| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Horizons The Nature of Islands To set foot on an island, especially for the first time, is almost always an exhilarating experience, all the more so if the island is remote and uninhabited. Such was the experience of Charles Darwin when he visited the Galapagos Islands in 1835 while serving as naturalist aboard HMS Beagle. He and the midshipmen, with an occasional assist from the austere Captain Fitzroy, seemed hell-bent on putting much of the islands' biota and rocks into specimen cases. And considering their relatively brief stay in the archipelago--only six weeks during September and October--they were remarkably successful. As the Beagle sailed westward from Galapagos, Darwin began to muse at the striking variation in the skins of mockingbirds collected on the different islands. He was prompted to write in his notebook what is generally considered to be the genesis of his thinking about biological evolution and insular geography: When I see these islands in sight of one another,
& possessed of but a scanty stock of animals, tenanted by these
birds, but slightly differing in structure & filling the same place
in Nature, I must suspect they are only varieties....If there is the
slightest foundation for these remarks the Zoology of Archipelagoes
will be well worth examining; for such facts would undermine the stability
of species. Clearly the biogeography of insular life has been of crucial significance since the study of natural selection began. And because of their abundance, variation in size, shape, amount of isolation, and ecology, islands continue to serve as natural laboratories for modern biologists seeking explanations for species diversity. From a geological perspective, the world's island masses fall into two broad categories: continental and oceanic. Continental islands rise on continental shelves and reflect the history of the Pleistocene when, with the melting of the great ice sheets, sea level rose and places such as Newfoundland, Trinidad, and the islands of Indonesia and Great Britain became stranded. The shallow seas embracing these marooned land masses attest to their former continental connection, as do the affinities of their biota and rock forms. A few continental islands such as Madagascar and New Zealand are not closely linked today to nearby land masses, and because of their remoteness and enduring isolation (since late Cretaceous times), their biotas show resemblances to certain features of oceanic islands. For example, on Madagascar there are few mammals, many endemic reptiles and amphibians, and only remnants of the nearby African continental fauna. True oceanic islands are initially born of volcanic eruptions. All the most lonely islands, such as the Azores, Easter Island, Pitcairn, the Galapagos, Bermuda, Tristan da Cunha, Mauritius, Reunion, Hawaii, as well as coral atolls, belong to this class. Their biotas consist of species derived entirely from outside sources. Their formation results from the effects of plate tectonics--the massive movement of ponderous slabs of rock in the Earth's crust, floating upon the denser, hotter subterranean mantle. In the layer beneath oceanic plates, molten rock develops at hot spots, and a linear chain of volcanoes forms as the plate slowly moves over it. The Galapagos and Hawaiian archipelagoes were formed in this fashion. Colorful, sun-blazed coralline islands, abundant in the tropical waters of the Pacific and Indian oceans, are formed, as Charles Darwin correctly deduced, when volcanic islands erode or subside in the ocean. Millions of tiny sea anemones grow toward the sunlight while their accumulated carbonate skeletons form a fringing reef. The central volcanic cone, the structural base of a developing atoll, may ultimately disappear, leaving a coral embankment around a central lagoon. The Aldabra group and Christmas Island are just such elevated coral islands. The question that once perplexed biologists was how it was possible for so many venturesome vagabond species to survive a trying ocean passage to an island when almost at the touch of seawater many would surely succumb. Exceptional handicaps must be overcome. Nonetheless, close scrutiny of the original flora and fauna of remote islands suggests that they were indeed derived by chance from weedy mainland colonists in what has been termed sweepstake dispersal. Flotation, driven by ocean currents, is probably the most common means of transport to a remote island. Many plant species are well adapted to dispersal by sea. The beach morning glory (Ipomea spp.) appears on the beaches throughout the Pacific, and the coral tree (Erythrina spp.) is widespread on oceanic islands, with its bean-like seeds adapted for floating and tolerance of extended periods of immersion in salt water. The seedlings of the red mangrove (Rhizophora) are preadapted to flotation and have reached sheltered lagoons of islands throughout the Pacific and beyond. Sea-surface transport of flightless insects between islands of the Galapagos has recently been documented by Canadian entomologist Stewart Peck from Carleton University. Some wasps and moths with tough, water-repellent chitinous exoskeletons have been seen to land on the surface of the ocean and then take flight again. Rafts made of a mat of vegetation or other debris can provide a suitable vehicle for snakes, lizards, and rodents, which can subsist without fresh water for a fairly long time. Logs and driftwood can serve as temporary homes for wood-boring insects such as termites, and bee and beetle larvae. Winds and jet streams scatter small dissemules such as spores of ferns and fungi, and even small living insects. One-hundred and eighty-six species of ferns and their relatives are found on Mauritius, about 450 miles east of Madagascar. Birds displaced from their traditional