Factors influencing vascular plant diversity on 22 islands off the coast of eastern North America

Aim The influence of physiographic and historical factors on species richness of native and non‐native vascular plants on 22 coastal islands was examined. Location Islands off the coast of north‐eastern USA and south‐eastern Canada between 41° and 45° N latitude were studied. Island size ranges from 3 to 26,668 ha. All islands were deglaciated between 15,000 and 11,000 yr bp ; all but the four New Brunswick islands were attached to the mainland until rising sea level isolated them between 14,000 and 3800 yr bp . Methods Island species richness was determined from floras compiled or revised since 1969. Simple and multiple regression and rank correlation analysis were employed to assess the relative influence of independent variables on species richness. Potential predictors included island area, latitude, elevation, distance from the mainland, distance from the nearest larger island, number of soil types, years since isolation, years since deglaciation, and human population density. Results Native vascular plant species richness for the 22 islands in this study is influenced most strongly by island area, latitude, and distance from the nearest larger island; richness increases with island area, but decreases with latitude and distance from the nearest larger island as hypothesized. That a similar model employing distance from the mainland does not meet the critical value of P confirms the importance of the stepping‐stone effect. Habitat diversity as measured by number of soil types is also an important predictor of native plant species richness, but at least half of its influence can be attributed to island area, with which it is correlated. Two historical factors, years since deglaciation and years since isolation, also appear to be highly correlated with native species richness, but their influence cannot be separated from that of latitude for the present sample size. Non‐native vascular plant species richness is influenced primarily by island area and present‐day human population density, although human population density may be a surrogate for the cumulative effect of several centuries of anthropogenic impacts related to agriculture, hunting, fishing, whaling, tourism, and residential development. Very high densities of ground‐nesting pelagic birds may account for the high percentage of non‐native species on several small northern islands. Main conclusions Many of the principles of island biogeography that have been applied to oceanic islands apply equally to the 22 islands in this study. Native vascular plant species richness for these islands is strongly influenced by physiographic factors. Influence of two historical factors, years since deglaciation and years since isolation, cannot be assessed with the present sample size. Non‐native vascular plant species richness is influenced by island area as well as by human population density; human population density may be a surrogate for other anthropogenic impacts.     

Genetic structure of the Canarian palm tree (Phoenix canariensis) at the island scale: does the ‘island within islands’ concept apply to species with high colonisation ability?

• Oceanic islands are dynamic settings that often promote within‐island patterns of strong population differentiation. Species with high colonisation abilities, however, are less likely to be affected by genetic barriers, but island size may impact on species genetic structure regardless of dispersal ability.
• The aim of the present study was to identify the patterns and factors responsible for the structure of genetic diversity at the island scale in Phoenix canariensis, a palm species with high dispersal potential. To this end, we conducted extensive population sampling on the three Canary Islands where the species is more abundant and assessed patterns of genetic variation at eight microsatellite loci, considering different within‐island scales.
• Our analyses revealed significant genetic structure on each of the three islands analysed, but the patterns and level of structure differed greatly among islands. Thus, genetic differentiation fitted an isolation‐by‐distance pattern on islands with high population densities (La Gomera and Gran Canaria), but such a pattern was not found on Tenerife due to strong isolation between colonised areas. In addition, we found a positive correlation between population geographic isolation and fine‐scale genetic structure.
• This study highlights that island size is not necessarily a factor causing strong population differentiation on large islands, whereas high colonisation ability does not always promote genetic connectivity among neighbouring populations. The spatial distribution of populations (i.e. landscape occupancy) can thus be a more important driver of plant genetic structure than other island, or species′ life‐history attributes.

     

Population demographic history of a temperate shrub,Rhododendron weyrichii (Ericaceae), on continental islands of Japan and South Korea

Continental islands provide opportunities for testing the effects of isolation and migration on genetic variation in plant populations. In characteristic of continental islands is that the geographic connections between these islands, which are currently distinguished by seaways, have experienced fluctuations caused by sea‐level changes due to climate oscillations during the Quaternary. Plant populations on the islands have migrated between these islands via the exposed seafloors or been isolated. Here, we examined the demographic history of a temperate shrub, Rhododendron weyrichii, which is distributed in the southwestern parts of the Japanese archipelago and on an island of South Korea, using statistical phylogeographic approaches based on the DNA sequences of two chloroplast and eight nuclear loci in samples analyzed from 18 populations on eight continental islands, and palaeodistribution modeling. Time estimates for four island populations indicate that the durations of vicariance history are different between these populations, and these events have continued since the last glacial or may have predated the last glacial. The constancy or expansion of population sizes on the Japanese islands, and in contrast a bottleneck in population size on the Korean island Jeju, suggests that these islands may have provided different conditions for sustaining populations. The result of palaeodistribution modeling indicates that the longitudinal range of the species as a whole has not changed greatly since the last glacial maximum. These results indicate that exposed seafloors during the glacial period formed both effective and ineffective migration corridors. These findings may shed light on the effects of seafloor exposure on the migration of plants distributed across continental islands.     

