Island Occupancy Research Articles

Time since first naturalization is key to explaining non‐native plant invasions on islands

AbstractAimInvestigating the extent of insular invasions by non‐native species (i.e., the number of islands they occupy) is central to island conservation. However, interrelationships among plant life history traits, naturalization histories, and island characteristics in determining island occupancy by non‐native plant species are poorly understood. We investigated whether island occupancy by different non‐native plant species declines in relation to their year of first naturalization and whether periods of first naturalization differ among growth forms, dispersal modes, and biogeographic origins. Then, we asked if non‐native plants that naturalized more recently occur more frequently on islands that are large, less isolated, and close to urban areas. We contrasted trends across growth forms, dispersal modes, and biogeographic origins.Location264 offshore islands in northern Aotearoa New Zealand.TaxaVascular plant species.MethodsWe combined field surveys and published data for 767 non‐native plant species on the islands. We categorized each species according to its growth form (n = 3), dispersal mode (n = 4) and biogeographic origin (n = 5) and identified its year of first naturalization in Aotearoa New Zealand. We tested our hypotheses using ANCOVA and generalized linear models (GLMs).ResultsThere were similar declines in island occupancy in relation to the year of first naturalization in Aotearoa New Zealand across all trait and biogeographic origin categories. First naturalization times of herbaceous species, those with unspecialized dispersal modes, and those originating from Eurasia and the Mediterranean basin were disproportionately earlier than other categories. Non‐native plants with more recent first naturalization occur more frequently on large islands close to urban areas, but not on less isolated ones. Relationships with island characteristics did not differ among trait and biogeographic origin categories.Main ConclusionsOverall, time of first naturalization was more important than trait and biogeographic origin categories in explaining non‐native plant invasion patterns on islands. Since there were similar relationships between island occupancy and the year of first naturalization in Aotearoa New Zealand for all categories, management bodies should focus on non‐native plant species of trait and biogeographic origin categories that have naturalized recently (e.g., woody species from other regions within Oceania), and on large islands close to urban areas. Introduction and naturalization histories provide essential context for interpreting the role of plant traits and biogeographic origin in understanding plant invasions on islands.

Functional traits explain non-native plant species richness and occupancy on northern New Zealand islands

Plant functional traits can greatly influence invasion success on islands. However, interrelationships between traits and invasion success are rarely integrated with the island biogeography theory. Here, we explored relations between functional traits and plant distributions to assess which traits are associated with invasion success (i.e. high island occupancy), test whether non-native richness and seed mass of species with distinct growth forms and dispersal modes vary differently with island characteristics (e.g. area, isolation, exposure to ocean-borne disturbances, distance from the nearest urban area, and whether islands were managed for conservation), and whether results differ from native species. We assembled a database of 264 northern New Zealand offshore islands, amalgamating species lists from field surveys and previously published data, and comprising 822 native and 855 non-native species. Non-native graminoids occurred on islands more frequently than forbs and woody species, and long-distance dispersal modes (wind, animal, unspecialized) more than species with short-distance dispersal modes. Most differences among trait categories of non-native species were associated with human-related variables (i.e. distance from the nearest urban area, and whether islands were conservation areas). Non-native plant species with high island occupancy were less commonly associated with human-related variables than non-native species with low island occupancy. Instead, they were more similar in their distributional patterns to native species within the same trait category, suggesting comparable processes regulate both sets of species. Our results illustrate that integrating trait-based approaches in the island biogeography framework can be a useful tool in understanding and predicting plant invasions.

