Woody Habitat Research Articles

Large-scale commonness is the best predictor of bird species presence in European cities

Urban bird communities are homogenized across large spatial scales, suggesting that the urban environment acts as an environmental filter. We hypothesize that large scale commonness is a better predictor of urban affinity of birds than any particular species trait. We estimated the relative importance of taxonomy, reproductive, ecological and morphological traits, and commonness of individual bird species. We compiled data on i) breeding bird communities of 41 European cities from urban bird atlases, and ii) regional bird assemblages defined by nine grid cells of the Atlas of European Breeding Bird around each city, and quantified the urban affinity of each species by comparing its incidence in cities and in randomly drawn communities from respective regional assemblages. Conditional inference tree-based random forest analysis was utilized to assess the importance of individual predictors. A sign test was used to detect differences between congeneric pairs of species with contrasting affinity to cities. Birds associated with woody habitats and those having altricial chicks had higher affinity for cities. Of the other reproductive traits, only clutch size showed an association with urban affinity. Different bird orders differed significantly in their urban affinity, exemplifying the homogenizing effect of cities. However, by far the most important factor associated with bird tolerance to the urban environment was species commonness, indicating that either the traits associated with commonness, or population effects driven by commonness, are responsible for their presence in cities.

Erratum to: Woody habitats promote pollinators and complexity of plant–pollinator interactions in homegardens located in rice terraces of the Philippine Cordilleras

Erratum to: Woody habitats promote pollinators and complexity of plant–pollinator interactions in homegardens located in rice terraces of the Philippine Cordilleras

Woody habitats promote pollinators and complexity of plant–pollinator interactions in homegardens located in rice terraces of the Philippine Cordilleras

Bees are important pollinators of wild plants and crops, but little is known about bee habitat requirements and pollinator management in tropical mountainous agricultural regions. Here, smallholder farmers produce fruits and vegetables in homegardens that depend upon or benefit from bee pollination. We hypothesized that abundance and richness of wild and domesticated bees and the complexity of plant–pollinator interactions are higher in homegardens surrounded by woody habitats than in homegardens found farther from woodlands. Bees were sampled in 20 homegardens in the rice terraces of the Philippine Cordilleras. We used linear mixed effect models to analyse effects of woody habitats around homegardens on bee richness and abundance. Based on pooled observations for each garden category, we built pollinator–plant interactions networks to illustrate shifts in interaction frequencies. We recorded 354 bee individuals of 13 wild and one domesticated bee species (Apis cerana). Wild bee richness was significantly higher in homegardens surrounded by woody habitats. Bee abundance increased significantly with increasing flower cover. Wild bees visited cultivated plants significantly more often than domesticated bees. Six vegetable species and 76% of all flower visits on cultivated plants in total were performed by wild bees and three plant species and 24% by domesticated bees. Pollinator–plant networks were more complex in homegardens surrounded by woody habitats. We conclude that woody habitats increase abundance and richness of wild and domesticated bees. Increasing availability of floral resources also promotes bee abundance. In order to promote pollination services in the landscape mosaic of smallholder rice farms, woody habitats and forest fragments together with numerous floral resources should be protected and restored.

A bustle in the hedgerow: Woody field margins boost on farm avian diversity and abundance in an intensive agricultural landscape

Considerable funding has been allocated to conservation management of non-crop habitat in agricultural landscapes, particularly field margin habitat such as hedgerows. Evaluation of the biodiversity benefits of non-crop habitat has lagged behind implementation, however, especially in the United States where this habitat has the potential to supply important resources for both common and rare species of birds. We examined the effects of woody field margin vegetation on winter and breeding season avian communities at 103 fields, row crops, and orchards in California's Central Valley, one of the most intensively-farmed landscapes on Earth. We found that margins with hedgerows, treelines or remnant riparian habitat harbored 2–3 times as many bird species, significantly greater species evenness, and 3–6 times higher maximum total abundances of birds than bare or weedy margins. The effect of margin type on richness was modulated by water year, whereas the effect of margin type on maximum total abundance was modulated by adjacent crop type. At the landscape scale, hedgerow and riparian margins that were further from woodland harbored greater species richness; a result that supports our recommendation for targeted development of hedgerows in simplified agricultural landscapes. These results demonstrate that non-crop woody habitats, both planted and remnant native patches, increase the biodiversity value of farms, providing support for policies to preserve remaining habitat and incentivize installation of woody hedgerows.

