What is Threatening Species? Non-Native Species Introductions Invasive species, or alien, non-native animals and plants introduced into ecosystems, present the most important threat to plants. The large numbers of threatened species that have limited distributions are highly vulnerable to the possibility of invasive species eliminating them from the wild. Islands have the greatest percentage of their native plants in danger of extinction. St. Helena, a small island in the South Atlantic, leads these in percentages, with 13 percent extinct or extinct in the wild, and 68 of 165 native plants and trees threatened (Walter and Gillett 1998). Livestock played a role in the extinctions and present status of many native plants (see Chapter 1). Mauritius follows, with 39.2 percent of native plants in danger, or 294 of 750 species (Walter and Gillett 1998). Other islands with many native plants threatened by exotic species include the Seychelles, Jamaica, French Polynesia, Pitcairn, Reunion and New Caledonia (Walter and Gillett 1998). Of the latter islands, all but French Polynesia and Pitcairn formed part of Gondwana, and many of the plants at stake are very ancient in origin.
One endemic Mauritian palm, Hyophorbe amaricaulis, has been reduced to a single plant. Although it grows vigorously and produces male and female flowers at different times, no fertile seeds are produced (Stearns and Stearns 1999). At the National Agricultural Station in Ireland, botanists cultivated the tiny embryonic growths found within the immature fruits; it is hoped that clones of the palm can be grown to adulthood (Parnell et al. 1986). Introduced animals and plants have taken over Mauritius and Rodrigues so completely that a number of species have been reduced to as few as two remaining plants.
Hawaii's native forests and plants have fared poorly, too. Native sandalwood forests were cut by early settlers, endangering several species. The most fragrant of these trees, the Iliahi (Santalum haleakalae), is now restricted to the dry lava slopes of eastern Maui. Its carmine red clusters of flowers have the same aroma as its fragrant heartwood (Daws 1993).
Seven or eight types of lobelia plants of the genus Delissea once flourished in Hawaii, and all became extinct by 1966 except one, Delissea undulata, which had become reduced to only a few plants. Conservationists fenced these plants in 1967 from the browsing cattle and rooting pigs, and they were thought secure (Carlquist 1980), but by 1995, the species' population had declined to a single plant. Botanists found it hanging by a few roots inside a sinkhole, the fence damaged and broken. The fence was repaired, and botanists are germinating seeds, hoping to prevent its extinction (Royte 1995). Another Hawaiian plant, Cyanea pennatifida, is in the same perilous status. Native to the mountains of Oahu, it also became reduced to a single plant, which was not producing its green flower. A botanist took a small sample of plant tissue and successfully cloned it in a test tube (Lipske 1997). Dozens of these plants have been cultivated from the slip, some of which were reintroduced into a preserve in 1995 (Lipske 1997).
Two hundred native Hawaiian plants are listed either on the US Endangered Species Act or are candidate species. Some 115 species have only 20 individual plants scattered in different areas or just one population of 50 or fewer plants in one location (Lipske 1997). Livestock and exotic plants have destroyed these plants and their habitats. One of these extraordinarily rare plants has been eliminated from all its original range, and the last members of the species cling to a single vertical cliff along the coast of Kauai, with its roots growing horizontally into rugged rocks (Carlquist 1980). This unusual lobelia, Brighamia insignis, has a thick, woody stem which tapers to a rosette of leaves, and it has lost its natural pollinator, which may have been a bird or an insect. Located 2,000 feet above crashing surf, it is out of reach of goats and pigs, but difficult for botanists to reach. This spectacular landscape is now this lobelia's sole habitat. Each year, botanists Steve Perlman and Ken Wood risk their lives by scaling the cliff, using climbers' ropes, to collect pollen from the plants with a brush. They then rappel to a neighboring plant to pollinate it, and months later, they must climb back up to collect the seeds, which are being placed into cultivation (Daws 1993; Royte 1995). Some of these seeds have successfully grown into plants at the National Tropical Botanical Garden on Kauai (Lipske 1997). This may be the most arduous and life-threatening plant conservation program in the world.
