Analysis of avian communities in Lake Guri,Venezuela, using multiple assembly rule models

This study analyzed the distribution of resident, forest-interior bird species nesting on islands in Lake Guri, Venezuela using several different community assembly rule models. The models that were tested included Diamond's Assembly Rules, Size Structure, Guild Proportionality, Favored States, and Nestedness. It was determined that the species composition of the study communities was only weakly influenced by competition, but that competition did appear to limit the size similarity which is permissible for co-occurring species. There was no tendency for the relative proportion of species within guilds (i.e. insectivore, omnivore, nectivore and frugivore) to remain stable among the islands. When only the insectivorous and omnivorous species were analyzed (using feeding strata as the functional groups) there was some support for the guild proportionality hypothesis. This study found no support for Fox's Favored State hypothesis, possibly due to the overrepresentation of insectivores and omnivores in the species pool. The island communities exhibit a highly nested structure. This high degree of nestedness supports the hypothesis that the assemblages are more strongly determined by differential extinction vulnerability and selective species loss than by interspecific or inter-guild competition. Understanding patterns of community assembly and their underlying forces has important implications for conservation ecology and reserve design.     

Testing multiple assembly rule models in avian communities on islands of an inundated lake,Zhejiang Province,China

Aim A fundamental question in community ecology is whether general assembly rules determine the structure of natural communities. Although many types of assembly rules have been described, including Diamond’s assembly rules, constant body‐size ratios, favoured states, and nestedness, few studies have tested multiple assembly rule models simultaneously. Therefore, little is known about the relative importance of potential underlying factors such as interspecific competition, inter‐guild competition, selective extinction and habitat nestedness in structuring community composition. Here, we test the above four assembly rule models and examine the causal basis for the observed patterns using bird data collected on islands of an inundated lake. Location Thousand Island Lake, China. Methods  We collected data on presence–absence matrices, body size and functional groups for bird assemblages on 42 islands from 2007 to 2009. To test the above four assembly rule models, we used null model analyses to compare observed species co‐occurrence patterns, body‐size distributions and functional group distributions with randomly generated assemblages. To ensure that the results were not biased by the inclusion of species with extremely different ecologies, we conducted separate analyses for the entire assemblage and for various subset matrices classified according to foraging guilds. Results The bird assemblages did not support predictions by several competitively structured assembly rule models, including Diamond’s assembly rules, constant body‐size ratios, and favoured states. In contrast, bird assemblages were highly significantly nested and were apparently shaped by extinction processes mediated through area effects and habitat nestedness. The nestedness of bird assemblages was not a result of passive sampling or selective colonization. These results were very consistent, regardless of whether the entire assemblage or the subset matrices were analysed. Main conclusions Our results suggest that bird assemblages were shaped by extinction processes mediated through area effects and habitat nestedness, rather than by interspecific or inter‐guild competition. From a conservation point of view, our results indicate that we should protect both the largest islands with the most species‐rich communities and habitat‐rich islands in order to maximize the number of species preserved.     

Patterns of bird invasion are consistent with environmental filtering

Predicting invasion potential has global significance for managing ecosystems as well as important theoretical implications for understanding community assembly. Phylogenetic relationships of introduced species to the extant community may be predictive of establishment success because of the opposing forces of competition/shared enemies (which should limit invasions by close relatives) versus environmental filtering (which should allow invasions by close relatives). We examine here the association between establishment success of introduced birds and their phylogenetic relatedness to the extant avifauna within three highly invaded regions (Florida, New Zealand, and Hawaii). Published information on both successful and failed introductions, as well as native species, was compiled for all three regions. We created a phylogeny for each avifauna including all native and introduced bird species. From the estimated branch lengths on these phylogenies, we calculated multiple measurements of relatedness between each introduced species and the extant avifauna. We used generalized linear models to test for an association between relatedness and establishment success. We found that close relatedness to the extant avifauna was significantly associated with increased establishment success for exotic birds both at the regional (Florida, Hawaii, New Zealand) and sub‐regional (islands within Hawaii) levels. Our results suggest that habitat filtering may be more important than interspecific competition in avian communities assembled under high rates of anthropogenic species introductions. This work also supports the utility of community phylogenetic methods in the study of vertebrate invasions.     

