Despotic, high-impact species and the subcontinental scale control of avian assemblage structure

Higher trophic levels and species with poorer dispersal traits are more susceptible to habitat loss on island fragments

Ongoing habitat loss and fragmentation caused by human activities represent one of the greatest causes of biodiversity loss. However, the effects of habitat loss and fragmentation are not felt equally among species. Here, we examined how habitat loss influenced the diversity and abundance of species from different trophic levels, with different traits, by taking advantage of an inadvertent experiment that created habitat islands from a once continuous forest via the creation of the Thousand Island Lake, a large reservoir in China. On 28 of these islands with more than a 9000-fold difference in their area (0.12-1154 ha), we sampled plants, herbivorous insects, and predatory insects using effort-controlled sampling and analyses. This allowed us to discern whether any observed differences in species diversity were due to passive sampling alone or to demographic effects that disproportionately influenced some species relative to others. We found that while most metrics of sampling effort-controlled diversity increased with island area, the strength of the effect was exacerbated for species in higher trophic levels. When we more explicitly examined differences in species composition among islands, we found that the pairwise difference in species composition among islands was dominated by species turnover but that nestedness increased with differences in island area, indicating that some species are more likely to be absent from smaller islands. Furthermore, by examining trends of several dispersal-related traits of species, we found that species with lower dispersal propensity tended to be those that were lost from smaller islands, which was observed for herbivorous and predatory insects. Our results emphasize the importance of incorporating within-patch demographic effects, as well as the taxa and traits of species when understanding the influence of habitat loss on biodiversity.

Interspecific behavioural interference and range dynamics: current insights and future directions

Novel biotic interactions in shifting communities play a key role in determining the ability of species' ranges to track suitable habitat. To date, the impact of biotic interactions on range dynamics have predominantly been studied in the context of interactions between different trophic levels or, to a lesser extent, exploitative competition between species of the same trophic level. Yet, both theory and a growing number of empirical studies show that interspecific behavioural interference, such as interspecific territorial and mating interactions, can slow down range expansions, preclude coexistence, or drive local extinction, even in the absence of resource competition. We conducted a systematic review of the current empirical research into the consequences of interspecific behavioural interference on range dynamics. Our findings demonstrate there is abundant evidence that behavioural interference by one species can impact the spatial distribution of another. Furthermore, we identify several gaps where more empirical work is needed to test predictions from theory robustly. Finally, we outline several avenues for future research, providing suggestions for how interspecific behavioural interference could be incorporated into existing scientific frameworks for understanding how biotic interactions influence range expansions, such as species distribution models, to build a stronger understanding of the potential consequences of behavioural interference on the outcome of future range dynamics.

Bird Communities in a Changing World: The Role of Interspecific Competition

Significant changes in the environment have the potential to affect bird species abundance and distribution, both directly, through a modification of the landscape, habitats, and climate, and indirectly, through a modification of biotic interactions such as competitive interactions. Predicting and mitigating the consequences of global change thus requires not only a sound understanding of the role played by biotic interactions in current ecosystems, but also the recognition and study of the complex and intricate effects that result from the perturbation of these ecosystems. In this review, we emphasize the role of interspecific competition in bird communities by focusing on three main predictions derived from theoretical and empirical considerations. We provide numerous examples of population decline and displacement that appeared to be, at least in part, driven by competition, and were amplified by environmental changes associated with human activities. Beyond a shift in relative species abundance, we show that interspecific competition may have a negative impact on species richness, ecosystem services, and endangered species. Despite these findings, we argue that, in general, the role played by interspecific competition in current communities remains poorly understood due to methodological issues and the complexity of natural communities. Predicting the consequences of global change in these communities is further complicated by uncertainty regarding future environmental conditions and the speed and efficacy of plastic and evolutionary responses to fast-changing environments. Possible directions of future research are highlighted.

