The form of the curves: a direct evaluation of MacArthur & Wilson's classic theory

1. We calculate the yearly numbers of bird species immigrating to – and becoming extinct on – 13 small islands of the British Isles, using a long and relatively complete data record.
2. We estimate the size of the colonist pool for each island using four methods.
3. We assume that immigrations and extinctions are distributed binomially, and use a maximum likelihood method to fit concave immigration and extinction functions to the data, utilizing all four species pool estimates.
4. Extinction rates increase significantly and consistently with increasing numbers of breeding species on each island. For nine of the 13 islands the extinction functions are significantly concave.
5. Immigration rates decrease consistently with increasing numbers of breeding species on each island. Seven islands have significantly concave immigration functions.
6. Immigration rates and extinction rates decline consistently, but not significantly, with island distance and island size, respectively. The number of breeding species does not always reflect the number of species likely to have reached an island. Moreover, some species may choose not to breed when their chance of extinction is high. These factors, plus the modest range of island areas and distances in our database, reduce our chances of finding the theoretically predicted effects of area and distance on extinction and immigration rates.     

Vascular plant species richness in relation to habitat diversity and island area in the Finnish Archipelago

Aim The aim of this study is to explore the interrelationships between island area, species number and habitat diversity in two archipelago areas. Location The study areas, Brunskär and Getskär, are located in an archipelago in south‐western Finland. Methods The study areas, 82 islands in Brunskär and 78 in Getskär, were classified into nine habitat types based on land cover. In the Brunskär area, the flora (351 species) was surveyed separately for each individual habitat on the islands. In the Getskär area, the flora (302 species) was surveyed on a whole‐island basis. We used standard techniques to analyse the species–area relationship on a whole‐island and a habitat level. We also tested our data for the small island effect (SIE) using breakpoint and path analysis models. Results Species richness was significantly associated with both island area and habitat diversity. Vegetated area in particular, defined as island area with the rock habitat subtracted, proved to be a strong predictor of species richness. Species number had a greater association with island area multiplied by the number of habitats than with island area or habitat number separately. The tests for a SIE in the species–area relationship showed the existence of a SIE in one of the island groups. No SIE could be detected for the species–vegetated area relationship in either of the island groups. The strength of the species–area relationship differed considerably between the habitats. Main conclusions The general principles of island biogeography apply well to the 160 islands in this study. Vascular plant diversity for small islands is strongly influenced by physiographic factors. For the small islands with thin and varying soil cover, vegetated area was the most powerful predictor of species richness. The species–area curves of various habitats showed large variations, suggesting that the measurement of habitat areas and establishment of habitat‐based species lists are needed to better understand species richness on islands. We found some evidence of a SIE, but it is debatable whether this is a ‘true’ SIE or a soil cover/habitat characteristics feature.     

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.     

Patterns of invertebrate density and taxonomic richness across gradients of area,isolation, and vegetation diversity in a lake‐island system

Over the past half century, ecologists have tried to unravel the factors that drive species richness patterns in ecological communities. One influential theory is island biogeography theory (IBT), which predicts that island or habitat area and isolation are drivers of species richness. However, relatively few studies testing IBT have considered invertebrate or belowground communities, and it is unclear as to whether the predictions made by IBT hold for these communities. Other theories predict that habitat characteristics such as vegetation diversity may be important drivers of invertebrate species richness. To investigate patterns of invertebrate density and species richness across gradients of area, isolation, and vegetation diversity, we used a system of 30 lake islands in the boreal zone of northern Sweden. We assessed density and taxonomic richness of ground‐dwelling spiders, web‐building spiders, beetles, collembolans, mites, and nematodes, for all islands during two consecutive summers. For all invertebrate groups, both density and taxonomic richness were either neutrally or negatively related to island size, and either neutrally or positively related to island isolation. Meanwhile the density and taxonomic richness for several groups was positively related to vegetation diversity (i.e. habitat heterogeneity). In multiple regression analyses, island size was often the single best predictor for both invertebrate density and taxonomic richness, but in some cases island size and isolation in combination explained more variation than each factor considered singly. Contrary to IBT predictions, invertebrate density and richness was never positively related to island size or negatively related to island isolation. Instead, our results suggest that plant diversity (and thus habitat heterogeneity) was the main driver of the patterns that we found, although other factors could have some influence. We conclude that several factors, but not necessarily those predicted as important by IBT, are important in determining invertebrate abundance and species richness in island systems.     