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

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

Re‐colonization by common eiders Somateria mollissima in the Aleutian Archipelago following removal of introduced arctic foxes Vulpes lagopus

Islands provide refuges for populations of many species where they find safety from predators, but the introduction of predators frequently results in elimination or dramatic reductions in island‐dwelling organisms. When predators are removed, re‐colonization for some species occurs naturally, and inter‐island phylogeographic relationships and current movement patterns can illuminate processes of colonization. We studied a case of re‐colonization of common eiders Somateria mollissima following removal of introduced arctic foxes Vulpes lagopus in the Aleutian Archipelago, Alaska. We expected common eiders to resume nesting on islands cleared of foxes and to re‐colonize from nearby islets, islands, and island groups. We thus expected common eiders to show limited genetic structure indicative of extensive mixing among island populations. Satellite telemetry was used to record current movement patterns of female common eiders from six islands across three island groups. We collected genetic data from these and other nesting common eiders at 14 microsatellite loci and the mitochondrial DNA control region to examine population genetic structure, historical fluctuations in population demography, and gene flow. Our results suggest recent interchange among islands. Analysis of microsatellite data supports satellite telemetry data of increased dispersal of common eiders to nearby areas and little between island groups. Although evidence from mtDNA is suggestive of female dispersal among island groups, gene flow is insufficient to account for recolonization and rapid population growth. Instead, near‐by remnant populations of common eiders contributed substantially to population expansion, without which re‐colonization would have likely occurred at a much lower rate. Genetic and morphometric data of common eiders within one island group two and three decades after re‐colonization suggests reduced movement of eiders among islands and little movement between island groups after populations were re‐established. We predict that re‐colonization of an island group where all common eiders are extirpated could take decades.     

Nested distribution of amphibians in the Zhoushan archipelago, China: can selective extinction cause nested subsets of species?

Because of their poor dispersal ability, amphibians are well suited for testing the selective extinction theory on islands. Amphibian fauna in the Zhoushan archipelago, China, exhibit a high level of nestedness (C = 0.893), and the species number is lower on islands than on similar sized areas on the mainland. No correlation was found between island-specific species richness and the nearest distance from a larger island, distance from the mainland or density of human population. These results suggest that no amphibian colonisation has occurred in the archipelago since island isolation 7000–9000 years ago. Furthermore, the results imply that selective extinction contributes to the nestedness of amphibians in the Zhoushan archipelago. The incidence of a species on the islands is significantly correlated with log area of the smallest island occupied by the species and the number of provinces on the Chinese mainland in which the species occur. However, there is no correlation with average body length of adults and island occurrence. It is concluded that (1) the area of the smallest island occupied by a species is a good estimate of the minimum area for a viable population of the species and a good predictor of species incidence on islands, (2) species with a restricted distribution range are more vulnerable to extinction from islands than those with a wide distribution range and (3) the effect of body size on occurrence on the islands is uncertain, and may be specific to the archipelago and taxa studied. The observed nestedness of amphibian assemblages has two implications for conservation: (1) not only can all the species found in several small reserves be found on a large reserve of the same total size, but additional species can be found on the single large reserve; (2) for a reserve to maintain viable populations of all species in a region it should be at least as large as the smallest island occupied by the most vulnerable species. Received: 16 December 1996 / Accepted: 22 September 1997     

Distribution and phylogenetic analyses of an endangered tick,Amblyomma sphenodonti

Abstract

In this study we investigate the geographic distribution, genetic diversity, and phylogenetic relationships of an endangered tick, Amblyomma sphenodonti (Family Ixodidae). Amblyomma sphenodonti and its host, the tuatara (Sphenodon), are found only on small offshore islands around New Zealand. Our results show that Amblyomma sphenodonti has a more severely restricted geographic distribution than its host, as it was found on only eight of 28 islands (four out of 12 island groups) where tuatara still live. The prevalence of A. sphenodonti is likely to have been affected by low host density and fluctuations in host population size as tuatara populations became isolated on offshore islands. Analysis of A. sphenodonti cytochrome oxidase 1 (CO1) sequences indicated a lack of gene flow between islands, with fixed differences in CO1 sequences between islands, but almost no genetic diversity within island populations. A similar phylogenetic pattern to that observed in tuatara mtDNA was observed, indicating co‐evolution of two species, at least since the Pleistocene. Phylogenetic analysis using 18S rRNA sequences suggest that A. sphenodonti is not closely related to other Amblyomma species, and that a separate genus for this species may be warranted. However, data from other ixodid ticks are required before the distinctiveness of A. sphenodonti can be confirmed and the phylogenetic relationships among ixodid ticks fully understood.     

