Patterns of ant species richness along elevational gradients in an arid ecosystem

Aim In this study, we examine patterns of local and regional ant species richness along three elevational gradients in an arid ecosystem. In addition, we test the hypothesis that changes in ant species richness with elevation are related to elevation‐dependent changes in climate and available area. Location Spring Mountains, Nevada, U.S.A. Methods We used pitfall traps placed at each 100‐m elevational band in three canyons in the Spring Mountains. We compiled climate data from 68 nearby weather stations. We used multiple regression analysis to examine the effects of annual precipitation, average July precipitation, and maximum and minimum July temperature on ant species richness at each elevational band. Results We found that patterns of local ant species richness differed among the three gradients we sampled. Ant species richness increased linearly with elevation along two transects and peaked at mid‐elevation along a third transect. This suggests that patterns of species richness based on data from single transects may not generalize to larger spatial scales. Cluster analysis of community similarity revealed a high‐elevation species assemblage largely distinct from that of lower elevations. Major changes in the identity of ant species present along elevational gradients tended to coincide with changes in the dominant vegetation. Regional species richness, defined here as the total number of unique species within an elevational band in all three gradients combined, tended to increase with increasing elevation. Available area decreased with increasing elevation. Area was therefore correlated negatively with ant species richness and did not explain elevational patterns of ant species richness in the Spring Mountains. Mean July maximum and minimum temperature, July precipitation and annual precipitation combined to explain 80% of the variation in ant species richness. Main conclusions Our results suggest that in arid ecosystems, species richness for some taxa may be highest at high elevations, where lower temperatures and higher precipitation may support higher levels of primary production and cause lower levels of physiological stress.     

Elevational species richness patterns for vascular plants on Mount Kinabalu, Borneo

Aim  We quantify the elevational patterns of species richness for all vascular plants and some functional and taxonomic groups on a regional scale on a tropical mountain and discuss some possible causes for the observed patterns.
Location  Mount Kinabalu, Sabah, Borneo.
Methods  A data base containing elevational information on more than 28,000 specimens was analysed for vascular plant distribution, taking into account sampling effort. The total species richness pattern was estimated per 300-m elevational interval by rarefaction analyses. The same methods were also applied to quantify species richness patterns of trees, epiphytes, and ferns.
Results  Total species richness has a humped relationship with elevation, and a maximum species richness in the interval between 900 and 1200 m. For ferns and epiphytes the maximum species richness is found at slightly higher elevations, whereas tree species did not have a statistically significant peak in richness above the lowest interval analysed.
Main conclusions  For the first time a rigorous estimate of an elevational pattern in species richness of the whole vascular plant flora of a tropical mountain has been quantified. The pattern observed depends on the group studied. We discuss the differences between the groups and compare the results with previous studies of elevational patterns of species richness from other tropical areas. We also discuss the methods used to quantify the richness pattern and conclude that rarefaction gives an appropriate estimate of the species richness pattern.     

Elevational diversity patterns of small mammals on Mount Kinabalu, Sabah, Malaysia

1 Diversity patterns of small mammals were studied along an elevational transect on Mount Kinabalu, the highest mountain in South‐east Asia, utilizing data from previously existing sources and a new field study. A mark‐and‐release study (conducted during wet and dry seasons between November 1994 and April 1995) resulted in captures of 12 small mammal species, including two species of squirrels, two tree shrews, seven murid rodents and one gymnure. 2 Based on data compiled from this survey, museum specimens, and published and unpublished literature (analysed by locally weighted sums of squares and quadratic polynomial regressions), species richness of small mammals formed a middle elevation bulge, highest at about 1200–1400 m and declining at lower and higher elevations. Trapping during two seasons did not change the assessment of the pattern. 3 A cluster analysis of these data indicated that there are two elevationally associated faunas, one in the highlands and another in the lowlands. The transition between these two assemblages is at 1700–1800 m elevation. The lowland faunal assemblage has the highest number of species, with maximum species richness at about 1300 m for total small mammal species, about 1200 m for arboreal species and about 1400 m for terrestrial species. 4 The areas where much overlapping of species occurs are the elevations where climate and vegetation change rapidly from lowland to montane types. Tree species, gymnosperms, orchids and ferns showed a similar curvilinear pattern along the same elevational gradient, with maximum species richness at about 1400–1500 m. Temperature declined progressively with increasing elevation, but rainfall and humidity reached their highest levels at about 1700 m. 5 Maximum diversity of small mammals thus occurred at the elevation where a highland and a lowland assemblage overlapped, where several types of plants reached their maximum diversity, and where rainfall and humidity reached their maxima. Similar patterns have been documented for small mammals, plants, and climate at sites scattered in Indo‐Australia from Taiwan to New Guinea.     

