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AMER. ZOOL., 34:134-144 (1994) Biodiversity on Oceanic Islands: Its Origin and Extinction1 GUSTAV PAULAY Marine Laboratory, University of Guam, Mangilao, Guam 96923 SYNOPSIS. The isolation and small size of oceanic islands make them attractive models for studies of diversification; the sensitivity of their biota makes them important subjects for studies of extinction. I explore the origin of island biotas through dispersal and in situ diversification, and examine the fate of these biotas since human contact. Island biotas start out depauperate and disharmonic, facilitating the survival of relict taxa and stimulating adaptive radiations. The often highly restricted range and small population size of insular species, together with their limited diversity of defenses, make island biotas particularly vulnerable to extinction, largely through habitat loss or interactions with introduced species. INTRODUCTION Islands, due to their discrete, isolated nature, provide excellent opportunities for understanding the origin, diversification and extinction of terrestrial biotas. Here I examine these processes on oceanic islands, focusing on those remote from continental source areas. Such islands can serve as model systems for addressing fundamental questions about biodiversity and conservation: what areas are most likely to develop high species diversity and endemicity, and what makes particular species or biotas vulnerable to extinction? In part, the biotas of oceanic islands diversify in situ, yielding insight into the evolutionary origin of diversity. Their biotas are usually endemic and among the most vulnerable in the world, and are currently undergoing unprecedented rates of extinction. An understanding of the processes leading to these extinctions is important in itself, and also serves as a model for conservation problems worldwide. ORIGIN OF ISLANDS Any isolated habitat can be considered insular. Because of their simplicity, oceanic islands are especially illuminating and are the focus of this review. By definition, oceanic islands never had connections to continental land masses. They are the products of volcanism or tectonic uplift, or the result of organic reef growth upon foundations formed by the first two processes. In contrast, most continental islands were joined 134 to other continental land masses in the past, having since become separated due to tectonics or sea level rise. Many oceanic islands are the products of volcanism from stationary, "hotspot," magma sources. As a tectonic plate drifts over such a source, volcanoes incorporated into the plate are carried away from the hotspot, and a linear chain of islands or seamounts results, with the youngest ones lying nearest the hotspot {e.g., Hawaiian, Society Islands) (see Menard, 1986 and Williamson, 1981 for reviews of island geology). Such hotspot islands usually have a well defined origin, and undergo predictable development with age. They change from high, undissected, shield volcanoes to smaller, highly eroded and topographically complex islands before final submergence of their volcanic portions, usually at 3-15 million years of age. Reef buildup around tropical islands may ultimately convert high islands into atolls, with low lying coral sand cays projecting a few meters above sea level. Although atolls may persist for tens of millions of years, they have a propensity for repeated submergence as sea levels rise and fall; thus their terrestrial biotas are ephemeral (Paulay, 1991a). Continued activity of a hotspot can yield an archipelago that is persistent through time, with new islands arising as older ones subside. The positions of islands on a single tectonic plate do not change relative to each other, although some islands may drown while others arise. Other oceanic islands are the result of island arc volcanism, formed where one tec- BIODIVERSITY ON ISLANDS 135 tonic plate is subducting under another (e.g., butes, and this is reflected in the composiSolomon, Tongan Islands). Such islands tion of island faunas. Thus land snail faunas generally undergo a variety of changes due on remote oceanic islands are generally limto recurrent volcanism, subsidence, uplift ited to minute species, a legacy of their origand erosion in these tectonically complex inating largely through aerial dispersal regions. In the Pacific, they often attain larger (Vagvolgyi, 1975). Of those species which sizes than hotspot islands. The age(s) of can disperse to an island, a subset will be emergence of an arc island is often difficult able to establish and maintain populations, to define, and the size and degree of emer- depending on attributes of the species and gence can vary greatly through time. Fur- the island. An island's