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Home , Endemism, Mainland

OCEANIC : MODELS OF DIVERSITY

Rosemary G. Gillespie University of , Berkeley

I. Introduction invasions of alien benefiting from the II. Characteristics of previous introduction of the other. III. Dynamics microcosm A small, representative system that has IV. Species Loss analogies to a larger system. Oceanic islands are V. The Future of Biodiversity on Oceanic Islands often considered microcosms of continental areas because the composition of species on islands, and the processes of formation and decline, are analo- gous but occur on a very small scale. GLOSSARY parapatric Occurring in geographical areas that abut but do not overlap. adaptive radiation Evolution of ecological and phe- phylogenetics The study of evolutionary relatedness notypic diversity within a rapidly multiplying among various groups of organisms (e.g., species lineage. and populations). Phylogenetics, also known as allopatric Occurring in separate, nonoverlapping phylogenetic systematics, treats a species as a group geographic areas. of lineage-connected individuals over time. dioecious In plants, having the male and female relictualization Survival of species on islands after reproductive organs borne on separate individuals those on adjacent mainland areas have suffered of the same species. . disharmony The nonrandom representation of biota sympatric Occurring in the same geographic areas among natural colonists of oceanic islands as com- without interbreeding. pared with mainland systems. Species that are more taxon cycle A hypothesis proposed by E. O. Wilson dispersive will have a higher representation in the to describe the common niche expansion among biota, the effect being accentuated by the tendency colonizers, and subsequent long term toward dif- of these few successful colonists to diversify into ferentiation, with isolated species narrowing their multiple species through adaptive radiation. niches and restricting their ranges resulting in ecomorph A local population or group associated eventual extinction. with a particular ; the terms has no systematic or taxonomic significance. endemic/endemism Unique to a given locale; occurs in the site at which it is endemic, and nowhere else. impoverishment On an island, paucity of species OCEANIC ISLANDS ARE UNUSUAL IN THEIR ISO- relative to adjacent mainland. LATION, which has allowed the formation of uniquely invasional meltdown The compounded effect on an evolved biotas that exhibit characteristics ecological community of successful and successive such as disharmony, high endemism, and sometimes

Encyclopedia of Biodiversity Copyright & 2007 Elsevier Inc. All rights of reproduction in any form reserved. 1 2 ______OCEANIC ISLANDS: MODELS OF DIVERSITY ______relictualism. Characteristics of species may include least a sample of what occurred on the be- reduced dispersal capabilities, evolutionary innovat- fore the fragment became isolated), and the number of ions, and size and reproductive changes. Ecological species, at least initially, will tend to decline as a result and evolutionary changes play a key role in successful of stochastic events. In contrast, oceanic islands are island colonization, and often result in adaptive radi- formed without life, so the number of species can only ation. While such circumstances may have facilitated increase, and will do so at a rate that will depend on rapid speciation, they also result in high rates of the interplay of isolation and time (Gillespie and extinction, greatly accelerated since the arrival of Roderick, 2002). If the islands are not extremely iso- humans. Human-mediated impacts on islands cannot lated, equilibrium will be reached between immigra- be held back; however, the islands present a relatively tion of colonists and extinction. With greater isolation simple system in which it is possible to understand the (and lower immigration rates), and given sufficient dynamics of species formation and loss. topographical diversity, local endemism will increase over time. Extreme isolation, with the frequency of colonization being insufficient to allow the occupation I. INTRODUCTION of available ecological niche space through migration, can provide the opportunity for adaptive radiation, There is a huge diversity of types of islands, but the and has been called as the ‘‘radiation zone’’ (Whittaker, common denominator is that they are isolated, well- 1998). Here, arises through the for- defined geographically, and have distinct boundaries. mation of multiple new species adapted to exploit the These characteristics can result in properties such as of available ecological space. a microcosmal nature and a uniquely evolved biota. Over the last few years, geological understanding of Indeed, the flora and fauna of oceanic islands inspired islands has improved immensely. For many oceanic scientific interest from the moment explorers first archipelagoes, the age of each island is often known began sailing the oceans. The importance of evolutio- with some level of precision. This, coupled with the nary processes on islands was first recognized in fact that diversification on these islands has a clearly ’s work on the Galapagos, which gave recognizable beginning (colonization from outside), rise to the theory of evolution by means of natural has given biologists a temporal framework within selection. Likewise, Alfred Russell Wallace contempo- which to examine ecological and evolutionary proc- raneously developed similar theories based on studies esses, and the formation of communities over time. in the Spice Islands of . Over the last century, research has focused on multiple facets of islands— from the mechanism of formation of the isolated hab- II. CHARACTERISTICS OF BIODIVERSITY itat and how its geological history has been shaped, to the colonization of species and the development of A. Community Characteristics uniquely evolved biotas in similarly unique commu- nities and (Keast and Miller, 1996). Island 1. Disharmony ecosystems tend to be simple when compared with On oceanic islands, the biota is often considered to be continental ecosystems; thus, the way ecosystems ‘‘disharmonic.’’ This phenomenon arises partly because function can be more easily studied. In essence, is- taxa differ considerably in their ability to colonize re- lands can be considered as Nature’s test tubes. Each mote islands, leading to attenuation in the number of island represents a trial in an experiment and each new groups represented on more isolated islands. More- island is the repeat of one of these experiments. over, as a consequence of the isolation, islands gene- Islands can be broadly classified as one of the two rally show a degree of impoverishment when types, continental or oceanic. Continental islands are compared with continental communities. Adaptive ra- fragments of a larger , often created by a rise diation accentuates this effect, with multiple species in sea level or when land breaks off and drifts from the arising from the few that successfully colonize, the mainland. Oceanic islands (e.g., Fig. 1) can result from result being very few families and genera compared volcanoes rising above the water on or near a mid- with overall diversity. This pattern has been noted in ocean ridge, or where lithospheric plates converge the , (Juan et al., 2000) but the effect is (Nunn, 1994). The key biological difference between a particularly well illustrated as one crosses the Pacific continental and an oceanic island is that the former, Ocean from west to east in the direction of increasing when created, has a full complement of species (or at isolation of an oceanic island (Fig. 2; it should be ______OCEANIC ISLANDS: MODELS OF DIVERSITY ______3