Ecological traits modulate bird species responses to forest fragmentation in an Amazonian anthropogenic archipelago

AbstractAimWe assessed patterns of avian species loss and the role of morpho‐ecological traits in explaining species vulnerability to forest fragmentation in an anthropogenic island system. We also contrasted observed and detectability‐corrected estimates of island occupancy, which are often used to infer species vulnerability.LocationTucuruí Hydroelectric Reservoir, eastern Brazilian Amazonia.MethodsWe surveyed forest birds within 36 islands (3.4–2,551.5 ha) after 22 years of post‐isolation history. We applied species–area relationships to assess differential patterns of species loss among three data sets: all species, forest specialists and habitat generalists. After controlling for phylogenetic non‐independence, we used observed and detectability‐corrected estimates of island occupancy separately to build competing models as a function of species traits. The magnitude of the difference between these estimates of island occupancy was contrasted against species detectability.ResultsThe rate of species loss as a function of island area reduction was higher for forest specialists than for habitat generalists. Accounting for the area effect, forest fragmentation did not affect the overall number of species regardless of the data set. Only the interactive model including natural abundance, habitat breadth and geographic range size was strongly supported for both estimates of island occupancy. For 30 species with detection probabilities below 30%, detectability‐corrected estimates were at least tenfold higher than those observed. Conversely, differences between estimates were negligible or non‐existent for all 31 species with detection probabilities exceeding 45.5%.Main conclusionsPredicted decay of avian species richness induced by forest loss is affected by the degree of habitat specialisation of the species under consideration, and may be unrelated to forest fragmentation per se. Natural abundance was the main predictor of species island occupancy, although habitat breadth and geographic range size also played a role. We caution against using occupancy models for low‐detectability species, because overestimates of island occupancy reduce the power of species‐level predictions of vulnerability.

Disturbance and diversity in a continental archipelago: a mechanistic framework linking area, height, and exposure

AbstractSpecies–area relationships (SARs) are among the most general patterns in nature. Yet, significant variation in species richness often remains after accounting for area, especially for small islands. One factor thought to influence species richness on small islands is disturbance from the combined influence of tides and waves. Here, we derive a quantitative framework for determining how ocean disturbance impacts island communities, which we then test in plant communities in a temperate island archipelago. We do so by applying some well‐developed techniques honed in the marine intertidal zone but rarely applied to studies on land. By estimating and adjusting for the effect of wave exposure on habitable island area, we dramatically improved the fit of the SAR for small islands, nearly doubling the amount of variation in species richness explained (from 37% to 69%). Our predictions of island occupancy also improved using this method. Our approach predicts that small islands (<100,000 m2) are the most affected by ocean‐borne disturbance. Given that many archipelagos are susceptible to wave disturbance, future studies should consider how ocean‐borne disturbances arising from the matrix might interact with sampling area to influence patterns of species richness on small islands.

Ecological correlates of vulnerability to fragmentation in forest birds on inundated subtropical land-bridge islands

Identifying the ecological and life-history traits that render species vulnerable to fragmentation is an important prerequisite for the development of effective conservation strategies to minimize future biodiversity losses. When determining how species traits influence vulnerability to fragmentation, however, several important confounding factors such as detectability and synergistic effects among traits are rarely considered. In this study, after controlling for these methodological shortcomings, we determined how species traits influenced fragmentation vulnerability using bird data collected from islands created by the inundation of the Thousand Island Lake, China. We obtained eight species traits from field surveys and from the literature: natural abundance, geographical range size, habitat specificity, body size, trophic level, mobility, fecundity, and nest type. After phylogenetic correction, these traits were used separately and in combination to assess their associations with the index of fragmentation vulnerability, the proportion of islands occupied. Inclusion of detectability in analysis resulted in considerable increases in overall island occupancy for all species in general and for cryptic species in particular. Accounting for detectability altered the rank of best models and thus influenced the identification of the relationships between species traits and fragmentation vulnerability. We found synergistic interactions between natural abundance and habitat specificity. Our findings highlight the importance of incorporating detectability and synergistic effects among traits into future studies. From a conservation perspective, our results suggest that we should give priority conservation efforts to rare species with low natural abundance and high habitat specificity.