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.

     

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.     

Annotated check-list of the family Tachinidae (Diptera: Calyptratae: Oestroidea) in the Canary Islands

Summary

A commented check-list of the tachinid species inhabiting the Canary Islands (Spain) is provided, with special emphasis on hosts species and floral resources. Nine new local records are provided and Cylindromyia rufipes is reported for the first time for the Canary Islands, thus increasing the species number to 52. Zoogeographic analysis of the area revealed that there are differences in chorotype composition among islands, with the easternmost islands being richer in Mediterranean species and poorer in endemics. These differences may be due to ecological differences between the easternmost island and the remaining islands. Relevance regarding influence of tachinid as pollinators is discussed.     

Integrated species delimitation and conservation implications of an endangered weevil Pachyrhynchus sonani (Coleoptera: Curculionidae) in Green and Orchid Islands of Taiwan

Oceanic islands are productive habitats for generating new species and high endemism, which is primarily due to their geographical isolation, smaller population sizes and local adaptation. However, the short divergence times and subtle morphological or ecological divergence of insular organisms may obscure species identity, so the cryptic endemism on islands may be underestimated. The endangered weevil Pachyrhynchus sonani Kôno (Coleoptera: Curculionidae: Entiminae: Pachyrhynchini) is endemic to Green Island and Orchid Island of the Taiwan‐Luzon Archipelago and displays widespread variation in coloration and host range, thus raising questions regarding its species boundaries and degree of cryptic diversity. We tested the species boundaries of P. sonani using an integrated approach that combined morphological (body size and shape, genital shape, coloration and cuticular scale), genetic (four genes and restriction site‐associated DNA sequencing, RAD‐seq) and ecological (host range and distribution) diversity. The results indicated that all the morphological datasets for male P. sonani, except for the colour spectrum, reveal overlapping but statistically significant differences between islands. In contrast, the morphology of the female P. sonani showed minimum divergence between island populations. The populations of P. sonani on the two islands were significantly different in their host ranges, and the genetic clustering and phylogenies of P. sonani established two valid evolutionary species. Integrated species delimitation combining morphological, molecular and ecological characters supported two distinct species of P. sonani from Green Island and Orchid Island. The Green Island population was described as P. jitanasaius sp.n. Chen & Lin, and it is recommended that its threatened conservation status be recognized. Our findings suggest that the inter‐island speciation of endemic organisms inhabiting both islands may be more common than previously thought, and they highlight the possibility that the cryptic diversity of small oceanic islands may still be largely underestimated.     

Island shape,size, and isolation affect nest‐site selection by Little Terns

ABSTRACT Populations of many seabirds and other species that nest along coasts are declining due to habitat degradation and loss. An improved understanding of the species‐specific factors that determine nest density across a landscape is therefore critical for conservation efforts. We examined factors that affected the density (number per hectare) and abundance (number at a sampling site) of nests of Little Terns (Sternula albifrons) on the Sinai Peninsula, Egypt. Terns preferred to nest on islands rather than the mainland, with islands constituting 64% of the area surveyed, but containing 99% of the 439 tern nests we found. Nest densities were highest on islands that were small, located at moderate distances from the mainland, and irregularly shaped or elongated. Most nests (69%) were on islands with areas < 3 ha, although these islands represented < 5% of total island area, and islands with the highest nest densities were 80–300 m from the mainland. Terrestrial predators were more likely to occur on larger islands, visiting three of the largest four islands. Most tern nests were within 1 m of shorelines, causing island perimeter to be a strong influence on nest density. Island shape was the only factor that significantly affected nest abundance, with more nests on islands with relatively long perimeters for their size. Our results suggest that protection or creation of relatively small, slender islands at moderate distances from shore may be an effective means of increasing the number of breeding sites for Little Terns. Although not generally considered a potential determinant of nest site preferences for seabirds, island shape is likely to be important for species that prefer sites adjacent to water, including species that nest on beaches and seaside cliffs.     