Mistletoes could moderate drought impacts on birds, but are themselves susceptible to drought-induced dieback

Mistletoes are hemiparasitic plants and keystone species in many ecosystems globally. Given predicted increases in drought frequency and intensity, mistletoes may be crucial for moderating drought impacts on community structure. Dependent on host vascular flows, mistletoes can succumb to stress when water availability falls, making them susceptible to mortality during drought. We counted mistletoe across greater than 350 000 km² of southeastern Australia and conducted standardized bird surveys between 2016 and 2021, spanning a major drought event in 2018-2019. We aimed to identify predictors of mistletoe abundance and mortality and determine whether mistletoes might moderate drought impacts on woodland birds. Live mistletoe abundance varied with tree species composition, land use and presence of mistletoebirds. Mistletoe mortality was widespread, consistent with high 2018/2019 summer temperatures, low 2019/2020 summer rainfall and the interaction between summer temperatures and rainfall in 2019/2020. The positive association between surviving mistletoes and woodland birds was greatest in the peak drought breeding seasons of 2018/2019 and 2019/2020, particularly for small residents and insectivores. Paradoxically, mistletoes could moderate drought impacts on birds, but are themselves vulnerable to drought-induced mortality. An improved understanding of the drivers and dynamics of mistletoe mortality is needed to address potential cascading trophic impacts associated with mistletoe die-off.

The role of invasion and urbanization gradients in shaping avian community composition

The extent to which native species utilize urban environments depends on species responses to multiple threatening processes. Here, we aimed to quantify changes in bird communities in response to changing habitat structure, invasive species and aggressive native species. We conducted surveys in two independently invaded regions with similar patterns of urban development. The study regions were New South Wales (NSW) and Queensland (QLD), Australia. We observed 127 species in NSW and 144 species in QLD. Most species (NSW 83 and QLD 84) are urban adapters making use of some or all urban sub-environments. Urban avoiders, species only found in remnant vegetation, were the second largest group (urban avoiders: NSW 23 and QLD 31). We found the lowest richness in the most urban sites (urban exploiters: NSW 10 and QLD 15). Using generalized linear mixed models, we found a non-significant relationship between species richness and the abundance of aggressive species like the common myna and noisy miners, Manorina melanocephala, but a significant positive correlation with the percentage of shrub cover at a site. As there is a gradual loss of species with increasing urbanization, retaining higher complexity in vegetation structure in urban areas will support large numbers of species and could help mitigate the potential impacts of aggressive urban-adapted species and habitat loss.

Competition and hybridization drive interspecific territoriality in birds

Costly interactions between species that arise as a by-product of ancestral similarities in communication signals are expected to persist only under specific evolutionary circumstances. Territorial aggression between species, for instance, is widely assumed to persist only when extrinsic barriers prevent niche divergence or selection in sympatry is too weak to overcome gene flow from allopatry. However, recent theoretical and comparative studies have challenged this view. Here we present a large-scale, phylogenetic analysis of the distribution and determinants of interspecific territoriality. We find that interspecific territoriality is widespread in birds and strongly associated with hybridization and resource overlap during the breeding season. Contrary to the view that territoriality only persists between species that rarely breed in the same areas or where niche divergence is constrained by habitat structure, we find that interspecific territoriality is positively associated with breeding habitat overlap and unrelated to habitat structure. Furthermore, our results provide compelling evidence that ancestral similarities in territorial signals are maintained and reinforced by selection when interspecific territoriality is adaptive. The territorial signals linked to interspecific territoriality in birds depend on the evolutionary age of interacting species, plumage at shallow (within-family) timescales, and song at deeper (between-family) timescales. Evidently, territorial interactions between species have persisted and shaped phenotypic diversity on a macroevolutionary timescale.