Butterflies of European islands: the implications of the geography and ecology of rarity and endemicity for conservation

Depending on their faunal content islands can function as important ‘vehicles’ for conservation. In this study, we examine data on 440 butterfly species over 564 European islands in 10 island groups. To determine the status of the butterfly fauna, we have adopted two approaches, island-focused and species-focused, examined using principal components analysis and regression modelling. In the former, we relate species richness, rarity and endemicity to island geography (area, elevation, isolation and location in latitude and longitude); in the latter, species occurrence on islands is examined in relation to distribution, range, range boundaries, and altitudinal limits on the continent as well as species’ ecology (number of host plants) and morphology (wing expanse). Species on islands are also assessed for their status on the continental mainland, their distributional dynamics (extinctions, distribution changes) and conservation status (Red Data Book, European Habitat Directive, Species of European Conservation Concern and Bern Convention listing. Unexpectedly, we find that a large fraction of the European butterfly species is found on the islands (63.4%; 59% on small islands) comprising some 6.2% of the land area of Europe. Although species occurring on the islands tend, on the whole, to have lower conservation status and are not declining over Europe, 45 species are endemics restricted to the islands. Species richness shows only a weak locational pattern and is related as expected to isolation from the continental source and island area; but, both rarity and endemicity have distinctive geographical bias to southern Europe, on islands now under increasing pressure from climate change and increasingly intensive human exploitation. The vulnerability of species on islands is emphasised in the relationship of island occurrence (% occurrence and presence/absence of species on any island) with continental distributions. A large proportion of the variation (84%) is accounted by continental distribution, the southern range limit and lower altitudinal limit. Most species (69%) occur on very few islands (<5%). In view of ongoing species dynamics on islands, migrations and extinctions of species, island repositories of species depend in large part on conservation of butterflies at continental sources. The unique faunas and rare species on islands also depend on appropriate concern being given to the island faunas. Conservation of European islands is thus a two-way process, sustaining sources and conserving island refuges. Residuals from the regressions (islands with more or fewer species, rare and endemic species; species occurring more or less frequently than expected on islands) provide warning signals of regions and islands deserving immediate attention.     

How island size and isolation affect bee and wasp ensembles on small tropical islands: a case study from Kepulauan Seribu,Indonesia

Aim To study the effects of isolation and size of small tropical islands on species assemblages of bees (superfamily Apoidea) and wasps (superfamily Vespoidea). Location Twenty islands in the Kepulauan Seribu Archipelago off the coast of west Java, Indonesia. The size of surveyed islands ranges between 0.75 and 41.32 ha; their distance from the coast of Java varies between 3 and 62 km. Methods Field work was conducted from February to May 2005. Bees and wasps were caught with a sweep net during sampling units of 15 min, continuing until four consecutive samples revealed no new species. Total species richness was quantified by the estimators Chao 2, first‐order jackknife and Michaelis–Menten. The software binmatnest was used to test for nestedness of species assemblages. Similarities of species composition between islands were quantified by Sørensen’s similarity index. Results Eighty‐two species were recorded on the 20 surveyed islands. Species richness declined with increasing isolation of islands from the source area, Java. Although the size of the largest island exceeded that of the smallest island by a factor of almost 60, island size only very weakly affected species richness of bees; no effect of island size was found for wasps. Mean body size of species decreased with increasing island isolation. Nestedness of island faunas was only weakly developed. Species composition of both superfamilies was affected by island isolation, but not by island size. Main conclusions While the species–isolation relationship on the very small islands of Kepulauan Seribu followed the prediction of MacArthur and Wilson’s equilibrium theory, the absence of a species–area relationship indicated a weak ‘small‐island effect’, at least in wasps. The combination of an only weakly developed pattern of nested species subsets, the shift in species compositions and the decline of mean body size with increasing island isolation from the source area indicates that biotic interactions and different species traits contribute to the shaping of communities of bees and wasps within the archipelago. The potential of biotic interactions for generating distribution patterns of species within the archipelago is also emphasized by the observed restriction of some species with apparently high dispersal abilities to outer islands.     