Dynamics,productivity and species richness of tropical rainforests along elevational and edaphic gradients on Mount Kinabalu,Borneo

We studied the dynamics of nine tropical rainforests on Mount Kinabalu, Borneo, at four elevations (700, 1,700, 2,700 and 3,100 m) on various edaphic conditions for four 2-year periods over 8 years (1995–2003), and examined the relationships with above-ground productivity. Mean growth rate of stem diameter, basal area turnover rate and estimated recruitment rate (using growth rate and size distribution) correlated with productivity among the nine forests in all periods. These rates based on growth rates of surviving stems appeared to be good measures of stand turnover. However, observed recruitment rate and mortality (and turnover rate as mean of these rates) based on direct observation of recruits and deaths did not correlate with productivity in some periods. These rates may not be useful as measures of stand turnover given small sample size and short census interval because they were highly influenced by stochastic fluctuation. A severe drought associated with the 1997–1998 El Niño event inflated mortality and depressed mean growth rate, recruitment rate and basal area turnover rate, but had little effect on the correlations between these rates (except mortality) and productivity. Across broad elevational and edaphic gradients on Mount Kinabalu, forest turnover, productivity and species richness correlated with each other, but the causal interpretation is difficult given the different histories and species pools among forests at different elevations.     

The mid‐domain effect matters: simulation analyses of range‐size distribution data from Mount Kinabalu,Borneo

Aim In simulation exercises, mid‐domain peaks in species richness arise as a result of the random placement of modelled species ranges within simulated geometric constraints. This has been called the mid‐domain effect (MDE). Where close correspondence is found between such simulations and empirical data, it is not possible to reject the hypothesis that empirical species richness patterns result from the MDE rather than being the outcome (wholly or largely) of other factors. To separate the influence of the MDE from other factors we therefore need to evaluate variables other than species richness. The distribution of range sizes gives different predictions between models including the MDE or not. Here, we produce predictions for species richness and distribution of range sizes from one model without the MDE and from two MDE models: a classical MDE model encompassing only species with their entire range within the domain (range‐restricted MDE), and a model encompassing all species with the theoretical midpoint within the domain (midpoint‐restricted MDE). These predictions are compared with observations from the elevational pattern of range‐size distributions and species richness of vascular plants. Location Mount Kinabalu, Borneo. Methods The data set analysed comprises more than 28,000 plant specimens with information on elevation. Species ranges are simulated with various assumptions for the three models, and the species simulated are subsequently subjected to a sampling that simulates the actual collection of species on Mount Kinabalu. The resulting pattern of species richness and species range‐size distributions are compared with the observed pattern. Results The comparison of simulated and observed patterns indicates that an underlying monotonically decreasing trend in species richness with elevation is essential to explain fully the observed pattern of richness and range size. When the underlying trend is accounted for, the MDE model that restricts the distributions of theoretical midpoints performs better than both the classical MDE model and the model that does not incorporate geometric constraints. Main conclusions Of the three models evaluated here, the midpoint‐restricted MDE model is found to be the best for explaining species richness and species range‐size distributions on Mount Kinabalu.     

