Model Of Island Biogeography Research Articles

Changes to plant species richness in forest fragments: fragment age, disturbance and fire history may be as important as area

AimThe impact of fragmentation on a eucalypt forest was investigated by examining the effects of fragment size, time since fragmentation, degree of anthropogenic disturbance to fragment interiors, and time since fire, on native and exotic plant species richness per unit area.LocationTwo areas of dry open‐forest were studied on the central coast of New South Wales in south‐eastern Australia. Fifty forest fragments were located at Tomago, an area progressively fragmented over the last 60 years, most recently by clearing for sand‐mining. Also at Tomago were six very large blocks of forest that were used as reference sites. The second area at Myall Lakes National Park (50 km north of Tomago) had four very large areas of intact forest that were also used as reference sites.MethodsFragments were allocated into (1) three size classes: small (<1 ha), medium (1 to <10 ha) and large (10 to <100 ha); (2) two classes of age since fragmentation: young (up to 10 years) and old (10 years or more); (3) two classes of disturbance (minor, major) based on the degree and extent of human‐induced disturbance to the fragment interior; and (4) two classes of time since fire (between 5 and 6 years; 10 years or more). Reference sites belonged to a very large size class (>100 ha). Mean plant species richness per 25 m2 area was determined for each fragment and analysed separately for native and exotic species.ResultsThe most significant effects observed resulted from anthropogenic disturbance. In fragments with major disturbance, native species richness per unit area was significantly reduced in small‐sized young and medium‐sized old fragments. A significant increase was also observed for exotic species richness in fragments with major disturbance. With minor disturbance, native species richness in small fragments declined significantly with time since fragmentation, in contrast to medium and large fragments. Among recently created fragments, those recently burned had significantly more native species per unit area than those burned 10 or more years ago.Main conclusionsAnthropogenic disturbance coupled with fragmentation had a stronger and more immediate effect in reducing native species richness and increasing exotic species richness than did fragmentation alone. In the absence of major disturbance, small fragments had fewer native species than larger size classes, but only after 10 or more years since fragmentation, confirming the importance of controlling for age of fragments when examining species–area relationships. This study has not tested whether differences in area were the direct cause of this loss of species over time; other factors that are correlated with area (such as edge effects) may also be involved. The increase in native species richness following fire was consistent with other studies of fire in unfragmented eucalypt forest. This study thus shows that in addition to the factors emphasized in classical island biogeography models, fragment age, disturbance and fire history are important in explaining species richness in fragmented eucalypt forests.

Potential mechanisms of phenotypic divergence in body size between Newfoundland and mainland black bear populations

Phenotypic variation in body size and degree of sexual size dimorphism of North American black bears (Ursus americanus) was quantified for populations from New Brunswick, Quebec, Ontario, Maine, Alaska, and the island of Newfoundland. Based on a model of island biogeography developed by Case, we predicted that body size should be larger in Newfoundland bears than in mainland populations. The presence of few large predators and minimal competition between herbivore prey on Newfoundland allow an appropriate test of the model (i.e., food availability for bears may differ between populations on the mainland and in Newfoundland). In addition, sexual-selection theory predicts that the coevolution of polygyny and large size will be coupled with an increase in sexual size dimorphism. Therefore, we also predicted that among the six populations, male body mass should scale hyperallometrically with female body mass (i.e., slope > 1). Analysis of deterministic growth curves indicated that bears from Newfoundland attained greater asymptotic body size than populations on the mainland, which supports our first prediction. On average, the relative difference in asymptotic body mass between females from the island and mainland populations was 55%, while the relative difference between males was 37%. However, we found that sexual size dimorphism did not increase disproportionately with body mass among the six populations, which refuted our second prediction. We discuss a range of abiotic and biotic selection pressures possibly responsible for larger body size in Newfoundland bears. We suggest that the ability to exploit seasonally abundant and spatially dispersed dietary protein by female and male black bears on the island has been and is still a primary environmental factor selecting for large body size in Newfoundland bears. Although the relationship between sexual size dimorphism and body size is tenuous (slope [Formula: see text] 1), it does suggest that (an)other adaptive mechanism(s), opposing sexual selection for extreme male size, explain(s) a large amount of the variation in sexual size dimorphism among black bear populations.

