Relationship between avian range limits and plant transition zones in Maine

Aim To determine if vegetation complexity associated with transition zones may be a contributing factor affecting bird species distributions in Maine, USA, and in increased numbers of bird species at about 45° north latitude in northeastern North America. Location Maine, USA; North America north of Mexico. Methods We delineated the ranges within Maine (86,156 km²) of 186 bird species and 240 woody plants using literature and expert review. Maps showing species richness and numbers of range limits, at 324 km² resolution, were developed for woody plants and groups of breeding birds: forest specialists, forest generalists, and those that used barren and urban habitats, early successional areas, and wetlands or open water. Two plant transition zones for Maine were identified previously, with the north–south transition zone mapped across eastern North America. Patterns in bird distribution maps were compared to woody plant maps and to transition zones. Results When the distributions of forest specialists were compared to the north–south vegetation transition zone in Maine, they were spatially coincident, but were not for other groups. Forest specialists had more species with range limits in the state (61%) than generalists (13%) or any other group. At a continental‐scale, the vegetation transition zone within eastern North America agreed fairly well with the areas of highest bird richness. Main conclusions A bird transition zone occurs in Maine and across eastern North America, akin to and overlapping the vegetation transition zone. Seasonality is likely the primary source of the inverse gradient in bird richness in the eastern USA, as reported by others. However, vegetation structure and habitat selection at very broad spatial scales appear to contribute to the reversed gradient. North of the vegetation transition zone, forest structure is simpler and coniferous forests more dominant, and this may contribute to reduced bird species richness. However, the northern (> 49°) typical gradient in bird species richness has been related to many hypotheses, and several are likely involved in the genesis of the gradient.     

Harlequin duck Histrionicus histrionicus population structure in eastern Nearctic

During May 1996 and April 1997, eight harlequin duck males were captured and fitted with satellite transmitters while migrating along the shores of Forillon National Park, Québec, Canada. Another 17 males were equipped with satellite transmitters in river systems of eastern Hudson Bay, Ungava Bay and northern Labrador in June 1997 and 1998. Our objectives were to determine relationships between breeding, moulting and wintering areas, and to determine whether distinct population segments existed among harlequin ducks in eastern North America. All birds tracked from Forillon migrated to Labrador. Moulting areas were identified for six birds. Forillon males were followed to the eastern North American major wintering site in Maine. Males captured in northern Québec and Labrador migrated to moult and winter in south-western Greenland. Our data suggest the presence of two demographically distinct population segments in eastern North America, perhaps originating from the Pleistocene glacial refuge in western Greenland and south of the Laurentide ice sheet in eastern Canada or United States.     

Diversity of birds in eastern North America shifts north with global warming

The distribution of diversity along latitudinal and elevation gradients, and the coupling of this phenomenon with climate, is a pattern long recognized in ecology. Hypothesizing that climate change may have altered this pattern over time, we investigated whether the aggregate of reported northward shifts of bird ranges in North America is now detectable in community‐level indices such as richness and diversity. Here, we report that bird diversity in North America increased and shifted northward between 1966 and 2010. This change in the relationship of diversity to the latitudinal gradient is primarily influenced by range expansions of species that winter in the eastern United States as opposed to species which migrate to this area from wintering grounds in the tropics. This increase in diversity and its northward expansion is best explained by an increase in regional prebreeding season temperature over the past 44 years.     

