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USDA Forest Service Proceedings RMRS-P-21. 2001 163

In: McArthur, E. Durant; Fairbanks, Daniel J., comps. 2001. Shrublandecosystem genetics and biodiversity: proceedings; 2000 June 13–15; Provo,UT. Proc. RMRS-P-21. Ogden, UT: U.S. Department of Agriculture, ForestService, Rocky Mountain Research Station.

Angela D. Yu is a Graduate Student, Department of Geography, Univer-sity of Colorado at Boulder, Boulder, CO 80309-0030. Simon A. Lei is aBiology and Ecology Professor at the Community College of SouthernNevada, 6375 West Charleston Boulevard, WDB, Las Vegas, NV 89146-1139.

Abstract—The topography, climatic pattern, location, and origin ofislands generate unique patterns of species distribution. The equi-librium theory of island biogeography creates a general frameworkin which the study of taxon distribution and broad island trendsmay be conducted. Critical components of the equilibrium theoryinclude the species-area relationship, island-mainland relation-ship, dispersal mechanisms, and species turnover. Because of thetheoretical similarities between islands and fragmented mainlandlandscapes, reserve conservation efforts have attempted to applythe theory of island biogeography to improve continental reservedesigns, and to provide insight into metapopulation dynamics andthe SLOSS debate. However, due to extensive negative anthropo-genic activities, overexploitation of resources, habitat destruction,as well as introduction of exotic species and associated foreigndiseases (biological invasions), island conservation has recentlybecome a pressing issue itself. The objective of this article is toanalyze previously published data, and to review theories fromnumerous research studies that attempt to explain species patternson islands. In effect, this analysis brings insight into current issuesof continental reserve design and island conservation efforts.

Introduction ____________________The equilibrium theory of island biogeography (ETIB),

proposed by MacArthur and Wilson, is a relatively recentdevelopment that has sparked a tremendous amount ofscientific controversy. Initially introduced to the public in1963 as “An Equilibrium Theory of Insular Zoogeography,”the idea was expanded in 1967 into a book publication. TheETIB implies that island fauna and flora (biota) eventuallyreach an equilibrium point between extinction and immigra-tion. Although species rarely reach equilibrium due to theextremely dynamic island system, MacArthur and Wilsonnote that the ETIB permits general predictions of futureisland biodiversity patterns. In this article, the theory ofisland biogeography is examined in reference to islandenvironments, including topographic origins and charac-teristics, as well as climatic patterns. A comprehensiveanalysis of the theory is discussed, such as species-area

Equilibrium Theory of IslandBiogeography: A Review

Angela D. YuSimon A. Lei

relationship, dispersal mechanisms and their response toisolation, and species turnover. Additionally, conservationof oceanic and continental (habitat) islands is examined inrelation to minimum viable populations and areas,metapopulation dynamics, and continental reserve design.Finally, adverse anthropogenic impacts on island ecosys-tems are investigated, including overexploitation of re-sources, habitat destruction, and introduction of exotic spe-cies and diseases (biological invasions). Throughout thisarticle, theories of many researchers are re-introduced andutilized in an analytical manner. The objective of this articleis to review previously published data, and to reveal if anyclassical and emergent theories may be brought into thestudy of island biogeography and its relevance to mainlandecosystem patterns.

Island Environments _____________

Island Formation and Topography

Island topography is primarily determined by the geo-physical origins of the island. Marine islands may be subdi-vided into two geophysically distinct categories: continentalshelf islands (land-bridge islands) and oceanic islands. Con-tinental shelf islands are likely to be physically connected tothe mainland during low sea level periods. Due to theirconnection, these islands have similar geological structureto the nearby mainland (Williamson 1981). This similartopography, coupled with the island’s close proximity to thecontinent, results in the proliferation of similar flora andfauna (biota).