Genetic and phylogenetic consequences of island biogeography.

Island biogeography theory predicts that the number of species on an island should increase with island size and decrease with island distance to the mainland. These predictions are generally well supported in comparative and experimental studies. These ecological, equilibrium predictions arise as a result of colonization and extinction processes. Because colonization and extinction are also important processes in evolution, we develop methods to test evolutionary predictions of island biogeography. We derive a population genetic model of island biogeography that incorporates island colonization, migration of individuals from the mainland, and extinction of island populations. The model provides a means of estimating the rates of migration and extinction from population genetic data. This model predicts that within an island population the distribution of genetic divergences with respect to the mainland source population should be bimodal, with much of the divergence dating to the colonization event. Across islands, this model predicts that populations on large islands should be on average more genetically divergent from mainland source populations than those on small islands. Likewise, populations on distant islands should be more divergent than those on close islands. Published observations of a larger proportion of endemic species on large and distant islands support these predictions.

Island Biogeography and the Multiple Domains of Models

This paper adopts a symmetrical approach to controversies over R.H. MacArthur and E.O. Wilson’s equilibrium model of island biogeography, in order to show how different interpretations of the model depend upon different philosophical understandings of the application of models and theories. In particular, there are quite distinct domains to which the model could apply; in addition, some equivocation among these domains is important to the model’s success. Therefore, apparently inconsistent interpretations, interpretations that fit into roughly instrumentalist, realist and rationalist conceptions of science, may be mutually supporting in practice. Descriptions of scientific practice, then, should not adjudicate among these interpretations, but should instead recognize ways in which successful models translate among domains, and in so doing can become realistic, instrumentally successful, or rationally established. As complex social objects, models can afford complex representational relations.

Dynamic disequilibrium: a long‐term, large‐scale perspective on the equilibrium model of island biogeography

Abstract The equilibrium model of island biogeography developed in the 1960s by MacArthur and Wilson has provided an excellent framework in which to investigate the dynamics of species richness in island and island‐like systems. It is comparable in many respects to the Hardy–Weinberg equilibrium model used in genetics as the basis for defining a point of reference, thus allowing one to discover the factors that prevent equilibrium from being achieved. Hundreds of studies have used the model effectively, especially those dealing with brief spans of time and limited geographical areas. In spite of this utility, however, there are important limitations to the MacArthur–Wilson model, especially when we consider long‐term and large‐scale circumstances. Although their general theory is more complex, the MacArthur–Wilson equilibrium model treats colonization and extinction as the only two processes that are relevant to determining species richness. However, it is likely that phylogenetic diversification (phylogenesis) often takes place on the same time‐scale as colonization and extinction; for example, colonization, extinction, and phylogenesis among mammals on oceanic and/or old land‐bridge islands in South‐east Asia are all measured in units of time in the range of 10 000–1 million years, most often in units of 100 000 years. Phylogenesis is not a process that can be treated simply as ‘another form of colonization’, as it behaves differently than colonization. It interacts in a complex manner with both colonization and extinction, and can generate patterns of species richness almost independently of the other two processes. In addition, contrary to the implication of the MacArthur–Wilson model, extinction does not drive species richness in highly isolated archipelagoes (those that receive very few colonists) to progressively lower values; rather, phylogenesis is a common outcome in such archipelagoes, and species richness rises over time. In some specific instances, phylogenesis may have produced an average of 14 times as many species as direct colonization, and perhaps 36 species from one such colonization event. Old, stable, large archipelagoes should typically support not just endemic species but endemic clades, and the total number of species and the size of the endemic clades should increase with age of the archipelago. The existence of long‐term equilibrium in actual island archipelagoes is unlikely. The land masses that make up island archipelagoes are intrinsically unstable because the geological processes that cause their formation are dynamic, and substantial changes can occur (under some circumstances) on a time‐scale comparable to the processes of colonization, phylogenesis, and extinction. Large‐scale island‐like archipelagoes on continents also are unstable, in the medium term because of global climatic fluctuations, and in the long term because of the geologically ephemeral existence of, for example, individual mountain ranges. Examples of these phenomena using the mammals of South‐east Asia, especially the Philippines, make it clear that the best conceptual model of the long‐term dynamics of species richness in island archipelagoes would be one in which colonization, extinction, and phylogenesis are recognized to be of equivalent conceptual importance. Furthermore, we should expect species richness to be always in a dynamic state of disequilibrium due to the constantly changing geological/geographical circumstances in which that diversity exists, always a step or two out of phase with the constantly changing equilibrium point for species richness.