Asymmetric boundary shifts of tropical montane Lepidoptera over four decades of climate warming

Aim To estimate whether species have shifted at equal rates at their leading edges (cool boundaries) and trailing edges (warm boundaries) in response to climate change. We provide the first such evidence for tropical insects, here examining elevation shifts for the upper and lower boundaries shifts of montane moths. Threats to species on tropical mountains are considered. Location Mount Kinabalu, Sabah, Malaysia. Methods We surveyed Lepidoptera (Geometridae) on Mount Kinabalu in 2007, 42 years after the previous surveys in 1965. Changes in species upper and lower boundaries, elevational extents and range areas were assessed. We randomly subsampled the data to ensure comparable datasets between years. Estimated shifts were compared for endemic versus more widespread species, and for species that reached their range limits at different elevations. Results Species that reached their upper limits at 2500–2700 m (n= 28 species, 20% of those considered) retreated at both their lower and upper boundaries, and hence showed substantial average range contractions (?300 m in elevational extent and ?45 km² in estimated range area). These declines may be associated with changes in cloud cover and the presence of ecological barriers (geological and vegetation transitions) which impede uphill movement. Other than this group, most species (n= 109, 80% of the species considered) expanded their upper boundaries upwards (by an average of 152 m) more than they retreated at their lower boundaries (77 m). Main conclusions Without constraints, leading margins shifted uphill faster than trailing margins retreated, such that many species increased their elevational extents. However, this did not result in increases in range area because the area of land available declines with increasing elevation. Species close to a major ecological/geological transition zone on the mountain flank declined in their range areas. Extinction risk may increase long before species reach the summit, even when undisturbed habitats are available.     

Comparative study of additive basal area of conifers in forest ecosystems along elevational gradients

We examined the basal area of two life forms (conifers vs. broadleaf trees) along elevational gradients on Yakushima Island, Japan and on two series of geological substrate on Mount Kinabalu, Borneo. On Yakushima, total stand basal area abruptly increased from 700 to 1,050 m in accordance with the high dominance of conifers, indicating the presence of additive basal area of conifers in conifer–broadleaf mixed forests at higher elevations (1,050–1,300 m). Along two substrate series on Kinabalu, some forests at higher elevations (1,860–3,080 m) showed relatively high dominance of conifers, but conifer basal area did not appear to be additive. Conifers were emergents above the canopy of broadleaf trees in mixed forests on Yakushima, but two life forms usually coexisted in the single-story canopy in mixed forests on Kinabalu. Litterfall rate as a surrogate of productivity decreased with decreasing temperature along elevation on both the sites, but the rate of decrease was slower on Yakushima, where mixed forests at higher elevations showed relatively high rates. Thus, we suggest that additive basal area of conifers was linked to their emergent status, and that it enhanced productivity by complementary use of light by two life forms that occupy different stories. On Yakushima, typhoons are a major disturbance, but do not severely limit the height growth of conifers, allowing the development of two-story mixed forests. On Kinabalu, a major disturbance is El Niño-driven drought, and hydraulic limitation to tree height may explain the non-additive and non-emergent nature of conifers.     

Elevation‐richness pattern of vascular plants in wadis of the arid mountain Gebel Elba,Egypt

Mountains provide a unique opportunity to study drivers of species richness across relatively short elevation gradients. However, few studies have reported elevational patterns for arid mountains. We studied elevation‐richness pattern along an elevational gradient at the arid mountain Gebel Elba, south‐east of Egypt, expecting a unimodal richness pattern. We sampled 133 vegetation plots (10 × 10 m) in four wadis along an elevational gradient from 130 to 680 m which represents the transition from desert to mountain wadi systems. We used generalised additive models to describe the relationship between elevation and plant species richness. We found a strong increase in species richness and Shannon diversity at low elevations followed by a plateau at mid‐ to high elevations. When we analysed each tributary as a single gradient, no pattern was found. The analysed elevational gradient seems to be a major stress gradient in terms of temperature and water availability, exhibiting a trend of increasing species richness that changes to a plateau pattern; a pattern rarely observed for wadi systems in arid mountains. We discuss the observed pattern with the climatic stress hypothesis and the environmental heterogeneity hypothesis as possible explanations for the pattern.     

