Island Biogeography: Topography, Climate, and Species Distribution, Study notes of Biology

Download Island Biogeography: Topography, Climate, and Species Distribution and more Study notes Biology in PDF only on Docsity! USDA Forest Service Proceedings RMRS-P-21. 2001 163 In: McArthur, E. Durant; Fairbanks, Daniel J., comps. 2001. Shrubland ecosystem genetics and biodiversity: proceedings; 2000 June 13–15; Provo, UT. Proc. RMRS-P-21. Ogden, UT: U.S. Department of Agriculture, Forest Service, 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 a Biology and Ecology Professor at the Community College of Southern Nevada, 6375 West Charleston Boulevard, WDB, Las Vegas, NV 89146- 1139. Abstract—The topography, climatic pattern, location, and origin of islands generate unique patterns of species distribution. The equi- librium theory of island biogeography creates a general framework in which the study of taxon distribution and broad island trends may be conducted. Critical components of the equilibrium theory include the species-area relationship, island-mainland relation- ship, dispersal mechanisms, and species turnover. Because of the theoretical similarities between islands and fragmented mainland landscapes, reserve conservation efforts have attempted to apply the theory of island biogeography to improve continental reserve designs, and to provide insight into metapopulation dynamics and the SLOSS debate. However, due to extensive negative anthropo- genic activities, overexploitation of resources, habitat destruction, as well as introduction of exotic species and associated foreign diseases (biological invasions), island conservation has recently become a pressing issue itself. The objective of this article is to analyze previously published data, and to review theories from numerous research studies that attempt to explain species patterns on islands. In effect, this analysis brings insight into current issues of continental reserve design and island conservation efforts. Introduction ____________________ The equilibrium theory of island biogeography (ETIB), proposed by MacArthur and Wilson, is a relatively recent development that has sparked a tremendous amount of scientific controversy. Initially introduced to the public in 1963 as “An Equilibrium Theory of Insular Zoogeography,” the idea was expanded in 1967 into a book publication. The ETIB implies that island fauna and flora (biota) eventually reach an equilibrium point between extinction and immigra- tion. Although species rarely reach equilibrium due to the extremely dynamic island system, MacArthur and Wilson note that the ETIB permits general predictions of future island biodiversity patterns. In this article, the theory of island biogeography is examined in reference to island environments, including topographic origins and charac- teristics, as well as climatic patterns. A comprehensive analysis of the theory is discussed, such as species-area Equilibrium Theory of Island Biogeography: A Review Angela D. Yu Simon A. Lei relationship, dispersal mechanisms and their response to isolation, and species turnover. Additionally, conservation of oceanic and continental (habitat) islands is examined in relation 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 this article, theories of many researchers are re-introduced and utilized in an analytical manner. The objective of this article is to review previously published data, and to reveal if any classical and emergent theories may be brought into the study of island biogeography and its relevance to mainland ecosystem 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: continental shelf islands (land-bridge islands) and oceanic islands. Con- tinental shelf islands are likely to be physically connected to the mainland during low sea level periods. Due to their connection, these islands have similar geological structure to the nearby mainland (Williamson 1981). This similar topography, coupled with the island’s close proximity to the continent, results in the proliferation of similar flora and fauna (biota). Oceanic islands are typically more isolated, and may have never been physically connected to a continental landmass. There are three main types of oceanic islands: oceanic ridge islands, hot-spot islands, and the individual islands of island arcs. Oceanic ridge islands and hot-spot islands are volcanic islands because they are formed from ocean-floor volcanoes. Islands that are part of island arcs also have a volcanic origin, involving the collision of continental and oceanic plates, resulting in islands that consist of both basalt and granite rock (Williamson 1981). All of the geological processes occurring volcanic islands can produce islands with high elevations, with peaks of at least 2,000 m (fig. 1) (Williamson 1981). Volcanic islands are typically steeper and become increasingly dissected with age. This phenomenon has important implications for island biota because a wide range of elevational gradients and associated ecological attributes allows for the persistence of diverse 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, and internal influences, such as island size, shape, and topogra- phy. Ocean circulation and atmospheric circulation consist of water currents and air currents, respectively, that have similar movements of upwelling and sinking. If an island is in the path of a moving current or is located where two currents intersect, this can alter the climate significantly. In addition to circulation influences, the proximity of an island to a continental