Morphological assortment in introduced Hawaiian passerines

Summary To evaluate the role of competition in structuring communities, we conducted morphological analyses on the surviving species of passerine birds that were successfully introduced to the Hawaiian islands. Forty-nine species have been introduced a total of 111 times to five of the six main islands. There have been 33 extinctions. Our analyses were done at two separate organizational levels: all species introduced to an island; and all forest-dwelling species. If competition determines which species can coexist, and the intensity of competition is correlated with morphological similarity, then the surviving species should be overdispersed in morphological space. Further, sets of surviving species that coexist should be regularly positioned in morphological space. At the island-wide organizational level, the surviving species were neither overdispersed nor regularly positioned in morphological space. However, at the forest-wide level the surviving species were not only highly overdispersed, they were also regularly positioned when compared to randomly assembled communities.     

Winter bird communities in the Åland archipelago: an island biogeographic point of view

The distribution of wintering land birds was examined in the archipelago of Åland (60°N, 20°E) along two ecological dimensions: (1) the island continuum from small skerries ((0.5 ha) to the mainland of Åland (97000 ha), and (2) the habitat range of the mainland of Åland. In the species-area relationship, both the exponential and power function models fitted well. The increase of the number of species with island size was interpreted as a consequence of increasing habitat diversity on larger islands. The smallest islands supported only few seed-eating species utilizing the islands in a fine-grained fashion; none of these birds belonged to the breeding fauna of the islands. With increasing island size and habitat diversity, insectivores and species tied to the human culture were added. On the mainland of Åland, more than half of the birds were observed near human habitations: bird communities wintering in pine forests were richer than in deciduous forests. Proportion of wintering species was greatest in the breeding communities of pine forests. The proportion of wintering species was presumably regulated by the degree of structural change in the habitat between summer and winter, this change being most drastic in deciduous habitats The species observed could be broadly classified into three feeding categories: opportunists ., omnivores and scavengers, dependent on the diverse food items provided by man; food specialists , species éating seeds and berries, ranging widely in the islands and habitats; and insectivorous habitat specialists , inhabiting coniferous habitats, and islands with coniferous (pine) forests. Opportunists, in particular, gel increasing wintering opportunities by the impact of man.     

Numerical and behavioural responses of migrant passerines to experimental manipulation of resident tits (Parus spp.): heterospecific attraction in northern breeding bird communites?

Summary We studied experimentally interspecific competition among foliage-gleaning passerine birds by manipulating the density of resident tits. In 1988 tit density was experimentally increased on three small islands in a central Finnish lake, and decreased on three other islands by tit removal. In order to avoid the effects of between-island differences in habitat quality, the role of the islands was reversed when the experiment was repeated in the following year. Censuses and observations on foraging and feeding behaviour were conducted to assess the numerical and behavioural responses of migrant conguilders (mainly chaffinches and willow warblers) with respect to the manipulated abundance of the tits. We also measured whether variation in food consumption of tits affected the frequency with which the migrants found food by calculating average intervals between successful prey captures, time lags to prey-capture and giving-up times. Our results indicate that interspecific competition is of minor importance in structuring breeding bird assemblages and species feeding ecologies on the study islands. No consistent difference in foraging or feeding niches of chaffinches and willow warblers was found between low and high tit density conditions. Niche overlap analysis showed no avoidance by chaffinches and willow warblers of the microhabitats which tits used. Tit abundance had no significant effect on feeding success or behaviour. Experimentally increased abundance of resident birds was associated with increased abundance of breeding migrants, however. This pattern was found not only in the foliage gleaning guild but also with all passerine birds, indicating that food was not an important contributor to this pattern. We elaborate a hypothesis suggesting heterospecific attraction in northern breeding bird assemblages. Habitat generalist migrants may use the presence of residents as an indicator of safe and/or productive breeding sites in northern unpredictable circumstances.     

Energy expenditure in Crocidurinae shrews (Insectivora): is metabolism a key component of the insular syndrome?

A cascade of morphological, ecological, demographical and behavioural changes operates within island communities compared to mainland. We tested whether metabolic rates change on islands. Using a closed circuit respirometer, we investigated resting metabolic rate (RMR) of three species of Crocidurinae shrews: Suncus etruscus, Crocidura russula, and C. suaveolens. For the latter, we compared energy expenditure of mainland and island populations. Our measurements agree with those previously reported for others Crocidurinae: the interspecific comparison (ANCOVA) demonstrated an allometric relation between energy requirements and body mass. Energy expenditure also scaled with temperature. Island populations (Corsica and Porquerolles) of C. suaveolens differed in size from mainland (gigantism). A GLM showed a significant relationship between energy expenditure, temperature, body mass and locality. Mass specific RMR allometrically scales body mass, but total RMR does not significantly differ between mainland and island, although island shrews are giant. Our results are consistent with other studies: that demonstrated that the evolution of mammalian metabolism on islands is partially independent of body mass. In relation to the insular syndrome, we discuss how island selective forces (changes in resource availability, decrease in competition and predation pressures) can operate in size and physiological adjustments.     