migratory routes may end up as stragglers on remote islands. This phenomenon is well documented in Galapagos, Hawaii, and Cocos Island, Costa Rica. Some of them, such as the African cattle egret, which only arrived in the Galapagos in the mid-1960s, have become permanent residents. For juvenile silk-spinning spiders, ballooning is an effective means of aerial transport. By hitchhiking on the feathers and feet of aquatic and semiaquatic birds, certain plants, sticky seeds, beetles, and young snails have colonized distant lands. The majority of terrestrial snail species on Pacific islands are small, most less than ten millimeters in length. On Aldabra Island in the Indian Ocean, British botanist G.E. Wickens has calculated that successful establishment of a new plant species by bird dispersal occurs every 650 years, and that two-thirds of the flora may have reached the island on or inside birds. He also estimates that natural avian introductions of new plant species occur, on average, every 8,000 years for the Galapagos and every 25,000 years in Hawaii. Of course, species are present in proportion not only to their capacity to disseminate, whether actively or passively, but also to their ability to establish themselves after arrival. This, in turn, is determined by ecological conditions and by the genetic constitution of the waif. Additionally, the need for an appropriate mate in sexually reproducing animals, or a compatible pollinator in out- crossing plants, poses a formidable challenge to long-term establishment. The idea that specific groups of organisms have different hurdle-values--a term coined by Sherwin Carlquist, a noted student of island life--which determine the limits of their dispersibility, is fundamental to the concept of disharmony in the biotas of oceanic islands. Disharmonic floras and faunas are characterized by the absence or poor representation of conventional groups such as large hoofed and carnivorous mammals, amphibians, primary division freshwater fishes--those that evolved in fresh water--and large-seeded forest trees. Out of an eclectic and impoverished mix of disparate colonists from mainlands, natural selection has, over time, molded an array of gap-filling, compromising bedfellows, which have established among themselves new ecologically and behaviorally based harmonic interrelationships. The extent of such realignments is often indicated by the level of endemism, that is, the extent to which various species have evolved to adapt their form and function to their new biological community. However, an isolated island or archipelago can be both harmonic and species-rich with respect to a single group, but disharmonic and species-poor with respect to other groups. This unbalanced condition has been well illustrated by population ecologists Robert MacArthur and Edward Wilson for the insect fauna of Hawaii. Certain families, such as those represented by crickets, damselflies, predaceous ground beetles, and fruit flies, among others, are characterized by a diverse assemblage of species derived from a few mainland stocks. In some cases these families have diversified, or radiated, much more than they have in the rest of the world, and have become harmonic. Other families found in the same orders elsewhere, such as the dung beetles and ants, are entirely absent. In fact, no less than 24 orders of the known insects of the world are underrepresented in Hawaii. Thus, like many oceanic archipelagos, the biota was originally depauperate but has been enriched through local adaptive radiation. Adaptive radiation is one of the most intriguing phenomena of island biology. It describes the diversification of a single lineage into a variety of species, each with different adaptive properties. Given enough time, all insular populations will evolve away from one another and from the ancestral colonizing population. Once a population has evolved to the full species level, the stage is set for adaptive radiation to begin. A classic example of adaptive radiation in birds, which has served generations of evolutionary biologists, is Darwin's finches. A total of 13 species evolved within the Galapagos archipelago from a common ancestor whose founding type and American continental source have not yet been identified. A single fourteenth species occurs on Cocos Island in Costa Rica, about five hundred miles northeast of Galapagos. That all the finches are closely related, and presumably evolved from the same progenitor stock, is indicated by a complement of characteristics common to all. These include such things as plumage (short tail and fluffy rump feathers), anatomy (musculature of the jaw and voice box, and the configuration of the bony palate), uniform chromosome number (karyotype), simple courtship display, basic song, and, recently confirmed, the DNA pattern. The principal feature of their adaptive radiation is the structure of the beak and the associated feeding behavior. There are various-sized vice-like beaks for cracking seeds of different hardness and size (found in six species of Geospiza ground-finches); a parrot-like beak for crushing fleshy fruits and soft seeds, flowers and buds (one species of Platyspiza tree-finch); pincer-like beaks for removing bark (three species of Camarhynchus tree-finches); elongate probing beaks for pecking into woody tissues for insects (two species of Cactospiza tool-using tree-finches); and a small, thin forceps-like beak for gleaning small insects from vegetation (one species of Certhidea warbler-finch). What is most remarkable about their beaks and feeding habits is that they seem to mimic ecological equivalents of continental species of songbirds belonging to such diverse families as sparrows, blackbirds, tanagers, warblers, and titmice. None of these was originally represented in the Galapagos avifauna. Almost equally as striking is