landmass also affects the island’s climate. Islands located close to a mainland, such as land-bridge islands, 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, can have a substantial impact on the precipitation regime on the island. Whittaker (1998) states that low islands typically have relatively dry climates and high islands are wetter through orographic rainfall, resulting in the creation of extensive arid regions due to the rain shadow effect. These higher islands often contain diverse habitats within a rela- tively small area. Due to the impact of elevation on island climate, research studies have indicated that elevation is a critical variable in analyzing species diversity on islands. Telescoping, a compression of elevational zones, is fairly common on small tropical islands. Leuschner (1996) proposes that forest lines on islands are generally 1,000 to 2,000 m lower than forest lines on continents. Hence, telescoping creates smaller patches from a variety of habitats favorable to many species, and permits high- and low-elevation inhab- iting species to coexist in a relatively small area (Whittaker 1998). 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 the static theory of islands (Dexter 1978), which hypothesizes that 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 to a limited number of ecological niches on the island (Lack 1976). Once these niches are completely filled, no space and resources are available for new immigrants, and they may not become successfully established on the island. The ETIB refutes the static theory, indicating that island communities exhibit a dynamic equilibrium between species colonization and extinction, or species turnover. The immigration curve is descending and is shaped concavely because the most successful 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 an ascending curve because as more species inhabit the island through time, more species would become extinct exponen- tially (fig. 2). Such a trend is amplified due to a combination of population size and negative biotic interactions among Figure 1—A map indicating all islands with a peak of 2000 m or higher. Larger islands are shaded, smaller islands are denoted by ∆ (Williamson 1981). USDA Forest Service Proceedings RMRS-P-21. 2001 167 Equilibrium Theory of Island Biogeography: A Review Yu and Lei Island Conservation _____________ The initial objective for the ETIB was to gain a better understanding of island ecosystems and their dynamic pro- cesses. Nevertheless, as studies continued, many ecologists, biogeographers, and conservation biologists discovered a parallel between oceanic islands and fragmented habitats on continental landmasses. Most species have a range of habitat in which they prefer to live, yet it has become evident that human activities such as deforestation, habitat de- struction, and urban and suburban development, have all contributed to a significant fragmentation of natural habi- tats. The continuation of such fragmentation has resulted in species extinction from local to global scales (Whitmore and Sayer 1992). The remaining patches of these relatively natural areas may be perceived as habitat islands (fig. 6). From the similarities between habitat islands and actual oceanic islands, biogeographers utilize the ETIB as a guide- line to preserve biodiversity in these patches (Whittaker 1998). This notion, ironically, spawned a great controversy in the field of biogeography and conservation biology. Janzen (1983) argues that natural habitats such as parks and nature preserves vary significantly from true oceanic is- lands, and thus the ETIB would not be completely applied to continental reserves. Unlike continental parks, Janzen (1983) points out that islands are encompassed by water, an ex- tremely different type of habitat. Habitat islands can be surrounded by a landscape containing a variety of species that are potentially capable of establishing populations within the habitat patch. This event often introduces nega- tive biotic interactions, such as competition, predation, and parasitism, within the habitat island that is not experienced by true oceanic island ecosystems. Likewise, species within the habitat island are capable of escaping the patch and influencing populations of species within the degraded or fragmented landscape. These are important biotic interac- tions that do not occur on oceanic islands, and must be considered when attempting to apply theories of island biogeography to habitat islands on continents. The scale and degree of insularity are critical components when making such comparisons. For these reasons, in addition to theories concerning oceanic island biogeography, issues such as mini- mum viable population and area, along with metapopulation dynamics must be evaluated when determining the most effective continental reserve design. Minimum Viable Population and Minimum Viable Area The study of population dynamics of a species is critical when attempting to support the future success of the species’ population. When evaluating populations of spe- cies inhabiting a reserve, survival pressures on these popu- lations are compounded. Smaller areas tend to support smaller numbers of species as noted by the species-area concept. Consequently, the proposal of the minimum viable population (MVP) concept emerged. Shaffer (1981) tenta- tively defines the minimum viable population for a given species in a given habitat regardless of the impacts of demographic stochasticity, environmental stochasticity, genetic stochasticity, and natural