Relative influences of current and historical factors on mammal and bird diversity patterns in deglaciated North America

Aim To investigate the relative contributions of current vs. historical factors in explaining broad‐scale diversity gradients using a combination of contemporary factors and a quantitative estimate of the temporal accessibility of areas for recolonization created by glacial retreat following the most recent Ice Age. Location The part of the Nearctic region of North America that was covered by ice sheets during the glacial maximum 20 000 BP. Methods We used range maps to estimate the species richness of mammals and terrestrial birds in 48 400 km² cells. Current conditions in each cell were quantified using seven climatic and topographical variables. Historical conditions were estimated using the number of years before present when an area became exposed as the ice sheets retreated during the post‐Pleistocene climate warming. We attempted to tease apart contemporary and historical effects using multiple regression, partial regression and spatial autocorrelation analysis. Results A measure of current energy inputs, potential evapotranspiration, explained 76–82% of the variance in species richness, but time since deglaciation explained an additional 8–13% of the variance, primarily due to effects operating at large spatial scales. Because of spatial covariation between the historical climates influencing the melting of the ice sheet and current climates, it was not possible to partition their effects fully, but of the independent effects that could be identified, current climate explained two to seven times more variance in richness patterns than age. Main Conclusions Factors acting in the present appear to have the strongest influence on the diversity gradient, but an historical signal persisting at least 13 000 years is still detectable. This has implications for modelling changes in diversity patterns in response to future global warming.     

Why do mountains support so many species of birds?

Although topographic complexity is often associated with high bird diversity at broad geographic scales, little is known about the relative contributions of geomorphologic heterogeneity and altitudinal climatic gradients found in mountains. We analysed the birds in the western mountains of the New World to examine the two‐fold effect of topography on species richness patterns, using two grains at the intercontinental extent and within temperate and tropical latitudes. Birds were also classified as montane or lowland, based on their overall distributions in the hemisphere. We estimated range in temperature within each cell and the standard deviation in elevation (topographic roughness) based on all pixels within each cell. We used path analysis to test for the independent effects of topographic roughness and temperature range on species richness while controlling for the collinearity between topographic variables. At the intercontinental extent, actual evapotranspiration (AET) was the primary driver of species richness patterns of all species taken together and of lowland species considered separately. In contrast, within‐cell temperature gradients strongly influenced the richness of montane species. Regional partitioning of the data also suggested that range in temperature either by itself or acting in combination with AET had the strongest “effect” on montane bird species richness everywhere. Topographic roughness had weaker “effects” on richness variation throughout, although its positive relationship with richness increased slightly in the tropics. We conclude that bird diversity gradients in mountains primarily reflect local climatic gradients. Widespread (lowland) species and narrow‐ranged (montane) species respond similarly to changes in the environment, differing only in that the richness of lowland species correlates better with broad‐scale climatic effects (AET), whereas mesoscale climatic variation accounts for richness patterns of montane species. Thus, latitudinal and altitudinal gradients in species richness can be explained through similar climatic‐based processes, as has long been argued.     

Taxonomic diversity of the terrestrial bird and mammal fauna in temperate and boreal biomes of the northern hemisphere

Abstract. Using comprehensive range information of northern Hemisphere birds and mammals, we assessed the taxonomic diversity of these two groups in four different regions: Europe, east Asia, and western and eastern North America. East Asia is the richest region in the number of bird and mammal species, genera, families and orders, except that mammal species richness is highest in western North America. Eastern North America is taxonomically the poorest region, but when only forest-associated taxa were considered in mammals taxonomic diversity is equally low in Europe and in eastern North America, and in birds, Europe is the least diverse region. Patterns in endemic taxa follow overall taxonomic diversity. The proportion of shared taxa between regions is higher among boreal species and genera than among all taxa. A comparison with tree species diversity underpins the role of east Asia as the most diverse of all northern biota. Largely congruent patterns at different taxonomic levels emphasizes the role of historical processes, such as differential extinction rate in response to paleoenvironmental fluctuations, in producing these patterns, but we stress the need for more research on the coevolution of species diversity and habitat diversity.     