Global variation in woodpecker species richness shaped by tree availability

AbstractAimSpecies richness patterns are generally thought to be determined by abiotic variables at broad spatial scales, with biotic factors being only important at fine spatial scales. However, many organism groups depend intimately on other organisms, raising questions about this generalization. As an example, woodpeckers (Picidae) are closely associated with trees and woody habitats because of multiple morphological and ecological specializations. In this study, we test whether this strong biotic association causes woodpecker diversity to be closely linked to tree availability at a global scale.LocationGlobal.MethodsWe used spatial and non‐spatial regressions to test for relationships between broad‐scale woodpecker species richness and predictor variables describing current and deep‐time availability of trees, current climate, Quaternary climate change, human impact, topographical heterogeneity and biogeographical region. We further used structural equation models to test for direct and indirect effects of predictor variables.ResultsThere was a strong positive relationship between woodpecker species richness and current tree cover and annual precipitation, respectively. Precipitation also showed a strong indirect effect on woodpecker richness via the effects on tree availability. Deep‐time tree availability, Quaternary climate change, human influence and other abiotic factors showed weaker direct effects. Human influence had a negative effect on tree availability, and hence a negative indirect effect on woodpecker species richness.Main conclusionsGlobal species richness of woodpeckers is primarily shaped by current tree cover and precipitation, reflecting a strong biotic association between woodpeckers and trees. Human influence can have a negative effect on woodpecker diversity when humans reduce tree availability. Hence, woodpeckers exemplify how broad‐scale diversity patterns are predominantly shaped by a biotic factor, and how climate and human influence can have indirect effects on animal biodiversity via the effects on tree availability and forest cover.

The value of abandoned olive groves for blackcaps (Sylvia atricapilla) in a Mediterranean agroecosystem: a year-round telemetry study

The European Common Agricultural Policy has promoted the intensification of productive olive orchards and the abandonment of the unproductive ones. This strategy has resulted in a significant loss of biodiversity and ecosystem functionality of this type of agroecosystem. Here, we studied the blackcap Sylvia atricapilla, a common frugivorous bird species, in an olive-dominated agroecosystem of southern Spain to assess the effects of the abandonment of olive groves and undergrowth structure on habitat selection in this species. By means of radiotelemetry, we determined habitat selection of blackcaps during both the breeding (N = 30) and the non-breeding (N = 27) seasons of 2011 and 2012. We found that outside the breeding season, olive groves were widely used by blackcaps, which invariably preferred abandoned olive groves over intensively managed ones. Additionally, deciduous woods and poplar plantation were positively selected, whereas open habitats and Pinus forests were avoided. Generally, woody habitats without undergrowth or with herbaceous undergrowth were avoided. During the breeding season, the species selected mainly riverine habitats and poplar plantations, favoring habitats with well-developed undergrowth. Our results suggest that the blackcap could benefit from maintaining patches of abandoned olive groves freely left to natural succession within intensive olive groves. In such a landscape, ecological services (i.e., seed dispersal provided by frugivorous birds) and functioning could also improve as a consequence of these measures.

Automated Identification and Mapping of Woody Habitat Using Digital Ortho Imagery

Northern bobwhite (Colinus virginianus) restoration efforts operate at multiple spatial scales, from landscape (regional) levels to farm level (local). Choosing proper data sources, analysis techniques, and accounting for differences in scale (minimum mapping unit) between sources are critical first steps to successfully delivering habitat information useful for broad regional planning efforts and site specific research and management activities. To this end, we compared 3 methods of creating a habitat map and associated data: National Landcover Dataset (NLCD) 2006, hand digitized from 2010 National Agriculture Imagery Program (NAIP) imagery, and an Interactive Supervised Classification of 1-m NAIP imagery using ArcGIS 10.1. We analyzed a 3,660-ha portion of Peabody Wildlife Management Area in Muhlenberg and Ohio counties in west central Kentucky. We also compared percent cover of forest canopy closure using 2011 NLCD Percent Canopy Closure along with 10-m and 30-m aggregated datasets derived from image classification. Office inspection of aerial imagery and field verification yielded a 94% positive identification of woody vegetation. We found good agreement between NLCD 2006 and Image Classification for habitat classes. Hand digitizing did not compare well and this method is not recommended for creating digital habitat data. Percent Canopy closure yielded similar results between data sources. We found the smaller pixel size of the 10-m aggregate data to better identify small woody patches in open matrix. Use of 30-m national datasets to compare basic habitat across large areas is well warranted. Site specific research and management activities will benefit from image classification of 1-m imagery. We recommend additional research into the relationship between varying pixel size and habitat classification.