FIGURE 1 Comparison of oceanic islands. (a) View from the top of Mt. Rotui, Moorea (Society Island archipelago, French Polynesia) showing topographical diversity characteristic of younger volcanic islands; higher elevation forested is shown in the foreground. (b) Atoll in Micronesia. Such oceanic islands, which show little topographic diversity, harbor relatively few terrestrial species, with little endemism. Photos by G. K. Roderick and R. G. Gillespie. noted that this figure excludes continental islands such leading to extensive adaptive radiation as a result of in as and , in which ancient situ evolution with associated adaptation to occupy the connections to continental also appear to available ecological space. On remote islands, the fre- have allowed high levels of endemism). quency of colonization becomes vanishingly rare. As a result, the biodiversity of remote islands has largely 2. Endemism arisen through evolution and adaptation of the few The pattern of species accumulation on oceanic is- initial colonists, and levels of endemism are conse- lands depends on the rate of evolution relative to the quently very high (see Table I for a comparison of frequency of island colonization, the extreme cases endemism among different groups of species in the 4 ______OCEANIC ISLANDS: MODELS OF DIVERSITY ______

Hawaii 3500 100

Indigenous spp. 90 3000 Area Papua % endemism New GuineaSolomon ls. 80 W. Sam.Am. Wallis Is. Samoa Marquesas Vanuatu 2500 Cook Is Tonga Societies 70 Fiji French Polynesia 60 2000 % endemism

50

1500 40 Area/ no. indigenous species 30 1000

20

500 10

0 0 Solomons Vanuata Wallis & Tonga Ind. Samoa Am. Samoa Cook Is. Societies Marquesas Hawaii Fatuna

FIGURE 2 Plant transect across selected Pacific islands: indigenous species, area, and endemism. Larger islands have dis- proportionately larger numbers of species, as documented in MacArthur and Wilson (1967)’s ‘‘Equilibrium Theory of Island .’’ The initial idea of equilibrium numbers of species on islands was based on a balance between immigration (decreases with distance from a source of colonists) and extinction (decreases with island size). However, the high levels of endemism on remote islands also reflects the role of evolution within these islands, which may compensate for low levels of immigration with numerous species arising through adaptive radiation.