The biogeography of aquatic macrophytes in North America since the Last Glacial Maximum

Aim To document the post‐glacial migration of the major aquatic macrophytes of North America. Location North America north of Mexico. Methods Aquatic macrophyte pollen were extracted from the North American Pollen Database. The modern pollen distribution was mapped and related to the climate to document the geographical and climatic constraints on these taxa. The fossil pollen were mapped at 2‐ka intervals for the past 21 ka. Results Numerous genera were present in ice‐free Alaska during the Last Glacial Maximum, and south of the Laurentide Ice Sheet in the southeast. Those taxa with the widest modern climatic ranges migrated rapidly into ice‐marginal areas, first in the west and then in the east of North America. Subsequent changes in the range and abundance were smaller. Main conclusions There were four migration routes of aquatic macrophytes during the late‐glacial and post‐glacial periods: a southward migration from Alaska between 14–13 and ka, a northern migration in the west at the same time into the ice‐free Cordilleran region, and movements east and west of Appalachia as early as 19 ka for some taxa into the lower Mississippi and into the upper Mississippi and Great Lakes by 11 ka. As the Laurentide ice sheet wasted, aquatic taxa with the broadest contemporary temperature tolerances rapidly occupied ice‐marginal environments.     

Energy availability, abundance, energy-use and species richness in forest bird communities: a test of the species–energy theory

Aim To test the ‘more individuals hypothesis’ as a mechanism for the positive association between energy availability and species richness. This hypothesis predicts that total density and energy use in communities is linearly related to energy availability, and that species richness is a positive function of increased density. We also evaluate whether similar energy–density patterns apply to different migratory groups (residents, short‐distance migrants and tropical migrants) separately. Location European and North American forest bird communities. Methods We collected published breeding bird census data from Europe and North America (n = 187). From each census data we calculated bird density (pairs 10 ha^?1), energy use by the community (the sum of metabolic needs of individuals, Watts 10 ha^?1) and geographical location with an accuracy of 0.5°. For each bird census data coordinate we extracted the corresponding monthly values of actual evapotranspiration (AET). From these values we calculated corresponding AET values that we expected to explain the density energy use of forest birds: total annual, breeding season (June) and winter AET. We used general linear modelling to analyse these data controlling for the area of census plots, forest type and census method. Results Total density and energy use in European and North American forest bird communities were linear functions of annual productivity, and increased density and energy use then translated into more species. Also resident bird density and energy consumption were positive functions of annual productivity, but the relationship between productivity and density as well as between productivity and energy use was weaker for migrants. Main conclusions Our results are consistent with the more individuals hypothesis that density and energy use in breeding forest bird communities is coupled tightly with the productivity of the environment, and that increased density and energy consumption results in more species. However, not all community members (migratory groups) are limited by productivity on the breeding grounds.     

Plants and landscape in the Vestfold Hills,Antarctica

Maps are presented showing the recorded distribution and species density of terrestrial plants in the Vestfold Hills. The distribution, biomass and species diversity of terrestrial lithic algae, mosses and lichens is influenced positively by availability of meltwater from drift snow and by additional nutrient supply (probably N and P) near bird nest sites. The terrestrial plants are affected negatively by exposure (including sand blast) and salinity. These four factors are probably the most important environmental characteristics exercising local control over plant distribution and abundance within the limits set by temperature.Large changes in salinity, degree of exposure, water supply and nutrient supply occur across the Vestfold Hills, with the most favourable conditions generally occurring in the eastern half fairly close to the ice sheet.Plant distribution and abundance are also discussed in relation to the length of time that particular areas have been exposed as a result of ice retreat. With increasing time of exposure, plant diversity and abundance rise but subsequently fall sharply as conditions become more arid, saline, or both. This temporal sequence can be explained by considering changes in the important factors that control plant growth.     