Geographical distribution pattern and interisland movements of Orii’s flying fox in Okinawa Islands, the Ryukyu Archipelago, Japan

The study of mobile animals such as flying foxes in insular habitats involves clarifying the population status on each island and determining the factors affecting movement patterns among the islands in their distributional range. We visited 25 of the Okinawa Islands and documented the number of Orii’s flying foxes Pteropus dasymallus inopinatus from August 2005 to May 2006. We also conducted a monthly road census on the main island (Okinawa-jima Island) and six adjacent islands from June 2006 to January 2007 and counted the number of fruit-bearing trees of the bats’ four main food plants. The results of classification and regression tree analysis suggested that distance from the main island was a primary factor in determining the distribution pattern and population size of this flying fox, whereas island area, number of plant species, and food availability did not directly affect population size. The number of flying foxes on each island tended to decrease with an increase in distance from the main island; no flying foxes existed on islands >30 km away from the main island. On the other hand, the results of the monthly census showed that the population size on each island fluctuated seasonally. Individuals may move between islands in response to seasonal changes in food availability. In conclusion, the distribution and abundance of Orii’s flying foxes in the Okinawa Islands may be determined by the rate of immigration/emigration, depending on each island’s distance from the main island. Seasonal changes in food availability may act as a trigger for interisland movement, but that movement may be restricted by island connectivity.     

On the insularity of islands

We propose that islands arc "less insular" than is generally perceived. This belief results, in part, from the paucity of studies on vagrant species that exploit islands but are not permanent residents with continual breeding populations. We show, via aneedotal evidence extracted from the literature and data acquired ON Gulf of California islands, that visitors to insular systems are fairly common. We delineate three types of events beyond the bounds of current biogeographical analysis that make islands less insular: I) migrants and "accidental" visitors. 2) individuals of a species whose foraging areas encompass many islands or the mainland and islands, and 3) species who "colonize" islands during opportune periods or years but become extinct during difficult times (source-sink situations). Such events potentially significantly affect the ecology and evolution of island inhabitants by such means as increased predation and/or competition, transport of parasites and pathogens, dispersal of seeds and eggs, and genetic introgression and hybridization. Discussion of other "insular" habitats such as freshwater lakes and wildlife refuges illustrate that vagrancy events may be nearly ubiquitous. Studies addressing the frequency and ecological and evolutionary significance of vagrants are required, especially in light of recent and rapid extinctions on islands and the increasing fragmentation of habitats.     

History and the species-area relationship in Lesser Antillean birds

We examined the species-area relationship for three historically distinct subsets of Lesser Antillean birds identified by molecular phylogenetic analysis of island and continental populations. The groups comprised recent colonists from continental or Greater Antillean source populations, old taxa having recently expanded distributions within the Lesser Antilles, and old endemic taxa lacking evidence of recent dispersal between islands. The number of young taxa was primarily related to distance from the source of colonists in South America. In a multiple regression, the logarithmic slope of the species-area relationship for this group was shallow (0.066+/-0.016). Old endemic taxa were restricted to islands with high elevation, and within this subset, species richness was related primarily to island area, with a steep slope (0.719+/-0.110). The number of recently spread endemic taxa was related primarily to island elevation, apparently reflecting the persistence of such populations on islands with large areas of forested and montane habitats. Historical analysis of the Lesser Antillean avifauna supports the dynamic concept of island biogeography of MacArthur and Wilson, rather than the more static view of David Lack, in that colonists exhibit dispersal limitation and extinction plays a role in shaping patterns of diversity. However, the avifauna of the Lesser Antilles is probably not in equilibrium at present, and the overall species-area relationship might reflect changing proportions of historically distinguishable subsets of species.     