Patch-scale culls of an overabundant bird defeated by immediate recolonization

Overabundant native animals cause a variety of human-wildlife conflicts that can require management to reduce their social, environmental, or economic impacts. Culling is an intuitively attractive management response to overabundance, but poor monitoring of results and costs means that evidence for successful outcomes is often lacking. Furthermore, many culls worldwide have been ineffective or counterproductive due to ecological release mechanisms or compensatory responses by the overabundant species. We completed a controlled, replicated, costed, and rigorously monitored experimental cull of the endemic Australian honeyeater, the Noisy Miner (Manorina melanocephala). Aggressive exclusion of birds from remnant woodland patches by overabundant Noisy Miners is listed as a Key Threatening Process under Australian conservation legislation due to its impacts on threatened birds. The problem is particularly prevalent in the highly modified agricultural landscapes of eastern Australia. The species impacts avian assemblages at low densities (0.6-0.8 birds/ha) and at a subcontinental scale (>1 million km² ). Some ecologists recommend culling as the only management response capable of timely reversal of declines of threatened small woodland birds. We monitored Noisy Miner abundance before and for 12 months after a culling program and found that immediate recolonization from the surrounding landscape negated the impact of the cull. We hypothesize that this is due to a vacuum effect; whereby, birds resident in more marginal habitat around treatment patches move into the vacant territory post-cull. Modeled mean abundance of Noisy Miners declined by 22% in treatment sites compared to an increase of 4% in control sites in the post-cull period. Abundance in all sites, however, remained three to five times higher than published ecological impact thresholds. Return on investment analysis indicated no relationship between culling effort and reduction in Noisy Miner abundance. We conclude that culling at a patch scale is not an efficient method of reducing Noisy Miner abundance to levels unlikely to impact threatened woodland birds in the highly modified study landscape, despite estimated costs 18 times lower than another potential management response of revegetation. Our study highlights the importance of building empirical evidence before intuitively attractive but not necessarily ecologically effective management responses are applied more widely.

Weather effects on birds of different size are mediated by long-term climate and vegetation type in endangered temperate woodlands

Species occurrence is influenced by a range of factors including habitat attributes, climate, weather, and human landscape modification. These drivers are likely to interact, but their effects are frequently quantified independently. Here, we report the results of a 13-year study of temperate woodland birds in south-eastern Australia to quantify how different-sized birds respond to the interacting effects of: (a) short-term weather (rainfall and temperature in the 12 months preceding our surveys), (b) long-term climate (average rainfall and maximum and minimum temperatures over the period 1970-2014), and (c) broad structural forms of vegetation (old-growth woodland, regrowth woodland, and restoration plantings). We uncovered significant interactions between bird body size, vegetation type, climate, and weather. High short-term rainfall was associated with decreased occurrence of large birds in old-growth and regrowth woodland, but not in restoration plantings. Conversely, small bird occurrence peaked in wet years, but this effect was most pronounced in locations with a history of high rainfall, and was actually reversed (peak occurrence in dry years) in restoration plantings in dry climates. The occurrence of small birds was depressed-and large birds elevated-in hot years, except in restoration plantings which supported few large birds under these circumstances. Our investigation suggests that different mechanisms may underpin contrasting responses of small and large birds to the interacting effects of climate, weather, and vegetation type. A diversity of vegetation cover is needed across a landscape to promote the occurrence of different-sized bird species in agriculture-dominated landscapes, particularly under variable weather conditions. Climate change is predicted to lead to widespread drying of our study region, and restoration plantings-especially currently climatically wet areas-may become critically important for conserving bird species, particularly small-bodied taxa.

Radical changes in the avifauna of a Sydney suburb, 1971–2014

We document changes in the avifauna of Longueville, a residential suburb of Sydney, between 1971–73 and 1977–79 based on daily bird lists, and make comparisons with the present avifauna based on surveys in 2013–14. Of the 31 most common native terrestrial bird species, 17 were ‘decreasers’, present in 1971–73 but not recorded in 2013–14 (although 16 of them were recorded in a large bushland remnant 5 km away), and 14 were ‘increasers’, seven of which were absent or very rare in 1971–73. Eleven species decreased during the 1970s, and six species afterwards. Eight species increased during the 1970s and six species afterwards. The decreasers were predominantly small insectivores and nectarivores. The increasers were of three main types: medium to large carnivores, large frugivores or granivores, and medium-sized nectarivores. Two of the nectarivores, the noisy miner (Manorina melanocephala) and rainbow lorikeet (Trichoglossus haematodus), now dominate the urban bird community of Longueville and are by far the most numerous birds. The increase of both species dates from the late 1970s. The noisy miner is highly aggressive towards other birds and its dramatic increase (it was absent in 1971–73) appears to be the chief cause of the decline of small native birds in Longueville.