catastrophes. Under this definition, the survival of the species must not only endure normal conditions, but also endure episodic catastrophes in order to persist through geological time. Shaffer (1981) attempts to give a definitive measure in order to conserve species living within a restricted area because this would allow conservation biologists to have a framework in which to proceed. However, Thomas (1990) argues that the MVP theory is not realistic with regard to actual population dynamics 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 the paucity of available information. The theory of minimum viable area (MVA) resembles the theory of minimum viable population. If a determined area of minimum size is conserved, the species inhabiting such area is conserved as well. The MVA often corresponds to the range size in which this particular species is found. Species located higher on the trophic level generally require more area or space to ensure maximum survival. Hence, for certain species, the MVA is considerably large in order for a MVP to persist within the designated area (Whittaker 1998). The MVA approach may be effective to help preserve entire ecosystems since various species coexist and interact closely within the MVA. However, this concept assumes that each area is discrete, and has no biotic (genetic) exchange with other surrounding areas. The MVA must account for the immigration and colonization of species in and out of the area (Whittaker 1998). Consequently, the MVA is difficult to quantify due to its extremely dynamic nature. Figure 6—The fragmentation of forested land in Cadiz Township Green County, Wisconsin (94.93 km2), into habitat “islands” during the era of European settlement (Shafer 1990). 168 USDA Forest Service Proceedings RMRS-P-21. 2001 Yu and Lei Equilibrium Theory of Island Biogeography: A Review Metapopulation Dynamics Metapopulation models first emerged in 1969, and have since evolved into a dynamic concept involving wildlife conservation and population turnover. A metapopulation is a discontinuous distribution of a population of species. This population is geographically spread over disjunct fragments of suitable habitat separated by intermixed fragments of unsuitable habitat through which little migration occurs (McCullough 1996). As a result, there is a limited movement of population among suitable patches, and populations re- main spatially separated. However, when a metapopulation becomes environmentally and physiologically stressed, the population crashes; the patch may be recolonized by a nearby metapopulation, and may eventually bring the popu- lation back up to threshold (fig. 7). Therefore, although populations of the same species are spatially separated, migration and gene flow still occur among suitable patches to ensure long-term survival of the species (Whittaker 1998). Gotelli (1991) realizes two main dilemmas in studying metapopulation models. Firstly, the dynamics of meta- populations may be difficult to replicate, especially when considering the temporal-scale in which metapopulation changes may occur under natural settings. Secondly, the subdivision of populations may occur at many levels, and the degree 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, interdependent island systems in which population fluctuations are deter- mined by the degree of isolation (Gotelli 1991; Whittaker 1998). These geographically distinct metapopulations may be viewed as habitat islands due to an extensive human colonization and development (McCullough 1996). Such metapopulation pattern is evident in both population ecol- ogy and island biogeography studies. Hence, the research used 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 metapopulations has many flaws. Most importantly, habitat patches on a continental landscape rarely, if ever, resemble true oceanic islands. Ecotones and edge effects tend to be less dramatic gradients of habitat than distinct changes from terrestrial landscapes to seascapes. This dissimilarity introduces two additional differences between these seemingly analogous habitats. First, the surrounding area represents a gradient of habitat, rather than a distinct boundary. The surrounding habitat may simultaneously offer both advantages, such as additional food sources, and disadvantages, such as com- petitors, predators, and pathogens. Second, through prop- erly designed corridors, the surrounding matrix permits periodic migrations among suitable patches; the degree of spatial isolation is considerably less on habitat islands than on true oceanic islands. Continental Reserve Design and the SLOSS Debate Metapopulation dynamic theory is utilized to find the most effective and efficient strategies for continental re- serve design, and has allowed for a better understanding of population ecology. Many opinions exist concerning the most effective and “natural” design theory, the size and shape of reserves, and the number of reserves necessary to maintain the optimum amount of biodiversity. The SLOSS debate (Single Large or Several Small) emerged in 1976, and proposed two schools of thoughts regarding reserve design. One extreme option was to create a single large reserve. The other option was to create several small reserves that, combined, would equal the same area as the large reserve. Diamond (1981) supports large reserves, using managerial considerations as a main determining factor. Nilsson (1978) is also in favor of large reserves, using field data on plant and bird observations as support. Conversely, Simberloff and