TABLE I because may not be as severe, the climate Approximate levels of endemism (%) for different groups of native is less extreme; in this case the species on the islands organisms on three of the better-known oceanic archipelagoes can be considered ‘‘relicts.’’ Clearly, this effect will be (all Flowering Terrestrial much more pronounced on older islands or archi- species) plants invertebrates pelagoes.

Canary Islands 45 40 58 Galapagos Islands 48 36 50 B. Species Characteristics Hawaiian Islands 81 91 98 1. Reduced Dispersal Capabilities Canary, Galapagos, and Hawaiian islands). The effect Loss of dispersal ability is a common feature of taxa on of isolation on endemicity is shown in Fig. 2, which oceanic islands (Williamson, 1981). The phenomenon illustrates how endemism increases with isolation in was first documented by Darwin, who suggested that the oceanic islands of the Pacific. ‘‘powers of flight would be injurious to insects inhab- iting a confined locality, and expose them to be blown to the sea.’’ For example, island plants are renowned 3. Relictualism for their large seed size and associated loss of dispersal The mainland taxa that give rise to the island colonists ability. Among endemic/indigenous insects, 90% of exist in an environment that is often very different those on Tristan de Cunha and 40% on Campbell from that of species on islands. Accordingly, species Island show some degree of wing reduction. Among that colonize islands may suffer extinction, though this birds, flightlessness has evolved repeatedly on islands cannot generally be distinguished from the effects of where predators are absent. Such reduction in dispersal disharmony. Likewise, island progenitors may become ability may accelerate species diversification. However, extinct on the mainland yet remain extant on islands, many of the flightless birds on islands have been driven ______OCEANIC ISLANDS: MODELS OF DIVERSITY ______5 to extinction as human occupation spread, for exam- low rates of reproduction, each of which may arise as a ple, in Hawaii the flightless rails and geese and the result of selection for loss of dispersal ability, together large flightless waterfowl the moa nalos, in with high selfing rates, after lineages have become es- the , and in the elephant . tablished on remote islands. The Hawaiian Islands, for example, are particularly rich in dioecious species with 2. Innovations about 15% dioecy in native species of flowering plants (compared with 6% in mainland species). The known disharmony of islands appears to stimulate the development of some unusual traits in island groups. For example, Hawaiian birds have developed a III. ISLAND DYNAMICS remarkable diversity of elongate and decurved bill shapes, presumably because of the availability of nec- A. Colonization tar resources. The Hawaiian silversword plant alliance includes trees, shrubs, subshrubs, rosette plants, cush- Given a suitable abiotic environment, the success of ion plants, and a vine, that occur in some of the wet- any given colonization event depends largely on the test sites recorded on Earth to extreme desert-like availability of ecological space, although chance clearly environments. Among insects, the absence of native plays a role. The importance of available ecological social insects in some of the more remote islands of the space is clearly illustrated on isolated archipelagoes Pacific may have been largely responsible for the ad- that are formed serially, some in a linear fashion as they aptive shift to in some terrestrial insect emanate from an oceanic hotspot. There is generally a groups and the diversity of other predatory arthropods heavy bias in the direction of more recent colonization on these islands. One of the most striking innovations toward younger islands (Wagner and Funk, 1995), is the development of predatory behavior in a lineage which suggests that, once ecological space is ‘‘filled’’ on of Hawaiian caterpillar moths in the genus Eupithecia these islands, there is little further colonization. (Howarth and Mull, 1992). Another innovation in Hawaii has been the development of odonate damsel- B. Species Change Subsequent to flies with terrestrial larvae, associated with the paucity of lakes in the upland yet moist forests of the islands. Colonization Similarly, the unusual habitat of cryptogram herbivory What happens to a population subsequent to success- in beetles in the islands is considered to ful colonization? Initially, the population may change be a result of the disharmony of the flora. ecologically, but over time evolutionary changes occur.