Exploring anthropogenic and natural processes shaping fern species richness along elevational gradients

Aim (1) To explore the impact of land use, climate and environmental heterogeneity on fern species richness along a complete elevational gradient, and (2) to evaluate the relative importance of the three groups of variables within different elevational intervals. Location A temperate mountain region (55,507 km²) of Italy on the southern border of the European Alps divided into a regular grid of 1476 cells (grain 35.7 km²). Methods We applied multiple regression (spatial and non‐spatial) to determine the relative influence of the three groups of variables on species richness, including variation partitioning at two scales. We considered the whole gradient (all 1476 cells) to explain the overall elevational pattern of species richness, and we grouped the cells into elevational intervals of 500 m in order to evaluate the explanatory power of the predictors within different zones along the gradient. Results Species richness showed a hump‐shaped pattern with elevation, forming a plateau between 800 and 1500 m. The lowest species richness was found in warm and relatively dry disturbed lowlands. Moving upwards, the greatest species richness was found in forest‐dominated mid‐elevations with high environmental heterogeneity. At high elevations dominated by open natural habitats, where temperature and precipitation were relatively low, species richness declined but less sharply than in the lowlands. Although it was impossible to separate the effects of the three groups of predictors along the whole gradient, the analysis of separate elevational intervals shed light on their relative importance. The decline of species richness within lowlands was mainly related to a combined effect of deforestation and low environmental heterogeneity. In the middle part of the gradient, habitat heterogeneity and topographic roughness were positively associated with species richness. The richness decline within high‐elevation areas was related mostly to climatic constraints. Main conclusions Human impact due to land‐use modifications strongly affects the elevational pattern of species richness. It is therefore increasingly important to adopt a multiple‐hypothesis approach, taking anthropogenic effects explicitly into account when describing ecological processes along elevational gradients.     

Elevational gradients of small mammal diversity on the northern slopes of Mt. Qilian, China

Aim Small mammal species richness and relative abundance vary along elevational gradients, but there are different patterns that exist. This study reports the patterns of distribution and abundance of small mammals along the broader elevational gradient of Mt. Qilian range. Location The study was conducted in the Mt. Qilian range, north‐western China, from June to August 2001. Methods Removal trapping was conducted using a standardized technique at 7 sites ranging between 1600 and 3900 m elevation within three transects. Correlation, regression and graphical analyses were used to evaluate the diversity patterns along this elevational gradient. Results In total, 586 individuals representing 18 nonvolant small mammal species were collected during 20 160 trap nights. Species composition was different among the three transects with 6 (33%) of the species found only within one transect. Elevational distribution and relative abundance of small rodents showed substantial spatial variation, with only 2 species showing nonsignificant capture frequencies across elevations. Despite these variations, some general patterns of elevational distribution emerged: humped‐shape relationships between species diversity and elevation were noted in all three transects with diversity peaks at middle elevations. In addition, relative abundance was negatively correlated with elevation. Conclusions Results indicate that maximum richness and diversity of nonvolant small mammals occurred at mid‐elevations where several types of plants reached their maximum diversity and primary productivity, and where rainfall and humidity reached a maximum. It is demonstrated that the mid‐elevation bulge is a general feature of at least a large portion of the biota on the Mt. Qilian range.     

What drives elevational patterns of diversity? A test of geometric constraints,climate and species pool effects for pteridophytes on an elevational gradient in Costa Rica

Aim We studied pteridophyte species richness between 100 m and 3400 m along a Neotropical elevational gradient and tested competing hypotheses for patterns of species richness. Location Elevational transects were situated at Volcán Barva in the Braulio Carrillo National Park and La Selva Biological Station (100–2800 m) and Cerro de la Muerte (2700–3400 m), both on the Atlantic slope of Costa Rica, Central America. Method We analysed species richness on 156 plots of 20 × 20 m and measured temperature and humidity at four elevations (40, 650, 1800 and 2800 m). Species richness patterns were regressed against climatic variables (temperature, humidity, precipitation and actual evapotranspiration), regional species pool, area and predicted species number of a geometric null model (the mid‐domain effect, MDE). Results The species richness of the 484 recorded species showed a hump‐shaped pattern with elevation with a richness peak at mid‐elevations (c. 1700 m). The MDE was the single most powerful explanatory variable in linear regression models, but species richness was also associated strongly with climatic variables, especially humidity and temperature. Area and species pool were associated less strongly with observed richness patterns. Main conclusions Geometric models and climatic models exclusive of geometric constraints explained comparable amounts of the elevational variation in species richness. Discrimination between these two factor complexes is not possible based on model fits. While overall fits of geometric models were high, large‐ and small‐ranged species were explained by geometric models to different extents. Species with narrow elevational ranges clustered at both ends of the gradient to a greater extent than predicted by the MDE null models used here. While geometric models explained much of the pattern in species richness, we cannot rule out the role of climatic factors (or vice versa) because the predicted peak in richness from geometric models, the empirical peak in richness and the overlap in favourable environmental conditions all coincide at middle elevations. Mid‐elevations offer highest humidity and moderate temperatures, whereas at high elevations richness is reduced due to low temperatures, and at low elevations by reduced water availability due to high temperatures.     

