Island inhabitants

Remote ocean islands are one of Earth's most creative habitats. Since Darwin, scientists have been intrigued by the thousands of unique and exotic species of plants and animals that may be found on them. But as human beings and their domestic animals continue to colonize these sanctuaries, we risk extinguishing many of our planet's rare and unusual life forms.

Island species are tragically prone to extinction. Their populations are generally small and cannot be replenished by immigration from other sites. In a 1977 article on species extinctions in the United States, biologist Paul Opler stated, ''It is . . . known that island forms are more susceptible to extinction than mainland forms. Fully 67 percent (351 of 518) of US extinctions to date have been island dwellers.''

Former President Carter's ''Global 2000 Report'' sees alarming rates of species extinctions during the next 20 years. Many of these will be island plants and animals. The problem of plant extinctions, whether on island or mainland, has serious practical implications, as many plant species have potential value as sources of food and medicines.

Daniel Simberloff, a Florida State University biologist and an expert on island biology, recognizes both intrinsic and extrinsic reasons for the fragility of island species.

The intrinsic reasons are related to acquired characteristics of the species themselves. ''Island species,'' Dr. Simberloff says, ''have evolved in isolation from the normal interactions with other species - both plant and animal - that mainland species always have to deal with. Consequently when they are faced - as they inevitably are - with the introduction of mainland species, they simply cannot cope with them - they may be out-competed, they may be easy prey for predators, or they may be easily parasitized. . . .''

Extrinsic reasons are related to the unique and fragile habitats in which island species live. ''Habitats on islands have been proportionately more devastated than on the mainland,'' says Dr. Simberloff. ''The areas of islands are smaller and they've been pillaged in a proportionately greater fashion.''

Isolation and unusual habitats are the keys to the great number of distinctive life forms on ocean islands. Most oceanic islands are volcanic in origin. They rise from the sea as barren, inhospitable masses of lava and over the course of millenia are colonized by vagrant plants and animals floated, blown, or carried over the intervening seas.

The chances of a particular plant seed or pair of mating animals (or single, pregnant female) successfully colonizing an oceanic island are extremely remote. This is reflected in the fact that most oceanic islands - particularly very isolated ones - have far fewer plant and animal species than similar areas of continental land. Certain groups of strong dispersers, such as birds and ferns (which reproduce by minute spores) are well-represented, while others, such as land mammals and freshwater fish, are usually entirely absent. New Zealand, for example, has 149 species of endemic birds but only one terrestrial mammal - a bat.

Once established on an island, a plant or animal species usually evolves to meet the demands of its new environment. The same isolation that limits colonization now favors evolution and speciation by restricting mating between island and continental populations. An island population, separated from the mainland and faced with unique selective pressures, gradually evolves until members of the island and mainland populations are unable to interbreed, even if given the opportunity. At this point a new species is born.

Divided among the many islands and habitats of an island archipelago, a single colonizing species may give rise to a great number of distinct endemic forms. In the Hawaiian Islands two dozen species of beautifully colored honeycreepers, each physically and ecologically distinct, evolved from what is believed to have been a single colonizing ancestral bird. A similar group of species - the Galapagos finches - led Darwin to conclude that species are not immutable, but change to meet the demands of their environments.

The archetypal example of an island species extinction is that of the St. Stephen's wren, a flightless bird driven to extinction by a lighthouse-keeper's cat on a small island near New Zealand. Others include New Zealand's giant flightless moas, some of which stood 10 feet high and weighed over 500 pounds, Madagascar's elephant bird, nearly twice as heavy as the largest moa, and the flightless dodo of Mauritius Island in the Indian Ocean. About 25 percent of more than 300 species of birds inhabiting New Zealand in the past few centuries now are extinct.

Man, either directly or indirectly, has been responsible for most extinctions. Early island settlers brought pigs, goats, sheep, dogs, and cats, which attacked native animals directly or damaged their habitats by grazing. Land clearing, by fire and axe, eliminated many native plants and habitats and the island animals that depended on them. Many island animals, having evolved in the absence of mammalian predators, lost their instinctive fear and such natural defenses as powers of flight.

In recent years, many island governments have taken action to protect their remaining native plants and animals. Dr. Ron Walker, chief wildlife biologist for the state of Hawaii, is in charge of protection efforts there. At present, conservation efforts in Hawaii center on the severely endangered ''alala,'' or Hawaiian crow, a large and very unusual member of the same genus as the common crow of North America. The alala is now restricted to the main island of Hawaii, where a population of 25 to 150 birds remains.

A captive breeding program during the past four years has provided encouraging preliminary results - the alala is an adaptive bird, willing to breed in captivity. Unfortunately, it may not get a chance to do so - budget cuts by the current administration have eliminated the grant that the US Fish and Wildlife Service provided the project.

As Dr. Walker summarizes the situation, ''When it comes to wildlife vs. welfare for the poor and aged, obviously wildlife is going to take a back seat . . . when money is tight, money for wildlife is much more difficult to get.'' State funds may replace previous federal support, and Dr. Walker retains cautious optimism about survival of the alala.

Another Hawaiian bird - the nene (pronounced ''nay-nay''), or Hawaiian goose - may well represent an important success in the field of island wildlife conservation. A captive breeding program - aided by the prodigious efforts of a gander who sired some 200 offspring - has helped replenish nene populations on the island of Hawaii.

A dryland goose with long legs and webless feet, the nene continues to wander the rough lava slopes of Hawaii. The latest studies by Hawaii's Division of Forestry and Wildlife indicate, however, that natural replacement is not sufficient to maintain nene populations in the wild. Continued captive breeding may be necessary to ensure survival of the species. Unfortunately, the commitment of scanty funds to save one species may imperil others that require similar attention.

The island of Puerto Rico faces similar conservation battles. US Department of Agriculture documents published in the mid-1970s list 126 species of endemic Puerto Rican plants and 14 endemic animals as endangered. Some are living so precariously that their continued existence, at this writing, is uncertain.

The world's oceans are scattered with similar mountainous islands, each with unique and irreplaceable plant and animal species. Without concerted efforts to preserve the natural habitats and native species of oceanic islands, the tragedy of the St. Stephen's wren will be repeated again.