Species richness, environmental heterogeneity and area: a case study based on land snails in Skyros archipelago (Aegean Sea, Greece)

Aim To test the performance of the choros model in an archipelago using two measures of environmental heterogeneity. The choros model is a simple, easy‐to‐use mathematical relationship which approaches species richness as a combined function of area and environmental heterogeneity. Location The archipelago of Skyros in the central Aegean Sea (Greece). Methods We surveyed land snails on 12 islands of the archipelago. We informed the choros model with habitat data based on natural history information from the land snail species assemblage. We contrast this with habitat information taken from traditional vegetation classification to study the behaviour of choros with different measures of environmental heterogeneity. R² values and Akaike's information criterion (AIC) were used to compare the choros model and the Arrhenius species–area model. Path analysis was used to evaluate the variance in species richness explained by area and habitat diversity. Results Forty‐two land snail species were recorded, living in 33 different habitat types. The choros model with habitat types had more explanatory power than the classic species–area model and the choros model using vegetation types. This was true for all islands of the archipelago, as well as for the small islands alone. Combined effects of area and habitat diversity primarily explain species richness in the archipelago, but there is a decline when only small islands are considered. The effects of area are very low both for all the islands of the archipelago, and for the small islands alone. The variance explained by habitat diversity is low for the island group as a whole, but significantly increases for the small islands. Main conclusions The choros model is effective in describing species‐richness patterns of land snails in the Skyros Archipelago, incorporating ecologically relevant information on habitat occupancy and area. The choros model is more effective in explaining richness patterns on small islands. When using traditional vegetation types, the choros model performs worse than the classic species–area relationship, indicating that use of proxies for habitat diversity may be problematic. The slopes for choros and Arrhenius models both assert that, for land snails, the Skyros Archipelago is a portion of a larger biogeographical province. The choros model, informed by ecologically relevant habitat measures, in conjunction with path analysis points to the importance of habitat diversity in island species richness.     

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.     

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).     

温州沿海小型海岛植物丰富度和β多样性及其影响因子

于2012-2015年调查了温州沿海20个小型无居民海岛的植物组成,共记录到维管束植物366种,隶属于95科244属,其中草本植物226种木本植物140种。拟合了5个种-面积关系模型,采用赤池信息量AIC对模型进行选择,发现种-面积-生境类型关系模型SAH_nR权重系数最大,为40.26%,两种断点回归种-面积关系模型BR-SAR权重系数分别仅为6.94%和0.43%,表明基于这20个海岛拟合的种-面积关系不存在小岛屿效应。岛屿植物物种丰富度主要受面积A影响,离大陆距离,I_m对丰富度无显著作用;偏相关分析表明除A外,周长/面积比PAR和岛屿生境多样性指数H_d显著影响了植物丰富度,其逐步回归方程分别为:植物总丰富度S=76.714+1.696A-0.046PAR,R~2=0.839;木本植物丰富度S_(-woody)=6.525+0.455A+24.544H_d,R~2=0.697;草本植物丰富度S_(-herbaceous)=66.899+1.285A-0.04PAR-23.434H_d,R~2=0.865。偏最小二乘回归PLS分析中岛屿空间特征参数对岛屿物种相似性指数重要性排序为:I_m(0.61)I_i(0.56)PAR(0.49)A(0.20)岸线长度Per(0.14)生境类型H(0.072)岛屿高程E(0.065)岛屿形状指数SI(0.05)。由此可见,近岸的小型海岛植物丰富度并不总是由岛屿面积来决定;隔离度对岛屿植物β多样性影响较大。     

Determinants of regional species richness: an empirical analysis of the number of hawkmoth species (Lepidoptera: Sphingidae) on the Malesian archipelago

Aims Major patterns and determinants of the species richness of Sphingidae in the Malesian archipelago were investigated, including a distinction of richness patterns between subfamilies and range‐size classes. Location Southeast Asia, Malesia. Methods Using a compilation of specimen‐label data bases, geographic information system (GIS)‐supported estimates of distributional ranges for all Sphingidae species of Southeast Asia were used to assess the species richness of islands. Range maps for all species and checklists for 114 islands can be found at http://www.sphingidae‐sea.biozentrum.uni‐wuerzburg.de . Potential determinants of the species richness of islands were tested with general linear models. Results The estimated species richness of islands in the region is determined by biogeographical association, seasonality, availability of rain forest and island size. Species–area relationships are linear on a semi‐logarithmic representation, but not on a double‐logarithmic scale. Species richness of all sphingid subfamilies is influenced by biogeography. The presence of large rain‐forest areas affects mainly Smerinthinae, whereas distance from continental Asia is conspicuously irrelevant for this group. Widespread rather than geographically restricted species shape the overall distribution patterns of species richness. The altitudinal range of islands does not significantly affect species‐richness patterns, but its potential effects on geographically restricted species are discussed. Main conclusions As well as being affected by climatic and vegetation parameters, sphingid species richness is strongly influenced by a historical, directional dispersal process from continental Southeast Asia to the Pacific islands. This process did not apply equally to species of different taxonomic groups or range sizes. Widespread species decline in species richness towards the south‐east, whereas geographically restricted species exhibit an inverse pattern of species richness, probably because speciation becomes more important in this group within the more isolated island groups.     