Elevational Gradient of Vascular Plant Species Richness and Endemism in Crete – The Effect of Post-Isolation Mountain Uplift on a Continental Island System

Understanding diversity patterns along environmental gradients and their underlying mechanisms is a major topic in current biodiversity research. In this study, we investigate for the first time elevational patterns of vascular plant species richness and endemism on a long-isolated continental island (Crete) that has experienced extensive post-isolation mountain uplift. We used all available data on distribution and elevational ranges of the Cretan plants to interpolate their presence between minimum and maximum elevations in 100-m elevational intervals, along the entire elevational gradient of Crete (0–2400 m). We evaluate the influence of elevation, area, mid-domain effect, elevational Rapoport effect and the post-isolation mountain uplift on plant species richness and endemism elevational patterns. Furthermore, we test the influence of the island condition and the post-isolation mountain uplift to the elevational range sizes of the Cretan plants, using the Peloponnese as a continental control area. Total species richness monotonically decreases with increasing elevation, while endemic species richness has a unimodal response to elevation showing a peak at mid-elevation intervals. Area alone explains a significant amount of variation in species richness along the elevational gradient. Mid-domain effect is not the underlying mechanism of the elevational gradient of plant species richness in Crete, and Rapoport''s rule only partly explains the observed patterns. Our results are largely congruent with the post-isolation uplift of the Cretan mountains and their colonization mainly by the available lowland vascular plant species, as high-elevation specialists are almost lacking from the Cretan flora. The increase in the proportion of Cretan endemics with increasing elevation can only be regarded as a result of diversification processes towards Cretan mountains (especially mid-elevation areas), supported by elevation-driven ecological isolation. Cretan plants have experienced elevational range expansion compared to the continental control area, as a result of ecological release triggered by increased species impoverishment with increasing elevation.     

Phylogenetic diversity of macromycetes and woody plants along an elevational gradient in Eastern Mexico

Phylogenetic information provides insight into the ecological and evolutionary processes that organize species assemblages. We compared patterns of phylogenetic diversity among macromycete and woody plant communities along a steep elevational gradient in eastern Mexico to better understand the evolutionary processes that structure their communities. Macrofungi and trees were counted and identified in eight sites from 100 to 3500 m asl, and sequence data retrieved from GenBank for the same or closely related species were used to reconstruct their phylogenies. Patterns of species richness and phylogenetic diversity were similar for both macrofungi and trees, but macromycete richness and diversity peaked at mid‐elevations, whereas woody plant richness and diversity did not show significant trends with elevation. Phylogenetic similarity among sites was low for both groups and decreased as elevational distance between sites increased. Macromycete communities displayed phylogenetic overdispersion at low elevations and phylogenetic clustering at high elevations; the latter is consistent with environmental filtering at high elevation sites. Woody plants generally exhibited phylogenetic clustering, consistent with the potential importance of environmental filtering throughout the elevational gradient.     

