ALIEN SPECIES
Vertebrate Herbivores
Herbivores (plant-eating animals) like pigs, goats, and deer can impact native Hawaiian ecosystems by grazing or browsing vegetation, trampling seedlings, disturbing soil, and spreading alien plant seeds. These can change normal light, moisture, and nutrient-cycling mechanisms in native ecosystems, resulting in changed plant and animal communities, decreased water retention of soils, and erosion.
Grazing and browsing animals often preferentially eat native plants because these have not evolved the thorns and toxins normally used by plants to protect themselves from this activity. Grazing and browsing can result in extinction of native plant populations, but under any level of intensity can select for plant species (typically alien) having some defenses from herbivore feeding. With time, this process alone is sufficient to alter plant communities from native to largely alien. Trampling and rooting of plants will also accelerate this effect, especially for species not otherwise targeted as food by the herbivores.
Soil disturbance by these animals' hooves typically favors the germination and establishment of alien plant species because many of these have evolved in the presence of such disturbance and now require it as a condition of successful completion of their life cycles. Native species have not evolved with such disturbance and, consequently, are negatively impacted by its appearance. Many alien plants brought to Hawai`i are specifically adapted to colonize relatively open habitats created by animal trampling and have a dependent relationship with introduced ungulates (hooved animals). Relatively few native plants can adapt to this type of early successional habitat; consequently, trampling by ungulates (and humans) selects for the replacement of native species by aliens. These disturbances can be quite important in determining the composition of plant communities and the communities of animals dependent upon them. Removal of ungulates typically results in recovery of native species and the decline of many alien plants, and is often the first step in ecosystem restoration.
Lastly, many alien plant species, including several of the most invasive in otherwise intact native ecosystems, are dependent upon animals for the spread of their seed. This includes plants having seeds that cling to fur or feathers as well as plants with seeds presented in an edible fruit which germinate upon excretion by herbivores. In both instances ungulates present important means of long-distance dispersal for many noxious alien plant species. Such dispersal then provides new weed infestations, which increase the rapidity with which alien plant populations spread and the total area that they encompass.
Vertebrate Predators
Except for a few hawks and owls, bird predators were originally lacking in Hawai`i. Thus, the native seabirds and forest birds evolved for millions of years in the absence of this threat. The subsequent introduction of a host of alien vertebrate predators by humans has changed this situation and resulted in massive mortality of native birds. The primary vertebrate predators introduced into Hawai`i are three species of rats (shown at left eating an endangered native tree snail), mongoose, cats, and dogs. Seabird colonies have been lost on many of the main Hawaiian islands because of these predators, and the small breeding colonies that remain must be protected by constant trapping for rats and mongoose. Losses due to free-roaming pets, such as dogs and cats, are another source of frequent mortality for these birds. Because of their large numbers, broad diets, and ability to climb easily, of all the introduced vertebrates rats have probably had the greatest effects on the native fauna. They eat forest bird and seabird nestlings, the colorful and rare endemic tree snails, and so many seeds from a variety of native trees that those species are unable to reproduce sufficiently to replace their numbers. Experiments on the Big Island have shown that rats have such a large effect on native forest birds that those birds are able to double the number of successfully reared young when rat numbers are controlled.
Other introduced vertebrates can be a problem as well. A variety of alien lizards and frogs now live in Hawai`i. Many of these negatively impact Hawai`i's unique insect communities, and a few may be able to eat young birds, but this has not been well studied. Introduced cattle egrets are reported to take nestlings of endangered native waterbirds, and soft-shelled turtles may do likewise. A variety of alien songbirds now inhabits native forests where they compete with the native birds, such as 'i'iwi and 'apapane, for food.
Social InsectsHawai`i originally had no social insects, such as ants, termites,
and many wasps (Yellowjacket, Vespula pensylvanica shown at
left) and bees. These aliens feed on native species and thereby exert
enormous predation pressure on native invertebrate communities by
virtue of their large numbers, broad diets, and predatory efficiency
. As a consequence of these attributes, social insects, especially
ants (shown below), have been strongly implicated in the loss of
native invertebrate faunas on oceanic islands and these losses, in
turn, result in a cascading series of changes in native plant and
bird communities dependent on the insects for pollination or food.
The sum result is large-scale changes in native forests caused by
tiny, seemingly insignificant, "bugs". Social insects have now
colonized most of the Hawaiian lowlands, contributing to dramatic
declines of native insects (and probably snails) in these areas. This
is especially true for ground-dwelling or ground-nesting species.
