Dr. James Kellner is undertaking research to understand the impacts of ungulate removal in dryland ecosystems of Hawaii. At more than 3,500 kilometers from the west coast of North America and over 6,000 from continental Asia and Australia, the Hawaiian Islands are the most isolated archipelago on earth. A consequence of their extreme isolation is that entire groups of plants and animals that are common on the continents never made it to Hawaii – at least not until humans arrived and began to transport new species, both deliberately and accidentally as stowaways. Hawaii contains no species of native ground-dwelling mammals, and ecosystems there evolved in isolation from typical herbivore pressures. They contain thorn-less raspberries, mint-less mints and nettle-less nettles, among diverse assemblages of snails, birds, insects and plants found nowhere else in the world.
Captain James Cook released goats (Capra hircus) and other ungulates in the Hawaiian Islands in 1778. The animals spread and consumed native vegetation to the point that populations for many plant species – and the animals that depend on them – have been decimated or are extinct in the wild. Impacts have been most severe in Hawaii’s tropical dry forests, which occupy the leeward flanks of the 8 main islands and were once the most diverse systems in the archipelago. Practical conservation and restoration of Hawaiian dry forests today requires eradication of ungulates to facilitate the recovery of threatened and endangered populations.
One high-profile example the Palila (pictured to the right, Loxioides bailleui), a critically-endangered honeycreeper that now numbers in the hundreds to thousands, and which depends on seed pods from a single species of tree called mamane (Sophora chrysophylla).
Removal of ungulates is a critical first step to conservation of threatened dry forests in Hawaii, but little is known about the consequences of abruptly removing an abundant grazing herbivore. Goats, for example, have been part of the landscape for more than 200 years, and it is unclear whether eradiation alone would adequately restore native ecosystems. To address this question, my colleagues and I used measurements from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) to characterize changes in vegetation cover during a 9-year period before and after a large-scale eradication effort in Pohakuloa Training Area, an active US Army installation on the Big Island and home to numerous species of threatened and endangered plants. We compared a site where goats had been removed to an adjacent area where ungulates were not manipulated.
Not surprisingly, there was an abrupt increase vegetation cover shortly after the removal of ungulates.
But on-the-ground studies and airborne remote sensing from the Carnegie Airborne Observatory led by Gregory P. Asner of the Carnegie Institution for Science demonstrated that most of the increase was due to population growth of introduced plant species not native to Hawaii.
The abundance of native plants did not change after ungulate removal. One exotic species in particular, the Madagascar ragwort (Senecio madagascariensis) accounted for about half of the increase in vegetation cover.
What can be made of the increase in introduced species while the abundance of native populations was unchanged? One possibility is the enemy-release hypothesis, which argues that introduced species are unlikely to transport their own ‘specialist enemies’ – things like pathogens, predators and herbivores that specialize on particular groups – when they are introduced to a novel geographic range. If this explanation is correct, then removing generalist herbivores like goats should facilitate the success of introduced species more easily than native populations because native species remain burdened by specialist enemies, but introduced species do not. A second possibility is that native populations are so rare, and their seed sources so impoverished, that they have lost the ability to quickly increase in population size. Obtaining definitive answers to these questions will require additional field studies that are now underway.
Whatever the case, it is clear that wholesale eradication of ungulates from dry forest ecosystems alone is unlikely to provide simple answers to pressing conservation problems. For additional information, see: J. R. Kellneret al., Remote analysis of biological invasion and the impact of enemy release. Ecol. Appl. 21, 2094 (Sep, 2011).
This work is supported by grants from the US Department of Defense.The Carnegie Airborne Observatory is made possible by the W. M. Keck Foundation and William Hearst III.