Insular Mammals Research Articles

The extent of extinctions of mammals on islands

Many of the world's oceanic and oceanic‐like islands possessed endemic mammal faunas before they were colonized by humans. These faunas, unbalanced and impoverished compared to continental faunas, usually lacked large mammalian carnivores. In virtually all cases, the arrival of humans and their domesticants and commensals on these islands is related to the extirpation of large numbers of endemic insular mammals. These extinction events affected at least 27% of autochthonous mammal species on the world's oceanic and oceanic‐like islands. This percentage rises the 35% when volant mammals are excluded. This reduction in the natural biodiversity brought about the disappearance of several unique biological types that apparently never existed on the continents.

Species Richness of Mammals Inhabiting Nearshore Archipelagoes: Area, Isolation, and Immigration Filters

Patterns in community structure of non-volant mammals on islands of the Drummond Archipelago (northwestern Lake Huron) were compared to those of insular mammals in-habiting three other nearshore archipelagoes to test the hypothesis that species-area and species-isolation relationships are strongly influenced by the nature of the immigration filter. All four archipelagoes shared similar habitats and nearly identical species pools, but differed substantially in characteristics of their filters. Islands of the Drummond Archipelago tended to have more species per area and more species per degree of geographic isolation. The rate of decline in species richness with increasing isolation was similar and relatively low for islands of the Drummond Archipelago and central Lake Michigan. In comparison to these lacustrine archipelagoes, which had relatively weak filters (high immigration rates), mammalian species richness declined much more rapidly with isolation of islands along the coast of Maine and the St. Lawrence River, where strong currents and instability of ice-cover impede immigration. These insights may have special relevance for conservation of endangered species, many of which are restricted to ecosystems fragmented and isolated at a scale similar to that of nearshore archipelagoes.

Immigrations and distribution patterns of insular mammals: studying fundamental processes in island biogeography

Immigrations and distribution patterns of insular mammals: studying fundamental processes in island biogeography

Winter filtering, immigrant selection and species composition of insular mammals of Lake Huron

Patterns in species composition of small mammals on islands of northwestern Lake Huron, USA were studied to test the hypothesis that insular community structure was strongly influenced by immigrant selection, l e, selection for the more vagile species Of 17 insectivores and rodents in the mainland pool nine were detected on at least one of the 25 islands studied These species comprised a biased subset of the mainland pool, biased in favor of species with relatively good winter immigration abilities The five true hibernators were absent from all islands studied and the two winter‐inactive species occurred on just two islands Frequency of insular occurrence increased with level of winter activity (hibernator vs winter‐inactive vs winter‐active) and with body size (p < 0.02)The winter filtering effect remains one of the best documented forms of immigrant selection In general however, insular community structure is influenced by the combined effects of selective extinction as well as selective immigration Common insular species are those which combine relatively high immigration abilities with low resource requirements

The Target Area Hypothesis: The Influence of Island Area on Immigration Rates of Non-Volant Mammals

Cross-ice movements of mammals in the Thousands Island Region of the St. Lawrence River, NY, USA, were analyzed to test for effects of island area on immigration rates, i.e., the target area hypothesis. The data were taken from 30 snow tracking surveys conducted during optimal tracking conditions over a four year period. Winter immigrations by the canids, non-canids and all mammals combined increased significantly with island area (P < 0.01). Thus, community structure of insular mammals in this and similar archipelagoes may be influenced by geometry of the islands and potential sources as well as the characteristics of the species and barrier in question.

Mammalian community structure on islands: the importance of immigration, extinction and interactive effects

A general review of the patterns of species richness of insular mammals (Lomolino, 1984a) indicated that richness is determined by interactive as well as additive effects of factors affecting immigration and extinction. The present paper reports that species composition of insular mammals is also influenced by such additive and interactive effects. Therefore, insular incidence should be high for those species whose (or on those islands where) (immigration rates) are high relative to extinction rates. The model presented in this paper predicts that species have high incidence on islands if low immigration rates (poor immigrators and/or distant islands) are compensated by low extinction rates (good survivors and/or large islands), or high extinction rates are compensated for by high immigration rates. Therefore, poor immigrators may be frequent inhabitants of distant islands if their extinction rates are compensatorily low (large islands and/or low resource requirements). Conversely, extinction-prone species (large, specialist carnivores) may be frequent inhabitants of small islands if their immigration rates are compensatorily high (near islands and/or good immigrators). These ‘compensatory effects’ were well evidenced by the mammalian faunas of the islands in the Thousand Islands Region, New York, and Lake Michigan (U.S.A.). ‘Compensatory effects’ are also evidenced by mammals of other archipelagos, as well as by birds inhabiting real and habitat islands. These results are consistent with the fundamental assumption of the equilibrium theory of island biogeography, i.e. insular community structure is the result of recurrent (rather than unique) immigrations and extinctions. Accordingly, I suggest that the concept of a fixed critical minimum area for isolated populations may be meaningless unless immigrations are unimportant with respect to the fauna under study. Finally, apparently anomalous or stochastic distribution patterns of insular species may readily be explained by the deterministic model presented here which incorporates the interactive as well as additive effects of immigration and extinctions on insular community structure.