the parallel acquisition of advertising songs typical of their familial counterparts. Two remarkable feeding habits, for which there is no obvious beak specialization, help three finch species exploit otherwise obscure food resources. The small-beaked ground-finch (Geospiza fuliginosa) has learned to puncture the skin at the base of the flight feathers of nesting boobies, feeding on the blood as it courses down the plume. Two species of finch misnamed "woodpecker-finches," (Cactospiza pallida and C. heliobates), employ a small stick or cactus spine as a tool in order to dislodge insects and other arthropods hiding in holes or under bark. Tool-use was first seen by E.W. Gifford, one of the members of the famous California Academy of Sciences Galapagos expedition of 1905-06. For fear of ridicule from his field companions, who were weary from many months of separation from families in San Francisco, where the 1906 earthquake had destroyed most of the city, he said nothing about his unusual sighting, and kept the matter secret for many years after returning home. By comparison with the Galapagos finches, the evolutionary response of the single species of Darwin's finch on the humid, tropical Cocos Island is much less imposing. The Cocos finch (Pinaroloxias inornata) sometimes referred to as a "honeycreeper-finch" is a full-time feeding generalist, eating insects, fruits, and nectar using its slender, slightly down-curved beak. In structure and behavior it parallels American mainland honeycreepers, and, in the absence of multiple islands on which diversification of isolated population might occur, only one uniform species has evolved. Whereas the Galapagos finches show a nearly unbroken series of intermediate forms that permits tracing the evolutionary pathways between adaptive ends with considerable certainty, the Hawaiian honeycreepers show greater extremes and variety in beak and plumage morphologies. In Hawaii, there are some 27 extant and 16 fossil species--a range befitting a group that is assuredly much older and which has evolved in a much more diverse setting. There appear to be some structural extremes exemplified in the honeycreepers, such as the long, sickle- shaped beaks used in nectar feeding, and a few evolutionary gaps which may be due to extinctions. The islands of Hawaii are some of the most isolated
in the world, about 2,500 miles from the nearest land mass. Collectively
very diverse in their habitats and geological age, they present another
fine natural laboratory for the study of evolutionary biology, especially
adaptive radiation, excellent examples of which are found among the
birds (honeycreepers), plants (silverswords), and insects. There are
over seven hundred species of fruit flies (Drosophila spp.)
described from the Hawaiian islands, and entomologists are still counting.
They have varied ecologies, unique behaviors, bizarre structures, and
specific differences that can be examined in minute detail through banding
patterns in their giant chromosomes, which are contained in cells of
the salivary glands and elsewhere. As a group, the Hawaiian fruit flies have features that set them apart from typical mainland relatives--diminutive red-eyed flies associated with fermenting fruits on which their eggs are laid. In Hawaii females and males are conspicuously different, with the males sporting extraordinary projections on their head and legs, and distinctive cross veins on their transparent wings, which gave rise to the group name picture-wing drosophilids. They also have modified mouthparts, leg appendages with variously shaped ends, (i.e. forked, spoon-shaped, etc.), a large body with a wingspan up to 20 millimeters, and a most unusual courtship behavior. This involves a highly ritualized combative stance in which competing males may physically wrestle with each other, thrusting with their legs and head with bodies upright. After copulation a pair just falls apart, and in some cases the male may remain immobile for a couple of minutes. Madagascar, a micro-continent in the Indian Ocean, presents quite a different picture of insular evolution than Galapagos or Hawaii. It is an expansive laboratory whose unique biological productions hinge on a form of double invasion. Fossil dinosaur remains speak to a very ancient connection with Africa. But because of its long and effective physical isolation from Africa beginning in mid- Permian times, its modern flora and fauna were randomly derived from just a fraction of the different kinds of organisms that have been stocking the African ecosystem over millions of years. Thus the ecological mix here was unusual from its very beginning and today the biota is quite distinct. The originality is evident from a high level of endemism--85 percent of the flora, 98 percent of the amphibians, and 99 percent of the reptiles. The island has evolved peculiar groups that are unknown elsewhere, such as vanga shrikes with their diverse beaks and now-extinct elephant birds which laid two-gallon eggs; it is rich in archaic forms such as lemurs and tenrecs yet evidences some surprising gaps in its representation--such as legless salamanders, egg-laying mammals, marsupials, true monkeys and apes, elephants, hyraxes, ruminants, and rabbits. Ecosystems are built by the available components. What goes on in an ecosystem determines the direction of natural selection that shapes future generations of every species. There is a tight give-and-take between ecology and evolution, and islands like Madagascar help us to analyze the nature of that relationship. For example, large carnivores of Africa, such as lions, are absent here, as are their prey, the antelopes. In their place is an adaptive array of lemurs and ground-dwelling birds that reflect, in part, the relative safety of a land free from feline and canine predators. Islands give a more focused