3. Size Changes 1. Ecological Change Species on islands show a tendency toward size ex- Subsequent to the initial establishment of species in tremes, both gigantism and dwarfism. Overall, among new , a common outcome is ecological (or mammal species that colonize islands, large species competitive) release, with colonizers expanding their tend to get smaller while small ones tend to get larger. range to fill the available ecological space. Regular cy- Included among the islands’ giants are the giant cles of distributional change following colonization of flightless birds of New Zealand (moa) and Hawaii, islands have been proposed several times in the liter- giant sheet-web spiders of the remote Pacific islands, ature, starting with the idea suggested by E. O. Wilson and hissing cockroaches of Madagascar. Dwarf species for a ‘‘taxon cycle’’ in Melanesian ants: widespread, di- include elephants, , rabbits, and snakes as well as spersive populations give rise to many more restricted the pygmy mammoth, which once inhabited Califor- and specialized populations or species (Wilson, 1961). nia’s Channel Islands. It appears that size changes are Although the generality of the taxon cycle has been associated with the very different conditions experi- questioned, the tendency for species to expand their enced by island species, often with fewer constraints range to fill the available ecological space upon initial on size extremes. colonization of an area is quite predictable.

4. Reproductive Changes 2. Evolutionary Change A common pattern seen in plants that colonize islands As described above, the remoteness of many oceanic is the development of dioecy, asexual reproduction, and islands allows evolution to play a prominent role in 6 ______OCEANIC ISLANDS: MODELS OF DIVERSITY ______the addition of species to such islands, and adaptive number of native species (B800 in two genera, Dro- radiation of species from single colonizers is a frequent sophila and Scaptomyza) (Carson and Kaneshiro, 1976). characteristic of the more remote archipelagoes. Mech- The initial processes of divergence leading to species anisms underlying adaptive radiation on oceanic formation on oceanic islands is still not entirely under- islands have been the subject of considerable interest stood, particularly with respect to the relative impor- ever since Darwin described the radiation of finches in tance of allopatric, parapatric, and sympatric speciation. the Galapagos (Grant, 1986). However, the extreme Adaptive shifts clearly underlie species diversification in diversity of form and modifications associated with many radiations of species in isolated islands, although island colonization, coupled with the superficially separation of gene pools is generally considered to be a similar appearances of many species on different requirement in initiating the process and has been dem- islands within an archipelago, made it very difficult onstrated in the initial diversification of finches in the to understand how, and from where and what the Galapagos, beetles in the Canary Islands, and spiders in diversity had arisen. The advent of molecular genetic the Hawaiian Islands, among other groups. In each case, methods has led to a huge growth in research on oce- subsequent sympatry appears to have led to ecological anic islands and has allowed the evolutionary history differentiation. In the Hawaiian Islands, differentiation of organisms to be assessed (Givnish and Sytsma, among marginal isolates or founder populations appears 1997). Given the microcosmal nature of islands, with a to have been involved in species formation within ra- finite pool of colonizers, and the diversification of diations of Drosophila and carabid beetles. Parapatric these into multiple closely related species confined to divergence through adaptive shifts may have played a small areas, knowledge of the evolutionary history of role in the initial divergence of cave-adapted species and the biotas has allowed some profound insights into the has been implicated in the formation of cave endemics mechanics of population divergence, and the interplay from surface relatives in the different Hawaiian Islands between isolation, sexual selection, and ecological (lycosid spiders and isopods) and the Canary Islands shifts in allowing species formation. (dysderid spiders) (Roderick and Gillespie, 1998). The Adaptive radiation, by definition, involves ecolo- idea that species on islands may be able to diverge in gical diversification (Schluter, 2000). On oceanic sympatry held considerable popularity for many years, islands, ecological isolation is clearly demonstrated by but increasing evidence from phylogenetic studies has numerous sympatric members of an adaptive radiation. provided little support to the idea. Cases where multiple Within a radiation of long-jawed spiny-legged spiders sister species co-occur within a single island are most in the Hawaiian Islands, 5–8 species co-occur at a given commonly found in arthropods, and may be attributed site, each species ecologically distinct from the others to parapatric or ‘‘micro-allopatric’’ speciation, although with which it co-occurs. Likewise, among the Hawaiian some authors have suggested that sympatric speciation honeycreepers, seven ecologically distinct species co- may have played a role in the diversification of palms on occur at a single location in high-elevation forests on (Savolainen et al., 2006) and lineage the island of Hawaii. The ecological differences among of tiny weevils (genus Miocalles) on the single small species are often profound, particularly on more island of Rapa in southern French Polynesia. Together, remote islands where a given lineage can sometimes these studies show that whatever the mechanism allo- monopolize the ecological arena that would be used by wing initial divergence of populations, ecological multiple genera, families, or even orders, in continental segregation clearly plays the primary role in allowing areas. coexistence of close relatives of an adaptive radiation. Adaptive radiation is frequently associated with an acceleration in rate of diversification. However, the rate of diversification within an adaptive radiation may be C. Species Diversification further enhanced by sexual selection. Recent research has shown that crickets in the Hawaiian Islands have Species and the communities in which they live are the highest documented rate of speciation known, dynamic entities, their existence, distribution, and abun- which has been attributed to the role of courtship or dance, dictated by evolutionary and ecological responses sexual behavior in accelerating species divergence to both natural and anthropogenic environmental (Mendelson and Shaw, 2005). Likewise, sexual selec- change. For hotspot archipelagoes—those in which is- tion is thought to have played a prominent role in the lands emanate from a single volcanic hotspot from diversification of Hawaiian drosophilid fruit flies which they progressively move away—afford a unique (Kaneshiro and Boake, 1987), leading to the large opportunity: they make it possible to visualize snapshots ______OCEANIC ISLANDS: MODELS OF DIVERSITY ______7 of evolutionary history. For example, the geological his- arrangement is found in the archipelagoes of both the tory of the Hawaiian archipelago is well understood, Marquesas and the Societies in French Polynesia. Be- with individual islands arranged linearly by age (Fig. 3), cause of this geological history, coupled with their ex- allowing early stages of diversification and community treme isolation, the Pacific hotspot archipelagoes afford formation to be studied on the island of Hawaii and the opportunity for phylogenetic study of community compared with progressively later stages on the older formation within a temporal context. Other hotspot islands of Maui, Lanai, Molokai, Oahu, and Kauai archipelagoes, such as the Galapagos and the Canary (Roderick and Gillespie, 1998). A similar chronological Islands, show temporal variation although islands in

