Species richness and structure of ant communities in a dynamic archipelago: effects of island area and age

Aim To assess how ant species richness and structure of ant communities are influenced by island age (disturbance history) in a dynamic archipelago. Location Cabra Corral dam, Salta Province, north‐west Argentina (25°08′ S, 65°20′ W). Methods Ant species richness on remaining fragments (islands) of a flooded forest was determined, as well as island area, isolation and age. Simple linear regressions were performed to assess relationships between ant species richness and those insular variables. Furthermore, a stepwise multiple linear regression analysis was conducted in order to determine the relative influence of each insular variable on ant species richness. Islands were categorized in two age classes (old and young) and co‐occurrence analyses were applied within each class to evaluate changes in community structure because of interspecific competition. Results Simple regression analyses indicated a moderate, positive effect of island area on ant species richness. Weak, marginally non‐significant relationships were found between ant species richness and both island isolation and island age, showing the tendency for there to be a decrease in ant species richness with island isolation and that ant species richness might be higher in old islands. The multiple regression analysis indicated that island isolation and age had no significant effects on the number of ant species, island area being the only independent variable retained in the analysis. On the contrary, whereas a random pattern of species co‐occurrence was found on young islands, ant communities in old islands showed a significantly negative pattern of species co‐occurrence, suggesting that the effect of competition on community structure was stronger on older islands than on younger islands. Main conclusions Island area was the most important variable explaining ant species richness on the islands of Cabra Corral dam. However, both island isolation and island age (or disturbance history) might also contribute to shape the observed community patterns. The present study also shows that island age significantly affects the strength with which interspecific interactions structure ant communities on islands.     

Effects of ecogeographic variables on genetic variation in montane mammals: implications for conservation in a global warming scenario

Aim Evolutionary theory predicts that levels of genetic variation in island populations will be positively correlated with island area and negatively correlated with island isolation. These patterns have been empirically established for oceanic islands, but little is known about the determinants of variation on habitat islands. The goals of this study were twofold. Our first aim was to test whether published patterns of genetic variation in mammals occurring on montane habitat islands in the American Southwest conformed to expectations based on evolutionary theory. The second aim of this research was to develop simple heuristic models to predict changes in genetic variation that may occur in these populations as a result of reductions in available mountaintop habitat in response to global warming. Location Habitat islands of conifer forest on mountaintops in the American Southwest. Methods Relationships between island area and isolation with measures of allozyme variation in four species of small mammal, namely the least chipmunk (Tamias minimus), Colorado chipmunk (Tamias quadrivittatus), red squirrel (Tamiasciurus hudsonicus), and Mexican woodrat (Neotoma mexicana), were determined using correlation and regression techniques. Significant relationships between island area and genetic variation were used to develop three distinct statistical models with which to predict changes in genetic variation following reduction in insular habitat area arising from global warming. Results Patterns of genetic variation in each species conformed to evolutionary predictions. In general, island area was the most important determinant of heterozygosity, while island isolation was the most important determinant of polymorphism and allelic diversity. The heuristic models predicted widespread reductions in genetic variation, the extent of which depended on the population and model considered. Main conclusions The results support a generalized pattern of genetic variation for any species with an insular distribution, with reduced variation in smaller, more isolated populations. We predict widespread reductions in genetic variation in isolated populations of montane small mammals in the American Southwest as a result of global warming. We conclude that climate‐induced reductions in the various dimensions of genetic variation may increase the probability of population extinction in both the short and long term.     

Island biogeography and the species richness of introduced mammals on New Zealand offshore islands