On the island of Guam, the forests have not been destroyed, but the bird life has been virtually eliminated by an insidious exotic animal that arrived in the 1960s or earlier. Several Brown Tree Snakes (Boiga irregularis) somehow secreted themselves in a shipment from their native Indonesia, and once on the island, they began to multiply. These snakes proved to be an environmental nightmare of the worst proportions, climbing trees and killing and consuming nestling and adult birds, and increasing at alarming rates. They have also caused major problems for the people of Guam. Ascending telephone poles, Brown Snakes short electrical wires. Between 1978 and 1990 alone, 1,000 power outages on the island were caused by these snakes (Jaffe 1994). A major threat to children, they enter homes, biting babies in cribs, and consuming pets. Untold millions of these snakes now live in virtually every environment in Guam. By 1981, these snakes had eliminated native birds from most of the island except for a remote part in the north. The snakes obliterated nearly the entire avifauna of Guam, an estimated 750,000 birds. Once a verdant tropical island teeming with birds, the forests have fallen silent. Three birds became extinct: the Guam Flycatcher (Myiagra freycineti) and two distinct subspecies of birds that survive on other islands: the Guam Bridled White-eye (Zosterops conspicillatus conspicillatus) and the striking chestnut, black-and-white Guam Rufous Fantail (Rhipidura rufifrons uraniae) (Jaffe 1994). The flycatcher was the greatest loss because it left no subspecies on neighboring islands. The Guam Kingfisher (Halcyon cinnamomina miyakoensis) became extinct on the island as well, but fortunately, a small number had been taken into captivity. These kingfishers have been kept at several US zoos, but although they have produced some young, they have exhibited abnormal behavior, such as cannibalizing their chicks, and some have succumbed to avian tuberculosis.
The Guam Rail (Gallirallus owstoni) escaped extinction by a hair's breadth. The rail's population declined to about 2,000 birds by 1981, and in 1983, fewer than 100 remained; the last wild birds disappeared by 1987. The Guam Division of Aquatic and Wildlife Resources captured 19 rails in 1984 and, after captive breeding and holding, began releases in 1995 of 30 to 50 rails every three months on the neighboring island of Rota (Line 1995). Rota, a 209-square-mile island, is snake-free, but only one-fourth the size of Guam. Its forest is mainly intact (Line 1995). In 1999, the introduced Guam Rails bred on Rota for the first time (BI 2000). A small area of 24 hectares on Guam has been fenced off from snakes, and Guam Rails introduced there have also bred (BI 2000). About 180 birds survive in 14 zoos in the United States (BI 2000).
To date, no effective control has been found to rid Guam of the Brown Tree Snake. Metal bands on nesting trees of endangered birds and high-voltage electrical wires meant to kill them on contact have hardly made a dent in their populations. A native bird at the edge of extinction, the Marianas Crow (Corvus kubaryi), declined on Guam from 351 birds in 1981 to just seven in 1999 (BI 2000). On neighboring Rota, only 592 survived in 1995, down from 1,318 in 1981 (BI 2000). Guam still has a high percentage of forest cover and many aquatic habitats intact. Among the few mammals on Guam, the Marianas Flying-fox (Pteropus mariannus mariannus) has also declined, with a population in the mid-1990s of only about 300 animals. They suffered the effects of heavy hunting by the people of Guam for food, and the Brown Tree Snake is now killing juvenile bats. The endemic Guam Flying-fox (Pteropus tokudae) is now extinct, last seen in 1968. It was a probable victim of unrelenting hunting by natives for food but may have been killed off by the Brown Tree Snake. Shrews and other rodents and monitor lizards are disappearing from Guam as well (Jaffe 1994). Since many of the birds and bats served as pollinators of native trees and plants, these species may die out as a result.
Invasive species, mainly those introduced by humans onto islands, caused virtually all avian extinctions over the past few hundreds years. Today, almost 30 percent of threatened birds, or 298 species, are affected by introduced predators, particularly cats, rats, mongooses and other animals (BI 2000). Livestock introduced into avian habitats represents a major threat to 72 species of birds, and 71 bird species have been adversely affected by the introduction of invasive plants that eliminated the food or habitat of plants on which these birds depended (BI 2000). Pathogens, such as diseases and parasites, brought into avian habitats by various means threaten an additional 69 species of birds (BI 2000).
Introduced species threaten fewer mammals, about 69 species, or 10 percent of those listed by the 2000 IUCN Red List (Hilton-Taylor 2000). This may be because far fewer mammals inhabit islands, compared to the number of native birds, especially flightless ones. Even sizeable islands, such as New Zealand, had very few native mammals prior to human colonization. Madagascar is an exception, with a very large diversity of primates and other mammals, a large number of which are now extinct because of invasive species, along with many other factors (See Islands chapter).