Kaua'i 5.1 My O'ahu 2.6 My 3.7 My Moloka'i Maui 1.9 My 1.3 My Lana'i 1.3 My 0.8 My Hawai'i 0.43 My 100 120 140 160 180 160 0.3860 My 0.40 My 0.20 My 40 40 0.10 My

United States

Hawaii 20 20

Nuku Hiva 2.9 My Galapagos 3.7 My Ua Huka 0 0

2.0 My Marquesas Ua Pou Hiva Oa

2.0 My Fiji Societies 20 Tahuata 20

Australia Fatu Hiva 1.4 My South Bora Bora America 3.3 My New Zealand 40 Tahaa 40 3.0 My 2.2 My

100 120 140 160 Huahine 160 140 120 100 80 60 Raiatea 2.3 My

Moorea Tahiti

1.9 My

1.0 My

FIGURE 3 Major hotspot archipelagoes in the Pacific. The archipelagoes of the Hawaiian Islands, Marquesas, and Society Islands are shown on the map of the Pacific, with approximate geological ages of each island indicated. Within each archipelago, the oldest island is to the northwest, the youngest to the southeast. 8 ______OCEANIC ISLANDS: MODELS OF DIVERSITY ______these groups are arranged as a cluster rather than a lin- E. Processes of Community Formation ear series, yet on these islands also the evolutionary progression of species tends to be from older to younger Over the last few years, studies have started to incor- islands, and it may be possible to use the history of these porate phylogenetic history into the investigation of islands in the same way as that in hotspot islands, to community assembly (Losos, 1992). While most examine stages in adaptive radiation and community of these studies argue for the nonrandom nature of formation. community composition, they have demonstrated opposing forces in shaping communities. For exam- D. Extinction ple, habitat filtering in which only taxa with certain traits colonize a community, can lead to phylogenetic Species on islands tend to have a limited geographic clustering within that community. However, compet- distribution and the naturally small population sizes itive exclusion, in which taxa that exploit similar means that most of these populations will not suffer resources will compete and exclude each other, can additional inbreeding depression. However, they are lead to phylogenetic overdispersion (Fig. 4). Oceanic vulnerable to extinction as a result of demographic islands have played a key role in the integration of stochasticity. Accordingly, natural extinction rates on phylogenetics and evolutionary biology into studies of islands are expected to be high. In the tropical islands community composition. Using the fact that a given of the Pacific, extinction has been much higher than on island system often includes multiple co-occurring the islands of the north Atlantic, where species ranges representatives of the same lineage it is possible to infer are much broader. The naturally small population sizes how characters have changed to allow multiple species and ranges make island species more vulnerable to ex- to co-occur in a community. In general, such studies tinction as a result of natural competition or habitat have highlighted the role of convergent evolution, and degradation due to geological and climatic changes, suggest that community formation is somewhat deter- and such effects have played a pivotal role in shaping ministic. Accordingly, in a radiation of Hawaiian island biotas. At the same time, these same attributes of Tetragnatha spiders, ecologically and morphologically island species makes them more vulnerable to human- similar sets of species (ecomorphs) occur in most mediated impacts (see below). habitats on all islands: one ‘‘green spiny,’’ one ‘‘large