size, its habitats, and thermore, because such islands lie in tec- its preexisting biota are all important in tonically active areas, their positions are determining the success of potential colocontinually changing relative to other land nists; preexisting species can place impormasses. Thus while the age, position, and tant limits on colonization via interactions physiographic evolution of hotspot islands such as competition, predation, and symare simple and readily predictable, those of biosis. arc islands are complex. This makes hotspot While the biodiversities of solitary, young islands more attractive as model systems for islands near continents can be solely attribthe study of diversification. uted to colonization events (e.g., Krakatau Island-Thornton et al, 1990), those of ORIGIN OF BIOTA—COLONIZATION remote, older archipelagoes often result from An island's biota is the product of oversea extensive radiations of a handful of original colonizations and local diversifications. colonists. Thus the estimated 10,000 Oceanic islands receive their biotas solely Hawaiian insect species evolved from ca. through dispersal. The probability of colo- 350-400 successful colonists (Howarth, nization is influenced by the island's geo- 1990), and the 1,138 indigenous Hawaiian graphical and environmental settings as well vascular plants are derived from 387-396 as by organismal attributes (MacArthur and founders (Wagner, 1991). Wilson, 1967). Because the probability of Although the immediate effects of disdispersal is inversely related to distance, an persal limitations are depauperate and disisland may receive more of its propagules harmonic biotas on remote islands, this from relatively depauperate nearby islands biotic poverty can lead to the survival of than from very diverse but remote conti- unusual, relict taxa, and stimulate the develnents. opment of adaptive radiations in successful Organisms vary greatly in dispersal abil- dispersers, so that high, remote oceanic ity, both with regards to distance traveled islands can become fairly species rich and method of transport (Carlquist, 1974). through time. The geographic range of a taxon is often RELICT TAXA correlated with its dispersal ability (Kohn and Perron, 1993), and the biotas of remote The view that evolution is an arms race islands are largely limited to species with among predators sensu lato (including parexcellent dispersal abilities and their de- asites, pathogens, herbivores) and their prey, scendents. In the island-rich oceanic Pacific, or among competitors (Van Valen, 1973), where the far majority of organisms origi- may explain the temporal or spatial restricnates from western (Austro-Asian) source tion of certain groups in the history of life areas, fewer and fewer taxonomic groups are (Vermeij, 1987). Species that cannot cope represented as one moves eastward through with escalating biotic pressures in most the basin (e.g., Zimmerman 1942), with environments become globally extinct or poorly dispersing taxa (e.g., mammals, may survive in unusual habitats or areas amphibians, gymnosperms) being especially that their adversaries have not penetrated, limited to western island groups. such as the deep sea, caves, or Australia Certain methods of dispersal are avail- (Vermeij, 1987). Islands frequently offer able only to organisms with particular attri- such refuge for relicts, and may also pro- 136 GUSTAV PAULAY FIG. 1. Distributions of the land snail families Endodontidae (solid line, based on Solem, 1976), Partulidae (dashed line, based on Cowie, 1992) and Amastridae (dotted line). mote the secondary, in situ evolution of species with limited defensive or competitive abilities (Solem, 1979; Carlquist, 1974). Thus many land crab and seabird species are widespread but strictly insular, restricted largely by mammalian predators (Hartnoll, 1988; Steadman, 1989). Numerous benthic marine species are restricted to, but widespread among, the small oceanic islands of the Pacific tectonic plate (Springer, 1982), an enigmatic distribution that similarly could be forced by biotic exclusion from more diverse western areas. Relict taxa are well known on large, old, continental islands, the lemurs of Madagascar and the monotremes and marsupials of Australia being classic (if not indisputable) examples. Although short-lived oceanic islands appear at first ill-suited for the long-term survival of relict lineages, groups with good dispersal abilities can survive by island hopping. Three (Partulidae, Achatinellidae, Amastridae) of the six families that comprise the most primitive order (Orthurethra) of pulmonate land snails, as well as the anatom- ically most generalized family (Endodontidae—Solem, 1976) of higher landsnails (Sigmurethra), are restricted to islands (Solem, 