Oceanic islands are typically more isolated, and may havenever been physically connected to a continental landmass.There are three main types of oceanic islands: oceanic ridgeislands, hot-spot islands, and the individual islands of islandarcs. Oceanic ridge islands and hot-spot islands are volcanicislands because they are formed from ocean-floor volcanoes.Islands that are part of island arcs also have a volcanicorigin, involving the collision of continental and oceanicplates, resulting in islands that consist of both basalt andgranite rock (Williamson 1981).

All of the geological processes occurring volcanic islandscan produce islands with high elevations, with peaks of atleast 2,000 m (fig. 1) (Williamson 1981). Volcanic islands aretypically steeper and become increasingly dissected withage. This phenomenon has important implications for islandbiota because a wide range of elevational gradients andassociated ecological attributes allows for the persistence ofdiverse habitats. The elevation of islands also has impor-tant influences on the climatic regime.

164 USDA Forest Service Proceedings RMRS-P-21. 2001

Yu and Lei Equilibrium Theory of Island Biogeography: A Review

Island Climate

Island climate is determined by both external influences,such as ocean circulation and atmospheric circulation, andinternal influences, such as island size, shape, and topogra-phy. Ocean circulation and atmospheric circulation consistof water currents and air currents, respectively, that havesimilar movements of upwelling and sinking. If an island isin the path of a moving current or is located where twocurrents intersect, this can alter the climate significantly. Inaddition to circulation influences, the proximity of an islandto a continental landmass also affects the island’s climate.Islands located close to a mainland, such as land-bridgeislands, are likely to be influenced by the continental cli-mate. Remote oceanic islands, on the contrary, are influ-enced by the maritime climate.

Internal influences, such as island size and elevation, canhave a substantial impact on the precipitation regime on theisland. Whittaker (1998) states that low islands typicallyhave relatively dry climates and high islands are wetterthrough orographic rainfall, resulting in the creation ofextensive arid regions due to the rain shadow effect. Thesehigher islands often contain diverse habitats within a rela-tively small area. Due to the impact of elevation on islandclimate, research studies have indicated that elevation is acritical variable in analyzing species diversity on islands.Telescoping, a compression of elevational zones, is fairlycommon on small tropical islands. Leuschner (1996) proposesthat forest lines on islands are generally 1,000 to 2,000 mlower than forest lines on continents. Hence, telescoping

creates smaller patches from a variety of habitats favorableto many species, and permits high- and low-elevation inhab-iting species to coexist in a relatively small area (Whittaker1998).

Island Patterns _________________The Equilibrium Theory of Island Biogeography (ETIB)

revolutionizes the way in which biogeographers and ecolo-gists viewed island ecosystems. Prior to the ETIB was thestatic theory of islands (Dexter 1978), which hypothesizesthat island community structures remain relatively con-stant over geological time. The only mechanism for biologi-cal change was the gradual evolutionary process of specia-tion. Few successful colonization events would occur due toa limited number of ecological niches on the island (Lack1976). Once these niches are completely filled, no space andresources are available for new immigrants, and they maynot become successfully established on the island. The ETIBrefutes the static theory, indicating that island communitiesexhibit a dynamic equilibrium between species colonizationand extinction, or species turnover. The immigration curveis descending and is shaped concavely because the mostsuccessful dispersing species would colonize initially, fol-lowed by a significant decrease in the overall rate of immi-gration (fig. 2). The extinction curve, on the contrary, is anascending curve because as more species inhabit the islandthrough time, more species would become extinct exponen-tially (fig. 2). Such a trend is amplified due to a combinationof population size and negative biotic interactions among

Figure 1—A map indicating all islands with a peak of 2000 m or higher. Larger islands areshaded, smaller islands are denoted by ∆ (Williamson 1981).


