Ecological drivers of avian diversity in a subtropical landscape: Effects of habitat diversity,primary productivity and anthropogenic disturbance

Understanding the roles of ecological drivers in shaping biodiversity is fundamental for conservation practice. In this study, we explored the effects of elevation, conservation status, primary productivity, habitat diversity and anthropogenic disturbance (represented by human population density and birding history) on taxonomic, phylogenetic and functional avian diversity in a subtropical landscape in southeastern China. We conducted bird surveys using 1‐km transects across a total of 30 sites, of which 10 sites were located within a natural reserve. Metrics of functional diversity were calculated based on six functional traits (body mass, clutch size, dispersal ratio, sociality, diet and foraging stratum). We built simultaneous autoregression models to assess the association between the ecological factors and diversity of the local avian communities. Local avian diversity generally increased with increasing habitat diversity, human population density and primary productivity. We also detected phylogenetic and functional clustering in these communities, suggesting that the avian assemblages were structured mainly by environmental filtering, rather than interspecific competition. Compared with sites outside the natural reserve, sites within the natural reserve had relatively lower avian diversity but a higher level of phylogenetic heterogeneity.     

Contrasting patterns in elevational diversity between microorganisms and macroorganisms

Aim Data and analyses of elevational gradients in diversity have been central to the development and evaluation of a range of general theories of biodiversity. Elevational diversity patterns have, however, been severely understudied for microbes, which often represent decomposer subsystems. Consequently, generalities in the patterns of elevational diversity across different trophic levels remain poorly understood. Our aim was to examine elevational gradients in the diversity of macroinvertebrates, diatoms and bacteria along a stony stream that covered a large elevational gradient. Location Laojun Mountain, Yunnan province, China. Methods The sampling scheme included 26 sites spaced at elevational intervals of 89 m from 1820 to 4050 m elevation along a stony stream. Macroinvertebrate and diatom richness were determined based on the morphology of the specimens. Taxonomic richness for bacteria was quantified using a molecular fingerprinting method. Over 50 environmental variables were measured at each site to quantify environmental variables that could correlate with the patterns of diversity. We used eigenvector‐based spatial filters with multiple regressions to account for spatial autocorrelation. Results The bacterial richness followed an unexpected monotonic increase with elevation. Diatoms decreased monotonically, and macroinvertebrate richness showed a clear unimodal pattern with elevation. The unimodal richness pattern for macroinvertebrates was best explained by the mid‐domain effect (r² = 0.72). The diatom richness was best explained by the variation in nutrient supply, and the increase in bacterial richness with elevation may be related to an increased carbon supply. Main conclusions We found contrasting patterns in elevational diversity among the three studied multi‐trophic groups comprising unicellular and multicellular aquatic taxa. We also found that there may be fundamental differences in the mechanisms underlying these species diversity patterns.     

Global patterns of Rhododendron diversity: The role of evolutionary time and diversification rates

Aim

Understanding the evolution of the latitudinal diversity gradient (i.e. increase in species diversity towards the tropics) is a prominent issue in ecology and biogeography. Disentangling the relative contributions of environment and evolutionary history in shaping this gradient remains a major challenge because their relative importance has been found to vary across regions and taxa. Here, using the global distributions and a molecular phylogeny of Rhododendron, one of the largest genera of flowering plants, we aim to compare the relative contributions of contemporary environment, evolutionary time and diversification rates in generating extant species diversity patterns.

Location

Global.

Time period

Undefined.

Major taxa studied

Rhododendron.

Methods

We compiled the global distributions of all Rhododendron species, and constructed a dated molecular phylogeny using nine chloroplast genes and seven nuclear regions. By integrating these two datasets, we estimated the temporal trends of Rhododendron diversification, and explored the global patterns of its species diversity, net diversification rates, and species ages. Next, we reconstructed the geographical ancestral area of the clade. Finally, we compared the relative contribution of contemporary environment, time‐for‐speciation, and diversification rates on the species diversity pattern of Rhododendron.

Results

In contrast to the predictions of the time‐for‐speciation hypothesis, we found that although Rhododendron originated at a temperate latitude, its contemporary species diversity is highest in the tropics/subtropics, suggesting an into‐the‐tropics colonization for this genus. We found that the elevated diversification induced by heterogeneous environmental conditions in the tropics/subtropics shapes the global pattern of Rhododendron diversity.