Fine‐scale determinants of butterfly species richness and composition in a mountain region

Aim Global patterns of species richness are often considered to depend primarily on climate. We aimed to determine how topography and land cover affect species richness and composition at finer scales. Location Sierra de Guadarrama (central Iberian Peninsula). Methods We sampled the butterfly fauna of 180 locations (89 in 2004, 91 in 2005) at 600–2300 m elevation in a region of 10800 km². We recorded environmental variables at 100‐m resolution using GIS, and derived generalized linear models for species density (number of species per unit area) and expected richness (number of species standardized to number of individuals) based on variables of topoclimate (elevation and insolation) or land cover (vegetation type, geology and hydrology), or both (combined). We evaluated the models against independent data from the alternative study year. We also tested for differences in species composition among sites and years using constrained ordination (canonical correspondence analysis), and used variation partitioning analyses to quantify the independent and combined roles of topoclimate and land cover. Results Topoclimatic, land cover and combined models were significantly related to observed species density and expected richness. Topoclimatic and combined models outperformed models based on land cover variables, showing a humped elevational diversity gradient. Both topoclimate and land cover made significant contributions to models of species composition. Main conclusions Topoclimatic factors may dominate species richness patterns in regions with pronounced elevational gradients, as long as large areas of natural habitat remain. In contrast, both topoclimate and land cover may have important effects on species composition. Biodiversity conservation in mountainous regions therefore requires protection and management of natural habitats over a wide range of topoclimatic conditions, which may assist in facilitating range shifts and alleviating declines in species richness related 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.     

Terrestrial gastropod diversity in an alpine region: disentangling effects of elevation,area, geometric constraints,habitat type and land‐use intensity

Elevational gradients have proven to be useful to examine key factors shaping species diversity patterns. This study examines the effects of elevation, area, geometric constraints, habitat type, environmental factors and land‐use intensity on terrestrial gastropod diversity patterns in Val Müstair, an alpine region influenced by different types of agricultural land use in the eastern Alps, Switzerland. Gastropods were sampled using a standardized method in 180 sites spanning an elevational range from 1215 to 2770 m and covering 11 different habitat types. A total of 11 102 specimens representing 70 species were recorded. Observed species richness, statistically estimated true richness (Chao) and geographically interpolated observed richness were used as measures of local species richness. The comparison of three alternative models (environmental, geometric constraints and gastropod abundance models) revealed that the environmental model explained most of the variation in all measures of local diversity. The best model combining the predictors of all three models showed that elevation, soil pH and habitat type affected all measures of local species richness. Similar analyses conducted at the level of 150‐m elevational bands showed that elevation was again the best predictor of species richness, while the area of the elevational band did not have any influence. However, in one out of the two measures of band species richness, the best model indicated that geometric constraints may also contribute to the observed pattern. At both spatial scales, all measures of species richness decreased with increasing elevation. An analysis of species‐specific life‐history traits showed that adult shell size of land snails decreased with increasing elevation. Most species with large shells were confined to lower elevations. The results indicate that environmental factors might be most important in shaping the observed patterns.     

Harvestman (Arachnida: Opiliones) species distribution along three Neotropical elevational gradients: an alternative rescue effect to explain Rapoport's rule?