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.     

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.     

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.

     

Factors affecting the distribution of vascular plants, springtails, butterflies and birds on small tropical islands

Aim

The primary objective of our study was to examine the factors affecting the distribution of vascular plants, springtails, butterflies and birds on small tropical islands to understand how different groups of organisms with distinct biological traits respond to biogeographical variables, such as island area.

Location

The Republic of Singapore (103°50′E, 1°20′N) located at the southern tip of Peninsular Malaysia.

Methods

Seventeen islands were surveyed for vascular plants, springtails, butterflies and birds. Correlation analysis, simple linear and multiple regression analyses and the nestedness index were used to test the hypotheses that (1) area is the best predictor of species/genus richness at both the community and specific/generic levels; (2) there is no correlation between population density and island area; and (3) species/genera are distributed as nested subsets.

Results

Area was the most significant factor in determining the island distribution of springtails, butterflies and birds at both the community and specific/generic levels, although there were disparate responses to the biogeographical variables between the three taxonomic groups, as well as between common species within each group. Individual species displayed disparate responses to biogeographical variables, suggesting that patterns of distribution at the community level may not be a good indicator of the population dynamics of individual species/genera. Plant species richness did not show any correlation with any of the tested variables. Population densities of springtails, butterflies and birds were positively correlated with area, contradicting the assumption of the equilibrium theory of island biogeography that population density of island species is independent of area. Population densities of plants showed no correlation with any of the tested biogeographical variables. Vascular plant, springtail, butterfly and bird communities on the islands showed significant patterns of nestedness, indicating there may be species/genus‐specific responses to biogeographical variables.

Main conclusions

We conclude that although area was the most important factor affecting the island distribution of springtails, butterflies and birds, conservation planning must take into consideration how target taxonomic groups respond to biogeographical variables, instead of relying on general principles (e.g. those derived from the equilibrium theory). On a local scale, in order to preserve the island biodiversity of Singapore, the highest priority should be given to preserving the larger islands (e.g. Pulau Ubin) which not only have higher numbers of species, but also species that are absent on smaller islands.

     

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.     

Richness and composition of plants and birds on land‐bridge islands: effects of island attributes and differential responses of species groups

Aim To test relationships between the richness and composition of vascular plants and birds and attributes of habitat fragments using a model land‐bridge island system, and to investigate whether the effects of fragmentation differ depending on species natural history traits. Location Thousand Island Lake, China. Methods We compiled presence/absence data of vascular plant and bird species through exhaustive surveys of 41 islands. Plant species were assigned to two categories: shade‐intolerant and shade‐tolerant species; bird species were assigned to three categories: edge, interior, and generalist species. We analysed the relationships between island attributes (area, isolation, elevation, shape complexity, and perimeter to area ratio) and species richness using generalized linear models (GLMs). We also investigated patterns of composition in relation to island attributes using ordination (redundancy analysis). Results We found that island area explained a high degree of variation in the species richness of all species groups. The slope of the species–area relationship (z) was 0.16 for all plant species and 0.11 for all bird species. The lowest z‐value was for generalist birds (0.04). The species richness of the three plant species groups was associated with island area per se, while that of all, generalist, and interior birds was explained mainly by elevation, and that of edge bird species was associated primarily with island shape. Patterns of species composition were most strongly related to elevation, island shape complexity, and perimeter to area ratio rather than to island area per se. Species richness had no significant relationship with isolation, but species composition did. We also found differential responses among the species groups to changes in island attributes. Main conclusions Within the Thousand Island Lake system, the effects of fragmentation on both bird and plant species appear to be scale‐dependent and taxon‐specific. The number of plant species occurring on an island is strongly correlated with island area, and the richness of birds and the species composition of plants and birds are associated with variables related to habitat heterogeneity. We conclude that the effects of fragmentation on species diversity and composition depend not only on the degree of habitat loss but also on the specific patterns of habitat fragmentation.     