Aim To investigate and establish the significance of various island biogeographic relationships (geographical, ecological and anthropological) with the species richness of introduced mammals on offshore islands. Location The 297 offshore islands of the New Zealand archipelago (latitude: 34–47°S; longitude: 166–179°E). Methods Data on New Zealand offshore islands and the introduced mammals on them were collated from published surveys and maps. The species richness of small and large introduced mammals were calculated for islands with complete censuses and regressed on island characteristics using a Poisson distributed error generalized linear model. To estimate the ‘z‐value’ for introduced mammals on New Zealand islands, least‐squares regression was used [log₁₀ S vs. log₁₀ A]. Results High collinearity was found between the area, habitat diversity and elevation of islands. The island characteristics related to the species richness of introduced mammals differed predictably between large and small mammals. The species richness of introduced large mammals was mostly related to human activities on islands, whereas species richness of introduced small mammals was mostly related to island biogeographical parameters. The ‘z‐value’ for total species richness is found to be expectedly low for introduced mammals. Main conclusions Distance appears to have become ecologically trivial as a filter for introduced mammal presence on New Zealand offshore islands. There is strong evidence of a ‘small island’ effect on New Zealand offshore islands. The species richness of both small and large introduced mammals on these islands appears to be most predominantly related to human use, although there is some evidence of natural dispersal for smaller species. The ecological complexity of some islands appears to make them less invasible to introduced mammals. Some human activities have an interactive effect on species richness. A small number of islands have outlying species richness values above what the models predict, suggesting that the presence of some species may be related to events not accounted for in the models.     

The maximal body mass–area relationship in island mammals

Aim A positive power relationship between maximal body mass and land area has previously been reported of the form M_max ∝ Area^0.5 whilst allometric scaling theory predicts either M_max ∝ Area^1.33 or M_max ∝ Area¹. We provide an analysis of the maximal mass–area relationship for four island systems, to test the hypothesis that community relaxation following isolation converges in each case to a slope of Area^0.5. Location Islands of the Japanese archipelago, the western Mediterranean, the Sea of Cortés and Southeast Asia. Methods We calculated the relationship between island area and the maximal body mass of the largest mammal species on the island using linear regression models with log‐transformed variables, and tested the hypothesis that the slopes were not significantly different from 0.5. Results We found a slope of 0.47 within the Japanese archipelago, 0.42 for western Mediterranean islands, 0.73 for the Sea of Cortés islands and 0.50 for Southeast Asian islands. None of these slopes were significantly different from 0.5. Main conclusions Our results provide further empirical support for previous findings of a general maximal body mass–area relationship of M_max ∝ Area^0.5, but they deviate from theoretical predictions. We hypothesize that this mass–area relationship was the ultimate end point of community relaxation initiated by the isolation of the mammal communities. Maximal body mass on each island today probably reflects the interaction between energetic constraints, home range size and island area.     

Small island biogeography in the Gulf of California: lizards, the subsidized island biogeography hypothesis, and the small island effect

Aim We used insular lizard communities to test the predictions of two hypotheses that attempt to explain patterns of species richness on small islands. We first address the subsidized island biogeography (SIB) hypothesis, which predicts that spatial subsidies may cause insular species richness to deviate from species–area predictions, especially on small islands. Next, we examine the small island effect (SIE), which suggests small islands may not fit the traditional log‐linear species–area curve. Location Islands with arthropodivorous lizard communities throughout the Gulf of California. Methods To evaluate the SIB hypothesis, we first identified subsidized and unsubsidized islands based on surrogate measures of allochthonous productivity (i.e. island size and bird presence). Subsequently, we created species–area curves from previously published lizard species richness and island area data. We used the residuals and slopes from these analyses to compare species richness on subsidized and unsubsidized islands. To test for an SIE, we used breakpoint regression to model the relationship between lizard species richness and island area. We compared results from this model to results from the log‐linear regression model. Results Subsidized islands had a lower slope than unsubsidized islands, and the difference between these groups was significant when small islands were defined as < 1 km². In addition to comparing slopes, we tested for differences in the magnitude of the residuals (from the species–area regression of all islands) for subsidized vs. unsubsidized islands. We found no significant patterns in the residual values for small vs. large islands, or between islands with and without seabirds. The SIE was found to be a slightly better predictor of lizard species richness than the traditional log‐linear model. Main conclusions Predictions of the SIB hypothesis were partially supported by the data. The absence of a significant SIE may be a result of spatial subsidies as explained by the SIB hypothesis and data presented here. We conclude by suggesting potential scenarios to test for interactions between these two small island hypotheses. Future studies considering factors affecting species richness should examine the possible role of spatial subsidies, an SIE, or a synergistic effect of the two in data sets with small islands.     