Aim Relationships between elevation and litter‐dweller harvestman (Arachnida: Opiliones) species richness along three elevational gradients in the Brazilian Atlantic Forest were evaluated. Specifically, three candidate explanatory factors for the observed patterns were tested: (1) the mid‐domain effect, (2) the Rapoport effect, and (3) the influence of environmental variables on species density and specimen abundance. Location Cuscuzeiro, Corcovado and Capricórnio mountains, in Ubatuba (23°26′ S, 45°04′ W), a coastal municipality in São Paulo state, south‐eastern Brazil. Methods We recorded harvestman species and abundance through active sampling using 8 × 8‐m plots in both summer and winter. At each plot we measured the temperature, humidity and mean litter depth. Harvestman species richness per elevational band was the sum of all species recorded in each band, plus the species supposed to occur due to the interpolation of the upper and lower elevational records. Differences between observed and expected species richness per elevational band, based on the mid‐domain effect, were examined through a Monte Carlo simulation. The Rapoport effect was evaluated using both the midpoint method and a new procedure proposed here, the ‘specimen method’. We applied multiple regression analysis to evaluate the contribution of each environmental variable (elevation, temperature, humidity and litter depth) on species density and specimen abundance per plot. Results Harvestman abundance and species richness decreased at higher elevations in the three mountains. The decrease in species richness was not monotonic and showed a plateau of high species richness at lower elevations. The number of harvestman species per elevational band does not fit that predicted by the mid‐domain effect based solely on geometric constraints assuming hard boundaries. Species with their midpoints at higher elevations tended to cover broader elevational range sizes. Both the midpoint method and the specimen method detected evidence of the Rapoport effect in the data. At fine spatial scales, temperature and humidity had positive effects on species density and specimen abundance, while mean litter depth had no clear effect. These relationships, however, were not constant between seasons. Main conclusions Our results suggest that harvestman species density declines at higher elevations due to restrictions imposed by temperature and humidity. We found a pattern in species range distribution as predicted by the elevational Rapoport effect. However, the usual rescue effect proposed to explain the Rapoport effect does not apply in our study. Since the majority of harvestman species covering broader elevational ranges do not exhibit reduced abundance at low elevations, an alternative rescue effect is proposed here. According to this alternative rescue effect, the decrease in species richness at higher elevations occurs due to differential upper limits of species with source populations below mid‐elevations. The seasonal differences in the relationships between environmental variables and species richness/specimen abundance per plot is an indication that species occurrence on elevational gradients is seasonally dependent. Thus relationships and hypotheses based on data recorded over short time periods, or in a single season, should be viewed cautiously.     

Invasive Temperate Species are a Threat to Tropical Island Biodiversity

To protect the remaining biodiversity on tropical islands it is important to predict the elevational ranges of non-native species. We evaluated two hypotheses by examining land snail faunas on the eastern (windward) side of the island of Hawaii: (1) the latitude of a species' native region can be used to predict its potential elevational range and (2) non-native temperate species, which experience greater climatic fluctuations in their native range, are more likely to become established at higher elevations and to extend over larger elevational ranges than non-native tropical species. All non-native tropical species were distributed patchily among sites ≤500 m and occupied small elevational ranges, whereas species introduced from temperate regions occupied wide elevational ranges and formed a distinct fauna spanning elevations 500–2000 m. Most native land snail species and ecosystems occur >500 m in areas dominated by temperate non-native snail and slug species. Therefore, knowing the native latitudinal region of a non-native species is important for conservation of tropical island ecosystems because it can be translated into potential elevational range if those species are introduced. Because temperate species will survive in tropical locales particularly at high elevation, on many tropical islands the last refuges of the native species, preventing introduction of temperate species should be a conservation priority.     

Different responses of avian feeding guilds to spatial and environmental factors across an elevation gradient in the central Himalaya

Although elevational patterns of species richness have been well documented, how the drivers of richness gradients vary across ecological guilds has rarely been reported. Here, we examined the effects of spatial factors (area and mid‐domain effect; MDE) and environmental factors, including metrics of climate, productivity, and plant species richness on the richness of breeding birds across different ecological guilds defined by diet and foraging strategy. We surveyed 12 elevation bands at intervals of 300 m between 1,800 and 5,400 m a.s.l using line‐transect methods throughout the wet season in the central Himalaya, China. Multiple regression models and hierarchical partitioning were used to assess the relative importance of spatial and environmental factors on overall bird richness and guild richness (i.e., the richness of species within each guild). Our results showed that richness for all birds and most guilds displayed hump‐shaped elevational trends, which peaked at an elevation of 3,300–3,600 m, although richness of ground‐feeding birds peaked at a higher elevation band (4,200–4,500 m). The Normalized Difference Vegetation Index (NDVI)—an index of primary productivity—and habitat heterogeneity were important factors in explaining overall bird richness as well as that of insectivores and omnivores, with geometric constraints (i.e., the MDE) of secondary importance. Granivore richness was not related to primary production but rather to open habitats (granivores were negatively influenced by habitat heterogeneity), where seeds might be abundant. Our findings provide direct evidence that the richness–environment relationship is often guild‐specific. Taken together, our study highlights the importance of considering how the effects of environmental and spatial factors on patterns of species richness may differ across ecological guilds, potentially leading to a deeper understanding of elevational diversity gradients and their implications for biodiversity conservation.