RETRACTED ARTICLE: Interspecific competition among ants in the boreal forest: Testing predictions from a linear hierarchical competition model

Competitive interactions often play an important role in local community structure, and particularly so in ant communities. We test predictions derived from a competition model proposed for Scandinavian ant communities by comparing ant nest densities inside and outside fifty randomly-selected territories of competitively dominant wood ants within 30 km of Ume?, northern Sweden. As predicted by the model, competitively-intermediate encounter species, as well as other territorial species, showed complementary occurrences with both studied wood ants, Formica aquilonia Yarrow and F. lugubris Zett.. In contrast, complementary abundances, i.e. lower density of competitively-inferior submissive species in the presence of wood ants, as predicted by the model, was not supported for all submissive species. Of the two studied wood ant species, the nest density of submissive species was negatively correlated only with F. aquilonia. Submissives as a group, as well as Myrmica-species, showed complementary abundances with F. aquilonia, but one Leptothorax and two Serviformica species had higher nest densities in the presence of this competitively-dominant species. We propose that, for Leptothorax, these deviations from the model predictions may be because of limited niche overlap with dominant wood ants, a small worker force and a timid behaviour which does not elicit aggression in wood ants. For the two Serviformica species, we propose a combination of protection against social parasites, inter-specific social control performed by F. aquilonia, and dominance relationships between competitively-inferior submissive species as reasons for the higher nest density inside F. aquilonia territories. Monogyny, and thereby smaller nests, lower worker force, smaller territory (as shown by this study) in F. lugubris, as compared to the polygyny in F. aquilonia, may also help explain the differences in their effects on subordinate species. Our analyses indicate that the linear competition hierarchy model proposed for Scandinavian ants accurately predicts the outcome of interaction between and community composition for dominant territorial and encounter species, but that it needs refinement with respect to the relationship between territorial and submissive species and the resulting community composition. Further studies are needed, especially addressing the complex relationships between these latter groups, and the effects of different competitively-dominant wood ants, to determine the mechanisms determining the outcome of these relationships and to more accurately predict community composition. Received 11 June 2007; revised 11 September 2007; accepted 17 September 2007.     

城市公园植被特征对陆生鸟类集团的影响

城市公共绿地是城市生态系统中重要的鸟类栖息地,其植被特征对鸟类集团存在显著影响。在通过分析植被特征对陆生鸟类集团的作用,从而为公园合理配置植被来提高其作为野生动物栖息地的生态服务功能提供理论基础。2009年10月至2011年10月,采用样线法对上海滨江森林公园进行鸟类调查,利用主成分分析划分鸟类集团,用高度定义植被层次,用卡方检验分析鸟类行为在植被层次上的差异。结果表明,滨江森林公园陆生鸟类群落在乔木层的栖息行为和运动行为频次显著多于其在灌木层和地被层的行为频次,在地被层的取食行为频次显著多于其在乔木层和灌木层的行为频次。陆生鸟类可划分为8个鸟类集团,鸟类集团之间存在栖息、运动和取食空间生态位的重叠。食虫拾取集团、杂食拾取集团、食肉飞取集团和植食拾取集团在栖息、运动和取食空间生态位上均存在较高的重叠度,其通过食性分离各自空间生态位。食虫探取集团和食虫飞取集团互为栖息空间生态位重叠度最高集团,其通过取食方式的不同来实现生态位的分离。根据公园植被特征对鸟类集团的影响结果对上海市公园绿地植被配置提出了建议。     

Interspecific competition between freshwater snails of medical importance: a Venezuelan example

Lake Valencia is located in the centre of the endemic area of the intestinal schistosimiasis in Venezuela. The dominance of two pulmonate species, Biomphalaria glabrata and B. prona., was observed in the lake. Both species are strongly associated with two distinct types of habitats suggesting that competition is occurring between these two species. B. glabrata and B. prona play the role of intermediate hosts of schistosomes in Venezuela. At the present time, parasite transmission is not occurring in the lake but the planning of important development programmes represents a risk of creation of active schistosomiasis foci. The knowledge of the importance and distribution of the snail host populations is therefore essential and must be taken into account for developing future control strategies.     