Body size evolution in insular vertebrates: generality of the island rule

Aim My goals here are to (1) assess the generality of the island rule – the graded trend from gigantism in small species to dwarfism in larger species – for mammals and other terrestrial vertebrates on islands and island‐like ecosystems; (2) explore some related patterns of body size variation in insular vertebrates, in particular variation in body size as a function of island area and isolation; (3) offer causal explanations for these patterns; and (4) identify promising areas for future studies on body size evolution in insular vertebrates. Location Oceanic and near‐shore archipelagos, and island‐like ecosystems world‐wide. Methods Body size measurements of insular vertebrates (non‐volant mammals, bats, birds, snakes and turtles) were obtained from the literature, and then regression analyses were conducted to test whether body size of insular populations varies as a function of body size of the species on the mainland (the island rule) and with characteristics of the islands (i.e. island isolation and area). Results The island rule appears to be a general phenomenon both with mammalian orders (and to some degree within families and particular subfamilies) as well as across the species groups studied, including non‐volant mammals, bats, passerine birds, snakes and turtles. In addition, body size of numerous species in these classes of vertebrates varies significantly with island isolation and island area. Main conclusions The patterns observed here – the island rule and the tendency for body size among populations of particular species to vary with characteristics of the islands – are actually distinct and scale‐dependent phenomena. Patterns within archipelagos reflect the influence of island isolation and area on selective pressures (immigration filters, resource limitation, and intra‐ and interspecific interactions) within particular species. These patterns contribute to variation about the general trend referred to as the island rule, not the signal for that more general, large‐scale pattern. The island rule itself is an emergent pattern resulting from a combination of selective forces whose importance and influence on insular populations vary in a predictable manner along a gradient from relatively small to large species. As a result, body size of insular species tends to converge on a size that is optimal, or fundamental, for a particular bau plan and ecological strategy.     

Not as the crow flies: assessing effective isolation for island biogeographical analysis

Aim To evaluate the role of island isolation in explaining the distribution of vascular plant species in a dense freshwater archipelago, specifically comparing conventional measures of island isolation with landscape measures of island isolation. Location Data were collected from 35 islands within Massasauga Provincial Park on the eastern shores of the Georgian Bay, Ontario, Canada. Methods Sampled islands were located using stratified random selection based on location and size variation. The number of species was recorded along stratified random transects. Island isolation variables included distance to the mainland, distance to the nearest island, largest gap in a stepping‐stone sequence, distance to the closest upwind point of land, and a landscape measure of island isolation. The landscape measure of isolation was quantified as the percentage of the land area within 100, 250, 500, 1000, 1500 and 2000 m of each island’s perimeter. The isolation variables were calculated within a geographical information system (GIS). Dependent variables in the regression analyses included species richness, the logarithm of species richness and residuals of the species–area relationship. Independent variables included island isolation variables and their logarithmic transformations. Results Isolation plays a role, albeit small, in explaining species richness in the study area. In the regression analyses, the landscape measure of isolation provided a better fit than conventional measures of island isolation. Islands with less land than water within a 250‐m buffer were more effectively isolated and had fewer species present than islands surrounded by a greater proportion of water. Main conclusions Consistent with the species–isolation relationship, fewer species were present on more isolated islands within the Massasauga study area, as elucidated using a series of island buffers in a GIS. Applying a landscape measure of isolation to similar dense, freshwater archipelagos may elucidate species–isolation patterns not evident through conventional, straight‐line distance measurements of island isolation. The low value of the regression coefficients as well as the isolation history and high density of the Massasauga islands suggests caution in extending the results, especially to dissimilar archipelagos.     

Of mice and mammoths: evaluations of causal explanations for body size evolution in insular mammals