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.     

The role of competition and introduction effort in the success of passeriform birds introduced to New Zealand

Abstract The finding that passeriform birds introduced to the islands of Hawaii and Saint Helena were more likely to successfully invade when fewer other introduced species were present has been interpreted as strong support for the hypothesis that interspecific competition influences invasion success. I tested whether invasions were more likely to succeed when fewer species were present using the records of passeriform birds introduced to four acclimatization districts in New Zealand. I also tested whether introduction effort, measured as the number of introductions and the total number of birds released, could predict invasion outcomes, a result previously established for all birds introduced to New Zealand. I found patterns consistent with both competition and introduction effort as explanations for invasion success. However, data supporting the two explanations were confounded such that the greater success of invaders arriving when fewer other species were present could have been due to a causal relationship between invasion success and introduction effort. Hence, without data on introduction effort, previous studies may have overestimated the degree to which the number of potential competitors could independently explain invasion outcomes and may therefore have overstated the importance of competition in structuring introduced avian assemblages. Furthermore, I suggest that a second pattern in avian invasion success previously attributed to competition, the morphological overdispersion of successful invaders, could also arise as an artifact of variation in introduction effort.     

Mechanisms of soil bacterial and fungal community assembly differ among and within islands

The study of islands has made substantial contributions to the development of evolutionary and ecological theory. However, we know little about microbial community assembly on islands. Using soil microbial data collected from 29 lake islands and nearby mainland, we examined the assembly mechanisms of soil bacterial and fungal communities among and within islands. We found that deterministic processes, especially homogeneous selection, tended to be more important in shaping the assembly of soil bacterial communities among islands, while stochastic processes tended to be more important within islands. Moreover, increasing island area increased the importance of homogeneous selection, but reduced the importance of variable selection, for soil bacterial community assembly within islands. By contrast, stochastic processes tended to dominate soil fungal community assembly both among and within islands, with dispersal limitation playing a more important role within than among islands. Our results highlight the scale- and taxon-dependence of insular soil microbial community assembly, suggesting that spatial scale should be explicitly considered when evaluating the influences of habitat fragmentation on soil microbial communities.     

Distribution patterns and insular biogeography of South Asian raptor communities

Ten diurnal raptor communities (Falconiformes) were studied in continental and peninsular situations, and on landbridge and oceanic islands of various sizes, from Southern India to Southern Vietnam and from Sri Lanka to Java. An index of abundance was derived from 1-km² sample plots. A consistent decrease of species richness occurred from continent to peninsulas and to large landbridge islands, then more abruptly to oceanic islands. The impoverishment process was much faster for open habitat raptors than for forest species, and for rarest and most specialized raptors than for common and more generalist species. Large taxa survived on islands as well as smaller species. Specific habitat requirements, historical factors and forest fragmentation were probably more important determinants of community composition than land area itself. An insular syndrome was documented in forest species on islands, including significant examples of habitat niche expansion, interspecific segregation and density compensation. Some cases suggested that interspecific competition was involved. Such relaxation of habitat and density constraints may enhance the survival probability of these species on islands.     

Patterns of species interaction strength in assembled theoretical competition communities

Strength of interactions between species may be an important tool in our effort to understand community structure. Recent theoretical and empirical findings suggest that despite the presence of some strong interactions, weak interactions prevail in communities. Here, we examine how mean interaction strengths change as theoretical competition communities assemble and what the distribution of interaction coefficients is in the communities that are formed during the assembly process. Our results show that the mean competition strengths fall as assembly progresses and that most interactions in the communities formed are weak. Communities that are invulnerable to further invasions are those where interspecific interactions are weaker than the average interaction strength between species in the pool. If these results can be generalized to more than one trophic level, implications for management and conservation of natural communities are substantial.     