Equilibrium Theory of Island Biogeography: A Review Yu and Lei

different species. This model graphically and mathemati-cally represents an equilibrium point, S, in which the immi-gration and extinction curves intersect (fig. 2). This pointrepresents the actual number of species present on an islandat equilibrium. Moreover, figure 3 accounts for trends inisland size and degree of island isolation (distance frommainland). Patterns regarding the species-area relation-ship and the species-isolation relationship have been ac-knowledged by biogeographers since the beginning of the19th century. Therefore, the contribution made to islandecosystem study by MacArthur and Wilson was a compila-tion of these well-known ideas into one, integrated paradigm(Brown and Lomolino 1998).

Species-Area Relationship

A positive relationship exists between island size andisland species richness (fig. 4). These two variables, never-theless, do not always exhibit a linear correlation. Arrheniusrepresented this non-linear trend in mathematical form in1920 with the following equation:

S = cAZ

In this equation, S = the species richness on an island, c = aconstant depending on biotic and biogeographic region,A = the area of the island, and z = a constant representing theslope of S and A on a logarithmic scale (z changes very littleamong different taxa or same taxon from different regions.).

There are two potential explanations for the positivecorrelation between island size and species diversity. First,large islands generally contain more diverse habitats and,thus, more biota compared to small islands. Although this isa logical explanation, ecologists and biogeographers have adifficulty to prove in the field because habitat diversity hasno distinct parameters and is a challenge to measure. Manyparameters are approximated to determine the number ofspecies inhabiting various islands. Second, the island size,rather than the habitat diversity, is the main factor indetermining the number of species on islands. This theory isalso difficult to prove in the field because habitat diversityand island size are strongly correlated, and it is nearlyimpossible to distinguish the two.

Species-Isolation Relationship(Distance Effect)

Isolation is a critical component when examining evolu-tionary processes since it allows for allopatric speciation tooccur. Islands offer prime examples of isolation effects onbiota (Cox and Moore 1993). Specific adaptations, such asseed parachutes, are necessary for plants to disperse acrosslarge bodies of water. Such dispersal invokes survival of asweepstakes route, an extensive barrier permitting only a

Figure 2—The equilibrium model of species presenton a single island. The equilibrium number of speciesis reached at the point of intersection between the rateof immigration and the rate of extinction (MacArthurand Wilson 1967).

Figure 3—The equilibrium models of species of severaldifferent islands with a various degrees of isolation andsize. An increase in isolation (near to far) decreases theimmigration curve, while an increase in area (small tolarge) decreases the extinction curve (MacArthur andWilson 1967).

Figure 4—The correlation between species number, S,and island area, A, for reptiles and amphibians of theWest Indies. Both axes are logarithmic and a best-fit lineis used. The inset represents this relationship arithmeti-cally (Brown and Lomolino 1998).



stochastic set of immigrants to successfully colonize theisland (Brown and Lomolino 1998). The more remote theisland is from a continental landmass, the more severe thesweepstakes route and the smaller the number of speciesthat would successfully colonize and become established onthe island. This is known as the species-isolation relation-ship on islands. The number of species would decreaseexponentially as a function of isolation because the species-isolation correlation should account for a pool of species(fig. 5) (Brown and Lomolino 1998). Successful colonizationover large bodies of water is potentially easier for plantsthan for animals. Many species of plants possess dispersalmechanisms, such as “parachutes and “wings,” to allow newgenerations of plants to be carried far from the parent plantby wind currents. Without having a mate to reproduce,plants only require one fertile seed or spore to colonize aremote island. Other mechanisms, such as sticky seeds,permit seeds to be transplanted by animals. Through animalassistance, the chance for successful plant immigration isgreatly enhanced. In addition, some plants are able towithstand long periods of seawater immersion, and success-fully colonize remote islands. Such adaptive characteristicsare essential in creating plant communities on both nearbyand remote islands.