Aim We investigated the hypothesis that the insular body size of mammals results from selective forces whose influence varies with characteristics of the focal islands and the focal species, and with interactions among species (ecological displacement and release). Location Islands world‐wide. Methods We assembled data on the geographic characteristics (area, isolation, maximum elevation, latitude) and climate (annual averages and seasonality of temperature and precipitation) of islands, and on the ecological and morphological characteristics of focal species (number of mammalian competitors and predators, diet, body size of mainland reference populations) that were most relevant to our hypothesis (385 insular populations from 98 species of extant, non‐volant mammals across 248 islands). We used regression tree analyses to examine the hypothesized contextual importance of these factors in explaining variation in the insular body size of mammals. Results The results of regression tree analyses were consistent with predictions based on hypotheses of ecological release (more pronounced changes in body size on islands lacking mammalian competitors or predators), immigrant selection (more pronounced gigantism in small species inhabiting more isolated islands), thermoregulation and endurance during periods of climatic or environmental stress (more pronounced gigantism of small mammals on islands of higher latitudes or on those with colder and more seasonal climates), and resource subsidies (larger body size for mammals that utilize aquatic prey). The results, however, were not consistent with a prediction based on resource limitation and island area; that is, the insular body size of large mammals was not positively correlated with island area. Main conclusions These results support the hypothesis that the body size evolution of insular mammals is influenced by a combination of selective forces whose relative importance and nature of influence are contextual. While there may exist a theoretical optimal body size for mammals in general, the optimum for a particular insular population varies in a predictable manner with characteristics of the islands and the species, and with interactions among species. This study did, however, produce some unanticipated results that merit further study – patterns associated with Bergmann’s rule are amplified on islands, and the body size of small mammals appears to peak at intermediate and not maximum values of latitude and island isolation.     

‘On being the right size’ – Do aliens follow the rules?

Aim

To assess whether mammalian species introduced onto islands across the globe have evolved to exhibit body size patterns consistent with the ‘island rule,’, and to test an ecological explanation for body size evolution of insular mammals.

Location

Islands worldwide.

Methods

We assembled data on body mass, geographical characteristics (latitude, maximum elevation) and ecological communities (number of mammalian competitors, predators and prey) for 385 introduced populations across 285 islands, comprising 56 species of extant, non‐volant mammals. We used linear regression, ANCOVA and regression tree analyses to test whether introduced populations of mammals exhibit the island rule pattern, whether the degree of body size change increased with time in isolation and whether residual variation about the general trend can be attributed to the geographical and ecological characteristics of the islands.

Results

Introduced populations follow the predicted island rule trend, with body size shifts more pronounced for populations with greater residence times on the islands. Small mammals evolved to larger body sizes in lower latitudes and on islands with limited topographic relief. Consistent with our hypothesis on the ecology of evolution, body size of insular introduced populations was influenced by co‐occurring species of mammalian competitors, predators and prey.

Conclusion

The island rule is a pervasive pattern, exhibited across a broad span of geographical regions, taxa, time periods and, as evidenced here, for introduced as well as native mammals. Time in isolation impacts body size evolution profoundly. Body size shift of introduced mammals was much more pronounced with increasing residence times, yet far less than that exhibited by native, palaeo‐insular mammals (residence times > 10,000 years). Given the antiquity of many species introductions, it appears that much of what we view as the natural character and ecological dynamics of recent insular communities may have been rendered artefacts of ancient colonizations by humans and commensals.     

Orchids of the West Indies: predictability of diversity and endemism

Aim We examined phytogeographical patterns of West Indian orchids, and related island area and maximum elevation with orchid species richness and endemism. We expected strong species–area relationships, but that these would differ between low and montane island groups. In so far as maximum island elevation is a surrogate for habitat diversity, we anticipated a strong relationship with maximum elevation and both species richness and endemism for montane islands. Location The West Indies. Methods Our data included 49 islands and 728 species. Islands were classified as either montane (≥ 300 m elevation) or low (< 300 m). Linear and multivariate regression analyses were run to detect relationships between either area or maximum island elevation and species richness or the number of island endemic species. Results For all 49 islands, the species–area relationship was strong, producing a z‐value of 0.47 (slope of the regression line) and explaining 46% of the variation. For 18 relatively homogeneous, low islands we found a non‐significant slope of z = −0.01 that explained only 0.1% of the variation. The 31 montane islands had a highly significant species–area relationship, with z = 0.49 and accounting for 65% of the variation. Species numbers were also strongly related to maximum island elevation. For all islands < 750 km², we found a small‐island effect, which reduced the species–area relationship to a non‐significant z = 0.16, with only 5% of the variation explained by the model. Species–area relationships for montane islands of at least 750 km² were strong and significant, but maximum elevation was the best predictor of species richness and accounted for 79% of the variation. The frequency of single‐island endemics was high (42%) but nearly all occurred on just nine montane islands (300 species). The taxonomic distribution of endemics was also skewed, suggesting that seed dispersability, while remarkable in some taxa, is very limited in others. Montane island endemics showed strong species–area and species–elevation relationships. Main conclusions Area and elevation are good predictors of orchid species diversity and endemism in the West Indies, but these associations are driven by the extraordinarily strong relationships of large, montane islands. The species richness of low islands showed no significant relationship with either variable. A small‐island effect exists, but the montane islands had a significant relationship between species diversity and maximum elevation. Thus, patterns of Caribbean orchid diversity are dependent on an interplay between area and topographic diversity.     

