanding the conservation standpoint more deeply because economic boundaries such as these stand in the way of timely and effective conservation projects.

The seedling ratio index is a method developed by Sweetapple et al. 2004 that evaluates forest understory condition by comparing the species richness of tall (30-200cm) and short (<30cm) seedlings in high, moderate and low ruminant feeding preference classes. This method is less time consuming and expensive than extensive forest biodiversity surveys, and provides valuable management information about the relative health of the forest understory and the impacts of browsing ungulates. “Vegetation monitoring usually involves labor-intensive establishment or re-measurement of large permanent plots on which a full demographic description of the forest is recorded (e.g. Allen 1993). While such information is required to monitor long-term chances in vegetation structure, the high cost means that sample sizes are usually small and re-measurements infrequent, leaving managers with little information to assess the short-term effectiveness of ungulate control. Another consequence of these costs is that the relationship between ungulate abundance and impacts remains largely unqualified (Sweetapple 2004).” This alternative method of quickly assessing the species richness and overall health of forest understory has direct conservation implications for forest managers because, “it provides a clear threshold point below which animal abundance must be reduced to achieve positive outcomes" (Sweetapple 2004).

Another method of more cost-effective conservation can be seen in the use of technologically improved electric wire fences for ungulate control. Reidy et al. 2008 studied the effectiveness of electric wire fencing on controlling feral pig populations in order to reduce their damage on agriculture and the environment. They also tested polywire fencing which is more flexible than ordinary wire fencing and can be reused and moved. They found that their electric fencing was very effective in controlling feral pig populations and should be utilized alongside other methods for management. Polywire electric fencing is a better solution than other methods such as “intensive harvest by recreational hunters, aerial harvest by helicopter, trapping, snaring and poisoning” because it is more “cost-effective, humane and socially acceptable (Reidy et al. 2008).” The aforementioned alternative methods are either too expensive, or harmful to other species besides the feral pigs.

Feral pigs are, perhaps, the oldest and most detrimental threat to Hawaiian biodiversity. Several studies cite their destructive behavior and its effects on local habitat and endangered endemic species. Controlling feral pigs is becoming a prevalent issue that demands immediate attention for many reasons. Reidy et al. 2008 cites some of these problems, “Feral pigs cause approximately $800 million in damages to livestock producers, farmers and wildlife managers in the United States each year. Furthermore, feral pigs have high productivity compared with other wild ungulates, can withstand intensive harvest, and are capable of overpopulating an area in a short time. Feral pigs are too prolific and elusive to be totally eliminated in most areas and agricultural and environmental damage will continue to increase as feral pigs flourish.” The effects of pig rooting are pervasive as Sweetapple et al. 2004 note, “In the closed canopy forest at Puuwaawaa Wildlife Sanctuary, pig-rooting exceeded 40% of plot area in 2000, and forest under stories had extensive areas of bare ground and rock throughout. Pigs are also known to facilitate the establishment and spread of weeds within indigenous ecosystems.”

Feral pigs have been shown to contribute to the habitat loss and endangerment of sixteen Hawaiian honeycreeper species. Honeycreepers are endemic to Hawaii and are unique in their morphology. “The Hawaiian honeycreepers have a spectacular range of morphological and behavioral adaptations to feed on foliage insects, timber-boring insects, nectar, fruits, seeds, mollusks and seabird eggs.” (Mountainspring 1986) The feral pig destroys their habitat, which has negative implications for population size. “In the wetter forests that most honeycreepers inhabit, the chief habitat modifier at present is the feral pig, whose rooting and wallowing activities drastically modify understory and eventually canopy composition.” (Mountainspring 1986) These effects are often drastic as Mountainspring 1986 documents, “In the upper Waimea Canyon study area on Kauai, Nukupuu densities are reduced by 91% due to pig activity…Pig activity also affects other trunk foragers, reducing densities by 36% for the Akiapolaau, 28% for the Kauai Creeper, and 25% for the Hawaii Creeper.” The cited reduced densities are dramatic and have direct implications for these species. The complete set of data for this study can be found in Appendix A.

