As with a number of other geographically isolated islands, Hawaii has problems with invasive species negatively affecting the natural biodiversity of the islands.
Historical examples
Hawaii is the most isolated major land mass in the world and that isolation has led to very high rates of endemism. Uniquely adapted endemic species are often sensitive to competition from invasive species and Hawaii has had numerous extinctions (List of extinct animals of the Hawaiian Islands). While not the only cause, introduction of invasive species can be a major cause of population decline and extinction. There are several routes for introduction of non-native species. Some species were accidentally introduced to Hawaii like the rat, fire ants, coqui frog, mosquitos, and coconut rhinoceros beetle. Some are species brought in for cultivation that spread to wild areas like miconia, pigs, and goats. Some species were intentionally introduced for sport like axis deer and some for pest control like mongoose or cane toad. The mongoose was introduced to Hawaii in the late-19th century in an attempt to control the large rat population in the sugar cane fields. However, since then, the mongoose population has grown to large numbers without controlling the rat population and has greatly diminished the population of native ground nesting birds, snails, and palms. Another example of an invasive species introduced in the 19th century is the fire tree, which is a small shrub that was brought from the Azores, Madeira, and the Canary Islands as an ornamental plant or for firewood. However, now it poses a serious threat to native plants on young volcanic sites, lowland forests, and shrublands, where it forms dense monocultural stands Another plant, the strawberry guava, was introduced in the early 19th century as an edible fruit. However, it now poses a major threat to Hawaii's rare endemic flora and fauna by forming shade-casting thickets with dense mats of surface feeder roots.
Potential harm from invasive species Invasive species threaten biodiversity by causing disease, acting as predators or parasites, acting as competitors, altering habitat, or hybridizing with local species.
Disease Invasive species often carry new diseases for native species. For example, the biting fly in Hawaii are small, even tiny, and include many species, some of which are vectors of diseases while others bite and cause considerable nuisance and health-related problems. The introduction of mosquitoes to Hawaii has resulted in the spread of avian malaria, and increases the risk of dengue and west Nile virus (not known to be in Hawaii yet). Other native species can be affected by invasive species diseases as well, such as the once-dominant koa tree being killed by koa wilt, which is believed to have been brought into Hawaii on an ornamental acacia plant, and the 'ohi'a tree, now being affected by Rapid Ohia Death.
Predators Invasive predators can severely reduce the population sizes of native species, or even drive them extinct, because native prey species may not have evolved defenses against the novel predators.
Competition Oftentimes the introduced species is better equipped to survive and competes with the native species for food or other resources. For example, the strawberry guava tree is one of Hawaii's worst invasive species. It is dangerous because it crowds out native plant species, breaks up natural areas, disrupts native animal communities, alters native ecosystem processes like water production, and provides refuge for alien fruit flies that are a major pest of Hawaiian agriculture.
Habitat alteration Invasive species can change the state of an environment in many ways based on how they feed and interact with their new surroundings. These interactions along with competition can limit the amount and type of resources for native species.
Hybridization Hybridization occurs when members of two different species mate with one another and produce viable offspring that carry genes from both parents. When an invasive species is much more abundant than a native relative, they may hybridize so often that the invaders genes "flood" the native species, such that no individuals contain the entire genotype of the native species, thus effectively driving the native species to extinction. For example, hybridization between Introduced mallards and the native Hawaiian duck (koloa maoli) and between the rarest European duck (the white-headed duck) and the invasive North American ruddy duck may result in the extinction of the native species.
Cultural Practice Impacts In Hawaii, the Hawaiian culture is closely connected to its environment and native species. Chants, ceremonies, hula, and other practices involve the use of plants (both native and Polynesian-introduced), traditional access to places of importance, and other activities that can be directly affected by invasive species. For example, taro (kalo, in Hawaiian) is defined in the Hawaiian Creation Chant as the plant from which Hawaiians were formed and is considered a sacred plant. The introduction of the golden apple snail, which attacks taro, threatens the very existence of Hawaiian ancestors.
Threats to Hawaii's ecosystems Hawaii has a growing invasive species crisis affecting the islands' endangered plants and animals, overall environmental and human health, and the viability of its tourism and agriculture-based economy. Invasive species occur globally, but Hawaii is more susceptible to invasive species because they are islands. The entire island chain of Hawaii has been devastated by invasive insects, plants, hoofed animals such as deer, goats, pigs and other pests. Feral pigs eat endangered birds' eggs and trample fragile native plants, rosy wolfsnails from Florida gorge themselves on the islands' native snails, weeds such as Australian tree fern and Miconia calvescens plants shade out native plants, and coqui tree frogs aggravate tourists, eat native insects and decrease home values with their piercing calls.
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