Patterns and processes in insular floras affected by hurricanes

Aim To investigate species compositions, rates of species turnover, species–area and species–distance relationships and patterns of nestedness in the floras of small Bahamian islands, by comparing two groups of islands that had been differentially affected by two hurricanes. Location Small islands occurring on either side of Great Exuma near Georgetown, Bahamas. Methods We surveyed the plant species of 44 small islands over a 5‐year period from 1998 to 2002. Hurricanes Lili and Michelle occurred in 1996 and 2001, respectively; both storms affected small islands on the more exposed south‐west side of Great Exuma to a greater degree than small islands on the more protected north‐east side. A set of 27 islands was surveyed in 1998 and 2002 to evaluate species turnover. Stepwise multiple linear regression analyses and an information‐theoretic approach (the Akaike information criterion) were used to elucidate the importance of area and distance as predictors of plant species number. We compared a piecewise linear regression model with a simple linear regression of species number against area to determine whether a small island effect existed. Nestedness patterns were evaluated by Wilcoxon two‐sample tests to analyse occurrence sequences. Results Species turnover was low in an absolute sense (overall = 0.74% year^?1), yet was over three times higher than that documented in a nearby archipelago in the absence of hurricanes. Both vegetated area and distance were important predictor variables for exposed islands but not for protected islands. Some support was found for a small island effect for the exposed islands based on a piecewise linear regression model. Both island groups revealed significant nestedness at the level of the assemblage (both P < 0.001). On exposed islands, 65–79% (depending upon the method of calculation) of all species were significantly nested, but only 47% of all species were significantly nested on protected islands. Main conclusions Overall, these insular floras seem highly resistant to hurricane‐force disturbances. Species turnover was low (< 1% year^?1) in an absolute sense, particularly in comparison with rates for other taxa. Higher degrees of nestedness and significant species–area and species–distance relationships for exposed islands indicated stronger patterns of community assembly. It is likely that disturbance is a major structuring force for the exposed islands, although the type of disturbances that mediate these patterns may not be primarily hurricane‐force storms.     

The island syndrome and population dynamics of introduced rats

The island syndrome predicts directional changes in the morphology and demography of insular vertebrates, due to changes in trophic complexity and migration rates caused by island size and isolation. However, the high rate of human-mediated species introductions to some islands also increases trophic complexity, and this will reduce the perceived insularity on any such island. We test four hypotheses on the role of increased trophic complexity on the island syndrome, using introduced black rats (Rattus rattus) on two isolated coral atolls in the Mozambique Channel. Europa Island has remained relatively pristine and insular, with few species introductions, whereas Juan de Nova Island has had many species introductions, including predators and competitors of rats, anthropogenically increasing its trophic complexity. In the most insular environments, the island syndrome is expected to generate increases in body size and densities of rodents but decreases in the rates of reproduction and population cycling. Morphology and reproduction were compared using linear regression and canonical discriminant analysis, while density and population cycling were compared using spatially explicit capture–recapture analysis. Results were compared to other insular black rat populations in the Mozambique Channel and were consistent with predictions from the island syndrome. The manifestation of an island syndrome in rodents depends upon the trophic composition of a community, and may not relate to island size alone when many species additions, such as invasions, have occurred. The differing patterns of rodent population dynamics on each island provide information for future rodent eradication operations.     