The parasites of Anolis lizards in the northern Lesser Antilles

Summary The helminth communities from ten species of lizard on seven islands in the Caribbean were sampled by collecting one hundred specimens of each species. Nine genera of parasites were identified; these included six nematodes, two digeneans and an acanthocephalan. No relationship was discernible between parasite density or abundance and island area or altitude, although dry islands tend to have fewer species of parasites. Anolis lizards of the bimaculatus and wattsi series share similar parasites with four out of nine species common to both series. The parasite community of lizards on these islands is depauperate with respect to similar surveys on the larger islands of the Greater Antilles.On three of the islands lizards were sub-sampled by collecting from moist woodland and more xeric habitats. These data suggest that differences between habitats are as significant as differences between islands in determining parasite burdens. Worm burdens of the commonest parasite species, T. cubensis, increased monotonically with host body size and no evidence was found to suggest that these parasites affect either host survival or fecundity. The sex-ratio of this species correlated with mean abundance of the parasite, with females the dominant sex on islands or in habitats where the parasite was common. This pattern may reflect haplodiploid sexual determination in this species.     

Island biogeography theory explains the genetic diversity of a fragmented rock ptarmigan (Lagopus muta) population

The island biogeography theory is one of the major theories in ecology, and its applicability to natural systems is well documented. The core model of the theory, the equilibrium model of island biogeography, predicts that species diversity on an island is positively related to the size of the island, but negatively related by the island's distance to the mainland. In recent years, ecologists have begun to apply this model when investigating genetic diversity, arguing that genetic and species diversity might be influenced by similar ecological processes. However, most studies have focused on oceanic islands, but knowledge on how the theory applies to islands located on the mainland (e.g., mountain islands, forest islands) is scarce. In this study, we examined how the size and degree of isolation of mountain islands would affect the genetic diversity of an alpine bird, the rock ptarmigan (Lagopus muta). Within our study area, we defined the largest contiguous mountain area as the mainland, while smaller mountains surrounding the mainland were defined as islands. We found that the observed heterozygosity (H_o) was significantly higher, and the inbreeding coefficient (F_is) significantly lower, on the mainland compared to islands. There was a positive significant relationship between the unbiased expected heterozygosity (H_n.b.) and island size (log km²), but a negative significant relationship between H_o and the cost distance to the mainland. Our results are consistent with the equilibrium model of island biogeography and show that the model is well suited for investigating genetic diversity among islands, but also on islands located on the mainland.     

Interactions between vegetation development and island formation in the Alpine river Tagliamento

Abstract. Early-successional stages of woody vegetation on gravel bars were studied in an island-braided section of the River Tagliamento in northeastern Italy. We mapped landscape-level changes in the study area (125 ha) by GIS-based analysis of aerial photographs for two time periods (1984–1986, 1986–1991); we surveyed island vegetation, and estimated island age by tree ring analysis. The study area experienced considerable changes between 1984 and 1991 due to at least two major floods in 1987 and 1990. The development of woody vegetation on bars follows three distinct phases: (1) gravel bars plus large woody debris (LWD), (2) pioneer islands, and (3) established islands. Established islands have sections dominated by shrubs of Salix elaeagnos, S. purpurea, S. daphnoides and S. triandra, and tree-dominated sections with Populus nigra, Salix alba and Alnus incana. Large woody debris seems to play a key role for plant colonization on gravel bars. The succession from bars to established islands took about 10 - 20 yr, and the probability of an island being washed away decreased with island age. Erosion produced new LWD which again initiated successional processes in the active zone of the river. Most species were already present in the early-successional stages, although the number of species increased with island development. Established islands were characterized by a distinctive species composition, including an assemblage of species less tolerant of inundation. The results are discussed within the framework of island dynamics and its significance for restoration of early-successional habitats in more regulated rivers.     

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

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

Mountain hare populations on islands: effects of predation by red fox

Summary On islands off the west coast of Sweden the density of mountain hares (Lepus timidus L.) is very high. One of the main predators on hares, the red fox (Vulpes vulpes L.), is only present during short periods. Data on hare density and predation by red fox and eagle owl (Bubo bubo (L.)) has been analyzed from five islands over several years. Winter mortality in years with low predation pressure was independent of hare density. But when red fox or eagle owl were present on islands (i.e., high predation pressure) winter mortality became density dependent. Thus, at low density, winter mortality did not increase through red fox predation. But at densities up to two hares/ha, predation pressure was increasing and could be limiting for these populations. At still higher hare density predation pressure became less intensive. The functional response for foxes preying on hares showed a type II or a sigmoid type III response pattern. In normal summers, the population increase due to reproduction was at least two-fold. When a fox was present there was instead a sharp decrease in hare numbers. Fox predation had a stronger effect in summer than in winter. By switching between islands and mainland areas from winter to summer, a fox can stabilize fluctuations in hare numbers on the islands. This is dependent on how often the ice permits a fox to reach an island and the lack of numerical response by predators.     