Interspecific comparison of the fecal microbiota structure in three Arctic migratory bird species

The gut microbiota of birds is known to be characterized for different species, although it may change with feeding items. In this study, we compared the gut microbiota of birds with different feeding behaviors in the same habitat. We collected fecal samples from three Arctic species, snow buntings Plectrophenax nivalis, sanderlings Calidris alba, and pink‐footed geese Anser brachyrhynchus that are phylogenetically quite distant in different families to evaluate effects of diet on gut microbiota. Also, we characterized the prevalence of fecal bacteria using the Illumina MiSeq platform to sequence bacterial 16S rRNA genes. Our NMDS results showed that fecal bacteria of snow buntings and sanderlings were significantly distant from those of pink‐footed geese. Although all three birds were occupied by three bacterial phyla, Proteobacteria, Firmicutes, and Bacteroidetes, dominant taxa still varied among the species. Our bacterial sequences showed that snow buntings and sanderlings were dominated by Firmicutes and Bacteroidetes, while pink‐footed geese were dominated by Proteobacteria. In addition, the bacterial diversity in snow buntings and sanderlings was significantly higher than that in pink‐footed geese. Our results suggest that insectivorous feeding diet of snow buntings and sanderlings could be responsible for the similar bacterial communities between the two species despite the distant phylogenetic relationship. The distinctive bacterial community in pink‐footed geese was discussed to be related with their herbivorous diet.     

Multiscale assessment of the influence of habitat structure and composition on bird assemblages in boreal forest

The relative contributions of habitat structure and composition to biodiversity are often scale-dependent. Although bird communities in boreal forest have been largely altered and threatened by forest harvesting, bird habitat selection in this ecosystem has not been fully understood. Our study aimed to assess the relative contributions of habitat structure and composition on the assemblages of boreal birds at multiple spatial scales characterized by radii ranging from 100 to 1,000?m. We recorded bird species occurrence at 96 stations located in an old-growth forest in the C?te-Nord region of Québec, Canada. We characterized habitat structure using the proportion of dense, open, and sparse stands, and habitat composition using the proportions of coniferous, mixedwood, and deciduous stands. We used partial canonical correspondence analyses and hierarchical variance partitioning to assess the relative contribution of habitat structure and composition on bird assemblage, and logistic regression to model the probability of occurrence for individual species in response to habitat variables. Our results revealed that habitat structure and composition explained similar proportions of the variance in bird assemblage (21.7 vs. 21.6?%), regardless of spatial scale. Whilst logistic regression yielded fair predictions in the occurrence of individual species (i.e., area under the receiver-operating characteristic curve >0.70 for 90?% of the species), it further confirmed our findings in community level analysis. Our study indicates that habitat structure and composition are both important in shaping bird assemblages, but spatial scale draws little influence on their relative contributions.     

Water bird guilds and their feeding connections in the Bodrogzug, Hungary

Thalli of the intertidal Phaeophyte Fucus spiralis L. and the subtidal Chlorophyte Ulva olivascens Dangeard were exposed to artificial UV-A, UV-B and photosynthetically active radiation (PAR) by combination of PAR + UV-A + UV-B (PAB), PAR + UV-A (PA) and PAR (P) treatments. UV-A enhanced photosynthesis and stimulated carbonic anhydrase (CA) and nitrate reductase (NR) in F. spiralis whilst PAR only had an inhibitory effect in this species. U. olivascens suffered chronic photoinhibition in all the treatments as evidenced by reduced maxima photosynthesis (P_max) and photosynthetic efficiency (α). Non stimulatory effect was observed upon CA and NR in this species. Our results showed that artificial UV radiation triggered opposite responses in both species. We suggest that differences shown by both species might be related to their location in the rocky shore and their ability to sense UV. We propose that the ratio UV:PAR acts as an environmental signal involved in the control of photosynthesis as shown by pronounced inhibition in samples exposed to only PAR. We also suggest that UV-regulated photosynthesis would be related to carbon (C) and nitrogen (N) cycles, regulating feedback processes that control C and N assimilation.     

Relationships of bird community structure and species distributions to two environmental gradients in the northern boreal forest

Few studies have explicitly examined avian community structure in the North American northern boreal forest Herein we report upon the results of bird surveys in mature stands of boreal forest in the Great Clay Belt Ontario. Canada We related trends in avian community structure and individual bird species abundance to two environmental gradients described in a landbase classification scheme called the Forest Ecosystem Classification a moisture and a nutrient-richness gradient Variation in environmental characteristics is limited in the Ontario Clay Belt and this was reflected in short environmental gradient lengths However major trends in avian community structure were strongly associated with the nutrient-richness gradient axis summarized by this scheme Analyses of avian community composition indicated a continuum from moist coniferous habitats to drier aspen-dominated mixed woodlands with several bird species occurring in varying abundances across the width of both gradients 48 of the 58 species examined showed statistically significant associations with at least one of the two multivariate gradient axes A smaller proportion of short-distance migrant species were associated with these gradients than were species in either the neotropical migrant or resident categories We used a multivariate variable (habitat breadth) to compare degree of habitat specialization across different migratory groups A large proportion of neotropical migrants showed a high degree of habitat specialization m mature Clay Belt forests, and neotropical species with small habitat breadths were more commonly associated with habitats dominated by broad-leaved deciduous tree species than either short-distance or permanent resident species We discuss our findings in relation to the post-glacial history of the Clay Belt region     