Social-ecological outcomes in recreational fisheries: the interaction of lakeshore development and stocking.

Many ecosystems continue to experience rapid transformations due to processes like land use change and resource extraction. A systems approach to maintaining natural resources focuses on how interactions and feedbacks among components of complex social-ecological systems generate social and ecological outcomes. In recreational fisheries, residential shoreline development and fish stocking are two widespread human behaviors that influence fisheries, yet emergent social-ecological outcomes from these potentially interacting behaviors remain under explored. We applied a social-ecological systems framework using a simulation model and empirical data to determine whether lakeshore development is likely to promote stocking through its adverse effects on coarse woody habitat and thereby also on survival of juvenile and adult fish. We demonstrate that high lakeshore development is likely to generate dependency of the ecosystem on the social system, in the form of stocking. Further, lakeshore development can interact with social-ecological processes to create deficits for state-level governments, which threatens the ability to fund further ecosystem subsidies. Our results highlight the value of a social-ecological framework for maintaining ecosystem services like recreational fisheries.

The role of novel forest ecosystems in the conservation of wood-inhabiting fungi in boreal broadleaved forests.

The increasing human impact on the earth's biosphere is inflicting changes at all spatial scales. As well as deterioration and fragmentation of natural biological systems, these changes also led to other, unprecedented effects and emergence of novel habitats. In boreal zone, intensive forest management has negatively impacted a multitude of deadwood‐associated species. This is especially alarming given the important role wood‐inhabiting fungi have in the natural decay processes. In the boreal zone, natural broad‐leaved‐dominated, herb‐rich forests are threatened habitats which have high wood‐inhabiting fungal species richness. Fungal diversity in other broadleaved forest habitat types is poorly known. Traditional wood pastures and man‐made afforested fields are novel habitats that could potentially be important for wood‐inhabiting fungi. This study compares species richness and fungal community composition across the aforementioned habitat types, based on data collected for wood‐inhabiting fungi occupying all deadwood diameter fractions. Corticioid and polyporoid fungi were surveyed from 67 130 deadwood particles in four natural herb‐rich forests, four birch‐dominated wood pastures, and four birch‐dominated afforested field sites in central Finland. As predicted, natural herb‐rich forests were the most species‐rich habitat. However, afforested fields also had considerably higher overall species richness than wood pastures. Many rare or rarely collected species were detected in each forest type. Finally, fungal community composition showed some divergence not only among the different habitat types, but also among deadwood diameter fractions. Synthesis and applications: In order to maintain biodiversity at both local and regional scales, conserving threatened natural habitat types and managing traditional landscapes is essential. Man‐made secondary woody habitats could provide the necessary resources and serve as surrogate habitats for many broadleaved deadwood‐associated species, and thus complement the existing conservation network of natural forests.

Early Spring Occurrence ofEmpoascaspp. (Hemiptera: Cicadellidae) in and Adjacent to Alfalfa Fields in Iowa

Abstract Adult Empoasca fabae (Harris) (Hemiptera: Cicadellidae) were monitored at three locations within four alfalfa fields (edge of field to 40 m into field) from mid-April to June and Empoasca spp. activity from adjacent tree and shrub species until July. Similar numbers of Empoasca spp. adults were collected from sticky traps placed on tree and shrub species in the four border areas. In three of the four fields examined, significantly more E. fabae were found at the edges of the alfalfa fields closest to the trees than at 40 m from the edge of the field during the first week of May. In subsequent samples, there was no difference in the distribution of E. fabae in the alfalfa fields (except Field 2, where there were more E. fabae near the field edge for the duration of the study). Based upon this study, Empoasca spp. (primarily E. fabae) are colonizing alfalfa fields from the field margins to the interior in early spring and may be using adjacent woody habitats (including Gleditsia, Prunus, Ulmus, Ti...

Influence of Agropastoral System Components on Mountain Grassland Vulnerability Estimated by Connectivity Loss.