Species Turnover

Species turnover on islands is also critical to the ETIBmodel. The cycle of extinction of certain established speciesand the immigration of new species permit the equilibriumpoint to maintain its value over geological time. Suchspecies turnover, however, can be extremely difficult toquantify in the field. Whittaker (1998) detects two mainproblems with quantifying extinction and immigration rateson islands. Firstly, species turnover is a situation in whichsurveys on islands are at irregular intervals. This phenom-enon allows for some species to become extinct and possiblyre-migrate during periods between the surveys, resulting ina depression of turnover rates (Simberloff 1976). Secondly,there is potential for pseudoturnover on islands, implying

that information with regard to breeding conditions isfragmentary or incomplete. Under such circumstances, spe-cies appear to have turned over when, in fact, they havesimply been overlooked. With such potential inaccuracies,the ETIB requires an extensive attention to census detail,causing it to be a relatively difficult theory to prove undernatural island conditions.

ETIB Strength and Weaknesses

In addition to the limitations in detecting species turn-over, Brown and Lomolino (1998) note a number of majorweaknesses in the theory initially proposed by MacArthurand Wilson. First, many island species rarely achieve equi-librium. The number of species at the equilibrium point maychange over geological time, especially when immigrationand extinction coincide with periods of anomalous climaticand geological (disturbance) events. Second, not all specieshave identical immigration and extinction rates. For in-stance, birds generally have a higher immigration rate thanlarge, heavy reptiles and mammals. Third, the ETIB modelregards immigration and extinction as being independentvariables when, in many cases, they are closely related. Forexample, when there is a depression or extinction of apopulation on an island, a rescue effect may occur, thusincreasing species numbers once again. Fourth, the modeldoes not consider specific characteristics of species, such asspecies fitness and dispersal ability. Fifth, species inhabit-ing a single island may be derived from several mainlandand island sources. It may be challenging to determine thesource pool for a species on an island without a comprehen-sive study on the historical and current distribution of thespecies. Sixth, the ETIB model does not incorporate specia-tion, which may exceed immigration rates as a potentialmechanism affecting an island system. Species present byspeciation do not add to the overall species diversity of theisland. This model should include the effects of both specia-tion and immigration. Seventh, anthropogenic-inducedhabitat fragmentation or destruction may isolate once mas-sive and continuous ecosystems on an island. Eighth, theETIB model does not consider periodic rescue effect andtarget area effect. The rescue effect reduces species extinc-tion, while the target area effect enhances immigration onlarge islands. Finally, the use of island size is extremelygeneral and does not consider the impact of habitat hetero-geneity (complexity of habitats and natural landscapes) onspecies diversity.

These main arguments against the ETIB are important toexamine. However, modifications of the ETIB are a possibil-ity to account for such problems. Despite such flaws, theETIB is still acceptable by some modern scientists in at leastthree ways. Firstly, the theory may be graphically simpli-fied, and makes logical sense to a wide array of audiences.Secondly, this theory forms a link between traditional no-tions of ecology and biogeography, thereby enriching theinformation of both types of study. And, thirdly, this theoryprovides a clear and testable strategy for population patternpredictions not only on islands, but also on localized conti-nental systems.

Figure 5—Display of a negative relationship betweenspecies richness (S) and island isolation (I). The insetrepresents the log-transformed equivalent of this rela-tionship (Brown and Lomolino 1998).



Island Conservation _____________The initial objective for the ETIB was to gain a betterunderstanding of island ecosystems and their dynamic pro-cesses. Nevertheless, as studies continued, many ecologists,biogeographers, and conservation biologists discovered aparallel between oceanic islands and fragmented habitatson continental landmasses. Most species have a range ofhabitat in which they prefer to live, yet it has become evidentthat human activities such as deforestation, habitat de-struction, and urban and suburban development, have allcontributed to a significant fragmentation of natural habi-tats. The continuation of such fragmentation has resulted inspecies extinction from local to global scales (Whitmore andSayer 1992). The remaining patches of these relativelynatural areas may be perceived as habitat islands (fig. 6).From the similarities between habitat islands and actualoceanic islands, biogeographers utilize the ETIB as a guide-line to preserve biodiversity in these patches (Whittaker1998). This notion, ironically, spawned a great controversyin the field of biogeography and conservation biology. Janzen(1983) argues that natural habitats such as parks andnature preserves vary significantly from true oceanic is-lands, and thus the ETIB would not be completely applied tocontinental reserves. Unlike continental parks, Janzen (1983)points out that islands are encompassed by water, an ex-tremely different type of habitat. Habitat islands can besurrounded by a landscape containing a variety of speciesthat are potentially capable of establishing populationswithin the habitat patch. This event often introduces nega-tive biotic interactions, such as competition, predation, and