FIGURE 4 Hypothetical scenario of phylogenetic relationships among spiders showing how different ecological forces give rise to opposite expectations about the similarity of species traits within communities. Phylogenetic clustering may occur if closely related species share similar physiological limitations; similar adaptations allow species with similar traits to co-occur in the same habitat. Phylogenetic overdispersion may occur as a result of competitive exclusion among related species preventing coexistence of close relatives that share a resource, leading to ecological divergence among close relatives. ______OCEANIC ISLANDS: MODELS OF DIVERSITY ______9 brown spiny,’’ one ‘‘maroon spiny,’’ and one ‘‘small Community dynamics, and the ability of invasive brown spiny.’’ These sets of species appear to have species to penetrate certain native communities, and of arrived in a habitat, either by direct colonization from certain native communities to withstand invasion, has an older island, or by evolution of one ecomorph from been the subject of numerous articles (Stone and Scott, another on the same island, suggesting convergence 1985). However, extensive research over the past few of the same set of ecological forms on each island decades has shown that multiple parameters may act (Gillespie, 2004). (or interact) to dictate invasion success, the most common being: (i) species diversity itself has been IV. SPECIES LOSS found, under some conditions, to serve as an ecological barrier to invasion; this effect has been used to explain While the isolation of oceanic islands has made them the higher frequency and impact of invasions on islands living laboratories for understanding species adapta- which are considered to be relatively species poor; tion and evolution, it has also made them extremely (ii) and the opening of ecological space can vulnerable to and other stresses. The facilitate invasion; (iii) propagule pressure, in particular vulnerability of islands to extinction is now widely the relative abundance of native versus nonnative recognized. Islands have suffered much higher rates of propagules, can permit infiltration of alien species into extinction than (Steadman, 1995). Of ap- native habitats; (iv) characteristics of propagules, such proximately 100 known land bird species in the Ha- as more generalist habitat requirements, can affect waiian Islands, almost 75% of the species present at invasion success; and (v) novelty of perturbations and first human contact have disappeared completely and ‘‘na¨ıvete´’’ of native biota can accentuate the impact of an many of those that survive are in danger of extinction invasive species. (James, 1995). The situation is similar for native It is widely cited that the worst kind of introduction plants, in which about 120 of 1000 flowering species to an isolated archipelago is that of a generalist pred- have fewer than 20 individuals left in the wild, and ator or competitor capable of exploiting a broad array Hawaii has lost more documented arthropod species of habitats and causing secondary impacts. This can than the entire continental United States. Indeed, the sometimes result in an effect known as ‘‘invasional Hawaiian Islands have been dubbed the ‘‘extinction meltdown.’’ For example, the crazy ant Anoplolepis capital of the world.’’ Given the information on species gracilipes has invaded , resulting in diversification that scientists have been able to deter- complete modification of the in just 2 years. mine from oceanic islands, can we also use the islands Supercolonies of the ants extirpate the dominant as tools to understand human-mediated extinction? native , a large land crab, which in turn Extinction is clearly more gradual in continents than leads to increased seedling but slows litter on many islands, but the trajectory is similar. Accord- . At the same time, the association of ingly, understanding of the processes involved in spe- the ants with plant-feeding insects increases the abun- cies extinction in the islands, and replacement of dance of Homoptera, with associated increased im- largely native environments by species from elsewhere, pacts on plants including honeydew productions, may allow the development of effective conservation which in turn enhances sooty moulds. strategies in mainland habitats. Devastating introductions by generalist predators/ competitors such as pigs are largely (though by no A. Alien Species Invasion means completely) historical, occurring before proto- cols were established for the introduction of nonnative One of the most severe impacts affecting many island species. However, abundant, small human commen- systems comes from nonnative species (Sax et al., sals, such as ants (Fig. 5), mosquitoes, and agricultural 2005). For insects, attributed to alien spe- and horticultural pests, are continually expanding cies invasion have been noted on Guam and the their range on to oceanic islands. Moreover, although Galapagos. However, perhaps because it is the most the introduction of highly specialized species for bio- remote archipelago, alien species in Hawaii appear to control purposes has been believed to be safe in that have taken a much greater toll on the native biota. The they are unlikely to go beyond the confines of their numbers now estimated for organisms purposely or specialized host or habitat, evidence is accumulating accidentally introduced into Hawaii and established in Hawaii to suggest that specialized species can ex- are 3046 arthropods, 20 reptiles, 46 land birds, 19 pand their range when faced with novel conditions, mammals, and 927 plants. and may do so quite dramatically. 1910−20 1978 1920−30 1998 1930−40 1999−2000 Native range 1970 2005