Over the last decades, global changes have altered the structure and properties of natural and semi-natural mountain grasslands. Those changes have contributed to grassland loss mainly through colonization by woody species at low elevations, and increases in biomass and greenness at high elevations. Nevertheless, the interactions between agropastoral components; i.e., ecological (grassland, environmental, and geolocation properties), social, and economic components, and their effects on the grasslands are still poorly understood. We estimated the vulnerability of dense grasslands in the Central Pyrenees, Spain, based on the connectivity loss (CL) among grassland patches that has occurred between the 1980s and the 2000s, as a result of i) an increase in biomass and greenness (CL-IBG), ii) woody encroachment (CL-WE), or iii) a decrease in biomass and greenness (CL-DBG). The environmental and grassland components of the agropastoral system were associated with the three processes, especially CL-IBG and CL-WE, in relation with the succession of vegetation toward climax communities, fostered by land abandonment and exacerbated by climate warming. CL-IBG occurred in pasture units that had a high proportion of dense grasslands and low current livestock pressure. CL-WE was most strongly associated with pasture units that had a high proportion of woody habitat and a large reduction in sheep and goat pressure between the 1930s and the 2000s. The economic component was correlated with the CL-WE and the CL-DBG; specifically, expensive pastures were the most productive and could maintain the highest rates of livestock grazing, which slowed down woody encroachment, but caused grassland degradation and DBG. In addition, CL-DBG was associated with geolocation of grasslands, mainly because livestock tend to graze closer to passable roads and buildings, where they cause grassland degradation. To properly manage the grasslands, an integrated management plan must be developed that includes an understanding of all components of the agropastoral system and takes into account all changes that have occurred in dense mountain grasslands. Addressing the problems individually risks the improvement of some grasslands and the deterioration of others.

Effects of landscape composition on carabids and slugs in herbaceous and woody field margins

Field margin vegetation can provide shelter and complementary resources for both agricultural pests and their natural enemies. Thus, simplification of agricultural landscapes can either reduce or enhance pest pressure, depending on the habitat preferences and mobility of the relevant organisms. The promotion of field margin vegetation that selectively enhances natural enemies but not pests, would be desirable for the optimization of pest control services. Unfortunately, the majority of existing studies on the effects of field margins and landscape focussed on either natural enemies or (less often) on pests, but very rarely on both. This study evaluates the influence of herbaceous and woody semi-natural habitats and landscape composition on carabid beetles as natural enemies of the pest slugs Deroceras reticulatum and Arion spp. We found effects of semi-natural habitats on carabids and slugs at the field and landscape scale. While most carabids and D. reticulatum preferred herbaceous over woody habitats, Arion spp. oppositely showed highest activity-densities in woody habitats. With increasing amount of semi-natural habitats in the landscape activity-densities of predatory carabids tended to increase while activity-densities of Arion spp. decreased. This, and a negative correlative trend between Arion spp. and carabid densities indicate favourable conditions for natural slug control by carabids in landscapes with a high amount of semi-natural habitats. Our results confirm that different response of pest and their natural enemies to the landscape context can indeed alter the predator-prey ratio. However, our study also revealed that pests such as Arion spp. and D. reticulatum prefer different types of field margins, complicating the optimization of agricultural landscapes for pest management.

Least Shrews in North-Central Kansas: Habitat and Individual Characteristics

We captured least shrews (Cryptotis parva; n = 16) in native grassland, anthropogenic and woody habitats in north-central Kansas. Total effort was ca. 100,000 trap nights [TN] in >200 herbaceous sites and ca. 20,000 TN in >20 shrubby and woody sites. Least shrews were rare in both habitat types but slightly more abundant in herbaceous (ca. 0.15 individuals/1,000 TN) than woody sites (ca. 0.05 individuals/1,000 TN). We captured 15 least shrews in grazed native mixed grass prairie, post-harvest wheat fields and fallow wheat fields combined as well as one in a shelterbelt of deciduous trees. Body mass ranged from 2.6 g–6.0 g; body length explained ∼55% of the variation in body mass. Two pregnant females (4.9–5.0 g) were trapped; a litter size of five was observed for the one necropsied female. Finally, we document the first occurrence of the least shrew in Lincoln County, Kansas.

Prediction of Extractable Cd, Pb and Zn in Contaminated Woody Habitat Soils Using a Change Point Detection Method