Fine-scale structure and cross-taxon congruence of bird and beetle assemblages in an old-growth boreal forest mosaic

Aim Nestedness occurs when species present in depauperate sites are subsets of those found in species‐rich sites. The degree of congruence of site nestedness among different assemblages can inform commonalities of mechanisms structuring the assemblages. Well‐nested assemblages may still contain idiosyncratic species and sites that notably depart from the typical assemblage pattern. Idiosyncrasy can arise from multiple processes, including interspecific interactions and habitat preferences, which entail different consequences for species co‐occurrences. We investigate the influence of fine‐scale habitat variation on nestedness and idiosyncrasy patterns of beetle and bird assemblages. We examine community‐level and pairwise species co‐occurrence patterns, and highlight the potential influence of interspecific interactions for assemblage structure. Location Côte‐Nord region of Québec, Canada. Methods We sampled occurrences of ground‐dwelling beetles, flying beetles and birds at sites within old‐growth boreal forest. We examined the nestedness and idiosyncrasy of sites and sought relationships to habitat attributes. We analysed non‐random species co‐occurrence patterns at pairwise and community levels, using null model analysis and five ‘association’ indices. Results All three assemblages were significantly nested. There was limited congruence only between birds and flying beetles whose nestedness was related to canopy openness. For ground‐dwelling beetles, nestedness was related to high stand heterogeneity and sapling density, whereas site idiosyncrasy was inversely related to structural heterogeneity. For birds, site idiosyncrasy increased with canopy cover, and most idiosyncratic species were closed‐canopy specialists. In all assemblages, species idiosyncrasy was positively correlated with the frequency of negative pairwise associations. Species co‐occurrence patterns were non‐random, and for flying beetles and birds positive species pairwise associations dominated. Community‐level co‐occurrence summaries may not, however, always reflect these patterns. Main conclusions Nestedness patterns of different assemblages may not correlate, even when sampled at common locations, because of different responses to local habitat attributes. We found idiosyncrasy patterns indicating opposing habitat preferences, consistent with antagonistic interactions among species within assemblages. Analysis of such patterns can thus suggest the mechanisms generating assemblage structures, with implications for biodiversity conservation.     

Scales of spatiotemporal variation in macroinvertebrate assemblage structure in monsoonal streams: the importance of season

1. We assessed the relative importance of different scales of spatial and temporal variability on benthic macroinvertebrate assemblage structure in six unpolluted streams in monsoonal Hong Kong using ordination and complementary multivariate analyses. The spatial scales were regions, sites (streams) and sections (riffles) within sites. The temporal scales were years (three, including one with unusually high rainfall), seasons (dry versus wet) and days within seasons. 2. Significant differences in assemblage structure were manifested at all temporal scales. Those at the site scale were most obvious, whereas demarcation of assemblage structure at the section (riffle) scale was smaller, and there was no significant regional differentiation in assemblage structure. Seasonal variability in assemblage structure was greater than that among years or days. 3. Inter‐year differences in assemblage structure were recorded at all sites, and were noted among all years at some sites but not at others. They were recorded more frequently during the dry season, although their occurrence (in pair‐wise comparisons between years) appeared to be related to differences in the monsoonal (wet season) rainfall. 4. Seasonal differences in assemblage structure were strongly evident at all sites. Inter‐site differences were more apparent during the dry season when local (site‐scale) influences on assemblages were stronger. By contrast, wet‐season samples were more variable because of spate‐induced disturbance, and inter‐site differentiation was less distinct. 5. Differences among days at all sites were relatively minor, but shifts attributable to repeated spate‐induced disturbance were evident at some sites during the wet season. 6. Differences at the section scale were recorded more frequently during the dry season, when the extent of within‐site variability among sections was higher, reflecting increased patchiness within sections resulting from increased substratum heterogeneity and/or greater intensity of biotic interactions. 7. Seasonal shifts in macroinvertebrate assemblage structure at a variety of scales in Hong Kong streams are likely to be attributable to monsoonal rains affecting the relative intensity of abiotic disturbance and biotic interactions in accordance with the harsh‐benign hypothesis.     