DRIVING FORCES OF ISLAND BIODIVERSITY: AN APPRAISAL OF TWO THEORIES

The avifauna of three oceanic islands of the Revillagigedo archipelago in the eastern Pacific Ocean consists of 14 endemic landbird taxa and 3 recent continental colonists. This study analyzes the origin, areography of continental relatives, ecological characteristics, and role of immigration and extinction factors as regulators of the islands' species. Only two natural extinctions have been detected on this archipelago in more than 130 years. The endemic nature of the pristine avifauna is interpreted as an indication of high stability over thousands of years. The MacArthur–Wilson (1967) “equilibrium theory of island biogeography” cannot explain or predict the persistence of this avifauna adapted to stable and unique island ecosystems. Lack's (1976) “theory of ecological poverty” regulating island avifaunas, however, predicts stability and a resource-limited bird community. The Revillagigedo avifauna can be understood as an interactive community of a few highly adapted and competitive ecological generalists that have effectively filled all niche spaces and closed off the islands to further colonization. Catastrophic change or anthropogenic landscape degradation may, however, create novel niche spaces and open affected islands to new colonists. This appraisal lends support to the call for a shift away from application of the equilibrium model of island biogeography in conservation science. [Key words: island biogeography, equilibrium theory, island stability, island avifauna, MacArthur-Wilson, Lack, Revillagigedo Islands, Socorro Island.]

Biogeography of Puerto Rican ants: a non-equilibrium case?

Ants were studied on Puerto Rico and 44 islands surrounding Puerto Rico. Habitat diversity was the best predictor of the number of species per island and the distributions of species followed a nested subset pattern. The number of extinctions per island was low, approximately 1–2 extinctions per island in a period of 18 years, and the rates of colonization seem to be greater than the extinction rates. Ant dynamics on these islands do not seem to support the basic MacArthur and Wilson model of island biogeography. The MacArthur and Wilson equilibrium is based on the notion that species are interchangeable, but some extinctions and colonizations can change the composition and number of species drastically.

A numerical model for tracking populations through a time-dependent stochastic network

Units from a population arrive from an external source in a random or fixed pattern at an initial compartment of a D-compartment, linked, network. Once inside the network, individuals move in a statistically independent manner among compartments, finally entering absorbing compartments from which they never depart. A numerical adaptation of the model is described and demonstrated. The model is compared to the MacArthur-Wilson model of island biogeography with respect to differences in statistics of movements that each model is capable of generating. A numerical example is given.

A system dynamics model of island biogeography

The MacArthur-Wilson equilibrium theory of island biogeography has been one of the more influential concepts in modern biogeography and ecology. In this paper, we synthesize the theory and examine effects of different immigration/extinction rate-species diversity curves on original predictions from the theory by using the System Dynamics simulation modeling approach. Moreover, we develop a comprehensive and generic System Dynamics model to incorporate a variety of recent modifications and extensions of the theory, including area effect, distance effect, competition effect, habitat diversity effect, target effect, and rescue effect. Through computer simulation with STELLA, a more profound understanding of the theory of island biogeography can be gained. The System Dynamics modeling approach is especially appropriate for such a study because it maximizes the utilization of the ecological data by incorporating qualitative information so that a complex, imprecisely-defined ecological system can be studied quantitatively, effectively, and comprehensively. Our simulation results show that different monotonic rate-species diversity curves do not affect the essence of the theory of island biogeography, while the magnitude of equilibrium species diversity may be greatly affected. Non-monotonic rate-species diversity curves may result in potential multiple equilibria of species diversity. In addition, our model suggests that a non-monotonic relationship may exist between the equilibrium turnover rate and island area and between the equilibrium turnover rate and distance.