From plots to islands: species diversity at different scales

Aim To investigate how plant diversity of whole islands (‘gamma’) is related to alpha and beta diversity patterns among sampling plots within each island, thus exploring aspects of diversity patterns across scales. Location Nineteen islands of the Aegean Sea, Greece. Methods Plant species were recorded at both the whole‐island scale and in small 100 m² plots on each island. Mean plot species richness was considered as a measure of alpha diversity, and six indices of the ‘variation’‐type beta diversity were also applied. In addition, we partitioned beta diversity into a ‘nestedness’ and a ‘replacement’ component, using the total species richness recorded in all plots of each island as a measure of ‘gamma’ diversity. We also applied 10 species–area models to predict the total observed richness of each island from accumulated plot species richness. Results Mean alpha diversity was not significantly correlated with the overall island species richness or island area. The range of plot species richness for each island was significantly correlated with both overall species richness and area. Alpha diversity was not correlated with most indices of beta diversity. The majority of beta diversity indices were correlated with whole‐island species richness, and this was also true for the ‘replacement’ component of beta diversity. The rational function model provided the best prediction of observed island species richness, with Monod’s and the exponential models following closely. Inaccuracy of predictions was positively correlated with the number of plots and with most indices of beta diversity. Main conclusions Diversity at the broader scale (whole islands) is shaped mainly by variation among small local samples (beta diversity), while local alpha diversity is not a good predictor of species diversity at broader scales. In this system, all results support the crucial role of habitat diversity in determining the species–area relationship.     

Biogeographical determinants for total and endemic species richness in a continental archipelago

We examined the relationship between plant species richness and biogeographical variables (island area, island maximum elevation, distance from nearest inhabited island, distance from nearest mainland) using a data set comprising 201 islands of the Aegean archipelago. We found that endemic species richness was strongly correlated to total species richness. Single-island endemic species richness was most strongly correlated to island maximum elevation, and then to island area, with an apparent small island effect for islands smaller than 47 km². Total species richness was most strongly correlated to island area (with no apparent small island effect), and less strongly correlated to island maximum elevation. Distance from the mainland or other inhabited islands displayed limited predictive value in our data set. The slope of the relationship between species richness and geographical factors (island area, elevation, distance from island/mainland) was steeper for endemic species richness than for total richness. Finally, the different scales of endemicity (single-island endemics, island group endemics and Aegean regional endemics) displayed similar qualitative trends and only differed quantitatively. Thus, we conclude that different biogeographical factors act as drivers for total species richness than for endemic species richness.     

Environmental correlates of body size distributions of European springtails (Hexapoda: Collembola)

Aim Species–body size distributions (SBDs) are plots of species richness across body size classes. They have been linked to energetic constraints, speciation–extinction dynamics and to evolutionary trends. However, little is known about the spatial variation of size distributions. Here we study SBDs of European springtails (Collembola) at a continental scale and test whether minimum, average and maximum body size and the shapes of size distributions change across latitudinal and longitudinal gradients and whether SBDs of islands and mainlands differ. We also test whether the island rule and the positive body size–range size relationship of vertebrates also holds for Collembola. Location Europe. Methods We use a unique data set on the spatial distributions of 2102 species of European springtails across 52 countries and larger islands together with associated data on body size, area, climate variables, longitude and latitude. Differences in the central moments of SBDs are inferred from simultaneous spatial autoregression models. Results The SBD of the European Collembola and its largest suborder Entomobryomorpha is unimodal and symmetrical. Average, minimum and maximum body weight and the skewness of the mainland/island SBDs peaked at intermediate latitudes. We could not find simple latitudinal gradients in minimum and maximum body weight. Average and maximum body size increased with country/island area in accordance with the island rule in vertebrates, while minimum body size did not significantly differ between islands and mainlands. Finally, we found a weak but statistically significant positive correlation of range size and body size. Main conclusions We provide evidence for differences in body size distributions between islands and mainlands that are in part in line with the island rule in invertebrates. We also find evidence for an interspecific body size–range size relationship similar to that of vertebrates although the vertebrate pattern is much stronger than the springtail pattern. Our results on latitudinal gradients of maximum and average body size imply the need to account for species richness and area effects in the study of latitudinal gradients in body size. We recommend implementing sample size and area effects in the study of body size distributions on islands and mainlands.     