1979). The fact that these land snails have reached even the most remote Pacific islands, yet are virtually unknown from continents, strongly implicates biotic factors rather than dispersal limitations as setting distributional boundaries (Fig. 1). The past continental occurrence of at least achatinellids (Solem, 1979) and the extreme vulnerability of all four families to certain introduced predators (see below) support the hypothesis that their relict distribution is the result of escalation. The primitive weevil family Aglycyderidae is similarly restricted to islands in the Pacific and Atlantic oceans, with one dubious exception (Zimmerman, 1948). Although aglycyderids occur on most central Pacific high islands, they are rare and species poor except on the Hawaiian Islands, where they have undergone an explosive diversification, resulting in over 170 described species (Nishida, 1992). Zimmerman (1970) argued that this Hawaiian BIODIVERSITY ON ISLANDS radiation was possible because of the absence of the very diverse and ecologically dominant Miocalles weevils that cooccur with the family elsewhere. The restriction to and survival on ephemeral islands by such primitive and presumably ancient groups depends upon their ability to continually disperse to new islands as older ones sink below sea level. For example, endodontoid land snails are known from Lower Miocene to Pleistocene deposits on three islands (Midway, Bikini, and Enewetak atolls) that have since subsided greatly and no longer support the snails (Solem, 1982); they persist on other, mostly younger islands throughout the Pacific. Similar dispersal from drowning to new islands has been demonstrated within drosophilids and honeycreepers in the Hawaiian island-chain (Sibley and Ahlquist, 1982; Beverley and Wilson, 1985; see below). Such relict taxa not only survive on Pacific islands, but can form a dominant portion of the indigenous fauna, especially among mollusks. The strictly insular land snail families discussed above constitute 79% of the Hawaiian and 70% of the Rapan (SE Polynesia) land snail fauna (Solem, 1982, 1990). ECOLOGICAL RELEASE AND LOCAL DIVERSIFICATION Island biotas are famous for large evolutionary radiations such as that of many hundreds of species of Hawaiian drosophilids (Carson, 1983; Nishida, 1992), and bizarre adaptations, like the boid snakes with jointed upper jaws in the Mascarenes (Frazetta, 1970). Studies of adaptive radiations on islands indicate that both diversification and adaptive evolution are favored by a low diversity of initial colonists, that both can be extremely rapid, and that diversification is stimulated by opportunities for isolation on topographically complex archipelagoes. The potential for taxa to diversify, to evolve unusual adaptations and to expand into novel adaptive zones can be thwarted in diverse biotas by the high degree of utilization of existing niche space and high levels of escalation that limit evolutionary experimentation. It has long been recognized that in low diversity settings, as after 137 mass extinctions or in remote insular settings, the potential of organisms to diversify and enter novel adaptive zones can be better realized (Simpson, 1953). Radiations are especially prevalent on remote islands lying at the edge of a group's dispersal range, in peripheral areas termed the "radiation zone" by MacArthur and Wilson (1967), where the low diversity of colonists facilitates in situ diversification (Diamond, 1977). The effects of certain "keystone" taxa in limiting the evolutionary options of others are especially striking, as demonstrated where such taxa are absent. Ants, for example, dominate arthropod communities through most of the world; their absence in Hawaii and SE Polynesia appears to be largely responsible for the great radiations of carabid beetles and spiders, and the exploitation of predatory niches by such unusual organisms as caterpillars, in those island groups (Wilson, 1990). That diversification can be very rapid is immediately suggested by the sheer diversity and high levels of single-island endemism exhibited by certain clades on islands a few million years old. Thus at least 70 species of Mecyclothorax beetles are endemic to 1 My old Tahiti (Perrault, 1987), 25 of the 26 species of picture-wing Drosophila on 600,000 yr old Hawaii are restricted to that island (Carson, 1983), and at least 11 lineages of insects and land snails have radiated to over 100 species each in the Hawaiian Islands. That morphological differentiation as well as speciation can be very rapid is further attested by the minimal genetic differentiation found within some striking adaptive radiations {e.g., Helenurm and Ganders, 1985; Lowrey and Crawford, 1985). The 19 species of Hawaiian Bidens exhibit much more