FIGURE5 Invasion of the little fire ant, Wasmannia auropunctata, in the Pacific. W. auropunctata is native to South and Central America. Over the last century, the species has invaded multiple locations worldwide, and in each location its invasion has been associated with substantial impacts on the native biota. (Note that lower green represents New Caledonia and higher green the Solomons; similarly 1999–2000 represents Fiji and Hawaii, and 2005, Tahiti.) ______OCEANIC ISLANDS: MODELS OF DIVERSITY ______11

B. Demographic Effects—Small Population units within which the biota can be quantified and compared, but the interplay of isolation and time has Sizes allowed the development of uniquely derived faunas The limited geographic range and small population on some islands. Understanding of the interplay be- sizes that characterize islands make them vulnerable to tween time and isolation in allowing these unique sets demographic stochasticity (see above). Species with of biota to develop is still limited. Even more limited is naturally small population sizes are similarly more our knowledge of the nebulous concept of ‘‘vacant vulnerable to habitat modification, simply because loss niche space,’’ which appears to play a critical role in of even a small amount of habitat for a geographically fostering colonization and diversification. There is restricted species could easily reduce numbers below much to be learned from the natural patterns of diver- sustainable levels. sification on islands. At the same time, the current rate of anthropogenically induced habitat degradation and homogenization of the world’s fauna threatens the C. Abiotic Effects—Climate Change biota of the world, particularly that of islands. So what are the priorities for addressing the ongoing extinction Islands can also be highly climate-sensitive. In the past crises on these islands? two decades, for example, short-term extreme high temperatures contributed to a decline of coral reefs throughout the tropics. Corals, stressed by high tem- A. Taxonomic Knowledge peratures, may eject their symbiotic algae resulting in coral bleaching, which renders the corals vulnerable to On many islands, the taxonomic impediment is huge. any further physiological stress and many of the colonies Accordingly, determining the status of a species, whether die. In 1994, elevated sea temperatures killed over 90% it is native or not, remains a major challenge for a of the living corals of American Samoa from the inter- conservation biologist on oceanic islands, particularly tidal zone to a depth of 10 m and fishing catches de- for those working on arthropods or other megadiverse clined drastically following the coral death. Likewise, groups. This problem is especially acute in tropical areas during the El Nin˜o of 1997–98, coral bleaching in Palau, where knowledge of both potentially native and the pool known for its spectacular coral reefs, was extensive. of nonnative species is frequently sparse and unreliable. Other concerns of climate change relate to the fact Research is needed to determine the identity and dis- that most of the island species have restricted distri- tribution of native and alien species on different islands, butions such that local vagaries of climate can elim- and subsequently to determine their evolutionary his- inate species so restricted as compared with more tory and assess their future trajectory. widespread taxa. In particular, the high-elevation cloud forests that characterize many oceanic islands have a narrow geographical and climatological niche B. Development of Realistic Evolutionary that may be eliminated with even a slight climatolo- Trajectories gical change. Climate change may also precipitate de- clines in forests due to floods, droughts, or increased In a world subject to accelerating global and local incidence of pests, pathogens, or fire. In addition, in- environmental change, policy makers and planners are creases in the frequency or intensity of hurricanes may struggling to maintain the status quo of diversity in cause disturbance that favors invasive species. natural systems. At the same time, there is growing Sea level rise is also a concern for oceanic islands, appreciation that biological diversity is inherently dy- resulting in coastal erosion and salt water intrusion into namic. Conservation strategies that do not incorporate freshwater lenses. However, it is the low-lying islands these evolutionary and ecological dynamics and that and atolls that are the most vulnerable from these effects. ignore the spatial flux in environmental quality will inevitably fail. Accordingly, an explicitly dynamic ap- proach is required to allow the assessment of the con- ditions under which areas of endemism are either V. THE FUTURE OF BIODIVERSITY ON sensitive or resilient to environmental change. Oceanic OCEANIC ISLANDS islands provide circumscribed areas of discrete, highly endemic, assemblages of organisms within which un- Oceanic islands have served as a source of intrigue for derstanding of evolutionary and ecological dynamics biologists for centuries. Not only are they discrete shaping the biota is an achievable goal. 12 ______OCEANIC ISLANDS: MODELS OF DIVERSITY ______