Short-term response of a declining woodland bird assemblage to the removal of a despotic competitor

Interspecific aggression by the noisy miner (Manorina melanocephala), a highly despotic species, is homogenizing woodland avifaunas across eastern Australia. Although a native species, the noisy miner's aggressive exclusion of small birds is a Key Threatening Process under national law. Large-scale removal of noisy miners has been proposed as a management response to this threat following increases in miner presence due to anthropogenic land use practices. We tested this proposal by experimentally removing noisy miners from eucalypt woodland remnants (16-49 ha), assigned randomly as control (n = 12) or treatment (miner removal) sites (n = 12). Standardized bird surveys were conducted before and after removal, and generalized linear mixed models were used to investigate the effect of miner removal on bird assemblage metrics. Despite removing 3552 noisy miners in three sessions of systematic shooting, densities of noisy miners remained similarly high in treatment and control sites, even just 14 days after their removal. However, there was evidence of an increase in richness and abundance of small birds in treatment sites compared to controls-an effect we only expected to see if noisy miner densities were drastically reduced. We suggest that miner removal may have reduced the ability of the recolonizing miners to aggressively exclude small birds, even without substantially reducing miner densities, due to the breakdown of social structures that are central to the species' despotic behaviour. However, this effect on small birds is unlikely to persist in the long term. Synthesis and applications: Despite evidence from other studies that direct removal of noisy miners can result in rapid and sustained conservation benefit for bird communities at small scales, our findings cast doubt on the potential to scale-up this management approach. The circumstances under which direct control of noisy miners can be achieved remain unresolved.

Causes and Consequences of Behavioral Interference between Species

Behavioral interference between species, such as territorial aggression, courtship, and mating, is widespread in animals. While aggressive and reproductive forms of interspecific interference have generally been studied separately, their many parallels and connections warrant a unified conceptual approach. Substantial evidence exists that aggressive and reproductive interference have pervasive effects on species coexistence, range limits, and evolutionary processes, including divergent and convergent forms of character displacement. Alien species invasions and climate change-induced range shifts result in novel interspecific interactions, heightening the importance of predicting the consequences of species interactions, and behavioral interference is a fundamental but neglected part of the equation. Here, we outline priorities for further theoretical and empirical research on the ecological and evolutionary consequences of behavioral interference.

How dependent are bell miners (Manorina melanophrys) on lantana (Lantana camara) as part of their habitat?

Context The bell miner (Manorina melanophrys) is thought to prefer the dense vegetation of Lantana camara, a dominating noxious weed, to nest in compared with native plant communities in northern New South Wales and south-eastern Queensland. As miner numbers seemed to have increased with increased availability of nesting habitat, larger numbers of despotic bell miners territorially reduce native avian diversity. The resultant high insect numbers in the canopy lead to dieback. It is unknown to what extent the native bell miner relies on lantana for nesting sites and if restoration through removal of lantana, resulting in an influx of native species, changes food availability and thus foraging behaviours. Aims The objective of our study was to understand the relationship between the presence of lantana and bell miners by determining: (1) bell miner behaviours in response to different understorey types; (2) bell miner nesting choices on a broad scale; (3) nesting preference for each site on a broad scale; and (4) whether nest height is related to fledging success. Methods Fifty individual bell miners were observed within five colonies (four containing comparison quadrats of treated and intact lantana as part of a restoration study and one containing water vine for comparison as a pristine site; n=10 per treatment) during four seasons. Five bouts of gleaning, probing or sallying were observed, noting height and plant species. Nest sites were examined over a large area, noting plant species present, nest height and plant height. Key results Bell miners used gleaning as the primary foraging tactic primarily in eucalypts (93%; n=600, mean=13.6m±0.29, s.e.). Bell miners generally nested in understorey vegetation (83.5% of nests observed) at a mean height of 2.7m in 37 different plant species, native and exotic. Nest height did not influence the probability of broods fledging. Conclusions Bell miners exhibit considerable nest plasticity, and the removal of lantana as an oft-used nesting habitat is unlikely to lead to relocation by this species. Implications Instead, future research needs to focus on canopy insect assemblages in bell miner colonies, a potentially more important determinant of bell miner distribution.