The Biogeography of the Lizards of the Seychelles Islands

The distributions of the eighteen species of lizards found in the Seychelles are given so far as they are known. The indigenous lizards of the granitic islands are endemic, at least at the specific level, and generally exhibit racial differentiation between island populations. The species-area relationship for the granitic islands is shallow, and there is little evidence for species turnover as predicted by the equilibrium theory of island biogeography. Low immigration rates arising from the isolation of the granitic Seychelles, together with low extinction rates due to climatic stability and high population densities may be responsible for this. The saurian faunas of the coralline islands are impoverished, with species not distinct from Madagascan and pantropical species. It is suggested that this has resulted from a recent sea-level stand higher than the present, which submerged the very low lying islands. Low rates of species immigration have not allowed the islands to attain a species equilibrium. This is supported by the presence of unfilled niches and the absence of a significant species-area relationship. The equilibrium model of island biogeography is of little relevance to the distribution of the Seychelles lizard fauna.

Biogeography of mammals in SE Asia: estimates of rates of colonization, extinction and speciation

Four categories of islands in SE Asia may be identified on the basis of their histories of landbridge connections. Those islands on the shallow, continental Sunda Shelf were joined to the Asian mainland by a broad landbridge during the late Pleistocene; other islands were connected to the Sunda Shelf by a middle Pleistocene landbridge; some were parts of larger oceanic islands; and others remained as isolated oceanic islands. The limits of late Pleistocene islands, defined by the 120 m bathymetric line, are highly concordant with the limits of faunal regions. Faunal variation among non-volant mammals is high between faunal regions and low within the faunal regions; endemism of faunal regions characteristically exceeds 70%. Small and geologically young oceanic islands are depauperate; larger and older islands are more species-rich. The number of endemic species is correlated with island area; however, continental shelf islands less than 125 000 km2 do not have endemic species, whereas isolated oceanic islands as small as 47 km2 often have endemic species. Geologically old oceanic islands have many endemic species, whereas young oceanic islands have few endemic species. Colonization across sea channels that were 5–25 km wide during the Pleistocene has been low, with a rate of about 1–2/500000 years. Comparison of species-area curves for mainland areas, late Pleistocene islands, and middle Pleistocene islands indicates that extinction occurs rapidly when landbridge islands are first isolated, with the extent of extinction dependent upon island size; extinction then slows to an average rate of 1–2%/10 000 years. The great majority of the non-volant Philippine mammals arrived from the Sunda Shelf, the geographically closest of the possible source areas. Speciation within the Philippines has contributed substantially to species richness, perhaps exceeding colonization by a factor of two or more as a contributor to species number. Colonization, extinction and speciation rates differ among taxonomic groups, with murid rodents being most successful and carnivores least successful. In order for any model of island biogeography to be widely applicable to insular faunas, the model must include speciation as a major variable. It is suggested that insular mammalian faunas typically are not in equilibrium, because geological and climatic changes can occur as rapidly as colonization and speciation.

Microcosms as Islands: A Test of the Macarthur‐Wilson Equilibrium Theory

A test of the MacArthur—Wilson equilibrium model of island biogeography was conducted using aquatic microcosms as island analogs. Fifty 400—mL beakers containing 300 mL of sterilized soil—water medium and 50 2000—mL beakers containing 1800 mL of medium were inoculated with replicated subsets of species from a species pool consisting of green, blue—green, and golden algae, ciliates, a rotifer, a dinoflagellate, and euglenoids. These species were added at two different average rates over a 28—wk period, resulting in four replicated sets of 25 insular communities. Community reorganization (turnover) was shown to occur, as well as relatively constancy in species richness for each beaker category, indicative of equilibrium. Contrary to the MacArthur—Wilson prediction, small microcosms had significantly higher species richness values than large microcosms. Microcosms subjected to the higher of the two invasion rates had significantly higher species richness values than those of the lower rate categories, primarily due to the influence of transient species. Early divergence into two broad community types is documented, suggestive of at least two stable community states that transcend microcosm categories.