Turnover of plants on small islets of the eastern Aegean Sea within two decades

Aim To estimate species turnover of plants on 32 small islands within a 20‐year period and to assess possible changes in community composition and properties, such as species richness and factors affecting it, nestedness, species co‐occurrence and overall community similarity. Additionally, to assess the possible effects of grazing, gull colonies and fire on turnover values. Location Thirty‐two islets in the eastern Aegean Sea (Greece). Methods Complete sampling of plants was performed in 1974 and in 1990–94 (mostly in 1994, which was used as the reference year). Species turnover rates were estimated using both per island and per species approaches. Multiple regression was used to evaluate factors affecting species richness. Chi‐square tests were applied to compare community composition among sampling periods. The effects of various factors on turnover rates and species richness were examined using one‐way anova and ancova . Mann–Whitney tests were applied in order to check for differences between frequencies of occurrence of extinct, immigrant and persisting species. Community nestedness was calculated using bitmatnest and the C‐score index for co‐occurrence was estimated using EcoSim7. Species similarities among islands in each of the 1974 and 1994 data sets were assessed using Jaccard’s index and the two similarity matrices were compared using a Mantel test. Results Of 391 species recorded on the islets, 334 were present in 1974, 301 in 1994 and 244 were common to both these periods. Species richness in the 1974 and 1994 data sets was significantly correlated with elevation and area, but not with distance from the nearest large island. Richness was positively affected by grazing, but not by fire or gull colonies. The slopes of species–area and species–elevation regressions were almost identical in 1974 and 1994. Mean relative turnover was 2.06 (species per islet) and 3.26 (islets per species). Turnover was not correlated with area, elevation or distance from the nearest large island. Nestedness and co‐occurrence levels were very similar. Tables of islet by islet floral similarity (Jaccard’s index) did not differ between the 1974 and 1994 data sets. Main conclusions The turnover rates found are among the highest recorded for plants; at the same time the islet communities exhibit notable stability in overall properties. Our results provide evidence for rapid shifts in species number that may nonetheless be considered as equilibrial dynamics, as these islets are able to respond rapidly to environmental change and disturbance. Human activities, notably the application of grazing, have a significant complicating effect on community dynamics, enhancing observed turnover rates.     

Multiple source pools for Galápagos plant species richness: a critical analysis of the line of sight connectivity index

An index (C_i*E) combining the number of line‐of‐sight islands (C_i) within a radius i and target island elevation (E) has been proposed as an improved predictive model of plant species richness (S_t) in the Galápagos Archipelago. We examined this index critically and found that several major flaws preclude it from being a useful predictive tool for the archipelago. Although the number of collecting trips to an island was reported over 20 years ago to have substantial predictive value for reported plant species richness in the Galápagos Islands, this relationship was ignored in multiple regression analyses of the index. When we included the number of collecting trips in different multiple regression analyses of the index, C_i*E had less predictive power than collecting trips or ceased to be significant at all. Additionally, the strong significant relationship between elevation and area in the Galápagos Archipelago results in area having a major confounding influence on the C_i*E index. When elevation is removed from the C_i*E index, the predictive power of C_i is far less than area alone. Finally, the data used to construct and correlate the C_i*E index with (S_t) were based only on a subset of the islands and species lists that were incomplete or out of date. Species richness on islands can be related to the interaction of different factors, so development and testing of indices like C_i*E is not inappropriate. However, it is important to examine the interrelationships among the components of these indices thoroughly, and not ignore the effect of factors already known to have high predictive power. We propose several ways in which more meaningful indices of source pool(s) capacity can be constructed.     

Lost and found: Short-term dynamics of the flora on 100 small islands in the White Sea

The aim was to uncover factors that influence short-term (decade) flora dynamics and species richness of northern marine islets characterized by poor flora and weak anthropogenic pressure. The study used presence–absence data of vascular plant species on 100 small uprising islets of the Kandalaksha Gulf of White Sea (Northern Karelia, Russia). We investigated the influence of islands' attributes on species richness and rates of flora dynamics. Two island types were analyzed separately: younger, stone-like and older, islet-like (which generally are larger and have higher diversity of habitats). Sampled islands were studied via classical biogeographical per island approach and metapopulation per species approach. Stone-like islands had noticeably poorer flora with higher rates of immigration and extinction when compared to those of islet-like islands. The species number for islet-like islands correlated positively with number of habitats, abundance of different habitat types and island area. Species richness of stone-like islands correlated positively only with number of habitat types. Plant species associated with birds, crowberry thickets and coastal rocks were the most stable, and the species of disturbed habitats were significantly less stable. Floristic changes that have occurred have been caused by the massive establishment of new species rather than the extinction of pre-existing taxa. Thus, most of these islands are still in the colonization (assortative) stage. While we found no relationship between island area and species number for stone-like islands, this relationship was seen on islet-like islands.     