Using empirical models of species colonization under multiple threatening processes to identify complementary threat-mitigation strategies

Approaches to prioritize conservation actions are gaining popularity. However, limited empirical evidence exists on which species might benefit most from threat mitigation and on what combination of threats, if mitigated simultaneously, would result in the best outcomes for biodiversity. We devised a way to prioritize threat mitigation at a regional scale with empirical evidence based on predicted changes to population dynamics-information that is lacking in most threat-management prioritization frameworks that rely on expert elicitation. We used dynamic occupancy models to investigate the effects of multiple threats (tree cover, grazing, and presence of an hyperaggressive competitor, the Noisy Miner (Manorina melanocephala) on bird-population dynamics in an endangered woodland community in southeastern Australia. The 3 threatening processes had different effects on different species. We used predicted patch-colonization probabilities to estimate the benefit to each species of removing one or more threats. We then determined the complementary set of threat-mitigation strategies that maximized colonization of all species while ensuring that redundant actions with little benefit were avoided. The single action that resulted in the highest colonization was increasing tree cover, which increased patch colonization by 5% and 11% on average across all species and for declining species, respectively. Combining Noisy Miner control with increasing tree cover increased species colonization by 10% and 19% on average for all species and for declining species respectively, and was a higher priority than changing grazing regimes. Guidance for prioritizing threat mitigation is critical in the face of cumulative threatening processes. By incorporating population dynamics in prioritization of threat management, our approach helps ensure funding is not wasted on ineffective management programs that target the wrong threats or species.

At the Crossroads: Does the Configuration of Roadside Vegetation Affect Woodland Bird Communities in Rural Landscapes?

In agricultural regions worldwide, linear networks of vegetation such as hedges, fencerows and live fences provide habitat for plant and animal species in heavily modified landscapes. In Australia, networks of remnant native vegetation along roadsides are a distinctive feature of many rural landscapes. Here, we investigated the richness and composition of woodland-dependent bird communities in networks of eucalypt woodland vegetation along roadsides, in an agricultural region in which >80% of native woodland and forest vegetation has been cleared. We stratified sites in a) cross sections and b) linear strips of roadside vegetation, to test the influence on woodland birds of site location and configuration in the linear network (the 'intersection effect'). We also examined the influence of tree size at the site, the amount of wooded vegetation surrounding the site, and the abundance of an aggressive native species, the noisy miner Manorina melanocephala. Birds were surveyed at 26 pairs of sites (cross section or linear strip) on four occasions. A total of 66 species was recorded, including 35 woodland species. The richness of woodland bird species was influenced by site configuration, with more species present at cross sections, particularly those with larger trees (>30 cm diameter). However, the strongest influence on species richness was the relative abundance of the noisy miner. The richness of woodland birds at sites where noisy miners were abundant was ~20% of that where miners were absent. These results recognise the value of networks of roadside vegetation as habitat for woodland birds in depleted agricultural landscapes; but highlight that this value is not realised for much of this vast vegetation network because of the dominance of the noisy miner. Nevertheless, roadside vegetation is particularly important where the configuration of networks create nodes that facilitate movement. Globally, the protection, conservation and restoration of such linear networks has an important influence on the persistence of biota within human-dominated landscapes.