It is not clear what such an assessment will tell us. only simply to be conducted, but also to be analyzed For example, considering the Hawaiian Islands, it is and interpreted to allow extrapolation of the param- estimated that the existing angiosperm flora developed eters to larger and more complex continental settings. from 265 colonization events (not including introduc- Recognition of the acute conservation concerns im- tions or doubtfully native taxa), with the number of pacting islands, and the lessons that they offer, is species arising per colonist varying from 1 to 91. If becoming increasingly recognized. A recent Confer- colonization commenced on the oldest current high- ence of the Parties (COP8) to the Convention on elevation island of Kauai (which appears to be true for Biological Diversity (CBD) placed island biodiversity most groups), 53 successful colonizations per million high on the international conservation agenda. years would be required to give rise to the current biota. Immediate action is clearly required, not only to In 2005, there were 1,162,000 aircraft operations (take- document and understand the unique biota that offs and landings) in the Hawaiian Islands, carrying characterizes oceanic islands, but also to use the approximately 33,300,000 passengers, 500,000 t of lessons learned from these relatively simple systems cargo, and 130,000 t of mail. If only one out of every potentially to address more complex global issues of 60 passengers plus one out of 200 packages (assuming conservation. the average package weighs 2 lb) each carried a stray seed, and just half of these seeds became established, this would still represent an increase in the rate of col- onization to the islands of 1 billion %! Clearly the Ha- See Also the Following Articles waiian Islands are no longer very isolated, and, even ADAPTATION ADAPTIVE RADIATION BIODIVERSITY,   with the utmost care in preventing the introduction of DEFINITION OF BIODIVERSITY, EVOLUTION AND   BIODIVERSITY, ORIGIN OF BIOGEOGRAPHY, OVERVIEW new species, the ancient processes of evolution in iso-   DARWIN, CHARLES (DARWINISM) DISPERSAL lation can never be recovered.  BIOGEOGRAPHY DIVERSITY, COMMUNITY/ REGIONAL  This does not mean, however, that the native LEVEL ENDEMISM EXTINCTION, CAUSES OF HOTSPOTS     environment on oceanic islands has ‘‘lost the game,’’ INTRODUCED PLANTS, NEGATIVE EFFECTS OF ISLAND  nor that we should abandon the idea of long-term BIOGEOGRAPHY LOSS OF BIODIVERSITY, OVERVIEW   MARKET ECONOMY AND BIODIVERSITY SPECIES-AREA preservation of the biota. It simply tells us that we  RELATIONSHIP SPECIES DIVERSITY, OVERVIEW need to act quickly, and that oceanic islands can   SPECIATION, PROCESS OF provide much-needed insights into factors underlying community resistance to change, and also its ability to recover completely under intensive restoration. Bibliography Carson, H. L., and Kaneshiro, K. Y. (1976). Drosophila of Hawaii: C. Restoration Systematics and ecological genetics. Annu. Rev. Ecol. Syst. 7, 311– 345. Again because of their small size and unique biota, Gillespie, R. G. (2004). 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