Variation in tropical bird survival across longitude and guilds: a case study from the Amazon

Survival estimates of tropical birds have been used to examine life‐history variation across latitudes and dietary guilds. Here, we provide apparent survival estimates of 31 rainforest species from central Amazonian Brazil and compare our results with estimates from Ecuador, Peru (western Amazonia) and French Guiana (eastern Amazonia) to examine geographic variation in demography. Our averaged annual survival estimate for central Amazonian species (?= 0.59, SE = 0.10) was concordant with previously published estimates from the western Amazon (?= 0.58, SE = 0.02), and eastern Amazon (?= 0.63, SE = 0.06). Two predominate demographic patterns were detected across the study areas: within species, survival was highest or lowest in eastern or western Amazonia, but rarely in central Amazonia. The most striking demographic variation was exhibited by Pithys albifrons, for which apparent survival estimates were nearly twice as high in eastern Amazonia (?= 0.80, SE = 0.06) than in western Amazonia (?= 0.42, SE = 0.06) but intermediate in central Amazonia (?= 0.54, SE = 0.04). Although variation in survival may be associated with differences in life history characteristics, our analysis of flocking guild, body size, and nest architecture revealed only moderate differences in survival associated with nest architecture. These results suggest that geographic variation in population processes may be significant for widespread Amazonian species.     

Interspecific competition in bats: state of knowledge and research challenges

1. Interspecific competition (IC) is often seen as a main driver of evolutionary patterns and community structure. Bats might compete for key resources, and cases of exaggerated divergence of resource-related characters or trait overdispersion in bat assemblages are often explained in terms of current or past interspecific competition. However, other pressures leading to patterns that mimic the outcome of competition cannot always be ruled out.
2. We present the state of knowledge on IC among bats, providing a critical evaluation of the information available and identifying open questions and challenges.
3. We reviewed 100 documents addressing potential or actual IC in bats and categorised them in terms of the resource for which bats compete (food, foraging habitat, roosts, water, and acoustic space). We also examined the ecomorphological and behavioural traits considered therein to highlight responses to IC or niche partitioning.
4. We found that: although resources should be limiting in order for competition to occur, this is seldom tested; sympatry is sometimes taken as synonymous of syntopy (yet sympatric species that are not syntopic will never experience competition); comparisons between sympatry and allopatry are rare; and testing of objective criteria exploring the existence of niche partitioning or character displacement is not commonly adopted.
5. While morphological examination of food remains in droppings has often led to coarse-grained analysis that proved insufficient to establish the occurrence of food niche overlap or partitioning, new frontiers are being opened by state-of-the-art molecular dietary analysis.
6. A better understanding of IC in bats is paramount, since distributional changes leading to novel bat assemblages driven by climate change are already taking place, and the dramatic decline in insect availability, as well as the global loss or alteration of foraging habitat, may generate new competitive interactions or exacerbate existing interactions in the Anthropocene, and into the future.

     

Foraging guilds of aquatic birds on productive boreal lakes: environmental relations and concordance patterns

Previous studies have shown that when an ecosystem consists of many interacting components it becomes impossible to understand how it functions by focussing only on individual relationships. Alternatively, one can attempt to quantify system behaviour as a whole by developing ecological indicators that combine numerous environmental factors into a single value. One such holistic measure, called the system ‘ascendency’, arises from the analysis of networks of trophic exchanges. It deals with the joint quantification of overall system activity with the organisation of the component processes and can be used specifically to identify the occurrence of eutrophication. System ascendency analyses were applied to data over a gradient of eutrophication in a well documented small temperate intertidal estuary. Three areas were compared along the gradient, respectively, non eutrophic, intermediate eutrophic, and strongly eutrophic. Values of other measures related to the ascendency, such as the total system throughput, development capacity, and average mutual information, as well as the ascendency itself, were clearly higher in the non-eutrophic area. When the whole-system properties of the three areas were compared, however, the values associated with the intermediate eutrophic area turned out to be the lowest, which possibly could be attributed to the unstable nature of this area. The current study provided an example of how the measures arising out of␣network analysis might lead to an improved understanding of the system functioning and of the eutrophication process itself.     