Predicting local extinctions of Amazonian vertebrates in forest islands created by a mega dam

Hydropower projects are rapidly expanding across lowland Amazonia, driving the conversion of large tracts of once-continuous forests into archipelagos embedded within a vast open-water matrix. Forest vertebrate populations thus become stranded in habitat islands, with their persistence governed by a combination of species life-history traits, habitat patch, and landscape context. We investigate the patterns of species extinction of 34 arboreal and terrestrial vertebrate species within three continuous forest sites and 37 land-bridge islands within one of the largest South American hydroelectric reservoirs, based on a combination of camera trapping, line-transect censuses, sign surveys, and armadillo burrow counts. Forest area was the best predictor of species persistence, so we classified all species into three levels of vulnerability to habitat insularization, with most species defined as ‘area-sensitive’. However, island occupancy was decisively determined by individual species traits, with wide-ranging species and poor dispersers showing high local extinction rates. We detected higher island occupancy rates of large vertebrate species compared to other Neotropical fragmented forest landscapes, suggesting that this is critically attributed to the absence of hunting pressure at Balbina. Nevertheless, most terrestrial and arboreal species have been driven to local extinction within the vast majority of islands, which have been largely defaunated. We predicted species composition across all 3546 islands within the reservoir, indicating that only ⩽2% of all islands continue to harbour at least 75% of all species. To minimise loss of vertebrate diversity, future hydroelectric dam projects in lowland tropical forests, if unavoidable, should consider their geographic location and landscape structure to maximise both island size and landscape connectivity, and set aside strictly protected reserves within reservoir areas.

Island tameness: living on islands reduces flight initiation distance

One of Darwin's most widely known conjectures is that prey are tame on remote islands, where mammalian predators are absent. Many species appear to permit close approach on such islands, but no comparative studies have demonstrated reduced wariness quantified as flight initiation distance (FID; i.e. predator-prey distance when the prey begins to flee) in comparison with mainland relatives. We used the phylogenetic comparative method to assess influence of distance from the mainland and island area on FID of 66 lizard species. Because body size and predator approach speed affect predation risk, we included these as independent variables. Multiple regression showed that FID decreases as distance from mainland increases and is shorter in island than mainland populations. Although FID increased as area increased in some models, collinearity made it difficult to separate effects of area from distance and island occupancy. FID increases as SVL increases and approach speed increases; these effects are statistically independent of effects of distance to mainland and island occupancy. Ordinary least-squares models fit the data better than phylogenetic regressions, indicating little or no phylogenetic signal in residual FID after accounting for the independent variables. Our results demonstrate that island tameness is a real phenomenon in lizards.

Nestedness in island faunas: novel insights into island biogeography through butterfly community profiles of colonization ability and migration capacity

AbstractAim To relate variation in the migration capacity and colonization ability of island communities to island geography and species island occupancy.Location Islands off mainland Britain and Ireland.Methods Mean migration (transfer) capacity and colonization (establishment) ability (ecological indices), indexed from 12 ecological variables for 56 butterfly species living on 103 islands, were related to species nestedness, island and mainland source geography and indices using linear regression models, RLQ analysis and fourth‐corner analysis. Random creation of faunas from source species, rank correlation and rank regression were used to examine differences between island and source ecological indices, and relationships to island geography.Results Island butterfly faunas are highly nested. The two ecological indices related closely to island occupancy, nestedness rank of species, island richness and geography. The key variables related to migration capacity were island area and isolation; for colonization ability they were area, isolation and longitude. Compared with colonization ability, migration capacity was found to correlate more strongly with island species occupancy and species richness. For island faunas, the means for both ecological indices decreased, and variation increased, with increasing island species richness. Mean colonization ability and migration capacity values were significantly higher for island faunas than for mainland source faunas, but these differences decreased with island latitude.Main conclusions The nested pattern of butterfly species on islands off mainland Britain and Ireland relates strongly to colonization ability but especially to migration capacity. Differences in colonization ability among species are most obvious for large, topographically varied islands. Generalists with abundant multiple resources and greater migration capacity are found on all islands, whereas specialists are restricted to large islands with varied and long‐lived biotopes, and islands close to shore. The inference is that source–sink dynamics dominate butterfly distributions on British and Irish islands; species are capable of dispersing to new areas, but, with the exception of large and northern islands, facilities (resources) for permanent colonization are limited. The pattern of colonization ability and migration capacity is likely to be repeated for mainland areas, where such indices should provide useful independent measures for assessing the conservation status of faunas within spatial units.