Spread of Argentine ants into Haleakala National Park now
threatens the recovering populations of silversword because of
predation on the plants native pollinators.
In addition to these negative effects, some social insects have also exacted high costs to humans. The Formosan ground termite, for example, is estimated to cause $150 million in treatment and damage-repair costs to buildings and other structures each year in Hawai`i. Three other species of termites have also been introduced to Hawai`i, and some 2000 other species exist elsewhere and could arrive unannounced at any time. Some of the worst social insects, such as fire ants, have not yet appeared in Hawai`i but have been stopped at the state's borders. A mail parcel from Florida infested with fire ants was intercepted by inspectors in Honolulu in 1991. These ants were accidentally introduced to Alabama in the 1950's. They have subsequently expanded across most of the southern U.S., where they inflict tremendous losses on wildlife each year. Their defensive swarming behavior and toxic stings send more than 20,000 people to the doctor each year. Approximately 30 people die each year in the southern U.S. from allergic reactions to the stings of these ants.
Fire-promoting
GrassesFire is an important means of determining the species composition of plant communities in many continental areas and, consequently, many plants from these areas have adapted to its presence. These adaptations include structural attributes such as thickened bark or underground shoots that enhance a plant's ability to survive fire and seed germination or new shoot growth after a fire. Other plants have adapted to take rapid advantage of the reduced competition and flush of nutrients that follow a burn. In many plants fire is not only tolerated but has become a requirement for successful completion of the life cycle, and these species often have attributes that promote the spread of fire.
Many of the alien grasses brought to Hawai`i have these requirements, burn readily, and serve as serious promoters of fire in native dryland ecosystems. Because fire was not important in the evolution of Hawaiian plants, fires promoted by alien grasses decimate native vegetative communities, which become gradually replaced by fire-adapted alien communities with each successive fire. In this way, former native dry forest has been replaced by grasslands in much of the leeward lowland areas of Hawai`i. Fires carried through these grasslands gradually erode any adjoining forested areas that may remain and may promote the spread of alien woody species that are also fire-adapted. Dense stands of alien grasses further prevent regeneration of native forest by forming a barrier that prevents native seeds from contacting the soil and stifling those few that may make contact and germinate.
Woody PlantsLarge woody plants threaten native forest vegetation in a number of ways, primarily by providing dense shade in which native species cannot germinate and grow (see at left). By this means, alien species adapted to shady conditions are able to gradually replace native forests. Also, physiological adaptations of alien plants may alter community structure or ecosystem processes in a number of ways other than by fire. The most important of these properties include nitrogen-fixing ability, which alters a critical nutrient-cycling regime; production of toxins, which inhibit the growth of native species; and evapotranspiration rates, which alter water flux within ecosystems. All of these attributes may be found in some alien plant species that occur in Hawai`i; a few exhibit all three.
No native plants that pioneer on young volcanic sites have the capability to fix atmospheric nitrogen, but some alien species, such as ironwood (Casuarina spp.) and faya (Myrica faya), have efficient symbioses with actinorhizal fungi that allow them to do so. This capability affords a competitive advantage to these species, such that they can often physically dominate and change the successional characteristics of such areas. The long-term effects of these changes are unknown, but may facilitate the establishment of additional alien species, many of which only invade relatively fertile sites.
Production of leaf litter that is slow to mineralize by plants such as pines and ironwood can also alter nutrient cycling regimes and physically interferes with the establishment of native seedlings . Such trees can often form single-species stands with little development of underlying shrub or herbaceous layers, reducing habitat available for native species and altering community structure.
Higher evapotranspiration and/or rainfall-interception rates characterize certain commonly planted alien plants and may lower water tables or reduce stream flows. Several tree species planted in Hawai`i, like pines, have led to these problems on a widespread scale elsewhere and may have similar drying effects locally. Furthermore, the open canopies of some of these species increase soil insolation, enhancing the drying out created by the increased transpiration rates. Conversely, other alien plants may increase water runoff during rains, leading to increased erosion and siltation.
Plant habitus may also alter native ecosystems. This is well illustrated by ironwoods, which shade out native shoreline vegetation and render beaches susceptible to erosion, resulting in habitat loss for nesting wildlife. Shading of native shrublands by temperate trees can also be a problem as can loss of wetlands to introduced mangroves.
Examples:
For photos, maps, and other information about a number of Hawai`i's greatest alien species pests, visit the following websites:
Hawaiian Ecosystems at Risk (HEAR) Project: http://www.hear.org
University of Hawai`i Department of Botany: http://www.botany.hawaii.edu/faculty/cw_smith/aliens.htm
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