Fitness effects of endemic malaria infections in a wild bird population: the importance of ecological structure

1.?Parasites can have important effects on host populations influencing either fecundity or mortality, but understanding the magnitude of these effects in endemic host-parasite systems is challenging and requires an understanding of ecological processes affecting both host and parasite. 2.?Avian blood parasites (Haemoproteus and Plasmodium) have been much studied, but the effects of these parasites on hosts in areas where they are endemic remains poorly known. 3.?We used a multistate modelling framework to explore the effects of chronic infection with Plasmodium on survival and recapture probability in a large data set of breeding blue tits, involving 3424 individuals and 3118 infection diagnoses over nine years. 4.?We reveal strong associations between chronic malaria infection and both recapture and survival, effects that are dependent on the clade of parasite, on host traits and on the local risk of infection. 5.?Infection with Plasmodium relictum was associated with reduced recapture probability and increased survival, compared to P.?circumflexum, suggesting that these parasites have differing virulence and cause different types of selection on this host. 6.?Our results suggest a large potential survival cost of acute infections revealed by modelling host survival as a function of the local risk of infection. 7.?Our analyses suggest not only that endemic avian malaria may have multiple fitness effects on their hosts and that these effects are species dependent, but also that adding ecological structure (in this case parasite species and spatial variation in disease occurrence) to analyses of host-parasite interactions is an important step in understanding the ecology and evolution of these systems.     

Interspecific competition between insect herbivores: asymmetric competition between cinnabar moth and the ragwort seed-head fly

ABSTRACT.

• 1 Removal field experiments and observational studies have been undertaken to determine whether feeding by cinnabar moth Tyria jacobaeae L. on the flower heads of ragwort Senecio jacobaea L. affects the abundance of the fly Pegohylemyia seneciella (Meade) that feeds in the flower heads as a larva.
• 2 Correlations between the population density of cinnabar moth and the population density of the fly were suggestive of habitat separation, but provided little evidence of exploitation competition.
• 3 Removal of cinnabar moth by hand from replicated plots over two years shows that, in years when ragwort flower production is consumed by cinnabar moth caterpillars, the fly may show no recruitment at all.
• 4 Fly populations persist in refugia, exploiting ragwort plants that grow in areas where there are no cinnabar moth.
• 5 Recruitment of ragwort is not seed limited, so the reduction in seed production caused by P. seneciella (maximum about 30%) has no impact on ragwort abundance, or on the abundance of cinnabar moth.
• 6 We conclude that there is strong interspecific competition between these two species, and that the competition is highly asymmetric. The cinnabar moth had a substantial effect on the recruitment of the fly in 1986, but the fly has no measurable impact on the recruitment of the moth. In six years out of seven in our long-term study, cinnabar moth reduced flower production to levels comparable to those measured in 1986, and we infer that strong competition with the fly was likely in six years out of seven.
• 7 One reason why there are so few published examples of asymmetric interspecific competition may be simply that the experiments are thought too obvious to be worth doing. We argue that this is not a good reason for eschewing manipulative field experiments, and that few processes in ecology are at all obvious when investigated in detail.

     

Competition and the structure of hydropsychid guilds in southern Sweden

Three stream in southern Sweden were sampled. The hydropsychid guild in the Lerbacken was found to consist of Hydropsyche saxonica, H. angustipennis and H. siltalai. In the Bråån, H. pellucidula replaces H. saxonica and in the Övedsån only H. angustipennis and H. siltalai are present. The age structure in the Ovedsån is advanced, suggesting niche shifts have occurred in the other streams.In-situ and laboratory studies at high densities show the inter-specific interactions between H. saxonica and H. pellucidula lead to a greater response than inter-specific interactions within either. No difference in interaction level was found for H. angustipennis and H. siltalaf. Behavioural studies into the reponses to net invasion, show differences in defence strategy between H. saxonica and H. pellucidula. Inter-specific encounters lead to the intruder leaving the net faster than encounters between conspecifics.Competition is suggested as leading to niche shifts and species exclusion and so is an important factor in guild structure.     