Nested subsets and the structure of insular mammalian faunas and archipelagos

The nested subset hypothesis was formulated to describe and explain patterns in the community structure of insular mammal faunas which are in the state of ‘relaxation’. The hypothesis states that the species comprising a depauperate fauna should constitute a proper subset of those in richer faunas, and that an archipelago of such faunas arranged by species richness should present a nested series. The non-randomness of this pattern is evaluated for montane mammals in the American Southwest using Monte Carlo simulations under two sets of conditions. First, we constructed model archipelagos with the observed distribution of species richnesses, drawing individual species at random (without replacement) from the species pool (RANDOM0). Secondly, we constructed model archipelagos having the observed distribution of species richnesses, but weighted the selection of species by their actual frequencies of occurrence (RANDOM1). The degree of nestedness in the model archipelagos was then used to assess the non-randomness of the observed structure. Actual Southwestern mammal faunas have a far more nested structure than model archipelagos produced by either RANDOM0 or RANDOM1, and there is virtually zero probability that observed structure is represented in the distribution of scores from either simulation run. Similar analyses were conducted on other archipelagos to determine the generality of this relationship and to identify variables putatively responsible for its production. Mammal faunas of large islands off the coast of Maine, U.S.A., studied by Crowell (1986) also comprise nested subsets, as do those inhabiting islands off the coast of Baja California, Mexico, studied by Lawlor (1983). Significantly, when the Baja archipelago is divided into landbridge islands (which are presumed to be relaxing to lower species level) and oceanic islands (where species number is limited by successful overwater dispersal), only the former show significant nestedness under the more stringent conditions of RANDOM1. These results and theoretical considerations suggest that selective extinction of species may be chiefly responsible for the nestedness in natural archipelagos. Our conclusions have obvious implications for the design of natural preserves (e.g. SLOSS): several small fragments of a single biota can be expected to support nested subsets of the species originally present or that would likely be retained in a single large preserve. Even more sobering are arguments raised which suggest that the faunas of preserves established in different habitats within the same biome might be expected to converge in composition via selective extinction.

A comparison of relict versus equilibrium models for insular mammals of the Gulf of Maine

For the mammalian faunas of 24 landbridge islands in the Gulf of Maine (0.003–279 km2 in size), area accounts for 86% of variance in species richness. The slope, z, of the species-area curve is 0.247. For the seven largest islands (>10km2), the non-equilibrium hypothesis of relaxation following saturation in the post-Pleistocene is suggested by (1) elevated slope of the species-area curve (0.353), (2) correlation of species richness with island age (r=- 0.81) and water depth to mainland (r= -0.70), (3) highly non-random nested subsets of species ranked by island area, and (4) discontinuity with the extremely depauperate faunas of oceanic islands of the eastern North Atlantic. The alternative hypothesis of a dynamic equilibrium determined by recurrent immigration and extinction is supported by (1) documented turnover in 16 species, (2) correlation of species-area residuals with distance (r= - 0.90), (3) distribution dependent upon vagility with reduction or absence of hibernators and other poor dispersers, (4) low levels of endemism, and (5) congruence of community structure with that of mainland fauna for both trophic level and body size. I conclude that while some insular populations may be relictual, the faunal composition of most of these islands is dependent on recurrent colonization, much of which takes place over ice bridges. However, true equilibrium is perturbed by climatic shifts, range expansions, and human disturbance.

Small mammals on Massachusetts islands: the use of probability functions in clarifying biogeographic relationships.

Thirty-three insular small mammal communities along the coast of Massachusetts (USA) were surveyed to investigate the biogeographic relationships of the insular communities and to examine the distribution patterns of individual species. Nine species of terrestrial small mammals were observed in the total insular fauna, whereas thirteen occurred on the mainland. The species-area relation yielded a z value of 0.06, which is the lowest value yet reported for insular mammal communities.Multiple logistic regression was used to calculate probability functions for each species in order to identify variables potentially important in determining a species' occurrence on islands and to estimate probabilities of occurrence on islands. Statistically significant and ecologically interpretable functions were obtained for all but one species. Occurrence on islands was positively related to increasing island size in four species and to decreasing island isolation in four species. The extremely low z value, negative correlations of species number with isolation variables, and the inclusion of an isolation variable in the logistic functions of four species indicated that immigration was an important determinant of small mammal occurrence on these islands. There was a positive relationship between population density and number of islands occupied.Logistic regression has several advantages over linear discriminant function analysis, and we suggest that it may be useful in other ecological studies and in the preservation of endangered species.