Species turnover and the order versus chaos controversy concerning reef fish community structure

Species turnover of coral reef fishes was examined on small artificial natural and coral heads. Data show that the time interval between samples has a small effect on measurements of absolute turnover but greatly affects estimates of turnover rate. Results suggest that differences between the order and chaos schools of reef fish community structure may have originated in part as an artifact of the time interval between samples. Calculated turnover rates were high and in general agreement with the MacArthur-Wilson model of island biogeography. The usefulness of island biogeographic models for understanding the dynamics of coral reef fishes is discussed.

Zoogeography of West Indian Vertebrates in Relation to Pleistocene Climatic Cycles

In the course of our inquiries into the fossil record of late Pleistocene terrestrial vertebrates of the West Indies, we have become impressed by the number of extinctions of species characteristic of arid habitats, savannas, or grasslands. The fossils themselves come mostly from areas now too mesic to support an extensive xerophilic fauna. This observation has led to the supposition that during the last glaciation, the West Indies were drier than they are now, and that those species presently restricted to xeric habitats are probably relicts of this period of aridity. The available data on late Pleistocene climates and sea levels are concordant with the hypothesis that the extinction of a considerable number of vertebrate species was a result of climatic changes since the end of the Pleistocene, 10-12,000 years ago. We find the application of this concept to be of great use in interpreting puzzling patterns of distribution. Because there is a compelling need for a better historical perspective in many ecological models of island biogeography, we have attempted to synthesize the literature on the late Pleistocene fossil record and to relate this to the influence of Pleistocene climatic and sea level changes on vertebrate distributions in the West Indies. In this regard the remarks of Ernest Williams (129) seem especially cogent: Until proper account is taken of the Pleistocene and its consequences, all discussions of island diversity, extinction, and related phenomena in this latest period of geologic time will lack in realism. Evidence that Pleistocene glacial climates significantly altered

Historic Sites Archaeology on the Western American Frontier: Theoretical Perspectives and Research Problems

Historic sites archaeology in the western United States is booming but continues to be conducted ad hoc. The demands of assessing significance for cultural resource management purposes suggests that integrative research problems must be identified. One set of such problems emerge from the frontier concept. The use of synecological models from general ecology is proposed as a new framework within which to better understand frontier phenomena. As an illustration, one aspect of Frederick Jackson Turner's “frontier thesis” — the homogenization of frontier behavior — is examined in this light and related to historic sites research. In addition patterns of frontier colonization are studied with models of island biogeography developed by the late Robert MacArthur and E. O. Wilson.

Immigration and extinction probabilities for individual species: relation to incidence functions and species colonization curves.

We develop a dynamic model of island biogeography based on immigration and extinction probabilities of individual species rather than on the usual biogeographic parameters of the number of species immigrating or going extinct per unit time. In the real world, these probabilities vary enormously among species, and three field studies suggest lognormal distributions. Based on such distributions of individual probabilities, our model proceeds by calculating the conditional probability of species occurrence--the likelihood that a given species will occur on a given island--as a function of total species number on the island. The model succeeds in predicting two observed features of species communities; (i) staggered, sigmoidally-shaped incidence functions ("differing area requirements of different species") and (ii) concave immigration and extinction curves.

An assortative model of island biogeography

Abstract A model for the assortative phase of island biogeography is proposed which emphasizes interspecific interactions. This model is based upon trends between complexity and both relative stability and invulnerability as observed from computer simulations. The model indicates that island community complexity decreases with decreasing island size, increasing insularity or increasing environmental stochasticity, and that community complexity varies most with area on distant islands. Differences between estimated colonization times and relaxation times observed for island communities are ascribed to the progressive assortment of these communities toward more complementary species compositions. Greater relative endemism on more isolated islands is partially attributed to reduced interspecific invasion pressure. Invasion pressure is emphasized as a significant force in community organization.