Intercontinental distribution of a new trypanosome species from Australian endemic Regent Honeyeater (Anthochaera phrygia)

Establishing a health screening protocol is fundamental for successful captive breeding and release of wildlife. The aim of this study was to undertake a parasitological survey focusing on the presence of trypanosomes in a cohort of Regent Honeyeaters, Anthochaera phrygia, syn. Xanthomyza phrygia (Aves: Passeriformes) that are part of the breeding and reintroduction programme carried out in Australia. We describe a new blood parasite, Trypanosoma thomasbancrofti sp. n. (Kinetoplastida: Trypanosomatidae) with prevalence of 24·4% (20/81) in a captive population in 2015. The sequence of the small subunit rRNA gene (SSU rDNA) and kinetoplast ultrastructure of T. thomasbancrofti sp. n. are the key differentiating characteristics from other Trypanosoma spp. T. thomasbancrofti sp. n. is distinct from Trypanosoma cf. avium found in sympatric Noisy Miners (Manorina melanocephala). The SSU rDNA comparison suggests an intercontinental distribution of T. thomasbancrofti sp. n. and Culex mosquitoes as a suspected vector. Currently, no information exists on the effect of T. thomasbancrofti sp. n. on its hosts; however, all trypanosome-positive birds remain clinically healthy. This information is useful in establishing baseline health data and screening protocols, particularly prior to release to the wild.

Social and environmental factors drive variation in plant and bird communities across urban greenspace in Sydney, Australia

We examined whether environmental or social factors alone, or a combination of social-ecological factors were more effective at explaining patterns in plant and bird assemblages across urban greenspaces. Thirty publicly accessible, passive recreation greenspaces provided by municipal councils (i.e. city parks) were surveyed in a highly urbanised city - Sydney, Australia. Plant communities were influenced most by topography and park management approach, and to a lesser extent by land-use history. Greenspaces with greater topographic variation and that were co-managed with local citizen groups hosted higher plant species richness and abundance. Bird species richness within greenspaces increased with increasing distance from the central business district and decreasing distance from freshwater. Bird abundance was best explained by a combination of social-ecological factors, with abundance increasing with increasing site (greenspace) age, increasing percent concrete groundcover of a site and increasing proximity to the central business district. We identified a group of 'rare city parks', dissimilar in ecological composition that hosted more complex and species rich plant communities compared to 'common city parks'. We suggest this difference is likely because rare city parks received management and maintenance input from local citizen groups, whereas common city parks were managed and maintained exclusively by local councils. How different social factors, such as management approach and ongoing maintenance, are linked to the ecology of urban greenspaces are key areas for future investigation to help create sustainable urban landscapes that provide maximum benefits to urban residents.

The control of rank-abundance distributions by a competitive despotic species

Accounting for differences in abundances among species remains a high priority for community ecology. While there has been more than 80 years of work on trying to explain the characteristic S shape of rank-abundance distributions (RADs), there has been recent conjecture that the form may not depend on ecological processes per se but may be a general phenomenon arising in many unrelated disciplines. We show that the RAD shape can be influenced by an ecological process, namely, interference competition. The noisy miner (Manorina melanocephala) is a hyperaggressive, 'despotic' bird that occurs over much of eastern Australia (>10(6) km(2)). We compiled data for bird communities from 350 locations within its range, which were collected using standard avian survey methods. We used hierarchical Bayesian models to show that the RAD shape was much altered when the abundance of the strong interactor exceeded a threshold density; RADs consistently were steeper when the density of the noisy miner ≥2.5 birds ha(-1). The structure of bird communities at sites where the noisy miner exceeded this density was very different from that at sites where the densities fell below the threshold: species richness and Shannon diversity were much reduced, but mean abundances and mean avian biomass per site did not differ substantially.