Geographic pattern of genetic diversity in Pinus resinosa: contact zone between descendants of glacial refugia

Although red pine (Pinus resinosa) generally has low or completely lacks variation for molecular markers, some variation is observed for chloroplast microsatellites (cpSSRs). We sampled and examined 10 cpSSRs for 19 populations. Analysis of these populations plus 10 previously studied populations shows that the geographic distribution of genetic diversity over the range of P. resinosa is markedly nonuniform. Although the pattern exhibits little isolation by distance, there is a region centered in northeastern New England where populations contain much greater chloroplast haplotype diversity than elsewhere. This area is band-shaped, with the longer axis nearly parallel with latitude, and very sharply delineated. The area of high diversity was buried by the Laurentide ice sheet. The geographic pattern indicates that P. resinosa is not at equilibrium, and the species has had a more complex postglacial history than typically purported for forest trees in eastern North America. The results suggest that the area of high diversity is a stable transition zone between descendants of two distinct refugia, one in the southern Appalachians and another near the North Atlantic coastline of the Wisconsinian glacial period. Plausible explanations are given that selection between two lineages, along latitudinal zones, may have maintained the transition zone.     

Large-scale biogeographic patterns of vascular plant richness in North America: an analysis at the generic level

The richness pattern of native vascular plants in North America (north of Mexico) was studied at the generic level. North America was divided into thirteen geographical regions, which were latitudinally grouped into four horizontal zones (northern, north-middle, south-middle, and southern zones); and longitudinally grouped into three vertical zones (eastern, central, and western zones). The native vascular flora of North America consisted of 1904 genera in 235 families and eighty-three orders. Along the latitudinal gradient, generic richness (in terms of the number of genera) showed a striking increase with decreasing latitude. The southern zone had more than four times as many genera as did the northern zone (with a difference of 1436 genera). 93.3% of genera in the northern zone also occurred in the southern zone. Along the longitudinal gradient, the central zone had the highest generic richness and the eastern zone had the lowest, but the difference in generic richness between the two zones was only sixty-one genera. The western and eastern zones shared 60% or more of their genera. Generic richness of vascular plants in North America was highly correlated ( r =0.965) to available environmental energy (expressed by annual potential evapotranspiration).     

Water availability drives aboveground biomass and bird richness in forest restoration plantings to achieve carbon and biodiversity cobenefits

To combat global warming and biodiversity loss, we require effective forest restoration that encourages recovery of species diversity and ecosystem function to deliver essential ecosystem services, such as biomass accumulation. Further, understanding how and where to undertake restoration to achieve carbon sequestration and biodiversity conservation would provide an opportunity to finance ecosystem restoration under carbon markets. We surveyed 30 native mixed‐species plantings in subtropical forests and woodlands in Australia and used structural equation modeling to determine vegetation, soil, and climate variables most likely driving aboveground biomass accrual and bird richness and investigate the relationships between plant diversity, aboveground biomass accrual, and bird diversity. We focussed on woodland and forest‐dependent birds, and functional groups at risk of decline (insectivorous, understorey‐nesting, and small‐bodied birds). We found that mean moisture availability strongly limits aboveground biomass accrual and bird richness in restoration plantings, indicating potential synergies in choosing sites for carbon and biodiversity purposes. Counter to theory, woody plant richness was a poor direct predictor of aboveground biomass accrual, but was indirectly related via significant, positive effects of stand density. We also found no direct relationship between aboveground biomass accrual and bird richness, likely because of the strong effects of moisture availability on both variables. Instead, moisture availability and patch size strongly and positively influenced the richness of woodland and forest‐dependent birds. For understorey‐nesting birds, however, shrub cover and patch size predicted richness. Stand age or area of native vegetation surrounding the patch did not influence bird richness. Our results suggest that in subtropical biomes, planting larger patches to higher densities, ideally using a diversity of trees and shrubs (characteristics of ecological plantings) in more mesic locations will enhance the provision of carbon and biodiversity cobenefits. Further, ecological plantings will aid the rapid recovery of woodland and forest bird richness, with comparable aboveground biomass accrual to less diverse forestry plantations.     