Insect biogeography in the south-western Sea of Korea with comments on the insect fauna of Kwanmae Island

The insect species richness of each island in the south‐western Sea of Korea was considered on the background of the equilibrium theory. The species number of insects on Kwanmae Island in the present study (140 species) was much higher compared with a previous survey. Based on a literature survey of island biota surveys published in the 1980s, of 47 islands, the lowest species number (12 species) was on Kwanmae Island, and the most diverse insect fauna (254 species) was on Baekryong Island. The mean species number of surveyed islands was approximately 54 (53.96 with a standard deviation of 46.95). The median species number was 38 with a skew of +2.56. Insects, including the orders Odonanta, Orthoptera, Hemiptera, Lepidoptera, Coleoptera, Hymenoptera and Diptera, occurred on 32 of the 47 surveyed islands, and had an occurrence rate of more than 0.68. This indicates that these insects are distributed widely on the islands in the south‐western Sea of Korea. The species number showed a significant linear relationship with both area of an island and its distance from the mainland (P < 0.05), with an extremely low determinant coefficient (r² = 0.13 for area vs species number and r² = 0.28 for distance vs species number). Other factors tested in the study failed to show a significant relationship with species number. A multiple‐regression model established using area and distance as independent variables showed significant relationship with species number, with a relatively higher determinant coefficient (r² = 0.70, P < 0.05). We present possible explanations to explain the difference between estimated and observed species number in Kwanmae Island.     

History, island area and habitat availability determine land snail species richness of Aegean islands

Aim We examined the species-area relation of Aegean land snails, comparing different models to describe the relation. By examining those factors other than area that may also affect species richness, we tested whether the Aegean land snail fauna was more influenced by equilibrial migration and colonization processes, or rather is conservative and relictual. Location The Aegean archipelago (Greece). Methods Sixty-five islands were examined. Data were taken from own collections and from literature sources. Multiple regression analysis was used to test the null hypothesis of no relationship between species richness and island area, elevation, distance to the next larger island, and the presence and extent of calcareous substrate. Results The single most important factor determining land snail species number was area. While colonization-extinction dynamics have frequently been cited to explain this result, this conclusion was not tenable in this study as it was contradicted by species number not being related to the islands’ distances to neighbouring larger islands, after accounting for other factors affecting species number. We also found that habitat diversity affected species richness even after accounting for the effects of area: both increased elevation and greater extent of calcareous substrate on islands resulted in higher species number. This effect was most likely due to the fact that particular ecological conditions increased the probability that particular species could survive on an island. We compared the utility of the power and extreme-value function models of the species-area relation and found that both gave substantially the same results. However, fitting the power function model using nonlinear regression was of questionable utility. Main conclusions We conclude that the snail fauna of the Aegean is relictual, not equilibrial. The unusually high number of land snail species found on Crete is consistent with this conclusion. Crete is a currently united island which was separated into at least six smaller islands for 7–9 million years during the Neogene. Our results are consistent with the hypothesis that Crete still hosts a large number of endemic species of these paleoislands, resulting in a total number of species in excess of what would be expected based on area alone.     

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.     