Accumulation of heavy metals in soils poses a potential risk to plant production, which is related to availability of the metals in soil. The phytoavailability of metals is usually evaluated using extracting solutions such as salts, acids or chelates. The purpose of this study was to identify the most significant soil parameters that can be used to predict the concentrations of acetic and citric acid-extractable cadmium (Cd), lead (Pb) and zinc (Zn) in contaminated woody habitat topsoils. Multiple linear regression models were established using two analysis strategies and three sets of variables based on a dataset of 260 soil samples. The performance of these models was evaluated using statistical parameters. Cation exchange capacity, CaCO3, organic matter, assimilated P, free Al oxide, sand and the total metal concentrations appeared to be the main soil parameters governing the solubility of Cd, Pb and Zn in acetic and citric acid solutions. The results strongly suggest that the metal solubility in extracting solutions is extractable concentration-dependent since models were overall improved by incorporating a change point. This change point detection method was a powerful tool for predicting extractable Cd, Pb and Zn. Suitable predictions of extractable Cd, Pb and Zn concentrations were obtained, with correlation coefficient (adjusted r) ranging from 0.80 to 0.99, given the high complexity of the woody habitat soils studied. Therefore, the predictive models can constitute a decision-making support tool for managing phytoremediation of contaminated soils, making recommendations to control the potential bioavailability of metals. The relationships between acetic and/or citric acid-extractable concentrations and the concentrations of metals into the aboveground parts of plants need to be predicted, in order to make their temporal monitoring easier.

Assessing the Distribution and Changes of Instream Woody Habitat in South-Eastern Australian Rivers

Managers and communities are now artificially reintroducing instream woody habitat (IWH) to rivers following historic large-scale removal. Riverscape-scale datasets that quantify existing habitat conditions are fundamental to setting the priorities and allocating resources for such programs. Unfortunately, such datasets are rare, primarily because existing assessment approaches are limited in their accuracy (remote sensing) or are costly and labour intensive (field assessments). This study used both field assessments and aerial data to improve the accuracy of remotely sensed measures of IWH and estimate current IWH volumes and subsequent condition (compared with reference levels) across approximately 28 000 km of stream in the south-eastern Australian state of Victoria. We found that aerial measures, when used in conjunction with measures of stream size and riparian overhang, produced significantly better estimates of IWH loads than using aerial data alone. The statewide assessment indicated that streams currently have IWH volumes, on average, 41% lower than reference levels that represented an average reduction of 0.0207 m3 m−2. The degree of IWH condition was highly variable across regions (20–95% reductions from reference levels), a likely reflection of regional variation in land use practices and past river work activities. This scale of IWH reduction may pose major negative impacts on the ecological integrity of these streams. Whilst the approach used during this study has temporal and spatial limitations, it was designed as a generalised, rapid and relatively inexpensive method to measure stream condition and assist with priority setting at state and regional levels. Copyright © 2016 John Wiley & Sons, Ltd.

Landscape level processes driving carabid crop assemblage in dynamic farmlands

AbstractLandscape heterogeneity has been shown to be a major factor in the maintenance of biodiversity and associated services in agricultural landscapes. Farmlands are mosaics of fields with various crop types and farming practices. Crop phenology creates asynchrony between fields sown and harvested in different periods (winter vs. spring crops). The present study was conducted to examine the influence of such spatio‐temporal heterogeneity on biodiversity, with the hypothesis that it would lead to spatio‐temporal redistribution (shifting) of species. Species richness and activity‐density of carabid beetles in winter cereal (winter) and maize (spring) crops were compared across 20 landscapes distributed along a double gradient of relative area and spatial configuration of winter and spring crops. Maize fields were sampled in spring and late summer for comparison over time. The response of carabid species richness to landscape heterogeneity was weak in spring, but maize field richness benefited from adjacencies with woody habitat, in late summer. In spring, increased length of interfaces between winter and spring crops lowered carabid activity‐density in winter cereal fields, suggesting that maize fields acted as sinks. Interfaces between woody habitats and crops increased activity‐density in both crop types. We found no evidence of spatio‐temporal complementation, but different species benefited from winter cereals and maize in spring and late summer, increasing overall diversity. These findings confirm the role of adjacencies between woody and cultivated habitats in the conservation of abundant carabid assemblage in winter cereals and maize. We conclude that between‐field population movement occurs, and advocate for better consideration of farmland heterogeneity in future research.

Spatial partitioning in the use of structural woody habitat supports the cohabitation of two cod species in a large lowland river

Many freshwater fish worldwide have been shown to use Structural Woody Habitat (SWH) for a variety of reasons. The mid reaches of the Murray River, a large lowland river in south-eastern Australia, was surveyed by boat electrofishing, to investigate the use of SWH type (hollows, rootmass and solids), SWH distance to bank (near bank, intermediate to bank and mid-channel) and the interaction between SWH type and distance to bank. The study found that Murray cod catch per unit effort (CPUE) increased in near-bank areas when hollows were a component of the SWH. The CPUE of trout cod was higher when hollows were present. However, the interactions between distance to bank and hollow SWH were complex and dependent on presence or absence of rootmass. The species-specific interactions between SWH microhabitat and distance to bank found within this study has important relevance for stream managers. The common practice of realigning SWH favours Murray cod over trout cod, which could have negative consequences for the endangered trout cod. More broadly, managers may need to consider a balance of SWH type and where it is placed in the river for the species they are targeting when rehabilitating rivers via the introduction of SWH.