morphological and ecological diversity than this diverse genus does in the Americas, yet the genetic diversity of this entire insular radiation is comparable to that found among populations within American species (Helenurm and Ganders, 1985). Within an oceanic archipelago, local diversification can occur by inter- or intra-island speciation. Only the former is generally available to organisms with good over-land dispersion abilities (especially 138 GUSTAV PAULAY vertebrates and plants). Diversifications among such organisms are relatively uncommon, with only a handful of isolated archipelagoes showing large endemic radiations of birds or plants. The number of adjacent islands appears to be more important than island size in facilitating bird radiations: the Hawaiian and Galapagos Archipelagoes have significant radiations in birds, while the larger and more ancient, continental islands of New Caledonia and New Zealand show little in situ diversification (Olson, 1991). Inter-island speciation can nevertheless lead to large radiations, as attested to by honeycreepers, several lineages of plants, and numerous lineages of highly volant insects in the Hawaiian Islands (Wagner, 1991; James and Olson, 1991; Nishida, 1992). Intra-island, or "continental," speciation is restricted to islands that are large enough to allow for effective isolation of populations. While intra-island speciation in birds requires an island the size of Madagascar, for small land snails and flightless insects a few square kilometers can be sufficient (Diamond, 1977; Paulay, 1985). Extreme geographic localization is especially prevalent on topographically complex, highly dissected volcanic islands, a reflection of the very steep topographic, climatic, and environmental gradients occurring across islands only a few kilometers in diameter yet hundreds or thousands of meters high. Such geographic localization can lead to remarkable radiations through intra-island speciation. Thus on isolated Rapa, a 40 km2, 650 m high, 5 My old island in SE Polynesia, local radiations resulted in 67 flightless Miocalles weevil, and 45 achatinellid and 24 endodontid land snail species (Solem, 1982; Paulay, 1985). Many of these species have restricted ranges on Rapa, some remarkably so {i.e., <1 km2). In contrast, islands with low topographic differentiation, such as the flat-topped, uplifted limestone islands of Henderson and Niue (S Polynesia), offer few opportunities for geographic restriction and generally lack intraisland diversifications (Paulay, 19916). The importance of topographic, climatic and environmental variation in limiting species ranges and facilitating diversification is also apparent in continental areas with steep topographic or environmental gradients {e.g., Tweedie, 1961). Both island size and age may limit diversification. The frequent correlation between species richness and island size may be the result of increasing habitat heterogeneity with island size, or an ecological or evolutionary equilibrium between origination (colonization and local speciation) and extinction rates (MacArthur and Wilson, 1967; Williamson, 1981). In remote islands, where much of the diversity is the result of local diversification, island age (the time available for diversification) may be an important control (Wagner, 1991). Because hotspot islands subside and shrink with age, older islands within an archipelago tend to be smaller. This may give rise to opposite diversity patterns, depending on whether time or area is limiting diversification. Groups capable of rapid rates of speciation (relative to island age and available niche space) may so rapidly saturate even young islands with species that they yield the expected positive correlation between species richness and island area. Such is the case for the highly volant, Hawaiian Plagithmysus beetles, which exhibit high rates of inter-island speciation (Fig. 2). In contrast, if diversification is relatively slow, then diversity may strongly correlate with island age and therefore be negatively correlated with island area. Such is the case for the flightless Hawaiian Rhyncogonus weevils, which appear to diversify slowly via intraisland speciation (Fig. 2). Age is important from both an evolutionary and conservation perspective, as old islands often have the most divergent species and for some lineages, the most diverse fauna. The largest land snail radiations among SE Polynesian and Micronesian hotspot islands, both in numbers of species and the development of endemic genera, occur on the relatively old islands of Gambier, Rapa and Pohnpei (all 5-7.5 My old) (Solem, 1982). The most interesting and divergent insect fauna among the major islands of the Hawaiian archipelago is on Kauai, the oldest island in the chain (Zimmerman, 