Drivers of diversity in Macaronesian spiders and the role of species extinctions

Aim To identify the biogeographical factors underlying spider species richness in the Macaronesian region and assess the importance of species extinctions in shaping the current diversity. Location The European archipelagos of Macaronesia with an emphasis on the Azores and Canary Islands. Methods Seven variables were tested as predictors of single‐island endemics (SIE), archipelago endemics and indigenous spider species richness in the Azores, Canary Islands and Macaronesia as a whole: island area; geological age; maximum elevation; distance from mainland; distance from the closest island; distance from an older island; and natural forest area remaining per island – a measure of deforestation (the latter only in the Azores). Different mathematical formulations of the general dynamic model of oceanic island biogeography (GDM) were also tested. Results Island area and the proportion of remaining natural forest were the best predictors of species richness in the Azores. In the Canary Islands, area alone did not explain the richness of spiders. However, a hump‐shaped relationship between richness and time was apparent in these islands. The island richness in Macaronesia was correlated with island area, geological age, maximum elevation and distance to mainland. Main conclusions In Macaronesia as a whole, area, island age, the large distance that separates the Azores from the mainland, and the recent disappearance of native habitats with subsequent unrecorded extinctions seem to be the most probable explanations for the current observed richness. In the Canary Islands, the GDM model is strongly supported by many genera that radiated early, reached a peak at intermediate island ages, and have gone extinct on older, eroded islands. In the Azores, the unrecorded extinctions of many species in the oldest, most disturbed islands seem to be one of the main drivers of the current richness patterns. Spiders, the most important terrestrial predators on these islands, may be acting as early indicators for the future disappearance of other insular taxa.     

Insular black files (Diptera: Simuliidae) of North America: tests of colonization hypotheses

Aim To understand factors that facilitate insular colonization by black flies, we tested six hypotheses related to life‐history traits, phylogeny, symbiotes, island area, and distance from source areas. Location Four northern islands, all within 150 km of the North American mainland, were included in the study: Isle Royale, Magdalen Islands, Prince Edward Island, and Queen Charlotte Islands. Methods Immature black flies and their symbiotes were surveyed in streams on the Magdalen Islands, and the results combined with data from similar surveys on Isle Royale, Prince Edward Island, and the Queen Charlotte Islands. Black flies were analysed chromosomally to ensure that all sibling species were revealed. Tests of independence were used to examine the frequency of life‐history traits and generic representation of black flies on islands vs. source areas. Results A total of 13–20 species was found on each of the islands, but no species was unique to any of the islands. The simuliid faunas of the islands reflected the composition of their source areas in aspects of voltinism (univoltine vs. multivoltine), blood feeding (ornithophily vs. mammalophily), and phylogeny (genus Simulium vs. other genera). Five symbiotic species were found on the most distant island group, the Magdalen Islands, supporting the hypothesis that obligate symbiotes are effectively transported to near‐mainland islands. An inverse relationship existed between the number of species per island and distance from the source. The Queen Charlotte Islands did not conform to the species–area relationship. Main conclusions The lack of precinctive insular species and an absence of life‐history and phylogenetic characteristics related to the presence of black flies on these islands argue for gene flow and dispersal capabilities of black flies over open waters, possibly aided by winds. However, the high frequency of precinctive species on islands 500 km or more from the nearest mainland indicates that at some distance beyond 100 km, open water provides a significant barrier to colonization and gene exchange. An inverse relationship between number of species and distance from the source suggests that as long as suitable habitat is present, distance plays an important role in colonization. Failure of the Queen Charlotte Islands to conform to an area–richness trend suggests that not all resident species have been found.     

Colonization of Sphagnum on land uplift islands in the Baltic Sea: time, area, distance and life history