Predicting natural instream woody-habitat loads across large river networks

Past waterway management practices worldwide involved extensive removal of instream woody habitat (IWH) and riparian vegetation. The importance of instream woody habitat for healthy aquatic ecosystems has now been recognised, with management approaches reversed to reintroduce instream woody habitat and replant riverbanks. Knowledge of natural or pre-disturbance IWH loads is useful to guide such restoration programs; however, such datasets are often unavailable. In this study, natural IWH loads were mapped along 105km of undisturbed rivers in south-eastern Australia. This field dataset was modelled, using boosted regression trees, against geomorphic, environmental and climatic variables to predict natural IWH loads in rivers across Victoria. Mapped natural IWH loads averaged 0.029m3m–2 (±0.005), ranging from 0.083 to 0.002m3m–2. Natural IWH volumes were predicted to range from 0 to 0.102m3m–2. Distinct IWH loading trends were noticeable over larger spatial scales. Eastern Victoria showed relatively lower natural IWH loads than did western Victoria. Because many stream restoration efforts do not have a quantifiable knowledge of natural IWH load, the results of the present study provide some guidance. The predicted IWH loadings are a useful first step in identifying broad areas for further investigation and a natural condition base for current IWH condition modelling.

Mosaic of grasslands and woodlands is more effective than habitat connectivity to conserve butterflies in French farmland

In agricultural landscapes, permanent grassy habitats are often fragmented and partly composed of linear elements. Landscape-level connectivity of both grassland patches and grassy linear elements actively contributes to biodiversity conservation in farmland. Nevertheless, their respective importance has rarely been tested. Butterflies form a suitable model group for this purpose; they have suffered a major overall decline in the last few decades and are known to be sensitive to landscape connectivity.We sampled 286 transects nested in 18 study landscapes in three regions. Using several metrics, we analyzed the response of butterflies to the connectivity to both grassland and woodland habitats, which may provide additional resources. The response to landscape connectivity appeared to depend on the type of the semi-natural elements (patch vs linear elements), and on species traits, i.e. the response was stronger for habitat specialists and species with low dispersal ability. We found that woody habitats close to grassland patches had a significant positive effect on butterfly communities, denoting potential mechanisms of resources supplementation and complementation. In grassy linear elements, butterfly communities were of lower conservation value, richness and abundance were much lower (−26 and −50%) than in grassland patches and decreased with isolation from grasslands, in line with the source–sink hypothesis.Although grassy linear elements probably play an important role in butterfly dispersal in agricultural landscapes, they may not be sufficient to preserve specialist and sedentary species. Conserving a mosaic of high quality grassland patches and woody habitats appears more effective than enhancing connectivity.

On Bird Functional Diversity: Species Richness and Functional Differentiation Show Contrasting Responses to Rainfall and Vegetation Structure in an Arid Landscape

Biodiversity affects ecosystem function through species’ functional traits. Although it is possible to predict species richness (SR) patterns along environmental gradients, whether functional diversity (FD) changes in predictable ways is not known. In arid environments, SR typically increases with rainfall. Aridity may limit functional differentiation by allowing only certain traits, but could also be associated with diverse traits associated with various strategies for coping with spatial and temporal variation in resources. Rare species may have unique traits, making them particularly important to continued ecosystem function. We investigated SR, FD, and functional differentiation in bird assemblages along an aridity gradient, with attention to functional uniqueness of rare species. We surveyed bird communities in open savanna, bush-thickened, and riparian habitats at five sites of increasing aridity (~150–400 mm rainfall year−1) in wet and dry seasons for 3 years in Namibia. We calculated the standardized effect size of FD (sesFD) as a measure of functional differentiation and used mixed models to ascertain how SR, FD, and sesFD relate to rainfall, season, and habitat type. SR and FD increased with increasing rainfall. Conversely, sesFD declined with increasing rainfall and was lower in woody habitats, suggesting habitat filtering and greater niche overlap. Rare species were more functionally unique than common species, in all three habitats, so loss of rare species could degrade ecosystem function. Our results are consistent with a linear diversity–productivity relationship maintained by regular disturbance (drought) preventing strong competitors from excluding weaker competitors in higher productivity environments.