The effect of island area on body size in a primate species from the Sunda Shelf Islands

Aim  We examine the effect of island area on body dimensions in a single species of primate endemic to Southeast Asia, the long-tailed macaque ( Macaca fascicularis ). In addition, we test Allen's rule and a within-species or intraspecific equivalent of Bergmann's rule (i.e. Rensch's rule) to evaluate body size and shape evolution in this sample of insular macaques.
Location  The Sunda Shelf islands of Southeast Asia.
Methods  Body size measurements of insular macaques gathered from the literature were analysed relative to island area, latitude, maximum altitude, isolation from the mainland and other islands, and various climatic variables using linear regression.
Results  We found no statistically significant relationship between island area and body length or head length in our sample of insular long-tailed macaques. Tail length correlated negatively with island area. Head length and body length exhibited increases corresponding to increasing latitude, a finding seemingly consistent with the expression of Bergmann's rule within a single species. These variables, however, were not correlated with temperature, indicating that Bergmann's rule is not in effect. Tail length was not correlated with either temperature or increasing latitude, contrary to that predicted by Allen's rule.
Main conclusions  The island rule dictating that body size will covary with island area does not apply to this particular species of primate. Our study is consistent with results presented in the literature by demonstrating that skull and body length in insular long-tailed macaques do not, strictly speaking, conform to Rensch's rule. Unlike previous studies, however, our findings suggest that tail-length variation in insular macaques does not support Allen's rule.     

Bird assemblages on Amazonian river islands: Patterns of species diversity and composition

The principles of island biogeography are rarely applied to the animal assemblages of Amazonian river islands. Here, we compare bird assemblages of Amazonian river islands with a variety of mainland habitats. We also examine how bird species diversity and composition are related to island physical attributes. Birds were sampled with mist nets and qualitative censuses on 11 river islands and 24 mainland sites on the lower reaches of the Rio Negro in the Brazilian Amazon. Island bird assemblages were characterized by lower species richness and a higher abundance of a few dominant species. Additionally, the species composition of the islands was distinct from that of the mainland, including the nearby floodplain habitats. The number of bird species increased with island size and habitat diversity, and decreased with degree of isolation. In addition, small islands tended to harbor an impoverished subset of the species present on larger ones. Bird species diversity and composition on Amazonian river islands are likely influenced by the ecological succession and historical events affecting island formation. Considering their small total area across the Amazon basin, these insular fluvial communities could be disproportionately threatened by river channel disturbances related to climate change or hydroelectric dam development. Abstract in Portughese is available with online material.     

Ant communities on small tropical islands: effects of island size and isolation are obscured by habitat disturbance and ‘tramp’ ant species

Aim Comparisons among islands offer an opportunity to study the effects of biotic and abiotic factors on small, replicated biological communities. Smaller population sizes on islands accelerate some ecological processes, which may decrease the time needed for perturbations to affect community composition. We surveyed ants on 18 small tropical islands to determine the effects of island size, isolation from the mainland, and habitat disturbance on ant community composition. Location Thousand Islands Archipelago (Indonesian name: Kepulauan Seribu) off Jakarta, West Java, Indonesia. Methods Ants were sampled from the soil surface, leaf litter and vegetation in all habitat types on each island. Island size, isolation from the mainland, and land‐use patterns were quantified using GIS software. The presence of settlements and of boat docks were used as indicators of anthropogenic disturbance. The richness of ant communities and non‐tramp ant species on each island were analysed in relation to the islands’ physical characteristics and indicators of human disturbance. Results Forty‐eight ant species from 5 subfamilies and 28 genera were recorded from the archipelago, and approximately 20% of the ant species were well‐known human‐commensal ‘tramp’ species. Islands with boat docks or human settlements had significantly more tramp species than did islands lacking these indicators of anthropogenic disturbance, and the diversity of non‐tramp species decreased with habitat disturbance. Main conclusions Human disturbance on islands in the Thousand Islands Archipelago promotes the introduction and/or establishment of tramp species. Tramp species affect the composition of insular ant communities, and expected biogeographical patterns of ant richness are masked. The island with the greatest estimated species richness and the greatest number of unique ant species, Rambut Island, is a forested bird sanctuary, highlighting the importance of protected areas in preserving the diversity of species‐rich invertebrate faunas.     