Diet variability among insular populations of Podarcis lizards reveals diverse strategies to face resource‐limited environments

Access to resources is a dynamic and multicausal process that determines the success and survival of a population. It is therefore often challenging to disentangle the factors affecting ecological traits like diet. Insular habitats provide a good opportunity to study how variation in diet originates, in particular in populations of mesopredators such as lizards. Indeed, high levels of population density associated with low food abundance and low predation are selection pressures typically observed on islands. In the present study, the diet of eighteen insular populations of two closely related species of lacertid lizards (Podarcis sicula and Podarcis melisellensis) was assessed. Our results reveal that despite dietary variability among populations, diet taxonomic diversity is not impacted by island area. In contrast, however, diet disparity metrics, based on the variability in the physical (hardness) and behavioral (evasiveness) properties of ingested food items, are correlated with island size. These findings suggest that an increase in intraspecific competition for access to resources may induce shifts in functional components of the diet. Additionally, the two species differed in the relation between diet disparity and island area suggesting that different strategies exist to deal with low food abundance in these two species. Finally, sexual dimorphism in diet and head dimensions is not greater on smaller islands, in contrast to our predictions.     

Genetic structure of Gymnures (genus Hylomys; Erinaceidae) on continental islands of Southeast Asia: historical effects of fragmentation

Eustatic sea level changes during Pleistocene climatic fluctuations produced several cycles of connection-isolation among continental islands of the Sunda shelf. To explore the potential effects of these fluctuations, we reconstructed a model of the vicariant events that separated these islands, based on bathymetric information. Among many possible scenarios, two opposite phylogenetic patterns of evolution were predicted for terrestrial organisms living in this region: one is based on the classical allopatric speciation mode of evolution, while the other is the outcome of a sequential dispersal colonization of the archipelago. We tested the applicability of these predictions with an analysis of sequence variation of the cytochrome b gene from several taxa of Hylomys. They were sampled throughout SE-Asia and the Sunda islands. High levels of haplotype differentiation characterize the different island taxa. Such levels of differentiation support the existence of several allopatric species, as was suggested by previous allozyme and morphological data. Also in accordance with previous results, the occurrence of two sympatric species from Sumatra is suggested by their strongly divergent haplotypes. One species, Hylomys suillus maxi, is found both on Sumatra and in Peninsular Malaysia, while the other, H. parvus, is endemic to Sumatra. Its closest relative is H. suillus dorsalis from Borneo. Phylogenetic reconstructions also demonstrate the existence of a Sundaic clade composed of all island taxa, as opposed to those from the continent. Although there is no statistical support for either proposed biogeographic model of evolution, we argue that the sequential dispersal scenario is more appropriate to describe the genetic variation found among the Hylomys taxa. However, despite strong differentiation among island haplotypes, the cladistic relationships between some island taxa could not be resolved. We argue that this is evidence of a rapid radiation, suggesting that the separation of the islands may have been perceived as a simultaneous event rather than as a succession of vicariant events. Furthermore, the estimates of divergence times between the haplotypes of these taxa suggest that this radiation may actually have predated the climatic fluctuations of the Pleistocene. Further refinement of the initial palaeogeographic models of evolution are therefore needed to account for these results.     

Why do some small islands lack vegetation? Evidence from long‐term introduction experiments

Classic island biogeography theory predicts that very small islands, near the extreme lower end of the species–area relationship, should support very few species. At times no species may be present, however, due to randomness in the immigration–extinction dynamics. Alternatively, a lack of vegetation on very small islands may indicate that such islands do not contain the appropriate habitat for the establishment or long‐term survival of plants, or that disturbances are too frequent or intense. These potential mechanisms were evaluated in the central Exumas, Bahamas, where surveys of 117 small islands revealed that over a third of the islands supported no terrestrial plant life. Area and exposure were significant predictors of whether a small island was vegetated or not in multiple logistic regressions. No islands naturally devoid of vegetation were colonized over a 17‐yr period, and only two naturally vegetated islands lost all vegetation. Experimental introductions of two species –Sesuvium portulacastrum and Borrichia arborescens– revealed that a number of islands naturally lacking vegetation were able to sustain introduced populations over the long term (up to 15 yr). Drought and hurricanes appeared to have reduced the establishment success and possibly long‐term survival of the introductions, although some populations survived four major hurricanes. Turnover rates of both introduced species were often an order of magnitude higher on the experimental introduction islands than on other islands in the archipelago. It appears many of the islands in this system that naturally lack vegetation may be physically capable of supporting terrestrial plant life, yet have no plants primarily due to barriers to colonization.