Equilibrium Theory of Island Biogeography: A Review Angela D. Yu Simon A. Lei Abstract—The topography, climatic pattern, location, and origin of relationship, dispersal mechanisms and their response to islands generate unique patterns of species distribution. The equi- isolation, and species turnover. Additionally, conservation librium theory of island biogeography creates a general framework of oceanic and continental (habitat) islands is examined in in which the study of taxon distribution and broad island trends relation to minimum viable populations and areas, may be conducted. Critical components of the equilibrium theory metapopulation dynamics, and continental reserve design. include the species-area relationship, island-mainland relation- Finally, adverse anthropogenic impacts on island ecosys- ship, dispersal mechanisms, and species turnover. Because of the tems are investigated, including overexploitation of re- theoretical similarities between islands and fragmented mainland sources, habitat destruction, and introduction of exotic spe- landscapes, reserve conservation efforts have attempted to apply cies and diseases (biological invasions). Throughout this the theory of island biogeography to improve continental reserve article, theories of many researchers are re-introduced and designs, and to provide insight into metapopulation dynamics and utilized in an analytical manner. The objective of this article the SLOSS debate. However, due to extensive negative anthropo- is to review previously published data, and to reveal if any genic activities, overexploitation of resources, habitat destruction, classical and emergent theories may be brought into the as well as introduction of exotic species and associated foreign study of island biogeography and its relevance to mainland diseases (biological invasions), island conservation has recently ecosystem patterns. 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 Island Environments _____________ on islands. In effect, this analysis brings insight into current issues Island Formation and Topography of continental reserve design and island conservation efforts. 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 Introduction ____________________ shelf islands (land-bridge islands) and oceanic islands. Con- tinental shelf islands are likely to be physically connected to The equilibrium theory of island biogeography (ETIB), the mainland during low sea level periods. Due to their proposed by MacArthur and Wilson, is a relatively recent connection, these islands have similar geological structure development that has sparked a tremendous amount of to the nearby mainland (Williamson 1981). This similar scientific controversy. Initially introduced to the public in topography, coupled with the island’s close proximity to the 1963 as “An Equilibrium Theory of Insular Zoogeography,” continent, results in the proliferation of similar flora and the idea was expanded in 1967 into a book publication. The fauna (biota). ETIB implies that island fauna and flora (biota) eventually Oceanic islands are typically more isolated, and may have reach an equilibrium point between extinction and immigra- never been physically connected to a continental landmass. tion. Although species rarely reach equilibrium due to the There are three main types of oceanic islands: oceanic ridge extremely dynamic island system, MacArthur and Wilson islands, hot-spot islands, and the individual islands of island note that the ETIB permits general predictions of future arcs. Oceanic ridge islands and hot-spot islands are volcanic island biodiversity patterns. In this article, the theory of islands because they are formed from ocean-floor volcanoes. island biogeography is examined in reference to island Islands that are part of island arcs also have a volcanic environments, including topographic origins and charac- origin, involving the collision of continental and oceanic teristics, as well as climatic patterns. A comprehensive plates, resulting in islands that consist of both basalt and analysis of the theory is discussed, such as species-area 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 In: McArthur, E. Durant; Fairbanks, Daniel J., comps. 2001. Shrubland ecosystem genetics and biodiversity: proceedings; 2000 June 13–15; Provo, typically steeper and become increasingly dissected with UT. Proc. RMRS-P-21. Ogden, UT: U.S. Department of Agriculture, Forest age. This phenomenon has important implications for island Service, Rocky Mountain Research Station. biota because a wide range of elevational gradients and 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 associated ecological attributes allows for the persistence of Biology and Ecology Professor at the Community College of Southern diverse habitats. The elevation of islands also has impor- Nevada, 6375 West Charleston Boulevard, WDB, Las Vegas, NV 89146- 1139. tant influences on the climatic regime. USDA Forest Service Proceedings RMRS-P-21. 2001 163 Yu and Lei Equilibrium Theory of Island Biogeography: A Review 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). Island Climate creates smaller patches from a variety of habitats favorable to many species, and permits high- and low-elevation inhab- Island climate is determined by both external influences, iting species to coexist in a relatively small area (Whittaker such as ocean circulation and atmospheric circulation, and 1998). 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 Island Patterns _________________ similar movements of upwelling and sinking. If an island is in the path of a moving current or is located where two The Equilibrium Theory of Island Biogeography (ETIB) currents intersect, this can alter the climate significantly. In revolutionizes the way in which biogeographers and ecolo- addition to circulation influences, the proximity of an island gists viewed island ecosystems. Prior to the ETIB was the to a continental landmass also affects the island’s climate. static theory of islands (Dexter 1978), which hypothesizes Islands located close to a mainland, such as land-bridge that island community structures remain relatively con- islands, are likely to be influenced by the continental cli- stant over geological time. The only mechanism for biologi- mate. Remote oceanic islands, on the contrary, are influ- cal change was the gradual evolutionary process of specia- enced by the maritime climate. tion. Few successful colonization events would occur due to Internal influences, such as island size and elevation, can a limited number of ecological niches on the island (Lack have a substantial impact on the precipitation regime on the 1976). Once these niches are completely filled, no space and island. Whittaker (1998) states that low islands typically resources are available for new immigrants, and they may have relatively dry climates and high islands are wetter not become successfully established on the island. The ETIB through orographic rainfall, resulting in the creation of refutes the static theory, indicating that island communities extensive arid regions due to the rain shadow effect. These exhibit a dynamic equilibrium between species colonization higher islands often contain diverse habitats within a rela- and extinction, or species turnover. The immigration curve tively small area. Due to the impact of elevation on island is descending and is shaped concavely because the most climate, research studies have indicated that elevation is a successful dispersing species would colonize initially, fol- critical variable in analyzing species diversity on islands. lowed by a significant decrease in the overall rate of immi- Telescoping, a compression of elevational zones, is fairly gration (fig. 2). The extinction curve, on the contrary, is an common on small tropical islands. Leuschner (1996) proposes ascending curve because as more species inhabit the island that forest lines on islands are generally 1,000 to 2,000 m through time, more species would become extinct exponen- lower than forest lines on continents. Hence, telescoping tially (fig. 2). Such a trend is amplified due to a combination of population size and negative biotic interactions among 164 USDA Forest Service Proceedings RMRS-P-21. 2001 Equilibrium Theory of Island Biogeography: A Review Yu and Lei Species-Area Relationship A positive relationship exists between island size and island species richness (fig. 4). These two variables, never- theless, do not always exhibit a linear correlation. Arrhenius represented this non-linear trend in mathematical form in 1920 with the following equation: S = cAZ In this equation, S = the species richness on an island, c = a constant depending on biotic and biogeographic region, A = the area of the island, and z = a constant representing the slope