Main conclusions

Our findings support tropical and subtropical mountains as not only biodiversity and endemism hotspots, but also as cradles for the diversification of Rhododendron. Our study emphasizes the need of unifying ecological and evolutionary approaches in order to gain comprehensive understanding of the mechanisms underlying the global patterns of plant diversity.     

Climate and food diversity as drivers of mammal diversity in Inner Mongolia

Traditionally, geographical distribution of biodiversity is assumed to be codetermined by multiple factors, for example, temperature, precipitation, environmental heterogeneity, and biotic interactions. However, few studies have simultaneously compared the relative roles of these factors in shaping the mammal diversity patterns for different feeding groups, that is, herbivores, insectivores, and carnivores. In this study, we assessed the relations between mammal diversity and current climate (mean annual temperature and precipitation), altitudinal range as well as mammal's food diversity in Inner Mongolia. Our results showed that the species richness for the three feeding guilds of mammals consistently increased with their food diversity, that is, species richness of plants, insects, and rodents. Mammal diversity also significantly decreased with mean annual temperature and precipitation. Random Forest models indicated that climate and food diversity were always included in the combinations of variables most associated with mammal diversity. Our findings suggest that while climate is an important predictor of large scale distribution of mammal diversity, biotic interactions, that is, food diversity, could also play important roles.     

Spatial diversity patterns of Pristimantis frogs in the Tropical Andes

Although biodiversity gradients have been widely documented, the factors governing broad‐scale patterns in species richness are still a source of intense debate and interest in ecology, evolution, and conservation biology. Here, we tested whether spatial hypotheses (species–area effect, topographic heterogeneity, mid‐domain null model, and latitudinal effect) explain the pattern of diversity observed along the altitudinal gradient of Andean rain frogs of the genus Pristimantis. We compiled a gamma‐diversity database of 378 species of Pristimantis from the tropical Andes, specifically from Colombia to Bolivia, using records collected above 500 m.a.s.l. Analyses were performed at three spatial levels: Tropical Andes as a whole, split in its two main domains (Northern and Central Andes), and split in its 11 main mountain ranges. Species richness, area, and topographic heterogeneity were calculated for each 500‐m‐width elevational band. Spatial hypotheses were tested using linear regression models. We examined the fit of the observed diversity to the mid‐domain hypothesis using randomizations. The species richness of Pristimantis showed a hump‐shaped pattern across most of the altitudinal gradients of the Tropical Andes. There was high variability in the relationship between area and species richness along the Tropical Andes. Correcting for area effects had little impact in the shape of the empirical pattern of biodiversity curves. Mid‐domain models produced similar gradients in species richness relative to empirical gradients, but the fit varied among mountain ranges. The effect of topographic heterogeneity on species richness varied among mountain ranges. There was a significant negative relationship between latitude and species richness. Our findings suggest that spatial processes partially explain the richness patterns of Pristimantis frogs along the Tropical Andes. Explaining the current patterns of biodiversity in this hot spot may require further studies on other possible underlying mechanisms (e.g., historical, biotic, or climatic hypotheses) to elucidate the factors that limit the ranges of species along this elevational gradient.     

The determinants of land snail diversity along a tropical elevational gradient: insularity,geometry and niches

Aim We investigated the patterns of species richness in land snails and slugs along a tropical elevational gradient and whether these patterns correlate with area, elevation, geographic constraints, and productivity. We did so both at the scale at which land snail population processes take place and at the coarser scale of elevational zones. Location Mount Kinabalu (4096 m) and the adjacent Mount Tambuyukon (2588 m) in Kinabalu Park, Sabah, Malaysian Borneo. Methods We used an effort‐controlled sampling protocol to determine land snail and slug species richness in 142 plots of 0.04 ha at elevations ranging from 570 to 4096 m. Extents of elevational ranges were determined by interpolation, extended where appropriate at the lower end with data from lowlands outside the study area. We used regression analysis to study the relationships between species density and richness on the one hand and elevation and area on the other. This was done for point data as well as for data combined into 300‐m elevational intervals. Results Species density (based on the individual samples) showed a decline with elevation. Elevational range length profiles revealed that range lengths are reduced at greater elevations and that a Rapoport effect is absent. Diversity showed a mild mid‐domain effect on Kinabalu, but not on Tambuyukon. When the data were combined into 300‐m elevational intervals, richness correlated more strongly with elevation than with area. Ecomorphospace was seen to shrink with increasing elevation. Main conclusions The elevational species richness patterns show the combined effects of (1) reduced niche diversity at elevations with lower productivity and (2) historical events in which the upward migration of lowland species as well as the speciation of highland endemics took place.     