Comparative Historical Demography of Migratory and Nonmigratory Birds from the Caribbean Island of Hispaniola

Islands offer unique opportunities for studies of evolution and historical demography. We hypothesized that wintering North American migrant bird species would show genetic evidence of population expansion over recent millennia due to the expansion of their breeding distributions following the retreat of the Laurentide ice sheet. In contrast, we presumed that non-migratory species would exhibit more stable historical demographies. We used mtDNA sequences from 649 individuals of 16 avian species on the Caribbean island of Hispaniola to test this prediction. Mismatch distributions did not differ significantly between migrants and non-migrants. However, neutrality indices indicated population expansion in the migrant species, as well as two non-migratory resident species with extensive distributions. Evidence of population expansion was less consistent in other non-migratory residents. We infer that climate prior to the Last Glacial Maximum significantly reduced effective population sizes of most migratory North American bird populations and some resident Hispaniolan bird populations. Our data further revealed that mismatch statistics were poorly correlated with and less informative than the neutrality test statistics, a consideration for future demographic studies.     

Explaining species richness from continents to communities: the time-for-speciation effect in emydid turtles

Speciation is the process that ultimately generates species richness. However, the time required for speciation to build up diversity in a region is rarely considered as an explanation for patterns of species richness. We explored this "time-for-speciation effect" on patterns of species richness in emydid turtles. Emydids show a striking pattern of high species richness in eastern North America (especially the southeast) and low diversity in other regions. At the continental scale, species richness is positively correlated with the amount of time emydids have been present and speciating in each region, with eastern North America being the ancestral region. Within eastern North America, higher regional species richness in the southeast is associated with smaller geographic range sizes and not greater local species richness in southern communities. We suggest that these patterns of geographic range size variation and local and regional species richness in eastern North America are caused by glaciation, allopatric speciation, and the time-for-speciation effect. We propose that allopatric speciation can simultaneously decrease geographic range size and increase regional diversity without increasing local diversity and that geographic range size can determine the relationship between alpha, beta, and gamma diversity. The time-for-speciation effect may act through a variety of processes at different spatial scales to determine diverse patterns of species richness.     

Perennial Agroenergy Feedstocks as En Route Habitat for Spring Migratory Birds

Increased production of bioenergy crops in North America is projected to exacerbate already heavy demands upon existing agricultural landscapes with potential to impact biodiversity negatively. Grassland specialist birds are an imperilled avifauna for which perennial-based, next-generation agroenergy feedstocks may provide suitable habitat. We take a multi-scaled spatial approach to evaluate the ability of two candidate second-generation agroenergy feedstocks (switchgrass, Panicum virgatum, and mixed grass–forb plantings) to act as spring migratory stopover habitat for birds. In total, we detected 35 bird species in mixed grass–forb plantings and switchgrass plantings, including grassland specialists and species of state and national conservation concern (e.g., Henslow’s Sparrow, Ammodramus henslowii). Some evidence indicated that patches with higher arthropod food availability attracted a greater diversity of migrant bird species, but species richness, total bird abundance, and the abundance of grassland specialist species were similar in fields planted with either feedstock. Species richness per unit area (species density) was relatively higher in switchgrass fields. The percent land cover of forest in landscapes surrounding study fields was negatively associated with bird species richness and species density. Habitat patch size and within-patch vegetation structure were unimportant in predicting the diversity or abundance of spring en route bird assemblages. Our results demonstrate that both switchgrass and mixed grass–forb plantings can attract diverse assemblages of migrant birds. As such, industrialized production of these feedstocks as agroenergy crops has the potential to provide a source of en route habitat for birds, particularly where fields are located in relatively unforested landscapes. Because industrialization of cellulosic biomass production will favor as yet unknown harvest and management regimes, predicting the ultimate value of perennial-based biomass plantings for spring migrants remains difficult.     