Species traits predict island occupancy in noctuid moths

Knowing how species’ traits relate to processes that underlie occupancy patterns such as colonisation and population persistence, is important for our understanding of how species survive in fragmented and changing landscapes. We used automatic UV light-traps to sample noctuid moths on two remote islands, and compared traits of island occupants with those of a species pool from mainland southeast Sweden. Widely distributed species, generalist species, species with a long adult activity period and species active late in the summer had higher probability of occupancy on the remote islands. The results were consistent between islands. The traits of host plant specificity and species with an adult activity period during late summer remained robust and were statistically significant after controlling for any possible phylogenetic bias. This indicates that species exhibiting those traits survive better when habitat and climate changes. It is crucial to include our results in; (1) conservation planning, e.g. when devising conservation measures in fragmented landscapes; (2) for predictions of future occupancy patterns; and (3) ecosystem impact assessments, e.g. the importance of moths as pollinators, herbivores and being the functional link between parasitoids, plants, consumers and predators.

Explanations for bird species range size: ecological correlates and phylogenetic effects in the Canary Islands

AbstractAim To explore the determinants of island occupancy of 48 terrestrial bird species in an oceanic archipelago, accounting for ecological components while controlling for phylogenetic effects.Location The seven main islands of the Canary archipelago.Methods We obtained field data on population density, habitat breadth and landscape distribution in Tenerife, Fuerteventura and La Palma, aiming to sample all available habitats and the gradient of altitudes. In total, 1715 line transects of 0.5 km were carried out during the breeding season. We also reviewed the literature for data on occupancy, the distance between the Canary Islands and the nearest distribution border on the mainland, body size and endemicity of the 48 terrestrial bird species studied. Phylogenetic eigenvector regression was used to quantify (and to control for) the amount of phylogenetic signal.Results The two measurements of occupancy (number of occupied islands or 10 × 10 km UTM squares) were tightly correlated and produced very similar results. The occupancy of the terrestrial birds of the Canary Islands during the breeding season had a very low phylogenetic effect. Species with broader habitat breadth, stronger preferences for urban environments, smaller body size, and a lower degree of endemicity had a broader geographical distribution in the archipelago, occupying a larger number of islands and 10 × 10 UTM squares.Main conclusions The habitat‐generalist species with a tolerance for novel urban environments tend to be present on more islands and to occupy a greater area, whereas large‐sized species that are genetically differentiated within the islands are less widespread. Therefore, some properties of the ranges of these species are explicable from basic biological features. A positive relationship of range size with local abundance, previously shown in continental studies, was not found, probably because it relies on free dispersal on continuous landmasses, which may not be applicable on oceanic islands.

Patterns of island occupancy in bats: influences of area and isolation on insular incidence of volant mammals