Geographic and Climatic Correlates of Morphological Variation in Eptesicus fuscus

Skull size in continental North American Eptesicus fuscus is inversely related to temperature as predicted by the classical form of Bergmann's rule, but it is positively related to moisture in contradiction to James' (1970) reformulation of the rule. Greater food availability in moister environments is suggested as the explanation of the latter relationship. Wing size is inversely related to temperature, but the relationship is not statistically significant. Wing size is significantly greater in moister environments, suggesting that evaporation from wings is an important selective factor. The geographic coordinates latitude, longitude, and elevation explain significant amounts of morphological variation, but differentiation among the insular subspecies of E. fuscus is not consistent with a gene flow model if a Central American ancestor is assumed. The relationship between body size and island area in the Antillean subspecies of E. fuscus supports Heaney's (1978) selective model of body size in insular mammals. Overall, the results suggest that both climate (natural selection) and geographic factors (barriers to gene flow) are important causes of differentiation in E. fuscus .

ISLAND AREA AND BODY SIZE OF INSULAR MAMMALS: EVIDENCE FROM THE TRI-COLORED SQUIRREL (CALLOSCIURUS PREVOSTI) OF SOUTHEAST ASIA.

The presence of Pleistocene elephants on Crete only 1 m in shoulder height, of giant rodents on Caribbean Islands, and of other equally remarkable dwarf and giant mammals, has prompted a variety of responses from their startled discoverers. Most of the mechanisms suggested for their origins are simplistic-usually a single factor, ranging from extreme food deprivation to isolation of antediluvian species. To my knowledge, no one has described adequately either the pattern of variation of mammals on islands, or synthesized the many suggested causes (Van Valen, 1973). This paper is an initial attempt to redress this deficiency. It is not intended as a thorough review; rather, it presents significant new data, briefly summarizes trends in body size of mammals, and presents a causal model of body size evolution in insular mammals. It is possible that the processes and patterns discussed here apply to other animals but I have directed my studies to maimmals alone. Each topic discussed below is worthy of a lengthy review, and it is my hope that this paper will stimulate additional research and discussion. During a study of the systematics of Callosciurus prevosti, the tri-colored squirrel of Malaysia, Indonesia, and peninsular Thailand, it became evident that body size of these squirrels was related to the size of the island on which the squirrels lived. This observation stimulated a more detailed study of size variation in these squirrels and led to the present paper. Body size of C. prevosti is discussed at the beginning of the paper, because variation in this species provided insight into the evolution of body size of insular mammals and formed the basis of my hypotheses and models. The biology of C. prevosti is poorly known. Externally tri-colored.squirrels resemble the familiar North American tree squirrels but are more brightly colored. Most populations have a black dorsum and tail, a broad white lateral stripe, and a red venter; many have a prominent shoulder patch which may be black, gray, red, white, or a mixture of these. In Selangor, Malaysia, tri-colored squirrels nested in trees and fed on coconut and rubber seeds, and oil palm fruit (Harrison and Traub, 1950; Harrison, 1962). Davis (1962) stated that in Sabah, North Borneo, they were found in primary and old logged forest where they inhabited chiefly the smaller trees of the middle story. Stomach contents consisted of pulpy fruit and about 5% insect matter, chiefly ants and termites. Three pregnant females taken on 20 June, 24 June, and 2 August had two, two, and three embryos, respectively. At Bukit Lanjong, West Malaysia, they have been seen feeding on the fruit of fig trees (Ficus) in the upper story (MacKinnon and MacKinnon, 1974:36). Nothing definite is known of the predators or competitors of these squirrels. ' Parts of this study were carried out while the author was resident at the Division of Mammals, Smithsonian Institution, Washington, D.C. 20560, and the Bell Museum of Natural History, University of Minnesota, Minneapolis, Minnesota 55455.

Island Area and Body Size of Insular Mammals: Evidence from the Tri-Colored Squirrel (Callosciurus prevosti) of Southeast Asia

Island Area and Body Size of Insular Mammals: Evidence from the Tri-Colored Squirrel (Callosciurus prevosti) of Southeast Asia

Variation Among Insular Mammals of Georgia Strait, British Columbia

Variation Among Insular Mammals of Georgia Strait, British Columbia