1948). The time available on an archipelago for evolutionary divergence and diversification of a lineage can be greater than the age of the oldest emergent island. Thus the Hawai- 139 BIODIVERSITY ON ISLANDS O Plagithmysus • Rhyncogonus O 50 to O o <D en 40 - - Q. in 30 - 0) -Q E o a> -Q 20 E 3 10 n O • i i 1 1 1 10° Island age (My) 10" Island area (km2) FIG. 2. Diversification of Plagithmysus (Cerambycidae) and Rhyncogonus (Curculionidae) among the major Hawaiian Islands. Species richness of Plagithmysus is strongly correlated with log island size (r = 0.93), but not island age (r = -0.29); that of Rhyncogonus with island age (r = 0.96) but not log island size (r = -0.19). Data from Nishida (1992) and Howarth and Mull (1992). ian chain, in existence for at least 70 My, was colonized by drosophilids and drepanidine honeycreepers well before the birth of the present major high islands (Sibley and Ahlquist, 1982; Beverley and Wilson, 1985). VULNERABILITY, EXTINCTION, CONSERVATION The biodiversity crisis is nowhere more apparent and in need of urgent attention than on islands. Approximately 90% of all bird extinctions during historic times have occurred on islands (Vitousek, 1988). More species of Polynesian landbirds appear to have gone extinct due to human agencies than survive today, and the survivors have greatly reduced ranges (Olson, 1989; Steadman, 1989). Among the historically known, indigenous plants of Hawaii, 10% are extinct and almost 40% threatened (Wagner, 1991). Comparable quantitative data on the extinction rates of terrestrial arthropods are not available, but indications such as the virtual absence of native insects below 600 m altitudes and the extinction of land crabs in Hawaii indicate similar tragedies (Zimmerman, 1948; Howarth, 1990). Land snails are perhaps the most vulnerable members of insular biotas, and the four relict families discussed above are at the greatest risk. Most of the 331 described spe- cies of Amastridae, a family endemic to the Hawaiian group, are extinct, with only a handful of arboreal species of this primarily ground-dwelling family surviving (M. G. Hadfield, personal communication, 1992). The spectacular radiations of endodontoid land snails are known to be largely or completely extinct in the Hawaiian Islands (195+ species), the Gambiers (25 species), Rarotonga (11 species), and St. Helena (12 species), these being the islands which have been recently searched for the snails (Solem, 1976, 1977, 1982). Achatinellid and partulid land snails in the Pacific are losing ground at a similarly alarming rate (Hadfield et al, 1993; Murray et al, 1988; Hopper and Smith, 1992). Direct habitat destruction The impact of humans on island habitats is usually devastating, with the most important causes of human-induced extinction on islands being habitat destruction and species introductions. All Polynesian islands were entirely or largely forested before man's arrival; fossil pollen and mollusks provide evidence for this even on islands or areas where no indigenous forests remain (e.g., Zimmerman, 1948;Paulay, 1985; Bahn and Flenley, 1992; Kirch et al, 1992). Humans have been 140 GUSTAV PAULAY responsible for the partial or entire loss of native forests from virtually all of these islands (Kirch, 1984; Kirch et al., 1992). The conversion of land for agriculture, construction, etc., and perturbations accompanying this process (pollution, erosion, etc.), directly destroy forests, while species introductions are further, indirect causes of habitat destruction (see below). An especially striking example of the environmental havoc created by complete deforestation is Easter Island, where destruction of the forests not only caused untold extinctions, but was apparently responsible for the collapse of the island's megalithic culture (Bahn and Flenley, 1992). The vulnerability of habitats is largely determined by their accessibility and suitability to humans, and by their resilience to disturbance. Dry, gentle sloping, lowland areas on volcanic terrain are among the most vulnerable, while steep, wet, mountainous, karstic areas are the least susceptible to human disturbance. In the Hawaiian Islands, humans had extensively altered 80% of all native habitats below 450 m in elevation by 1600 A.D. (Simon et al, 1984). Dry forests, which may host an even more diverse and endemic biota than rain forests on islands, are especially vulnerable, with but fractions of their former cover remaining on all but the most pristine islands (Olson, 1989). Olson's (1989) discovery, based on subfossil remains, that bird faunas of dry forests were much more diverse before human occupation, led him to question whether we are similarly underestimating the diversity of mesic and arid habitats on continents, an idea which