parasitism, within the habitat island that is not experiencedby true oceanic island ecosystems. Likewise, species withinthe habitat island are capable of escaping the patch andinfluencing populations of species within the degraded orfragmented landscape. These are important biotic interac-tions that do not occur on oceanic islands, and must beconsidered when attempting to apply theories of islandbiogeography to habitat islands on continents. The scale anddegree of insularity are critical components when makingsuch comparisons. For these reasons, in addition to theoriesconcerning oceanic island biogeography, issues such as mini-mum viable population and area, along with metapopulationdynamics must be evaluated when determining the mosteffective continental reserve design.

Minimum Viable Population and MinimumViable Area

The study of population dynamics of a species is criticalwhen attempting to support the future success of thespecies’ population. When evaluating populations of spe-cies inhabiting a reserve, survival pressures on these popu-lations are compounded. Smaller areas tend to supportsmaller numbers of species as noted by the species-areaconcept. Consequently, the proposal of the minimum viablepopulation (MVP) concept emerged. Shaffer (1981) tenta-tively defines the minimum viable population for a givenspecies in a given habitat regardless of the impacts ofdemographic stochasticity, environmental stochasticity,genetic stochasticity, and natural catastrophes. Under thisdefinition, the survival of the species must not only endurenormal conditions, but also endure episodic catastrophes inorder to persist through geological time.

Shaffer (1981) attempts to give a definitive measure in orderto conserve species living within a restricted area because thiswould allow conservation biologists to have a framework inwhich to proceed. However, Thomas (1990) argues that theMVP theory is not realistic with regard to actual populationdynamics within a limited geographical range. Additionally,Thomas (1990) states that in certain large-scale, remote areas,the MVP concept would be too difficult to quantify due to thepaucity of available information.

The theory of minimum viable area (MVA) resembles thetheory of minimum viable population. If a determined areaof minimum size is conserved, the species inhabiting sucharea is conserved as well. The MVA often corresponds to therange size in which this particular species is found. Specieslocated higher on the trophic level generally require morearea or space to ensure maximum survival. Hence, forcertain species, the MVA is considerably large in order for aMVP to persist within the designated area (Whittaker 1998).

The MVA approach may be effective to help preserveentire ecosystems since various species coexist and interactclosely within the MVA. However, this concept assumes thateach area is discrete, and has no biotic (genetic) exchangewith other surrounding areas. The MVA must account forthe immigration and colonization of species in and out of thearea (Whittaker 1998). Consequently, the MVA is difficult toquantify due to its extremely dynamic nature.

Figure 6—The fragmentation of forested land in CadizTownship Green County, Wisconsin (94.93 km2), intohabitat “islands” during the era of European settlement(Shafer 1990).



Metapopulation Dynamics

Metapopulation models first emerged in 1969, and havesince evolved into a dynamic concept involving wildlifeconservation and population turnover. A metapopulation isa discontinuous distribution of a population of species. Thispopulation is geographically spread over disjunct fragmentsof suitable habitat separated by intermixed fragments ofunsuitable habitat through which little migration occurs(McCullough 1996). As a result, there is a limited movementof population among suitable patches, and populations re-main spatially separated. However, when a metapopulationbecomes environmentally and physiologically stressed, thepopulation crashes; the patch may be recolonized by anearby metapopulation, and may eventually bring the popu-lation back up to threshold (fig. 7). Therefore, althoughpopulations of the same species are spatially separated,migration and gene flow still occur among suitable patchesto ensure long-term survival of the species (Whittaker 1998).