Aim To test whether species richness of Sphagnum mosses on islands in a land uplift archipelago is related to island age, area or connectivity, and whether the frequency of different species can be predicted by their life history and autecology. Location The northern Stockholm archipelago in the Baltic Sea, east‐central Sweden, with a current land uplift rate of 4.4 mm year^?1. Methods We sampled 17 islands differing in area (0.55–55 ha), height (3.6–18 m, representing c. 800–4000 years of age) and distance from mainland (1.6–41 km). For each Sphagnum patch we measured area, height above sea level, horizontal distance from the shore and shading from vascular plants. Factors affecting island species richness, species frequency and habitats on the islands were tested by stepwise regressions. Species frequency was tested on nine life history and autecological variables, including estimated abundance and spore output on the mainland, habitat preference and distribution. Results We recorded 500 patches of 19 Sphagnum species, distributed in 83 rock pools on 14 islands. Island species richness correlated positively with island area and with degree of shelter by surrounding islands, while distance from the mainland, connectivity, height or age did not add to the model. Species frequency (number of colonized islands and rock pools) was mainly predicted by spore output on the mainland and by habitat preference (swamp forest species were more frequent than others), while spore size, for example, did not add to the model. Species differed in mean height above and horizontal distance from the shore, area of occupied rock pools and in the degree of shading of patches. The mean horizontal distance from the shore and the area of occupied rock pools correlated positively with the normal growth position above the water table among species. Spore capsules were found in only 2% of patches, mostly in the bisexual Sphagnum fimbriatum. Main conclusions The presence of Sphagnum in the Stockholm archipelago seems to be governed by regional spore production and habitat demands. Sphagnum does not appear to be dispersal limited at distances up to 40 km and time spans of centuries. Species with a high regional spore output have had a higher colonization rate, which, together with the rarity of spore capsules on the islands, indicate the mainland as a source for colonization rather than dispersal among islands. Swamp forest species seem more tolerant to the island conditions (summer droughts and some salt spray) than open mire species. The different distances from the sea occupied by the species indicate a slow, continuous succession and species replacement towards the island interior as islands are being uplifted and thus expand in area. This partly explains why larger islands harbour more species. Our results thus support some of the island biogeographical theories related to the species–area relationship.     

The influence of geographical and ecological factors on island beta diversity patterns

Aim To investigate the importance of various island characteristics in determining spatial patterns of variations in beta diversity for various animal groups. Location Analyses are presented for 10 animal groups living on the Aeolian Islands, a volcanic archipelago in the central Mediterranean, near Sicily. Methods Three hypotheses were formulated to explain patterns of beta diversity: the target‐area–distance effect, stepping stone dispersal and island age. Matrices of inter‐island dissimilarities were constructed under each hypothesis and correlated with matrices of faunal dissimilarities using Mantel tests. For the ‘target‐area–distance effect’ hypothesis, inter‐island dissimilarities were calculated using island sizes and distances to nearest mainland areas. For the ‘stepping stone dispersal’ hypothesis, inter‐island distances were measured. Finally, for the ‘island age’ hypothesis, inter‐island dissimilarities were calculated on the basis of the geological age of the islands. Cluster analysis was used to investigate inter‐island faunal relationships. Results Support for a target‐area–distance effect was found only for birds. For these highly mobile animals, inter‐island distances had no significant effects on beta diversity. Birds are known to colonize islands by crossing large sea barriers and thus they can easily reach the Aeolian Islands, which are close to source areas (notably Sicily). Inter‐island distances had a significant role in determining patterns of beta diversity in most invertebrates. For Mollusca, Opiliones, Chilopoda, Heteroptera, coprophagous Scarabaeoidea, and Tenebrionidae, even relatively short distances preclude invertebrates from colonizing an island regularly from the mainland, and most colonization probably results from inter‐island faunal exchanges. Island age was proved to be important only for orthopterans. Main conclusions The origin of most of the Aeolian invertebrate fauna is quite recent, and species appear to have established on the islands predominantly by stepping stone dispersal. Birds, which are highly mobile organisms, follow more direct mainland–island dynamics. As further studies on other islands become available, comparative analyses will confirm whether the factors influencing variations in beta diversity in this study and their relationships with species dispersal ability are consistent across scales and geographical context.     

A hump-shaped density–area relationship for island lizards

We evaluated the density of Anolis lizards as a function of island area, habitat diversity, and topographic diversity on the Grenadines islands. We found a hump-shaped density–area relationship. Density at sea level increased with island area until an island was sufficiently large to contain multiple habitat types. Beyond this area threshold, average sea level density declined with island area. Increased species interactions on larger islands may explain the density decline.