Testing the Context and Extent of Host-Parasite Coevolution

Brooks, D. R. (Department of Biology, University of Notre Dame, Notre Dame, Indiana 46556) 1979. Testing the context and extent of host-parasite coevolution. Syst. Zool. 28:299307.-Coevolution is defined as a combination of two processes: co-accommodation between host and parasite with no implication of host or parasite speciation and co-speciation, indicating concomitant host and parasite speciation. Parasite speciation in is viewed as primarily the result of allopatric speciation processes regardless of host speciation or changes in host type. The observation that co-speciation of hosts and parasites forms a predominant pattern relates to a more principle of biotic allopatric speciation explicit in the vicariance biogeography model, more than to an assumption that host speciation somehow causes parasite speciation. The close ecological relationship between hosts and parasites, which may be depicted using a variation of the MacArthur-Wilson island biogeography model, explains their spatial proximity at any time during which an isolating event occurs and thus may be necessary in some cases, but is not sufficient to explain parasite speciation, coevolution, or parasite phylogeny. [Parasitology; coevolution; allopatric speciation; island biogeography; vicariance biogeography; speciation.] No science can free itself from its traditions; it is a difficult matter suddenly and entirely to give up doctrines which for generations have been regarded almost as axioms, even when the erroneous character of the doctrines in question can no longer be doubted. (Arthur Looss, 1911:185.) Given the name of a parasite species, a parasitologist can tell, with a great deal of consistency, the type of host and geographical locality in which the parasite occurred. Virtually any biologist, given a species of plant or animal with which he is familiar, can also predict accurately the geographical distribution and of that species, but this has assumed greater significance with parasites because their general habitat is another living organism. Parasitologists have assumed that, whereas free-living organisms might be free to move from one locality or to another, parasites are so dependent upon their hosts that evolutionarily they have no life of their own. Historically, parasites have often been studied in an attempt to elucidate aspects of host paleobiology (phylogeny or biogeography). Only in a few cases have hosts been utilized to examine aspects of parasite phylogeny and biogeography, but the conclusions have been the same whether one takes the host's view or the parasite's view, namely that host and parasite phylogenies and geographical distributions are highly correlated. Those conclusions led to the formulation of several Rules (see Inglis, 1971) concerning host-parasite coevolu-

Urban Ecosystems and Island Biogeography

In urban areas, continuing fragmentation of natural habitat, disturbance, and increasing isolation of individual ‘habitat islands’, has brought on almost general reduction in species richness. Sensitive species are being replaced by aggressive synanthropic ones. Continued loss seems inevitable, but reducing the rate of that loss is a worthwhile conservation goal. It is imperative that the numerous and varied habitats within each urban area be considered as interrelated and not as separate units. Island biogeography models supply the means for unifying the disparate elements in urban ecological studies and can provide a useful strategy for conservation.It is difficult to define colonization, extinction, and equilibrium, in disturbed, often transient, urban habitats. Pseudo- and successional turnover dominate. The occupation of each ‘habitat island’ is a function of its own geography and of its position relative to other ‘islands’. Each habitat may serve as isolated ‘island’, ‘stepping-stone’, or ‘corridor’, depending on its spatial relationships with other ‘habitat islands’ and with the nature of each organism present.‘Habitat islands’ in small cities appear to function like large or near oceanic islands, while those in large cities seem to respond like small or distant oceanic islands. In all cases, the analogy with land-bridge islands is appropriate. Continuing urbanization is leading to reduced ‘habitat island’ size, and increasing the isolation of units from one another and from the surrounding rural ‘reservoir’.

Turnover Rates in Insular Biogeography: Effect of Immigration on Extinction

Demographic and genetic contributions from conspecific immigrants tend to reduce extinction rates of insular populations. The MacArthur—Wilson model of island biogeography is modified to provide for this effect of immigration on extinction, which we call the rescue effect. This new model predicts that when immigration rates are high relative to extinction rates, turnover rate is directly related to the distance between an island and the source of colonizing species. A field study of the distribution of arthropods among isolated plants supports the model.