Gotelli (1991) realizes two main dilemmas in studyingmetapopulation models. Firstly, the dynamics of meta-populations may be difficult to replicate, especially whenconsidering the temporal-scale in which metapopulationchanges may occur under natural settings. Secondly, thesubdivision of populations may occur at many levels, and thedegree of separation in metapopulations is often subjective.

Despite potential weaknesses as suggested by Haila (1990),the metapopulation concept forms a link between popula-tion ecology studies and island biogeography theory. Meta-population models are comprised of dynamic, interdependentisland systems in which population fluctuations are deter-mined by the degree of isolation (Gotelli 1991; Whittaker1998). These geographically distinct metapopulations maybe viewed as habitat islands due to an extensive humancolonization and development (McCullough 1996). Suchmetapopulation pattern is evident in both population ecol-ogy and island biogeography studies. Hence, the researchused for island biogeography study may allow for the postu-lation of a unified concept concerning isolation, area, and

species number on mainland metapopulation systems(McCullough 1996). However, the application of island bio-geography theory to the patch dynamics of metapopulationshas many flaws. Most importantly, habitat patches on acontinental landscape rarely, if ever, resemble true oceanicislands. Ecotones and edge effects tend to be less dramaticgradients of habitat than distinct changes from terrestriallandscapes to seascapes. This dissimilarity introduces twoadditional differences between these seemingly analogoushabitats. First, the surrounding area represents a gradientof habitat, rather than a distinct boundary. The surroundinghabitat may simultaneously offer both advantages, such asadditional food sources, and disadvantages, such as com-petitors, predators, and pathogens. Second, through prop-erly designed corridors, the surrounding matrix permitsperiodic migrations among suitable patches; the degree ofspatial isolation is considerably less on habitat islands thanon true oceanic islands.

Continental Reserve Design and theSLOSS Debate

Metapopulation dynamic theory is utilized to find themost effective and efficient strategies for continental re-serve design, and has allowed for a better understanding ofpopulation ecology. Many opinions exist concerning themost effective and “natural” design theory, the size andshape of reserves, and the number of reserves necessary tomaintain the optimum amount of biodiversity. The SLOSSdebate (Single Large or Several Small) emerged in 1976, andproposed two schools of thoughts regarding reserve design.One extreme option was to create a single large reserve. Theother option was to create several small reserves that,combined, would equal the same area as the large reserve.Diamond (1981) supports large reserves, using managerialconsiderations as a main determining factor. Nilsson (1978)is also in favor of large reserves, using field data on plant andbird observations as support. Conversely, Simberloff andGotelli (1984) argue that several small reserves would maxi-mize local biodiversity. Shafer (1990) and Simberloff andGotelli (1984) utilize plant data to reveal that small reservesare as effective as a large reserve in maintaining biodiversity.Like true oceanic islands, a higher degree of isolation oncontinental reserves would result in decreased migrationlevels to and from the reserve. Furthermore, the destructionand degradation of habitat surrounding the reserve wouldincrease extinction rates, as habitat becomes unsuitable tosustain high biodiversity. A new equilibrium number wouldbe reached as suitable habitats become less available (fig. 8).While the reserve is being properly designed and developed,supersaturation may result primarily due to an excess ofspecies as displaced populations flee into the still relativelypristine reserve system. The reserve may not have thecapacity to sustain such high biodiversity, and relaxationof species numbers into a new point of equilibrium mayeventually occur (fig. 8). The new equilibrium number maybe estimated after information regarding area and degree ofisolation is gathered. Diamond and May (1981) use theequilibrium point from ETIB to determine what type ofreserve would maximize the species richness and abun-dance. Whittaker (1998) postulates that larger reserves are

Figure 7—The classic metapopulation model in whichoccupied patches (shaded) will re-supply patches in whichthe population has decreased or vanished (unshaded).Movement is denoted by the arrows (Whittaker 1998).