The effects of climate change and land-use change on demographic rates and population viability

Understanding the processes that lead to species extinctions is vital for lessening pressures on biodiversity. While species diversity, presence and abundance are most commonly used to measure the effects of human pressures, demographic responses give a more proximal indication of how pressures affect population viability and contribute to extinction risk. We reviewed how demographic rates are affected by the major anthropogenic pressures, changed landscape condition caused by human land use, and climate change. We synthesized the results of 147 empirical studies to compare the relative effect size of climate and landscape condition on birth, death, immigration and emigration rates in plant and animal populations. While changed landscape condition is recognized as the major driver of species declines and losses worldwide, we found that, on average, climate variables had equally strong effects on demographic rates in plant and animal populations. This is significant given that the pressures of climate change will continue to intensify in coming decades. The effects of climate change on some populations may be underestimated because changes in climate conditions during critical windows of species life cycles may have disproportionate effects on demographic rates. The combined pressures of land-use change and climate change may result in species declines and extinctions occurring faster than otherwise predicted, particularly if their effects are multiplicative.

Visualization of species pairwise associations: a case study of surrogacy in bird assemblages

Quantifying and visualizing species associations are important to many areas of ecology and conservation biology. Species networks are one way to analyze species associations, with a growing number of applications such as food webs, nesting webs, plant–animal mutualisms, and interlinked extinctions. We present a new method for assessing and visualizing patterns of co-occurrence of species. The method depicts interactions and associations in an analogous way with existing network diagrams for studying pollination and trophic interactions, but adds the assessment of sign, strength, and direction of the associations. This provides a distinct advantage over existing methods of quantifying and visualizing co-occurrence. We demonstrate the utility of our new approach by showing differences in associations among woodland bird species found in different habitats and by illustrating the way these can be interpreted in terms of underlying ecological mechanisms. Our new method is computationally feasible for large assemblages and provides readily interpretable effects with standard errors. It has wide applications for quantifying species associations within ecological communities, examining questions about particular species that occur with others, and how their associations can determine the structure and composition of communities.

Influence of interspecific competition and landscape structure on spatial homogenization of avian assemblages

Human-induced biotic homogenization resulting from landscape change and increased competition from widespread generalists or ‘winners’, is widely recognized as a global threat to biodiversity. However, it remains unclear what aspects of landscape structure influence homogenization. This paper tests the importance of interspecific competition and landscape structure, for the spatial homogeneity of avian assemblages within a fragmented agricultural landscape of eastern Australia. We used field observations of the density of 128 diurnal bird species to calculate taxonomic and functional similarity among assemblages. We then examined whether taxonomic and functional similarity varied with patch type, the extent of woodland habitat, land-use intensity, habitat subdivision, and the presence of Manorina colonies (a competitive genus of honeyeaters). We found the presence of a Manorina colony was the most significant factor positively influencing both taxonomic and functional similarity of bird assemblages. Competition from members of this widespread genus of native honeyeater, rather than landscape structure, was the main cause of both taxonomic and functional homogenization. These species have not recently expanded their range, but rather have increased in density in response to agricultural landscape change. The negative impacts of Manorina honeyeaters on assemblage similarity were most pronounced in landscapes of moderate land-use intensity. We conclude that in these human-modified landscapes, increased competition from dominant native species, or ‘winners’, can result in homogeneous avian assemblages and the loss of specialist species. These interacting processes make biotic homogenization resulting from land-use change a global threat to biodiversity in modified agro-ecosystems.

Is there an ecological basis for species abundance distributions?

Community ecologists have attempted to explain species abundance distribution (SAD) shape for more than 80 years, but usually without relating SAD shape explicitly to ecological variables. We explored whether the scale (total assemblage abundance) and shape (assemblage evenness) of avifaunal SADs were related to ecological covariates. We used data on avifaunas, in-site habitat structure and landscape context that were assembled from previous studies; this amounted to 197 transects distributed across 16,000 km(2) of the box-ironbark forests of southeastern Australia. We used Bayesian conditional autoregressive models to link SAD scale and shape to these ecological covariates. Variation in SAD scale was relatable to some ecological covariates, especially to landscape vegetation cover and to tree height. We could not find any relationships between SAD shape and ecological covariates. SAD shape, the core component in SAD theory, may hold little information about how assemblages are governed ecologically and may result from statistical processes, which, if general, would indicate that SAD shape is not useful for distinguishing among theories of assemblage structure.