Perennial biomass feedstocks enhance avian diversity

Federal mandates to increase biofuel production in North America will require large new tracts of land with potential to negatively impact biodiversity, yet empirical information to guide implementation is limited. Because the temperate grassland biome will be a production hotspot for many candidate feedstocks, production is likely to impact grassland birds, a group of major conservation concern. We employed a multiscaled approach to investigate the relative importance of arthropod food availability, microhabitat structure, patch size and landscape‐scale habitat structure and composition as factors shaping avian richness and abundance in fields of one contemporary (corn) and two candidate cellulosic biomass feedstocks (switchgrass and mixed‐grass prairie) not currently managed as crops. Bird species richness and species density increased with patch size in prairie and switchgrass, but not in corn, and was lower in landscapes with higher forest cover. Perennial plantings supported greater diversity and biomass of arthropods, an important food for land birds, but neither metric was important in explaining variation in the avian community. Avian richness was higher in perennial plantings with greater forb content and a more diverse vegetation structure. Maximum bird species richness was commonly found in fields of intermediate vegetation density and grassland specialists were more likely to occur in prairies. Our results suggest that, in contrast to corn, perennial biomass feedstocks have potential to provide benefits to grassland bird populations if they are cultivated in large patches within relatively unforested landscapes. Ultimately, genetic improvement of feedstock genets and crop management techniques that attempt to maximize biomass production and simplify crop vegetation structure will be likely to reduce the value of perennial biomass plantings to grassland bird populations.     

Shifts in trait means and variances in North American tree assemblages: species richness patterns are loosely related to the functional space

One of the key hypothesized drivers of gradients in species richness is environmental filtering, where environmental stress limits which species from a larger species pool gain membership in a local community owing to their traits. Whereas most studies focus on small‐scale variation in functional traits along environmental gradient, the effect of large‐scale environmental filtering is less well understood. Furthermore, it has been rarely tested whether the factors that constrain the niche space limit the total number of coexisting species. We assessed the role of environmental filtering in shaping tree assemblages across North America north of Mexico by testing the hypothesis that colder, drier, or seasonal environments (stressful conditions for most plants) constrain tree trait diversity and thereby limit species richness. We assessed geographic patterns in trait filtering and their relationships to species richness pattern using a comprehensive set of tree range maps. We focused on four key plant functional traits reflecting major life history axes (maximum height, specific leaf area, seed mass, and wood density) and four climatic variables (annual mean and seasonality of temperature and precipitation). We tested for significant spatial shifts in trait means and variances using a null model approach. While we found significant shifts in mean species’ trait values at most grid cells, trait variances at most grid cells did not deviate from the null expectation. Measures of environmental harshness (cold, dry, seasonal climates) and lower species richness were weakly associated with a reduction in variance of seed mass and specific leaf area. The pattern in variance of height and wood density was, however, opposite. These findings do not support the hypothesis that more stressful conditions universally limit species and trait diversity in North America. Environmental filtering does, however, structure assemblage composition, by selecting for certain optimum trait values under a given set of conditions.     

Ants and plants as indicators of biodiversity,ecosystem services,and conservation value in constructed grasslands

Grasslands are constructed for soil and wildlife conservation in agricultural landscapes across Europe and North America. Constructed grasslands may mitigate habitat loss for grassland-dependent animals and enhance ecosystem services that are important to agriculture. The responses of animal species richness and abundance to grassland habitat quality are often highly variable, however, and monitoring of multiple taxa is often not feasible. We evaluated whether multiple animal taxa responded to variation in constructed grassland habitats of southwest Ohio, USA, in ways that could be predicted from indicators based on quality assessment indices, Simpson diversity, and the species richness of ants and plants. The quality assessment indices included a widely used Floristic Quality Assessment (FQA) index, and a new Ant Quality Assessment (AntQA) index, both based on habitat specificity and species traits. The ant and plant indicators were used as predictor variables in separate general linear models of four target taxa—bees, beetles, butterflies and birds—with response variables of overall species richness and abundance, and subsets of taxa that included the abundance of ecosystem-service providers and grassland-associated species. Plant Simpson diversity was the best-fitting predictor variable in models of overall bee and beetle abundance, and the abundance of bees classified as ecosystem-service (ES) providers. FQA and plant richness were the best predictors of overall butterfly species richness and abundance. Ant species richness was the best predictor of overall bird species richness and abundance as well as the abundance of ES birds, while the AntQA index was the best predictor for the abundance of grassland bird and butterfly species. Thus, plant Simpson diversity and ant species richness were the most effective indicators for complementary components of grassland animal communities, whereas quality assessment indices were less robust as indicators and require more knowledge on the habitat specificity of individual ant and plant species.     