ABSTRACTAim The influence of landscape structure on the distribution patterns of bats remains poorly understood for many species. This study investigates the relationship between area and isolation of islands and the probability of occurrence of six bat species to determine whether persistence and immigration abilities vary among bat species and foraging guilds.Location Thirty‐two islands in the Gulf of California near the Baja California peninsula in north‐west Mexico.Methods Using logistic regression and Akaike information criterion (AIC) model selection, we compared five a priori models for each of six bat species to explain patterns of island occupancy, including random patterns, minimum area effects, maximum isolation effects, additive area and isolation effects and compensatory area and isolation effects.Results Five species of insectivorous bats (Pipistrellus hesperus, Myotis californicus, Macrotus californicus, Antrozous pallidus and Mormoops megalophylla) displayed minimum area thresholds on incidence. The probability of occurrence tended to decrease at moderate distances of isolation (c. 10–15 km) for these species (excepting A. pallidus). The distributions of two non‐insectivorous species (Leptonycteris curasoae and Myotis vivesi) were not influenced by island size and isolation.Main conclusions Minimum area thresholds on incidence suggest that island occupancy by insectivorous bats may be limited by resource requirements. Islands smaller than 100 ha typically did not support occupancy or use by insectivorous bats, except at minimal isolation distances. Insectivorous bat species may also be more sensitive to moderate levels of habitat isolation in some landscape contexts than previously expected. Our results suggest that differences in foraging habits may have important implications for understanding the distribution patterns of bats.

Are ground beetles neutral?

Species–abundance distribution; Nestedness; Colonisation Summary The neutral model approach to community ecology is currently intensively discussed but has mainly been tested for its ability to mimic observed patterns like species–abundance or abundance–occupancy distributions. Less well studied is the question whether neutral models are able to explain these distributions and other ecological patterns like species co-occurrences simultaneously. Here we use a random draw and a spatially explicit neutral model to test whether both models are able to predict patterns of island colonisation by ground beetles. We show that island colonisation is not a simple random draw from the mainland metacommunity. The neutral model was able to mimic basic patterns of species diversity, island occupancy, and island abundances. Both models, however, were unable to produce observed non-random patterns of species co-occurrences and nestedness while predicting random distributions of species across islands. The neutral model produced patterns of nestedness and abundance–occupancy relationships that were intermediate between the observed patterns and the predictions from a simple random draw. Our results add further to the view that a neutral model might be the appropriate null model for the study of ecological communities. Its ability to mimic basic patterns of biodiversity helps to sharpen our perception of niche differences and competitive effects that might cause deviations from the model. It seems better suited to distinguish between random effects and species interactions than simple random draw models.

Extinction, colonization, and distribution patterns of common eider populations nesting in a naturally fragmented landscape

Spatial distribution, patchy environments, and population turnover have many fundamental implications for conservation ecology. Common eider ( Somateria mollissima L., 1758) population processes were investigated in Labrador, Canada, between 1998 and 2003. We predicted that local colonies would exhibit population turnover, that extinction would be negatively related to colony and patch size, that colonization would be negatively related to island isolation, and that intraspecific incidence–abundance relationships would be positive. We found that small colonies were prone to extinction, but patch size was not a significant predictor of extinction, nor was colonization related to isolation. The overall observed annual extinction and colonization rates were 0.11 ± 0.02 and 0.41 ± 0.06, respectively, and showed variation across archipelagos. At two spatial scales we found that mean colony size was a positive predictor of island occupancy (incidence), and these relationships were maintained across years. Our findings show that common eider colonies in Labrador are dynamic and have greater turnover rates than previously expected in a species that is considered highly philopatric. Our findings support the notion that highly mobile organisms such as migratory birds can exhibit characteristics associated with metapopulation processes.

Facts from Faeces: Prey Remains in Wolf, <em>Canis lupus</em>, Faeces Revise Occurrence Records for Mammals of British Columbia's Coastal Archipelago

Archipelagos often harbour taxa that are endemic and vulnerable to disturbance. Conservation planning and research for these areas depend fundamentally on accurate and current taxonomic inventories. Although basic ecological information is in its infancy, the temperate rainforest islands of coastal British Columbia are undergoing rapid human-caused modification, particularly logging. We report herein new mammal records for these islands as determined by prey remains in the faeces of Wolves (Canis lupus), the area’s apex mammalian terrestrial predator. Of particular interest is our detection of Marten (Martes americana) on islands previously inventoried and island occupancy by Moose (Alces alces), which have apparently migrated recently to coastal British Columbia. Remains in faeces provided valuable new species occurrence information, but more extensive and focused inventories are required to generate predictions of island occupancy by mammals based on biotic and abiotic landscape features.