Vanishing bird species in the Atlantic Forest: relative importance of landscape configuration, forest structure and species characteristics

Patch size, isolation, and vegetation structure are expected to strongly affect species persistence in fragmented landscapes, particularly for those with <30% of native habitat remaining. Those influences should be modulated by species characteristics, resulting in complex relationships. In order to investigate how species, habitat structure and landscape factors are related and how they affected species persistence, we studied bird communities in a fragmented Atlantic Forest region. Patch size strongly affected species richness and population abundances. However, some functional groups were more affected than others, particularly endemic and understory insectivores, species that are near the limits of their geographical distribution, those using few forest types, and those with their center of abundance in high altitude tropical forests. The effect of vegetation structure was mainly at the species level, reflecting specific responses to habitat quality. The importance of landscape variables varies according to the species group. For the most affected ones, which usually have low dispersal capacity, patch size and quality were the most relevant factors, whereas patch isolation was associated with the richness of groups with more generalist species. This pattern is due to the limited structural connectivity in the study region, composed of low matrix permeability (e.g. pastures and sugar cane), which isolate the most affected species, making them more dependent on local factors. In such a fragmented landscape, the largest patches should be prioritized for conservation purposes, as they aggregate the most vulnerable species and present the highest alpha diversity. Landscape management, as such, should also reconnect large fragments through corridors or matrix improvements, promoting better conditions for long-term persistence of the most affected species.     

Vegetation structure influences foraging decisions in a declining grassland bird: the importance of fine-scale habitat and grazing regime

Capsule: Whinchat Saxicola rubetra foraging behaviour was significantly influenced by habitat structure and grazing.

Aims: To assess how foraging habitats selected by breeding Whinchats differed from wider territory attributes under contrasting grazing management in multiple upland areas in Scotland: principally sheep grazed, Red Deer grazed or ungrazed, and to identify how differing land use may limit suitable foraging areas.

Methods: We compared fine-scale vegetation structure in patches chosen for foraging by Whinchats in contrasting grazing management regimes.

Results: Whinchats were less likely to forage in patches with a greater cover of bracken and tall non-bracken vegetation, regardless of grazing regime. Grass cover influenced foraging behaviour in ungrazed habitats only, where Whinchats were less likely to forage in areas with high grass cover.

Conclusion: Whinchats appear to require a mosaic or range of sward structures within breeding territories, highlighting the importance of establishing how vegetation structure influences breeding birds at different spatial scales. Our results suggest that suitable foraging patches were plentiful within grazed habitats but potentially limited in ungrazed habitats. Further work is needed to identify management regimes and interventions to maintain conditions suitable for breeding Whinchats that are compatible with other land use and conservation objectives.     

Interspecific variation in egg testosterone levels: implications for the evolution of bird song

Although interspecific variation in maternal effects via testosterone levels can be mediated by natural selection, little is known about the evolutionary consequences of egg testosterone for sexual selection. However, two nonexclusive evolutionary hypotheses predict an interspecific relationship between egg testosterone levels and the elaboration of sexual traits. First, maternal investment may be particularly enhanced in sexually selected species, which should generate a positive relationship. Secondly, high prenatal testosterone levels may constrain the development of sexual characters, which should result in a negative relationship. Here we investigated these hypotheses by exploring the relationship between yolk testosterone levels and features of song in a phylogenetic study of 36 passerine species. We found that song duration and syllable repertoire size were significantly negatively related to testosterone levels in the egg, even if potentially confounding factors were held constant. These relationships imply that high testosterone levels during early development of songs may be detrimental, thus supporting the developmental constraints hypothesis. By contrast, we found significant evidence that song-post exposure relative to the height of the vegetation is positively related to egg testosterone levels. These results support the hypothesis that high levels of maternal testosterone have evolved in species with intense sexual selection acting on the location of song-posts. We found nonsignificant effects for intersong interval and song type repertoire size, which may suggest that none of the above hypothesis apply to these traits, or they act simultaneously and have opposing effects.     

Large size in an island-dwelling bird: intraspecific competition and the Dominance Hypothesis

Differences between island‐ and mainland‐dwelling forms provide several classic ecological puzzles. Why, for instance, are island‐dwelling passerine birds consistently larger than their mainland counterparts? We examine the ‘Dominance hypothesis’, based on intraspecific competition, which states that large size in island passerines evolves through selection for success in agonistic encounters. We use the Heron Island population of Capricorn silvereyes (Zosterops lateralis chlorocephalus), a large‐bodied island‐dwelling race of white‐eye (Zosteropidae), to test three assumptions of this hypothesis; that (i) large size is positively associated with high fitness, (ii) large size is associated with dominance, and (iii) the relationship between size and dominance is particularly pronounced under extreme intraspecific competition. Our results supported the first two of these assumptions, but provided mixed evidence on the third. On balance, we suggest that the Dominance Hypothesis is a plausible mechanism for the evolution of large size of island passerines, but urge further empirical tests on the role of intraspecific competition on oceanic islands versus that on mainlands.