环境梯度下蒙古栎群落的物种多样性特征

通过样地法研究了东北地区处于不同经度、纬度和海拔的 13个地点蒙古栎群落的物种丰富度、Gini指数、PIE指数、Shannon指数和 Pielou指数 ,利用相关和回归的统计方法分析了不同地点物种的丰富度指数、Simpson多样性指数和 Shannon多样性指数与各地所处的经度、纬度和海拔的关系。结果发现 :不仅不同地点 (较大尺度 )的物种丰富度和多样性指数均有差异 ,即使在相同的地点 (较小尺度 ) ,物种丰富度及多样性指数也有差异 ,有时还具有很大的差异 ,呈现空间异质性分布的特征 ;因为影响这些多样性指数的环境因子更加复杂 ,不仅受经度、纬度和海拔的影响 ,也受地形、群落的年龄、干扰史等多种生态因子影响。不同地点的物种丰富度与海拔和纬度都具有明显的相关性 (p<0 .0 5 ) ,物种丰富度随海拔和纬度的升高而降低 ,依据显著度的大小可以推测物种丰富度与海拔的相关性比与纬度的相关性更密切 ;蒙古栎群落不同类群的植物种的丰富度具有不同的分布格局 ,木本植物的丰富度与当地纬度具有明显的相关性 (p<0 .0 5 ) ,而与所在地的海拔没有显著的关系 (p>0 .0 5 ) ,而草本植物受海拔的影响更显著 (p<0 .0 5 ) ,而与纬度之间没有显著的关系 (p>0 .0 5 )。群落的 Gini指数、PIE指数、Shannon指数和Pielou指数未发现与海     

Global analysis of bird elevational diversity

Aim Elevational gradients distributed across the globe are a powerful test system for understanding biodiversity. Here I use a comprehensive set of bird elevational gradients to test the main drivers of diversity, including sampling, area, mid‐domain effect, temperature, temperature and water availability, and hypotheses of evolutionary history. Location Seventy‐eight elevational gradients of bird diversity from mountains in both hemispheres spanning 24.5° S to 48.2° N, including gradients from various climates, biogeographical regions and habitat types. Methods Data on bird elevational diversity were taken from the literature. Of the 150 datasets found or compiled, only those with a high, unbiased sampling effort were used in analyses. Datasets sampled all birds, all breeding birds or all forest birds; a few studies detailed seasonal, elevational shifts. Eighteen predictions of diversity theory were tested, including three sets of interactions. Results Birds display four distinct diversity patterns in nearly equal frequency on mountains: decreasing diversity, low‐elevation plateaus, low‐elevation plateaus with mid‐peaks, and unimodal mid‐elevational peaks. Bird elevational diversity strongly supports current climate as the main driver of diversity, particularly combined trends in temperature and water availability. Bird diversity on humid mountains is either decreasing or shows a low‐elevation plateau in diversity, while on dry mountains it is unimodal or a broad, low‐elevation plateau usually with a mid‐elevation maximum. The predictions of sampling, area and mid‐domain effect were not consistently supported globally. The only evolutionary hypothesis with preliminary support was niche conservatism. Main conclusions Both water and temperature variables are needed to comprehensively predict elevational diversity patterns for birds. This result is consistent for breeding and forest birds, for both hemispheres, and for local‐ or regional‐scale montane gradients. More analyses are needed to discern whether the mechanism underlying these relationships is ecological, based on direct physiological limitations or indirect food resource limitations, or historical, based on phylogenetic niche conservation or other evolutionary trends related to climate. The species–area and mid‐domain effects are not supported as primary drivers of elevational diversity in birds.     