The concept of the taxon cycle in biogeography

AbstractTaxon cycles are sequential phases of expansion and contraction of the ranges of species, associated generally with shifts in ecological distribution. The important contribution of the taxon cycle to biogeographical analysis is its emphasis on evolutionary and ecological interactions among colonizing and resident species, which influence their extinction dynamics and establish patterns of geographical distribution. Taxon cycles were inferred originally from the distribution of species across island archipelagos, where a correlation was noted between gaps in island occupancy and the degree of phenotypic differentiation. This pattern implied that phases of colonization were followed by range contraction, while endemic Antillean species that were undifferentiated between islands suggested secondary expansion and the beginning of a new cycle. This interpretation was met with scepticism, but reconstruction of phylogenetic relationships from gene sequences has now permitted us to characterize taxon cycles in Lesser Antillean birds. The relative timing of phases of the cycle can be deduced from genetic divergence between island populations. We have found that taxon cycles have periods in the order of 106 years and that cycles in different lineages occur independently of each other and independently of Pleistocene climate cycles. Individual island populations may persist for several millions of years on the larger islands of the Lesser Antilles; occasional expansion phases lead to the replacement of island populations that have disappeared, thus reducing the archipelago‐wide rate of extinction to nil. What drives taxon cycles is unknown, but we speculate that they may be caused by co‐evolution with enemy populations, and a probable mechanism would involve infrequent mutations influencing parasite virulence and avian host disease resistance. Taxon cycles undoubtedly occur on continents, but the geographical configuration of island archipelagos reveals more clearly their presence and invites their study.

Body size and migration rate in moths

Migration rate is often thought to be affected by species distribution, abundance, body size, and niche width, but empirical results are controversial and fragmentary. In this study we examined these relationships in a large assemblage of noctuid moths. Migration rate was measured using two approaches, directly with a mark‐recapture study in a network of small islands, and indirectly on the basis of the occurrence of moths outside their breeding habitat. The effects of the factors assumed to affect migration rate were adjusted for taxonomy using a simple yet novel approach based on logistic regression. Both with and without adjusting for taxonomy, the results indicate that abundance and body size influence migration rate, that the effects of abundance and body size have a negative interaction, and that the effects of ecological specialization on migration rate are evident (monophagous species migrate less than oligophagous or polyphagous species). The incidence of island or habitat patch occupancy was not affected by body size, most likely because body size has several contrasting consequences on the processes that determine island occupancy. Migration rate appears to be an evolutionarily labile character, which can readily transform in different phylogenetical lineages of moths.

Metapopulations of moths on islands: a test of two contrasting models

1. We describe a generalized mainland‐island metapopulation model which includes migration among the island populations. We test model predictions with quantitative data on more than 200 species of moths in two contrasting networks of small islands. The data include a direct measure of migration rate, based on trapping of moths on rocky skerries with no local populations of the vast majority of species. 2. We predicted that moths which are strong fliers but uncommon on the islands have a higher incidence on scattered islands than on islands in a group, because the latter ‘compete’ for immigrants from the mainland. In contrast, we predicted that weakly flying species with potentially large local populations on the islands occur more frequently on islands in a group due to enhanced colonization rate. 3. Both predicted patterns were observed. Island occupancy increased significantly with the number of individuals caught on the rocky skerries, which is our measure of migration rate from the mainland, supporting the basic assumption that the species occur on the islands in a balance between colonizations and extinctions. 4. These results demonstrate that the moth metapopulations on islands represent a mixture of Levins’s and mainland‐island metapopulations, and that the mixture is different for different species in the same landscape.