has gained recent support in the Americas (Mares, 1992). As a result of millions of years of erosion and subsidence, the oldest oceanic hotspot islands tend to have low relief, elevation and (therefore) precipitation, traits which make them especially vulnerable to human disturbance. Thus these islands, which likely held the most interesting products of evolution, are also among the most disturbed (e.g., Eiao, Chuuk, Tubuai, Gambier). Among Pacific hotspot islands > 5 My old, only Pohnpei (Carolines), Rapa (Australs), Kauai (Hawaii) and the small rock-like, leeward Hawaiian high islands retain a reasonable cover of native vegetation; these islands, because of their spectacular biotas, should be among the highest conservation priorities. Introduced species Introduced species threaten island biodiversity both indirectly through habitat alteration and directly through interactions with native species. Their effects can be even greater than those of anthropogenic habitat destruction; in this islands differ from continental environments, where exotics appear to be much less often the cause of extinction (Vitousek, 1988). Introduced species, especially certain herbivorous mammals and plants, are among the worst culprits in causing large-scale habitat destruction and alteration. Insular floras have few defenses against herbivorous mammals which, unchecked by predators, can destroy most of the vegetation and lead to massive erosion. Paradoxically, uninhabited islands can be among the most affected by introduced feral mammals, due to lack of human predation. The large-scale devegetation by mammalian herbivores of uninhabited Laysan (Hawaiian Is.) and Eiao (Marquesas), and of St. Helena prior to human settlement, led to the loss of much oftheir endemic biotas (Ely and Clapp, 1973; Montgomery et al., 1979; Cronk, 1989). Another serious threat to native vegetation is certain introduced plants, such as Miconia in Tahiti, Psidium in Tubuai, Leucaena in the Marquesas, which have the ability to crowd out native vegetation with their monospecific stands. In addition to destroying habitat, introduced species can wreak havoc in direct interactions (competition, predation) with native species. Low diversity biotas such as those on islands, whose species tend to have less highly evolved defensive, competitive and reproductive capabilities, are especially vulnerable to biological invasion (Vermeij, 1991). As they evolved, insular organisms were exposed to but a fraction of the diversity of predators sensu lato and competitors experienced by mainland species; thus the modern arrival of certain exotics, against which the indigenous biota lack defenses, can be devastating. As closely related species can all be at risk from a single exotic, considerable portions of the biota can 141 BIODIVERSITY ON ISLANDS become rapidly endangered on islands where large radiations occurred (Simon et ai, 1984). The accidental introduction of the snake Bioga irregularis to Guam has led to the extermination of practically the entire native bird fauna (Savidge, 1987); avian malaria and avian pox have decimated Hawaiian birds (Riper, 1991). Among insects, social Hymenoptera are an especially serious threat, and certain introduced ant species have been implicated in the extinction of large portions of the invertebrate faunas of several Pacific island groups, especially on some remote islands (see above) which lacked indigenous ants {e.g., Cole et ai, 1992; Lubin, 1984; Howarth, 1990). Humans themselves are a predatory threat. Recent work on Holocene extinctions clearly implicates Homo sapiens in the demise of insular birds. The arrival of humans to Polynesia led to the extinction of generally more than half, and in some cases up to 80%, of each island's avifauna, with large, predatory and flightless birds being most vulnerable (Olson, 1989; Steadman, 1989). Similarities between insular bird extinctions and the late Quaternary demise of continental vertebrate megafaunas lends strong support to the interpretation that the latter were also human-caused {e.g., Olson, 1989; Martin, 1990). Although it may be impossible to arrest the arrival of all introduced species to islands with large human populations without causing extreme restrictions to travel and shipping, it is critical that we stem this flow as much as possible. The scale of this ongoing biotic interchange is astounding. Almost 3,000 species of terrestrial arthropods are known to have been introduced to Hawaii, the vast majority since European contact (Nishida, 1992). Control can be more easily implemented for islands with few or no human inhabitants, by restricting and carefully screening access. Control of especially harmful exotics should be a priority, and is increasingly being pursued (Gillis, 1992). One