Turnover of passerine birds on islands in the Aegean Sea (Greece)

Aim We wish to determine the effect of migratory status on turnover rates in island birds. Because turnover is influenced by factors other than migratory status, we also considered the influence of body size and physical characteristics of the islands inhabited on the probabilities of extinction and immigration. Location The Mediterranean islands of Delos, Astypalea, Paros, Naxos and Lesvos in the Aegean Sea, Greece. Methods The passerine birds of these islands were surveyed between 1954 and 1961 by G.E. Watson, and were resurveyed between 1988 and 1992. The effects of migratory status and body size on the probabilities of extinction and immigration were examined by G‐tests of linear trend in proportion, and analysis of variance, respectively. A combined analysis of migratory status, body size and physical characteristics of the islands was carried out using logistic regressions of the probabilities of extinction and immigration on these factors. Results Species number on each island changed little between surveys, with no island's species number changing by more than one species. Twelve population extinctions and 11 immigrations were recorded. The smallest island, Delos (6 km²), had the highest annualized relative turnover rate (1.08), while the four larger islands (96–1614 km²) had lower and mutually similar rates (0.21–0.27). Populations on higher elevation islands were less likely to go extinct. There is no evidence for an effect of body size on the probabilities of extinction or immigration. Migratory status affected extinction and immigration probabilities differently: migratory species were more likely to immigrate, but less likely to go extinct. Main conclusions The position of the Aegean islands along a major north–south flyway may account for the observed effects of migratory status. The annual passage of large numbers of migrants may, via the rescue effect, decrease the chances of extinction, while at the same time increasing the chances of colonization of unoccupied islands. The likelihood of both extinction and immigration involves a complex interaction between life‐history traits and island characteristics. The effects of migratory status will depend not only on consideration of vagility, vulnerability and stochasticity identified by previous authors, but also upon the location of the islands in relationship to migratory pathways.     

Patterns of bird species richness and composition on islands off Arnhem Land,Northern Territory,Australia

The bird faunas of the adjacent Wessel and English Company island chains were sampled at two scales (0.25 ha quadrats and entire islands). Ninety‐six species were recorded from 226 quadrats, with the most frequently recorded species being mistletoebird Dicaeum hirundinaceum, brown honeyeater Lichmera indistincta, silver‐crowned friarbird Philemon argenticeps, bar‐shouldered dove Geopelia humeralis, northern fantail Rhipidura rufiventris and yellow white‐eye Zosterops lutea. At the quadrat scale, vegetation type was a major determinant of the abundance of individual species (and hence species composition), species richness and total bird abundance. Bird species composition and richness at the quadrat scale was also significantly affected by island isolation (particularly the amount of land within 20 km of the island perimeter). Island size had no effect on quadrat‐scale richness or total abundance. However, the abundance of 10 of the 38 most frequently recorded individual species was significantly related to island size, in most cases even when the comparison was restricted to similar habitats. The most striking cases were rufous fantail Rhipidura rufifrons, mangrove golden whistler Pachycephala melanura, brown honeyeater and yellow white‐eye, which were all significantly more abundant on smaller islands. One hundred and seventy‐one species were recorded from the 62 islands sampled. There was a very tight relationship between island size and the number of terrestrial species (73% of deviance explained) and of all species (84% of deviance explained). This relationship was improved (marginally) when isolation was included in the model. Ordination of islands by their terrestrial bird species composition was related to island size and isolation, and suggested an erratic species composition on small islands.     

Disentangling the effects of area, energy and habitat heterogeneity on boreal forest bird species richness in protected areas

Aim  One of the few general laws in ecology is that species richness is a positive function of area. However, it has been proposed that area would merely be a proxy for energy. Additionally, habitat heterogeneity has been found to be an important factor determining species richness. Yet the relative importance of those relationships is little known, and it is still unclear how they are brought about. We aimed to dissect which factors drive the species richness of boreal forest birds, and to identify the most probable mechanisms.
Location  Forested protected areas in Finland.
Methods  Using bird line census data collected in 104 protected areas, we ran simultaneous autoregressive models to explain the species richness of forest birds. We explored the value of forest area, tree volume, tree growth, mean degree days and habitat heterogeneity as explanatory variables and used the species richness within different species groups, based on the predictions of hypothesized mechanisms, as a response variable.
Results  Energy, rather than area or habitat heterogeneity, seems to be the main driver of species richness in boreal forest birds. More specifically, productive energy was a better predictor of total species richness than solar energy. Among the tested hypothetical mechanisms, the sampling hypothesis received strong support. After accounting for sampling, solar energy had an effect on species richness.
Main conclusions  As productive energy, such as tree volume, is associated with species richness, high-energy areas should be prioritized in forest conservation planning. Reductions in productive energy may first lead to the disappearance of the rarest species due to the random sampling process. Climate change may result in increased species richness due to increasing amount of productive and solar energy in forests. However, the range shifts of bird species may not be fast enough to keep up with the temperature increases.