Spatial patterns of woody plant and bird diversity: functional relationships or environmental effects?

Aim To understand cross‐taxon spatial congruence patterns of bird and woody plant species richness. In particular, to test the relative roles of functional relationships between birds and woody plants, and the direct and indirect environmental effects on broad‐scale species richness of both groups. Location Kenya. Methods Based on comprehensive range maps of all birds and woody plants (native species > 2.5 m in height) in Kenya, we mapped species richness of both groups. We distinguished species richness of four different avian frugivore guilds (obligate, partial, opportunistic and non‐frugivores) and fleshy‐fruited and non‐fleshy‐fruited woody plants. We used structural equation modelling and spatial regressions to test for effects of functional relationships (resource–consumer interactions and vegetation structural complexity) and environment (climate and habitat heterogeneity) on the richness patterns. Results Path analyses suggested that bird and woody plant species richness are linked via functional relationships, probably driven by vegetation structural complexity rather than trophic interactions. Bird species richness was determined in our models by both environmental variables and the functional relationships with woody plants. Direct environmental effects on woody plant richness differed from those on bird richness, and different avian consumer guilds showed distinct responses to climatic factors when woody plant species richness was included in path models. Main conclusions Our results imply that bird and woody plant diversity are linked at this scale via vegetation structural complexity, and that environmental factors differ in their direct effects on plants and avian trophic guilds. We conclude that climatic factors influence broad‐scale tropical bird species richness in large part indirectly, via effects on plants, rather than only directly as often assumed. This could have important implications for future predictions of animal species richness in response to climate change.     

Elevational gradients in bird diversity in the Eastern Himalaya: an evaluation of distribution patterns and their underlying mechanisms

Background

Understanding diversity patterns and the mechanisms underlying those patterns along elevational gradients is critically important for conservation efforts in montane ecosystems, especially those that are biodiversity hotspots. Despite recent advances, consensus on the underlying causes, or even the relative influence of a suite of factors on elevational diversity patterns has remained elusive.

Methods and Principal Findings

We examined patterns of species richness, density and range size distribution of birds, and the suite of biotic and abiotic factors (primary productivity, habitat variables, climatic factors and geometric constraints) that governs diversity along a 4500-m elevational gradient in the Eastern Himalayan region, a biodiversity hotspot within the world''s tallest mountains. We used point count methods for sampling birds and quadrats for estimating vegetation at 22 sites along the elevational gradient. We found that species richness increased to approximately 2000 m, then declined. We found no evidence that geometric constraints influenced this pattern, whereas actual evapotranspiration (a surrogate for primary productivity) and various habitat variables (plant species richness, shrub density and basal area of trees) accounted for most of the variation in bird species richness. We also observed that ranges of most bird species were narrow along the elevation gradient. We find little evidence to support Rapoport''s rule for the birds of Sikkim region of the Himalaya.

Conclusions and Significance

This study in the Eastern Himalaya indicates that species richness of birds is highest at intermediate elevations along one of the most extensive elevational gradients ever examined. Additionally, primary productivity and factors associated with habitat accounted for most of the variation in avian species richness. The diversity peak at intermediate elevations and the narrow elevational ranges of most species suggest important conservation implications: not only should mid-elevation areas be conserved, but the entire gradient requires equal conservation attention.