Mammals of Australian islands: factors influencing species richness

Distribution patterns of indigenous non-volant terrestrial mammals on 257 Australian islands were examined in relation to environmental parameters and the effects of human-induced disturbance during prehistoric and historic times on island species numbers. Species occurrence for individual species, for taxonomic and trophic groups, and for all species together was related to environmental parameters using regression analysis and the extreme-value function model. Patterns of occurrence were examined separately within three major biogeographic regions derived by pattern analysis. The number of species known to have occurred on these islands during historic times was adequately predicted from area alone. No statistically significant improvement in predicted species number was gained by including island elevation, mean annual rainfall, isolation from the mainland or the number of potentially competing species present on the island. Similarly, no single factor other than area was found to influence consistently the presence of individual species. We conclude that the occurrence of indigenous non-volant terrestrial mammal species on these islands indicates a relictual rather than equilibrial fauna. Visitation by Aboriginal people during prehistoric times did not significantly increase mammal extinctions on islands. Examination of patterns of species richness for a given area on a regional basis showed that islands in and around Bass Strait and Tasmania (Bass Region) were the most species-rich, islands off the northern coasts were slightly less rich, and islands off the south western coasts had fewest species. This is in contrast to the usual latitudinal gradient in species richness patterns. However, islands off the northern and eastern coasts had an overall greater number of different species. When considered in relation to the number of different species of mammals occurring within each region, islands of a given size in Bass Region typically bore a higher proportion of this species pool than other regions. The Bass Region was found to be particularly rich in macropoid herbivores and dasyurid carnivores and insectivores. Analyses indicated that there is a very strong relationship between the presence of exotics as a whole and the local extinction of native mammals. Many mammal species formerly widespread on the Australian mainland are now restricted totally to islands (nine species) or are threatened with extinction on the mainland and have island populations of conservation significance (ten species). In all, thirty-five islands protect eighteen taxa of Australian threatened mammals. The land-use and management of these islands is of considerable importance to nature conservation. The introduction of exotic mammals to these islands should be prevented; any introductions that occur should be eradicated immediately.     

A global model of island biogeography

Aim The goal of our study was to build a global model of island biogeography explaining bird species richness that combines MacArthur and Wilson's area–isolation theory with the species–energy theory. Location Global. Methods We assembled a global data set of 346 marine islands representing all types of climate, topography and degree of isolation on our planet, ranging in size from 10 ha to 800,000 km². We built a multiple regression model with the number of non‐marine breeding bird species as the dependent variable. Results We found that about 85–90% of the global variance in insular bird species richness can be explained by simple, contemporary abiotic factors. On a global scale, the three major predictors — area, average annual temperature and the distance separating the islands from the nearest continent — all have constraining (i.e. triangular rather than linear) relationships with insular bird species richness. We found that the slope of the species–area curve depends on both average annual temperature and total annual precipitation, but not on isolation. Insular isolation depends not only on the distance of an island from the continent, but also on the presence or absence of other neighbouring islands. Range in elevation — a surrogate for diversity of habitats — showed a positive correlation with bird diversity in warmer regions of the world, while its effect was negative in colder regions. We also propose a global statistical model to quantify the isolation‐reducing effect of neighbouring islands. Main conclusions The variation in avian richness among islands worldwide can be statistically explained by contemporary environmental variables. The equilibrium theory of island biogeography of MacArthur and Wilson and the species–energy theory are both only partly correct. Global variation in richness depends about equally upon area, climate (temperature and precipitation) and isolation. The slope of the species richness–area curve depends upon climate, but not on isolation, in contrast to MacArthur and Wilson's theory.     

Island biogeography of bats in Baja California, Mexico: patterns of bat species richness in a near-shore archipelago