aspect of this biotic interchange that can be controlled is the purposeful introduction of scores of species for biological control. Many of these are fairly generalist predators and parasites that can lead to the extinction of numerous endemics (Howarth, 1991). The purposeful and thoughtless introduction of the "cannibal snail," Euglandina rosea, has led to the extinction of large suites of endemic land snails on one island after another, providing some of the most detailed case-studies of insular extinction (Hadfield et al, 1993; Murray et ah, 1988). Apparent relict groups, such as many land snails, are among the most sensitive to introduced predators, and the malacofauna of Pacific islands is undergoing a veritable mass extinction, with only an estimated 2535% of Hawaii's 1,461 described land snails remaining extant (Solem, 1990, see also above). CONCLUSIONS The isolation and small size of oceanic islands yield differences in the dynamics of diversification and extinction when compared to continents. These attributes, however, also allow a more intimate understanding of insular biotas: diversity on islands is limited and the geographic and ecologic ranges of species can be precisely determined, providing excellent grounds for studies on the distribution and origin of biological diversity. The extreme vulnerability of insular biotas not only makes their study and conservation ever more urgent, but the mass extinctions occurring on many islands facilitate analysis of numerous well defined examples of human-caused extinctions, studies which can lead to better conservation policies. Low primary (founding) diversity promotes in situ diversification and the evolution of unusual adaptations. The most isolated islands, those with the lowest number of original colonists, often have the largest adaptive radiations and most peculiar species. Whether intra-island and inter-island speciation is more important depends on the dispersion ability of the taxon and opportunities for isolation. Evidence from islands shows the importance of topographic and environmental variability in limiting species ranges and promoting in situ diversification. Diversification can be very rapid and result in large speciesflockswithin the normally 3-15 My lifespan of oceanic hotspot islands. The small size and isolation of islands result in relatively small and localized pop- 142 GUSTAV PAULAY ulations, and low primary diversity. Small population size, especially prevalent among species with large body size or those at high trophic levels, makes many island species especially vulnerable to extinction. The often highly restricted ranges of flightless invertebrates exacerbate this problem, because entire species can be vulnerable to even limited habitat destruction. Data from islands also imply that mesic and xeric environments may have been equally or more diverse, and certainly much more vulnerable, than moist habitats. The relatively low levels of escalation resulting from limited diversity of founders allow for the survival of relicts and the development of species with limited repertoires of defenses; thus island ecosystems are especially vulnerable to invasion and extinctions caused by introduced species. Introduced species are much more devastating on islands than continents, and are presently the greatest threat on many islands. The most devastating exotics include plants that can literally push out native forests with their monospecific stands, social insects (especially certain ants) and herbivorous mammals. Studies of vertebrate extinctions on islands strongly implicate humans as the cause of late Quaternary extinctions worldwide. Purposeful introductions of generalist predators for biological control have also led to large scale extinction. The 50-95 + % levels of extinctions exhibited by taxa with a fossil record, like birds and land snails, on oceanic islands, are comparable to the most severe mass extinctions in earth history (Jablonski, 1991). Arresting habitat destruction, reducing chances of future introductions, and controlling harmful exotic species are urgently needed to prevent the annihilation of these most spectacular showcases of evolution. ACKNOWLEDGMENTS I thank Bern Holthuis, Frank Howarth, Alex Kerr, Alan Kohn, Scott Miller, and Barry Smith for comments on the manuscript and Trish Morse for organizing the symposium. This is contribution 331 from the University of Guam Marine Laboratory. REFERENCES Bahn, P. and J. Flenley. 1992. Easter Island, Earth Island. Thames and Hudson, London. Beverley, S. M. and C. A. Wilson. 1985. Ancient origin for Hawaiian Drosophilinae inferred from protein comparisons. Proc. Nat. Acad. Sci. U.S.A. 82:4753-4757. Carlquist, S. 1974. Island biology. 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