Landscape Ecology of the Wart-Biter Decticus verrucivorus in a Patchy Landscape

1. Populations of wart-biter Decticus verrucivorus living on habitat islands in an intensively exploited agricultural landscape in south-eastern Norway were studied. Auditory recordings of singing males indicated that 27 (39%) of 70 habitat islands contained populations in the first year of the study, while 2 years (one wart-biter generation) later the number was 16 (23%). Dispersing male wart-biters were also observed, and the dispersal distances measured. 2. Predictors of island occupancy were identified by applying logistic regression models to the pattern of presence/absence, probing the predictive power of both landscape variables (island size and isolation) as well as various microhabitat quality variables (vegetation, aspect and slope). The isolation of habitat islands relative to surrounding wart-biter populations was measured by two types of connectivity indices: one based on the empirical probability distribution of dispersal distances, the other on the negative-exponential dispersal function. 3. Habitat area and the connectivity indices were by far the best predictors of occupancy. The probability of occupation increased with increasing area and decreasing isolation. Of the unoccupied habitat islands, the ones with observations of single males were less isolated than the ones without. From the pattern of occupancy, the average dispersal distance was estimated to be 40m, while the observed average dispersal distance of males was 37 m. 4. In addition to the effects of area and connectivity, the probability of occupation was positively influenced by increasing slope to the south, increasing amounts of the plant Achillea millefolium, and decreasing fraction of vegetation lower than 10 cm. The probability of local population extinction increased with declining habitat area and increasing fraction of low vegetation. 5. The situation of the wart-biter in this highly fragmented study area seems to largely fit a metapopulation model: extinctions and absence of wart-biters were adequately predicted by habitat island size and isolation, and observed dispersal distances closely matched estimated dispersal distances from the model.

Risk of Population Extinction in Moths: Effect of Host Plant Characteristics

The relationship between the risk of population extinction and population size was inferred from the parameters of an incidence function model, fitted to data on island occupancy of moths in an archipelago in SW Finland. The extinction risk declined faster with increasing population size in moths feeding on coniferous than deciduous woody plants, on perennials than annuals, and on habitat generalist than habitat specialist plant species. Coniferous woody plants, perennials and habitat generalist plants represent more stable resources for moths than deciduous woody plants, annuals and habitat specialist plants, which appears to explain the observed pattern. The relationship between extinction risk and population size was not affected by the characteristics of the moths, with the possible exception of more rapidly declining extinction risk with increasing population size in polyphagous than monophagous species. These results suggest that patterns in the risk of extinction of herbivorous moths are significantly affected by the host plant characteristics rather than by the characteristics of the moths themselves.

Patterns of Island Occupancy Explained by Colonization and Extinction Rates in Shrews

This is the first study reporting patterns of island occupancy in a guild of mammals with detailed information on the processes affecting these patterns: dispersal, colonization, and extinction. We surveyed the presence of three species of shrews, Sorex araneus, S. caecutiens, and S. minutus, on 108 islands in three lakes in Finland. A significant compensatory effect between island area and isolation was detected in one species, but most islands were so little isolated (< 1 km) that isolation played no great role in this study. The position and the slope of the incidence function varied greatly among the species. Fitting the model J = 1/(1 / y/Ax) to the observed incidence functions, and assuming that the annual extinction probability is 1 on islands of A = 0.5 ha, yielded quantitative predictions of the rates of colonization and extinction in the three species. Observed rates agreed remarkably well with the predicted ones (median difference 20%). Overwater dispersal rate and/or ability was lowest in the smallest species (S. minutus), probably because of its short starvation time and slow swimming rate. Unexpectedly, this species had the highest colonization ability (rate of successful colonization conditional on arriving at an island). There were no significant differences between the species in colonization rate, but interspecific differences in extinction rate were highly significant and negatively correlated with body size. We suggest that these differences are due to increasing importance of environmental stochasticity with decreasing body size in shrews. This hypothesis is supported by larger coefficients of variation of abundance in small than in large species on the mainland.