Figure 9—The simplified geometric principles for naturereserve designs derived from island biogeography research(Diamond 1975; Whittaker 1998).

Figure 8—According to the assumptions made by the ETIB,as reduction in area will cause supersaturation as immigra-tion rates decrease and extinction rates increase. Thiscauses a “relaxation” into a lower species equilibrium point.Under extreme circumstances, “biotic collapse” may occurand the results may be an immigration rate so low that theequilibrium number is zero.

more effective than smaller reserves, shorter distances be-tween reserves are better than longer distances, circularshaped reserves are better at maintaining reserve speciesthan elongated shaped reserves due to a reduction of edgeeffects, and corridors connecting large reserves would bemore favorable than without corridors (fig. 9).

The use of island biogeography theory has two mainlimitations when applying it to the continental reservedesign. Firstly, the ETIB focuses on overall species richnessof a habitat island by using species-area equations. TheETIB does not allow for the prediction of species with thehighest probability of becoming extirpated from the remain-ing patch (Saunders and others 1991; Whitmore and Sayer1992; Whittaker 1998; Worthen 1996). This approach then

does not permit the investigation of specific species circum-stances within the reserve, and may prove harmful if requir-ing in-depth analysis. Secondly, the ETIB model itself isflawed. Any application of this concept to the continentalreserve design and conservation policy also contains suchflaws (Whittaker 1998). If the use of this theory is notmeticulously studied on a case-by-case basis, certain flawsare not only represented in reserve design, but also perhapseven amplified.

Human Impact on IslandEcosystems ____________________

The utilization of island biogeography theory in determin-ing the most effective reserve design has recently been animportant issue in conservation. Yet, islands themselveshave also been an issue in conservation biology, mainly dueto detrimental human impacts in island environments.There are numerous heated debates as to what type ofimpact the earliest human colonizers had on island ecosys-tems. Some ecologists and biogeographers argue that mostof the earliest island colonizers were respectful of the islandecosystem, and that negative impacts occurred only aftersecondary arrivals of colonizers conflicted with the interestsof the initial inhabitants. Others argue that earliest inhab-itants of some islands devastated the environment becauseof their ignorance and negligence concerning island ecosys-tems. One rather undisputed fact is that as human commu-nities on islands reached the carrying capacity, humansoften modified island landscapes to support the rapidlygrowing population. A classic example is the terracing ofsteep terrain on islands in order to maximize agriculturalproductivity (Nunn 1994). Through history and into themodern age, negative anthropogenic impacts have contin-ued and increased. Humans can easily damage pristineisland environments in four ways: overexploitation andpredation by humans, habitat loss, fragmentation, and deg-radation, as well as introduction of exotic species and dis-eases (biological invasions).

Overexploitation and Predationby Humans

Many islands contain unique endemic species because theremote quality of islands allows for the speciation of floraand fauna to be considerably different from mainland taxa.A classic example of predation of island species by humansis that of the dodo bird (Raphus cucullatus), once populatedon the island of Mauritius, located east of Madagascar in theIndian Ocean. Dodo birds were endemic and were highlyadapted to island conditions. By the early 17th century,Dutch settlers began to colonize the island, hunting bothdodo birds and tortoises as food sources. Dodo birds becameextinct by the year 1690. Predation of species by humans notonly occurred for food sources, but also for tribal (in otherwords, vibrantly colored bird feathers) and exportation rea-sons. Moreover, fruit bats are currently being exported fromthe Polynesian islands. Not only is the declining populationof fruit bats an issue of conservation, but also these bats playan imperative role in pollination and seed dispersal of island



flora. Therefore, the absence of fruit bat population can alsoharm the proliferation of many plant species (Whittaker1998). A secondary extinction can occur under extremecases.