Aim We studied the relationship between the size and isolation of islands and bat species richness in a near‐shore archipelago to determine whether communities of vagile mammals conform to predictions of island biogeography theory. We compared patterns of species richness in two subarchipelagos to determine whether area per se or differences in habitat diversity explain variations in bat species richness. Location Islands in the Gulf of California and adjacent coastal habitats on the Baja California peninsula in northwest Mexico. Methods Presence–absence surveys for bats were conducted on 32 islands in the Gulf of California using acoustic and mist‐net surveys. We sampled for bats in coastal habitats of four regions of the Baja peninsula to characterize the source pool of potential colonizing species. We fitted a semi‐log model of species richness and multiple linear regression and used Akaike information criterion model selection to assess the possible influence of log₁₀ area, isolation, and island group (two subarchipelagos) on the species richness of bats. We compared the species richness of bats on islands with greater vegetation densities in the southern gulf (n = 20) with that on drier islands with less vegetation in the northern gulf (n = 12) to investigate the relationship between habitat diversity and the species richness of bats. Results Twelve species of bats were detected on islands in the Gulf of California, and 15 species were detected in coastal habitats on the Baja peninsula. Bat species richness was related to both area and isolation of islands, and was higher in the southern subarchipelago, which has denser vegetation. Log₁₀ area was positively related to bat species richness, which increased by one species for every 5.4‐fold increase in island area. On average, richness declined by one species per 6.25 km increase in isolation from the Baja peninsula. Main conclusions Our results demonstrate that patterns of bat species richness in a near‐shore archipelago are consistent with patterns predicted by the equilibrium theory of island biogeography. Despite their vagility, bats may be more sensitive to moderate levels of isolation than previously expected in near‐shore archipelagos. Differences in vegetation and habitat xericity appear to be associated with richness of bat communities in this desert ecosystem. Although observed patterns of species richness were consistent with those predicted by the equilibrium theory, similar relationships between species richness and size and isolation of islands may arise from patch‐use decision making by individuals (optimal foraging strategies).     

Factors determining mammal species richness on habitat islands and isolates: habitat diversity, disturbance, species interactions and guild assembly rules

• 1 For over three decades the equilibrium theory of island biogeography has galvanized studies in ecological biogeography. Studies of oceanic islands and of natural habitat islands share some similarities to continental studies, particularly in developed regions where habitat fragmentation results from many land uses. Increasingly, remnant habitat is in the form of isolates created by the clearing and destruction of natural areas. Future evolution of a theory to predict patterns of species abundance may well come from the application of island biogeography to habitat fragments or isolates.
• 2 In this paper we consider four factors other than area and isolation that influence the number and type of mammal species coexisting in one place: habitat diversity, habitat disturbance, species interactions and guild assembly rules. In all examples our data derive from mainland habitat, fragmented to differing degrees, with different levels of isolation.
• 3 Habitat diversity is seen to be a good predictor of species richness. Increased levels of disturbance produce a relatively greater decrease in species richness on smaller than on larger isolates. Species interactions in the recolonization of highly disturbed sites, such as regenerating mined sites, is analogous to island colonization. Species replacement sequences in secondary successions indicate not just how many, but which species are included. Lastly, the complement of species established on islands, or in insular habitats, may be governed by guild assembly rules. These contributions may assist in taking a renewed theory into the new millennium.

     

Predators in natural fragments: foraging ecology of wolves in British Columbia's central and north coast archipelago

Aim Predator–prey dynamics in fragmented areas may be influenced by spatial features of the landscape. Although little is known about these processes, an increasingly fragmented planet underscores the urgency to predict its consequences. Accordingly, our aim was to examine foraging behaviour of an apex mammalian predator, the wolf (Canis lupus), in an archipelago environment. Location Mainland and adjacent archipelago of British Columbia, Canada; a largely pristine and naturally fragmented landscape with islands of variable size and isolation. Methods We sampled 30 mainland watersheds and 29 islands for wolf faeces in summers 2000 and 2001 and identified prey remains. We examined broad geographical patterns and detailed biogeographical variables (area and isolation metrics) as they relate to prey consumed. For island data, we used Akaike Information Criteria to guide generalized linear regression model selection to predict probability of black‐tailed deer (main prey; Odocoileus hemionus) in faeces. Results Black‐tailed deer was the most common item in occurrence per faeces (63%) and occurrence per item (53%) indices, representing about 63% of mammalian biomass. Wolves consumed more deer on islands near the mainland (65% occurrence per item) than on the mainland (39%) and outer islands (45%), where other ungulates (mainland only) and small mammals replaced deer. On islands, the probability of detecting deer was influenced primarily by island distance to mainland (not by area or inter‐landmass distance), suggesting limited recolonization by deer from source populations as a causal mechanism. Main conclusions Although sampling was limited in time, consistent patterns among islands suggest that population dynamics in isolated fragments are less stable and can result in depletion of prey. This may have important implications in understanding predator–prey communities in isolation, debate regarding wolf–deer systems and logging in temperate rain forests, and reserve design.