sense of give-and-take between ecology and evolution than do continents. In parallel fashion, these predaceous types are absent from the Indonesian island of Komodo, where the world's largest lizard, the huge Komodo dragon (Varanus komodoensis) is the predator of deer and pigs and the occasional attacker of humans. In the Galapagos, the grazing hoofed mammal niche is occupied by other giants, the tortoises (Geochelone elephantopus), which are free of natural predators but which have become vulnerable to attacks by introduced feral pigs and dogs. Introduction of each new species to an island ecosystem disrupts the ecological stability that has evolved. To survive, insular organisms must cope with the limited habitats available. Many have adapted to become evolutionary "dead ends," trapped in their isolated worlds where they have evolved traits that preclude their return to their mainland origins or their survival in the now foreign competitive mainland milieu. The greater the degree of endemic specialization, the greater the likelihood of extinction should the insular environment change, even in rather subtle ways. The smaller the land area, the greater the impact on the endemic biota of introduced parasites, competitors, predators, and weeds. The apparent tameness of insular endemic animals in the presence of humans has been misinterpreted to mean that these animals live in a "world without fear." Darwin was careful to point out that the "wildness of birds with regard to man, is a particular instinct directed against him, and not dependent on any degree of caution arising from other sources of danger." Galapagos finches will perch on a human's head, the buteo hawk can be coaxed from its perch to sit on a hand-held stick. But the finches will quickly take flight or utter warning notes in the presence of the endemic hawk, short-eared owl or colubrid snake, even the mockingbird, which is an effective nest predator. But an array of now unrecognized enemies would thwart any resettlement attempt on the mainland. But, in any case, the loss of dispersal mechanisms is another feature of the island syndrome dilemma. Animals, typically perceived to be highly mobile creatures, especially volant--flying--birds and insects, tend to develop diminished powers of flight. This amounts to burning one's bridges and forecloses on the possibility of a return to the mainland by the very means they used to leave. Seeds with special devices for attachment to animals may develop reduced spikes and barbs and can no longer depend on hitching a ride for dispersal. Species of the beggar tick (Bidens) are widely distributed throughout the South Pacific, but the seeds of only a few of the island forms retain a full complement of the features that facilitated their original dispersal from the probable mainland form B. pilosa, which has spines armed with recurved barbs and numerous upward-pointing hairs on the flat-sided seed coat--features that facilitate attachment to feathers and hair. Humans have been the greatest single modern destroyer of island habitats and their biota, causing the extinction of thousands of species. David Steadman of the New York State Museum, an indefatigable student of fossil birds, claims that the first human inhabitants of Pacific islands wiped out as many as two thousand different avian species, or roughly 20 percent of all recent bird species on Earth. The major difference between ancient and modern humans is not in their attitude toward nature but their technology--and their numbers. What is now done with guns and chain saws was once accomplished more slowly with snares and fire. The close correlation among organisms in an ecosystem is well illustrated by an example from Mauritius, once the home of the extinct dodo, Raphus cuculatus, an aberrant, flightless relative of pigeons about the size of a turkey, and a ready source of fresh meat for early colonists. The eminent ecologist Stanley A. Temple of the University of Wisconsin was perplexed by the lack of natural reproduction in the endemic tree Calvaria major, formerly widespread on the island. His studies of historical records suggested that the dodo fed on the large fleshy fruit which contained a hard pit that fails to germinate if not mechanically abraded, presumably by the gravel-gorged gizzard of the dodo. With the loss of this cooperative relationship, termed mutualism, the tree fails to reproduce. The last surviving Calvaria trees are of an age that matches the time when the dodo became extinct, about three hundred years ago. Calvaria seeds were recently fed to domestic turkeys, which have digestive mechanics similar to dodos, and some of the defecated seeds did germinate. After the extinction of the dodo, apparently no other Mauritian animal was capable of ingesting and scarring the large Calvaria pits. For three centuries the tree had failed to reproduce and was on the verge of extinction. Rescue efforts by nursery technicians of the Mauritian Forestry have been successful. Even without the intrusion of humanity, the expectation of species extinction is much greater on islands than on continents. This is in part because of their lack of resilience and partly because they are younger ecosystems with less stability. But one of our major challenges is to clarify the natural biological processes that have in the past led to extinction. The rules of biology governing the course of events on islands are universal rules. The evolutionary process is the same, only the ecological theater and the cast of characters are changed. Man has inherited this biodiversity--nature's richest gift. We can only hope that we will prove ourselves worthy of such largess. Robert Bowman is professor emeritus at San Francisco State University. He serves on the Academy's Board of Trustees. |
Summer 1995
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