In 1967, Charles Yokom conducted an intensive study of the population dynamics of feral goats in Haleakala National Park in Maui, Hawaii. He observed their behaviors, especially the degradation of native ecosystems and came to the conclusion that feral goats should be eliminated from the island in order to preserve the local flora and fauna for future generations. He noted that a predominant issue is that local flora are not adapted to deal with grazing because they evolved in the absence of grazing ungulates while invasive flora have developed methods of protecting themselves against extinction by ungulate grazing. As a result, the introduced species are more prevalent and healthy than the more fragile, native species. He also discussed erosion as an issue exacerbated by the feral goats and cites the following examples of how the goats increase erosion (Yocom 1967):

  1. Overgrazing of the native plants
  2. Elimination of some native plants, thus eliminating ground cover
  3. Disturbance of the soil by sharp hooves
  4. Complete elimination of plants from saddles, hogback ridges, goat trails, and along the rim of the crater by feeding or loitering herds of goats
  5. Pawing of the ground by billies before lying down
  6. Increased erosion by rainfall of soils disturbed by goats
  7. Increased erosion by wind of soils disturbed by goats
  8. Slides started by grazing goats
  9. Slides started by goats of all ages playing on the exposed cinders and basalt
  10. Slides started by rocks dislodged by feeding goats

This list is fairly extensive and characterizes the extent of damage done by feral goats specifically in the area of increased erosion. As previously mentioned, Yokom suggested that all goats in Hawaii be fenced in or killed in order to stop the damage they are doing to the local biodiversity and that conservation parks be enlarged. He wrote, “The rate at which native trees, shrubs and other plants are being destroyed on the Island of Maui makes the purchase of such lands a moral obligation so that future generations of people can see, appreciate and study these endangered species” (Yocom 1967).

Cattle play a complicated role in Hawaiian ecosystems that must be considered in depth before significant control efforts are established for cattle. They play the dual role of opening the canopy through grazing, and then maintaining the grass that grows in the open space. “If grazing is continued, forest cover can be expected to continue a slow decline. If grazing is removed, however, the consequent accumulation of grass biomass can increase fuel loads and greatly augment the probability of catastrophic fire and rapid loss of the residual forest.” (Blackmore et al 2000) This is a problem that must be considered in depth because as the forest becomes more degraded as a result of cattle grazing, grazing becomes more important for regulating the invasive grasslands because they pose the risk of fire. Management action is necessary because of the extent of degradation occurring. Blackmore et al. 2004 found that the dry forest decreased 62 percent between 1954 and 1994 and the area covered in invasive grassland increased by 237%. Blackmore et al. 2000 claims, “To restore forest at Pu’u Wa’aWa’a, or any dry forest grazed by cattle and invaded by grasses, we must find ways to break the grass/fire cycle or find a balance between the opposing tendencies of grazing to degrade and protect forest.” This problem will pose the need for additional research about the fire cycle and the number of cattle needed to manage the grassland sustainably without degrading further forest cover.

A case study on the small island of Lana’i provides a microcosm portrait of biodiversity loss on Hawaii. Hobdy in 1993 reports that a mere 2% of dry forest community remains, 3% of mesic forest community and 30% of the cloud forest community.  20% of native vascular plants have disappeared along with most of the native land snails. A large part of this biodiversity loss can be attributed to feral ungulates on Lana’i. Hobdy observed damage done by ungulate populations, “They were running free over the Island because few fences had been constructed and severe damage was being done. Native cover was stripped from most of the lowlands and central plateau, allowing advanced wind and water erosion to begin. The animals were pushing back the mesic forests and cloud forests of the main mountain ridge and denuding the middle-elevation canyon lands of the wind-ward slope.” (Hobdy 1993) The extensive damage inflicted by the grazing ungulates can be seen through Hobdy’s observations on even the smallest of Hawaiian Islands and shows the need for ungulate control and barriers.