Habitat Loss, Fragmentation, andDegradation on Island Systems

For centuries, island ecosystems have been the target forhabitat loss, fragmentation, and degradation primarily foragricultural reasons. In addition, the wood from the forestedtrees was used for fuel and residential cooking (Heywood1979). Such habitat destruction not only directly damagesthe island flora, but also reduces the faunal biodiversity. Asforested areas are diminished, suitable habitats and foodresources for fauna also diminish. As habitat fragmentation(deforestation) continues at an alarming rate, it createsmajor ecological dilemmas on islands such as Madagascar.

Another mechanism of habitat degradation or simplifica-tion is human-induced fires. Fires are often utilized forhunting purposes or to clear a plot for agricultural purposes.Frequent fires in areas with a low natural fire frequency canbe tremendously destructive. Such prescribed burning canclearly destroy the present island ecosystem and perma-nently transform the island landscape.

Impact of Exotic Species and AssociatedDiseases on Island Ecosystems

As humans travel the globe, different species have beenintentionally and inadvertently introduced into new ecosys-tems. Many island species, also known as the native biota,are particularly vulnerable to biological invasions due totheir isolation through evolutionary time. The absence ofherbivory, for instance, has resulted in the persistence ofmany island flora with no defensive mechanisms againstgrazing pressures. Isolation, in a sense, has protected suchplant species that, otherwise, would have become extinct onthe mainland (Melville 1979). Nevertheless, the presence ofabundant herbivores, such as cattle, sheep, goats, and pigs,has led to extreme habitat degradation. Historically, hu-mans intentionally introduced these grazing animals toensure abundant food supply, without even considering thenegative ecological consequences. However, many of theseanimals have become feral and extremely detrimental to theisland landscape. Overgrazing has caused massive erosionon the hillsides, leading to large-scale landslides. Similarly,heavy grazing has encouraged the proliferation of exoticspecies. The introduction of exotic species into an islandecosystem is typically irreversible. Some exotic species, infact, are more successful in these foreign environments thanin their native landscape. Once those exotic species haveestablished, their populations rapidly proliferate, making itnearly impossible to completely extirpate them from theisland.

Foreign diseases on islands are closely associated withbiological invasions onto islands. An exotic species is fre-quently infected with a disease, and can introduce addi-tional harmful diseases onto the island. If island biota isimmune to the ill effects of the disease, there is little or no

negative impact on the native biota. Conversely, if nativeisland biota is susceptible to foreign diseases, the results canbe disastrous. The entire island biota is adversely affected,and many species can be extinguished from the island. If thisspecies is endemic with a narrow geographic range, theentire population of this species is likely to become extinct.

Conclusions and Implications _____This article re-examines the equilibrium theory of island

biogeography initially proposed by MacArthur and Wilsonin 1963. Island environments including topographic originsand characteristics, along with climatic patterns have beendiscussed. The ETIB is analyzed using essential compo-nents, such as species-area relationship, island isolation,dispersal mechanisms, and species turnover. The strengthsand weaknesses of the ETIB model are evaluated. Addition-ally, continental conservation theories that apply the ETIBconcept are assessed in terms of minimum viable popula-tions and areas, metapopulation dynamics, continentalreserve design, and the SLOSS debate. Lastly, adverseanthropogenic impacts on island ecosystems, such asoverexploitation of resources, habitat destruction, and in-troduction of exotic species and diseases, are examined. Thisarticle has primarily re-investigated the research performedby ecologists, biogeographers, and conservation biologists atthe forefront of island study. The ETIB has been an issue ofheated debate since its emergence nearly four decades ago,and still today, the vitality of the concepts is evolving asresearchers gain better understanding of island ecosystemsand their pertinence to mainland habitats. This trend willundisputedly continue, but hopefully researchers will beable to unlock all of the knowledge to be gained by thesedelicate ecosystems before they become relics of the past.

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