Incisor size and community structure in rodents: Two tests of the role of competition

We used two different approaches to test for the effect of interspecific competition on community-wide patterns in the size of the upper incisor in six rodent communities. One tests for constancy of size ratios between adjacent species (Barton and David's test), and the other tests for minimum mean size overlap between species pairs (randomization test). The results of the two tests were more congruent for the radius of the upper incisor than for its diameter. Although a number of tests gave significant results, they led to a consistent rejection of the null hypothesis of a randomly generated pattern only in two communities from an African rainforest. Many factors are likely to disturb community structure, among which we identify recent species introductions and the presence of rare species. Our results also show that Hutchinson's rule of a limiting size similarity does not hold; the mean size ratio between adjacent species was correlated with the overall size range and the number of species in a community.     

Overdispersion of body size in Australian desert lizard communities at local scales only: no evidence for the Narcissus effect

Both local and regional processes may contribute to community diversity and structure at local scales. Although many studies have investigated patterns of local or regional community structure, few have addressed the extent to which local community structure influences patterns within regional species pools. Here we investigate the role of body size in community assembly at local and regional scales in Ctenotus lizards from arid Australia. Ctenotus has long been noted for its exceptional species diversity in the Australian arid-zone, and previous studies have attempted to elucidate the processes underlying species coexistence within communities of these lizards. However, no consensus has emerged on the role of interspecific competition in the assembly and maintenance of Ctenotus communities. We studied Ctenotus communities at several hundred sites in the arid interior of Australia to test the hypothesis that body sizes within local and regional Ctenotus assemblages should be overdispersed relative to null models of community assembly, and we explored the relationship between body size dispersion at local and regional scales. Results indicate a striking pattern of community-wide overdispersion of body size at local scales, as measured by the variance in size ratios among co-occurring species. However, we find no evidence for body size overdispersion within regional species pools, suggesting a lack of correspondence between processes influencing the distribution of species phenotypes at local and regional scales. We suggest that size ratio constancy in Ctenotus communities may have resulted from contemporary ecological interactions among species or ecological character displacement, and we discuss alternative explanations for the observed patterns. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Predicting community structure in snakes on Eastern Nearctic islands using ecological neutral theory and phylogenetic methods

Predicting species presence and richness on islands is important for understanding the origins of communities and how likely it is that species will disperse and resist extinction. The equilibrium theory of island biogeography (ETIB) and, as a simple model of sampling abundances, the unified neutral theory of biodiversity (UNTB), predict that in situations where mainland to island migration is high, species-abundance relationships explain the presence of taxa on islands. Thus, more abundant mainland species should have a higher probability of occurring on adjacent islands. In contrast to UNTB, if certain groups have traits that permit them to disperse to islands better than other taxa, then phylogeny may be more predictive of which taxa will occur on islands. Taking surveys of 54 island snake communities in the Eastern Nearctic along with mainland communities that have abundance data for each species, we use phylogenetic assembly methods and UNTB estimates to predict island communities. Species richness is predicted by island area, whereas turnover from the mainland to island communities is random with respect to phylogeny. Community structure appears to be ecologically neutral and abundance on the mainland is the best predictor of presence on islands. With regard to young and proximate islands, where allopatric or cladogenetic speciation is not a factor, we find that simple neutral models following UNTB and ETIB predict the structure of island communities.     

Interspecific competition and the structure of bird guilds in boreal Europe: the importance of doing fieldwork in the right season

Bird studies have gained a central role in the debate on the importance of interspecific competition in nature. Thus, the negative results reported from a breeding bird community in a North American shrubsteppe area have created ripples throughout community ecology. However, the set of coexisting breeding birds might be an inappropriate operational definition of a bird community, because the intensity of interspecific competition can be expected to peak in autumn-winter. A review of North European data on wintering birds suggests that the case for the competition theory remains strong when bird communities are defined on the basis of winter coexistence.     

Competition matters: Determining the drivers of land snail community assembly among limestone karst areas in northern Vietnam