The tropical dry forest of Hawaii is the main target for conservational efforts according to US Forest service plant ecologist Robert Cabin who conducted a three year project in Kaupulehu mauka, a six acre fenced remnant of tropical dry forest. Dry forests have been depleted by feral ungulates and alien grasses by 90%. This makes the dry forest more endangered than the tropical rain forest which is depleted by 40%. Dry forests are culturally important for Hawaiian locals who gather materials from the forests for medicine and building materials (Allen 2000). The restoration project by Cabin served to compare the ability of native flora to regenerate in an environment where ungulate disturbance was taken out of the equation by comparing the preserved six acre patch with an adjacent patch that had been continuously grazed by ungulates. The preserved patch had been isolated for 40 years. They found that alien grasses and rodent predation on canopy tree seeds also played an important role in suppressing canopy tree growth and thus ungulate control was an important first step, but not a complete solution (Cabin et al. 2000). A key finding was that fire is a new problem for native species that they did not evolve to face. A fire is likely to destroy a plant population because, “Even if the plants do come back, they must contend with goats, fountain grass and other invasive alien species (Allen 2000).” Cabin’s findings were important because they emphasize the importance of establishing solid data about what factors are causing the ecosystem degradation and how to go about solving the problems. “In Hawaii it has become increasingly clear that without active management, remnant stands of native vegetation will suffer further deterioration, ranging from gradual loss of native species to complete ecosystem destruction. Although the costs of aggressive intervention may be high initially, over time these costs often decrease dramatically as major threats are controlled or at least mitigated. Once preserved, these remnant communities may then be utilized for biological and cultural education, as habitat for rare species introductions, and as a model and propagule [sic]source for future, larger-scale eco-system restoration" (Cabin et al. 2000).

Feral ungulates have caused severe and extensive habitat degradation and biodiversity loss in Hawaii that has been documented across islands an in a variety of habitats.  Management strategies for ungulates are varied in their scope and effectiveness, but have been generally focused towards solutions that are “cost effective, humane and socially acceptable.” (Reidy et al. 2008) New technology and increasing knowledge about the life history of ungulates and their specific relationships with the environment will increase the ability of scientists to establish effective management strategies and protect the remaining biodiversity in danger.


Allen, W. 2000. Restoring Hawaii's Dry Forests. BioScience 12th ser. 50. 1037-040.

Blackmore, M., P.M. Vitousek. 2000. Cattle Grazing, Forest Loss, and Fuel Loading in a Dry Ecosystem at Pu’u Wa’aWa’a Ranch, Hawai’i. Biotropica. 32:625-632.

Cabin, R. J., S. G. Weller, D. H. Lawrence, T. W. Flynn, A. K. Sakai, D. Sandquist L. J. Hadway. 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 2nd ser. 14. 439-53.

Goldstein, J., G. Daily, J. Friday, P. Matson, R. Naylor, P. Vitousek. 2006. Business Strategies for Conservation on Private Lands: Koa Forestry as Case Study. PNAS 26th ser. 103. 10140-0145.

Hawaii Conservation Alliance. 2007. Controlling Wild Sheep and Deer on Conservation Lands in Hawaii. Available at: http://hawaiiconservation.org/_library/documents/ungulates_2007_web.pdf.

Hawaii Conservation Alliance. 2005. Controlling Ungulate Populations in Native Ecosystems in Hawaii. Available at: http://hawaiiconservation.org/_library/documents/ungulates.pdf. Last viewed on October 27, 2009.

Hobdy, R.1993. Lana'i--A case study; The Loss of Biodiversity on a small Hawaiian Island. Pacific Science 47.3 201-10.

Reidy, M.M., T.A. Campbell, D.G. Hewett. 2008.Evaluation of Electric Fencing to Inhibit Feral Pig Movements. Journal of Wildlife Management. 72: 1012-1018.

Sweetapple, P.J., G.Nugent. 2004. Seedling Rations: A Simple Method for Assessing Ungulate Impacts on Forest Understories. Wildlife Society Bulletin. 32: 137-147.

Maguire, L.A., P. Jenkins, G. Nugent. 1997. Research as  a Route to Consensus? Feral Ungulate Control in Hawaii.  62nd North American Wildlife and Natural Resource Conference. 135-145.

Mountainspring, S. 1986. An Ecological Model of the Effects of Exotic Factors on Limiting Hawaiian Honeycreeper Populations. Ohio Journal of Science. 86: 95-100

Wilson, E.O. 2002. The Future of Life. Knopf. New York.

Yocom, C. F. 2009. Ecology of Feral Goats in Haleakala National Park, Maui, Hawaii. American Midland Naturalist 2nd ser. 77. 418-51. JSTOR. Web. 29 Oct. 2009.

Appendix A

Table 4: Percent reduction in population densities for 15 Hawaiian honeycreepers due to feral pig activity. Values represent data popled for all populations. No data are available for the Oahu Creeper. Negative values indicate a greater population due to the factor. (..) indicates insufficient data for calculations due to small sample size.



Mean Effect











Maui Parrotbill




Common Amakihi
















Kauai Creeper




Hawaii Creeper




Maui Creeper












Crested Honeycreeper











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