Mammals on mountainsides: elevational patterns of diversity

The four major papers in this special feature present and interpret data from field studies on the distributions and diversity of small mammals in elevational gradients on mountains in the Philippines, Borneo, southern Mexico and western United States. In the introductory paper, Lomolino places these studies in the context of historical, methodological and conceptual themes in contemporary biogeography. In this final paper, I focus on some important similarities and interesting differences among the four case studies. All of the studies provide evidence for the influence of ecological factors, such as climate, productivity and habitat heterogeneity, on mammalian diversity. All also provide evidence for the influence of historical dispersal, extinction, and speciation events. Perhaps the most interesting result is the documentation of a frequent, but not universal, peak in species diversity at some elevation intermediate between the base and peak of a mountain. Efforts to understand the mechanistic basis for this pattern — and why it differs from the continuous decrease in diversity from the equator to the poles — promise to contribute to developing a general theoretical explanation for the major patterns of biodiversity on earth.     

Small mammal species richness and turnover along elevational gradient in Yulong Mountain,Yunnan, Southwest China

Understanding the species diversity patterns along elevational gradients is critical for biodiversity conservation in mountainous regions. We examined the elevational patterns of species richness and turnover, and evaluated the effects of spatial and environmental factors on nonvolant small mammals (hereafter “small mammal”) predicted a priori by alternative hypotheses (mid‐domain effect [MDE], species–area relationship [SAR], energy, environmental stability, and habitat complexity]) proposed to explain the variation of diversity. We designed a standardized sampling scheme to trap small mammals at ten elevational bands across the entire elevational gradient on Yulong Mountain, southwest China. A total of 1,808 small mammals representing 23 species were trapped. We observed the hump‐shaped distribution pattern of the overall species richness along elevational gradient. Insectivores, rodents, large‐ranged species, and endemic species richness showed the general hump‐shaped pattern but peaked at different elevations, whereas the small‐ranged species and endemic species favored the decreasing richness pattern. The MDE and the energy hypothesis were supported, whereas little support was found for the SAR, the environmental stability hypothesis, and the habitat complexity. However, the primary driver(s) for richness patterns differed among the partitioning groups, with NDVI (the normalized difference vegetation index) and MDE being the most important variables for the total richness pattern. Species turnover for all small mammal groups increased with elevation, and it supported a decrease in community similarity with elevational distance. Our results emphasized for increased conservation efforts in the higher elevation regions of the Yulong Mountain.     

Functional traits reveal the expansion and packing of ecological niche space underlying an elevational diversity gradient in passerine birds

Variation in species richness across environmental gradients may be associated with an expanded volume or increased packing of ecological niche space. However, the relative importance of these alternative scenarios remains unknown, largely because standardized information on functional traits and their ecological relevance is lacking for major diversity gradients. Here, we combine data on morphological and ecological traits for 523 species of passerine birds distributed across an Andes-to-Amazon elevation gradient. We show that morphological traits capture substantial variation in species dietary (75%) and foraging niches (60%) when multiple independent trait dimensions are considered. Having established these relationships, we show that the 14-fold increase in species richness towards the lowlands is associated with both an increased volume and density of functional trait space. However, we find that increases in volume contribute little to changes in richness, with most (78%) lowland species occurring within the range of trait space occupied at high elevations. Taken together, our results suggest that high species richness is mainly associated with a denser occupation of functional trait space, implying an increased specialization or overlap of ecological niches, and supporting the view that niche packing is the dominant trend underlying gradients of increasing biodiversity towards the lowland tropics.     

Animal species diversity driven by habitat heterogeneity/diversity: the importance of keystone structures

Aim In a selected literature survey we reviewed studies on the habitat heterogeneity–animal species diversity relationship and evaluated whether there are uncertainties and biases in its empirical support. Location World-wide. Methods We reviewed 85 publications for the period 1960–2003. We screened each publication for terms that were used to define habitat heterogeneity, the animal species group and ecosystem studied, the definition of the structural variable, the measurement of vegetation structure and the temporal and spatial scale of the study. Main conclusions The majority of studies found a positive correlation between habitat heterogeneity/diversity and animal species diversity. However, empirical support for this relationship is drastically biased towards studies of vertebrates and habitats under anthropogenic influence. In this paper, we show that ecological effects of habitat heterogeneity may vary considerably between species groups depending on whether structural attributes are perceived as heterogeneity or fragmentation. Possible effects may also vary relative to the structural variable measured. Based upon this, we introduce a classification framework that may be used for across-studies comparisons. Moreover, the effect of habitat heterogeneity for one species group may differ in relation to the spatial scale. In several studies, however, different species groups are closely linked to ‘keystone structures’ that determine animal species diversity by their presence. Detecting crucial keystone structures of the vegetation has profound implications for nature conservation and biodiversity management.     