The insular limestone karsts of northern Vietnam harbor a very rich biodiversity. Many taxa are strongly associated with these environments, and individual species communities can differ considerably among karst areas. The exact processes that have shaped the biotic composition of these habitats, however, remain largely unknown. In this study, the role of two major processes for the assembly of snail communities on limestone karsts was investigated, interspecific competition and filtering of taxa due to geographical factors. Communities of operculate land snails of the genus Cyclophorus were studied using the dry and fluid‐preserved specimen collections of the Natural History Museum, London. Phylogenetic distances (based on a Bayesian analysis using DNA sequence data) and shell characters (based on 200 semilandmarks) were used as proxies for ecological similarity and were analyzed to reveal patterns of overdispersion (indicating competition) or clustering (indicating filtering) in observed communities compared to random communities. Among the seven studied karst areas, a total of 15 Cyclophorus lineages were found. Unique communities were present in each area. The analyses revealed phylogenetic overdispersion in six and morphological overdispersion in four of seven karst areas. The pattern of frequent phylogenetic overdispersion indicated that competition among lineages is the major process shaping the Cyclophorus communities studied. The Coastal Area, which was phylogenetically overdispersed, showed a clear morphological clustering, which could have been caused by similar ecological adaptations among taxa in this environment. Only the community in the Cuc Phuong Area showed a pattern of phylogenetic clustering, which was partly caused by an absence of a certain, phylogenetically very distinct group in this region. Filtering due to geographical factors could have been involved here. This study shows how museum collections can be used to examine community assembly and contributes to the understanding of the processes that have shaped karst communities in Vietnam.     

Community organization in a recently assembled fauna: the case of Polynesian ants

The ant species inhabiting three remote Polynesian islands were censused by hand collecting techniques. Three commonly occurring species, which exhibited complementary distribution patterns, were numerically and behaviorally dominant to other species in the communities in which they occurred. Field observations and laboratory experiments showed that negative interspecific interactions were important in maintaining the exclusive territories of the dominant species. Habitat also played a role in determining the distribution of dominant species. Pairwise tests of association among all species revealed dominant species were commonly positively associated with their own non-overlapping group of subordinate species, forming a large-scale mosaic distribution pattern of dominant and subordinate species. Baiting experiments, conducted with and without the dominant species removed, demonstrated that the presence of the dominant species decreased: (1) the proportion of baits occupied by other species, and (2) overall ant species diversity, at rich food resources. In ant communities where one of the three recognized dominant species was present, the outcome of negative interspecific interactions among species was more predictable than in communities without a recognized dominant. The same patterns were evident regardless of which of the three dominant species was present in the community. These patterns of community organization observed in this relatively newly assembled fauna are very similar to those reported from studies of older ant communities in mainland areas.     

Species matter: the role of competition in the assembly of congeneric bacteria

Interspecific competition is an important driver of community assembly in plants and animals, but phylogenetic evidence for interspecific competition in bacterial communities has been elusive. This could indicate that other processes such as habitat filtering or neutral processes are more important in bacterial community assembly. Alternatively, this could be a consequence of the lack of a consistent and meaningful species definition in bacteria. We hypothesize that competition in bacterial community assembly has gone undetected at least partly because overly broad measures of bacterial diversity units were used in previous studies. First, we tested our hypothesis in a simulation where we showed that how species are defined can dramatically affect whether phylogenetic overdispersion (a signal consistent with competitive exclusion) will be detected. Second, we demonstrated that using finer-scale Operational Taxonomic Units (OTUs) (with more stringent 16S rRNA sequence identity cutoffs or based on fast-evolving protein coding genes) in natural populations revealed previously undetected overdispersion. Finally, we argue that bacterial ecotypes, diversity units incorporating ecological and evolutionary theory, are superior to OTUs for the purpose of studying community assembly.     

Are island plant communities more invaded than their mainland counterparts?

Questions: Are island vegetation communities more invaded than their mainland counterparts? Is this pattern consistent among community types? Location: The coastal provinces of Catalonia and the para‐oceanic Balearic Islands, both in NE Spain. These islands were connected to the continent more than 5.35 million years ago and are now located <200 km from the coast. Methods: We compiled a database of almost 3000 phytosociological relevés from the Balearic Islands and Catalonia and compared the level of invasion by alien plants in island versus mainland communities. Twenty distinct plant community types were compared between island and mainland counterparts. Results: The percentage of plots with alien species, number, percentage and cover percentage of alien species per plot was greater in Catalonia than in the Balearic Islands in most communities. Overall, across communities, more alien species were found in the mainland (53) compared to the islands (only nine). Despite these differences, patterns of the level of invasion in communities were highly consistent between the islands and mainland. The most invaded communities were ruderal and riparian. Main conclusion: Our results indicate that para‐oceanic island communities such as the Balearic Islands are less invaded than their mainland counterparts. This difference reflects a smaller regional alien species pool in the Balearic Islands than in the adjacent mainland, probably due to differences in landscape heterogeneity and propagule pressure.