Gotelli (1984) argue that several small reserves would maxi- mize local biodiversity. Shafer (1990) and Simberloff and Gotelli (1984) utilize plant data to reveal that small reserves are as effective as a large reserve in maintaining biodiversity. Like true oceanic islands, a higher degree of isolation on continental reserves would result in decreased migration levels to and from the reserve. Furthermore, the destruction and degradation of habitat surrounding the reserve would increase extinction rates, as habitat becomes unsuitable to sustain high biodiversity. A new equilibrium number would be 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 of species as displaced populations flee into the still relatively pristine reserve system. The reserve may not have the capacity to sustain such high biodiversity, and relaxation of species numbers into a new point of equilibrium may eventually occur (fig. 8). The new equilibrium number may be estimated after information regarding area and degree of isolation is gathered. Diamond and May (1981) use the equilibrium point from ETIB to determine what type of reserve would maximize the species richness and abun- dance. Whittaker (1998) postulates that larger reserves are Figure 7—The classic metapopulation model in which occupied patches (shaded) will re-supply patches in which the population has decreased or vanished (unshaded). Movement is denoted by the arrows (Whittaker 1998). USDA Forest Service Proceedings RMRS-P-21. 2001 169 Equilibrium Theory of Island Biogeography: A Review Yu and Lei Figure 9—The simplified geometric principles for nature reserve 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. This causes a “relaxation” into a lower species equilibrium point. Under extreme circumstances, “biotic collapse” may occur and the results may be an immigration rate so low that the equilibrium number is zero. more effective than smaller reserves, shorter distances be- tween reserves are better than longer distances, circular shaped reserves are better at maintaining reserve species than elongated shaped reserves due to a reduction of edge effects, and corridors connecting large reserves would be more favorable than without corridors (fig. 9). The use of island biogeography theory has two main limitations when applying it to the continental reserve design. Firstly, the ETIB focuses on overall species richness of a habitat island by using species-area equations. The ETIB does not allow for the prediction of species with the highest probability of becoming extirpated from the remain- ing patch (Saunders and others 1991; Whitmore and Sayer 1992; 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 is flawed. Any application of this concept to the continental reserve design and conservation policy also contains such flaws (Whittaker 1998). If the use of this theory is not meticulously studied on a case-by-case basis, certain flaws are not only represented in reserve design, but also perhaps even amplified. Human Impact on Island Ecosystems ____________________ The utilization of island biogeography theory in determin- ing the most effective reserve design has recently been an important issue in conservation. Yet, islands themselves have also been an issue in conservation biology, mainly due to detrimental human impacts in island environments. There are numerous heated debates as to what type of impact the earliest human colonizers had on island ecosys- tems. Some ecologists and biogeographers argue that most of the earliest island colonizers were respectful of the island ecosystem, and that negative impacts occurred only after secondary arrivals of colonizers conflicted with the interests of the initial inhabitants. Others argue that earliest inhab- itants of some islands devastated the environment because of their ignorance and negligence concerning island ecosys- tems. One rather undisputed fact is that as human commu- nities on islands reached the carrying capacity, humans often modified island landscapes to support the rapidly growing population. A classic example is the terracing of steep terrain on islands in order to maximize agricultural productivity (Nunn 1994). Through history and into the modern age, negative anthropogenic impacts have contin- ued and increased. Humans can easily damage pristine island environments in four ways: overexploitation and predation by humans, habitat loss, fragmentation, and deg- radation, as well as introduction of exotic species and dis- eases (biological invasions). Overexploitation and Predation by Humans Many islands contain unique endemic species because the remote quality of islands allows for the speciation of flora and fauna to be considerably different from mainland taxa. A classic example of predation of island species by humans is that of the dodo bird (Raphus cucullatus), once populated on the island of Mauritius, located east of Madagascar in the Indian Ocean. Dodo birds were endemic and were highly adapted to island conditions. By the early 17th century, Dutch settlers began to colonize the island, hunting both dodo birds and tortoises as food sources. Dodo birds became extinct by the year 1690. Predation of species by humans not only occurred for food sources, but also for tribal (in other words, vibrantly colored bird feathers) and exportation rea- sons. Moreover, fruit bats are currently being exported from the Polynesian islands. Not only is the declining population of fruit bats an issue of conservation, but also these bats play an imperative role in pollination and seed dispersal of island