Global patterns of diversity among the specialist birds of riverine landscapes

1. Despite the growing view that biodiversity provides a unifying theme in river ecology, global perspectives on richness in riverine landscapes are limited. As a result, there is little theory or quantitative data on features that might have influenced global patterns in riverine richness, nor are there clear indications of which riverine landscapes are important to conservation at the global scale. As conspicuous elements of the vertebrate fauna of riverine landscapes, we mapped the global distributions of all of the world's specialist riverine birds and assessed their richness in relation to latitude, altitude, primary productivity and geomorphological complexity (surface configuration). 2. Specialist riverine birds, typical of high‐energy riverine landscapes and dependent wholly or partly on production from river ecosystems, occur in 16 families. They are represented by an estimated 60 species divided equally between the passerines and non‐passerines. Major radiation has occurred among different families on different continents, indicating that birds have evolved several times into the niches provided by riverine landscapes. 3. Continental richness varies from four species in Europe to 28 in Asia, with richness on the latter continent disproportionately larger than would be expected from a random distribution with respect to land area. Richness is greatest in mountainous regions at latitudes of 20–40°N in the riverine landscapes of the Himalayan mountains, where 13 species overlap in range. 4. Family, genus and species richness in specialist riverine birds all increase significantly with productivity and surface configuration (i.e. relief). However, family richness was the best single predictor of the numbers of species or genera. In keeping with the effect of surface configuration, river‐bird richness peaks globally at 1300–1400 m altitude, and most species occur typically on small, fast rivers where they feed predominantly on invertebrates. Increased lengths of such streams in areas of high relief and rainfall might have been responsible for species–area effects. 5. We propose the hypothesis that the diversity in channel forms and habitats in riverine landscapes, in addition to high temperature and primary productivity, have been prerequisites to the development of global patterns in the richness of specialist riverine organisms. We advocate tests of this hypothesis in other taxonomic groups. We draw attention, however, to the challenges of categorically defining riverine organisms in such tests because (i) rivers grade into many other habitat types across several different ecotones and (ii) `terrestrialisation' processes in riverine landscapes means that they offer habitat for organisms whose evolutionary origins are not exclusively riverine.     

Factors influencing vascular plant diversity on 22 islands off the coast of eastern North America

Aim The influence of physiographic and historical factors on species richness of native and non‐native vascular plants on 22 coastal islands was examined. Location Islands off the coast of north‐eastern USA and south‐eastern Canada between 41° and 45° N latitude were studied. Island size ranges from 3 to 26,668 ha. All islands were deglaciated between 15,000 and 11,000 yr bp ; all but the four New Brunswick islands were attached to the mainland until rising sea level isolated them between 14,000 and 3800 yr bp . Methods Island species richness was determined from floras compiled or revised since 1969. Simple and multiple regression and rank correlation analysis were employed to assess the relative influence of independent variables on species richness. Potential predictors included island area, latitude, elevation, distance from the mainland, distance from the nearest larger island, number of soil types, years since isolation, years since deglaciation, and human population density. Results Native vascular plant species richness for the 22 islands in this study is influenced most strongly by island area, latitude, and distance from the nearest larger island; richness increases with island area, but decreases with latitude and distance from the nearest larger island as hypothesized. That a similar model employing distance from the mainland does not meet the critical value of P confirms the importance of the stepping‐stone effect. Habitat diversity as measured by number of soil types is also an important predictor of native plant species richness, but at least half of its influence can be attributed to island area, with which it is correlated. Two historical factors, years since deglaciation and years since isolation, also appear to be highly correlated with native species richness, but their influence cannot be separated from that of latitude for the present sample size. Non‐native vascular plant species richness is influenced primarily by island area and present‐day human population density, although human population density may be a surrogate for the cumulative effect of several centuries of anthropogenic impacts related to agriculture, hunting, fishing, whaling, tourism, and residential development. Very high densities of ground‐nesting pelagic birds may account for the high percentage of non‐native species on several small northern islands. Main conclusions Many of the principles of island biogeography that have been applied to oceanic islands apply equally to the 22 islands in this study. Native vascular plant species richness for these islands is strongly influenced by physiographic factors. Influence of two historical factors, years since deglaciation and years since isolation, cannot be assessed with the present sample size. Non‐native vascular plant species richness is influenced by island area as well as by human population density; human population density may be a surrogate for other anthropogenic impacts.