Patterns of species richness in a limestone grassland under different treatments in relation to spatial scale

Abstract. In an experiment in a limestone grassland on the Baltic island of Öland, SE Sweden, nutrient and water supply, light intensity and grazing regime were altered in 10 combinations during four years in 10 plots of 0.25 m² with subplots of 0.01 m² and 0.0004 m². Only the combined application of fertilizer and shade led to a strong decrease in average species richness (S₁) at all scales. When comparing species numbers summed up over all 10 replicates of each treatment (S_n) at the three quadrat sizes, differences in effect of these treatments were much smaller, and were so already at the finest scale. α-diversity, measured as (S_n - S₁ was quite constant over different scales for most treatments, i.e. diversity did not increase with an increase in scale. The ‘richness ratio’S_n/S₁ decreased with increasing scale, indicating an increasing degree of homogeneity at larger scales. Treatments which only included fertilizer or shade, maintained high species richness; this high richness was also maintained in combination with grazing and could then be explained by the denser packing of vegetation. Patterns of species richness were correlated at the large scale, but not at the finer scales, indicating a high degree of spatial and temporal heterogeneity at the finer scales. With increasing quadrat size species persistence increased which explains the small effect of certain treatments. Clearly, a range of scales has to be sampled in this type of vegetation to be able to measure different patterns, which may occur under different experimental treatments. The finest scale in this study can become too small, when certain treatments result in a coarse-grained vegetation pattern. The quadrat size of 10 cm x 10 cm should be included in the range of scales. It combines accuracy in sampling with efficiency in time effort, a reasonably large number of species sampled, and a strong differentiation in the effects of the various treatments.     

Habitat heterogeneity,temperature, and primary productivity drive elevational gradients in avian species diversity

AimAnticipating and mitigating the impacts of climate change on species diversity in montane ecosystems requires a mechanistic understanding of drivers of current patterns of diversity. We documented the shape of elevational gradients in avian species richness in North America and tested a suite of a priori predictions for each of five mechanistic hypotheses to explain those patterns.LocationUnited StatesMethodsWe used predicted occupancy maps generated from species distribution models for each of 646 breeding birds to document elevational patterns in avian species richness across the six largest U.S. mountain ranges. We used spatially explicit biotic and abiotic data to test five mechanistic hypotheses proposed to explain geographic variation in species richness.ResultsElevational gradients in avian species richness followed a consistent pattern of low elevation plateau‐mid‐elevation peak (as per McCain, 2009). We found support for three of the five hypotheses to explain the underlying cause of this pattern: the habitat heterogeneity, temperature, and primary productivity hypotheses.Main ConclusionsSpecies richness typically decreases with elevation, but the primary cause and precise shape of the relationship remain topics of debate. We used a novel approach to study the richness‐elevation relationship and our results are unique in that they show a consistent relationship between species richness and elevation among 6 mountain ranges, and universal support for three hypotheses proposed to explain the underlying cause of the observed relationship. Taken together, these results suggest that elevational variation in food availability may be the ecological process that best explains elevational gradients in avian species richness in North America. Although much attention has focused on the role of abiotic factors, particularly temperature, in limiting species’ ranges, our results offer compelling evidence that other processes also influence (and may better explain) elevational gradients in species richness.     

Environmental predictors of global parrot (Aves: Psittaciformes) species richness and phylogenetic diversity

Aim Spatial patterns of phylogenetic diversity (PD) aid our ability to discern diversification rate mechanisms underlying hypotheses for the large‐scale distribution of biodiversity. We develop a predictive framework for the way in which spatial patterns of PD vary with those of species richness, depending on the balance between speciation and extinction rates. Within this framework, diversification processes thought to underlie the productive energy, ambient energy, topographic variability and habitat variety hypotheses predict that gradients of increase in species richness will be associated with: (1) decreasing extinction rates where driven by productive energy, hence increasing relative PD (i.e. PD controlling for species richness, or PD_rel); (2) a similar positive relationship between ambient energy and PD_rel; (3) increasing speciation rates where driven by topographic variability, hence decreasing PD_rel; and (4) no consistent relationship between PD_rel and habitat variety when driven by the latter. We test these predictions using distributional data on parrots. Location Neotropical, Afrotropical, Indo‐Malayan and Australasian realms. Methods Spatial models were used to test the predictions. Results Globally, a positive association between productive energy and PD_rel confirms prediction (1). However, within realms, hump‐shaped relationships suggest the importance of decreasing extinction rates up to a threshold level of productive energy, and the increasing importance of speciation rates thereafter. Ambient energy is positively associated with PD_rel in Australasia, Indo‐Malaya, and globally, supporting prediction (2). However, this is driven by the coincidence of highest PD_rel in areas of high ambient energy and intermediate productive energy (i.e. in seasonal tropical environments), which may be characterized by relatively low speciation and extinction rates. In the Neotropics, increasing topographic variability is associated with decreasing PD_rel and increasing species richness, suggesting an increasing gradient of speciation, supporting prediction (3). Elsewhere, the signal of this mechanism may be obscured by collinearities with energy gradients. The lack of an overall relationship between habitat diversity and PD_rel confirms prediction (4). Main conclusions Spatial patterns of PD_rel in relation to environmental gradients may be sensitive to collinearities among those gradients. Nevertheless, patterns emerge which have implications for the relative importance of speciation and extinction processes in generating latitudinal diversity gradients.     

The relationship between species richness and productivity in metazoan parasite communities

Biodiversity is not distributed homogeneously in space, and it often covaries with productivity. The shape of the relationship between diversity and productivity, however, varies from a monotonic linear increase to a hump-shaped curve with maximum diversity values corresponding to intermediate productivity. The system studied and the spatial scale of study may affect this relationship. Parasite communities are useful models to test the productivity-diversity relationship because they consist of species belonging to a restricted set of higher taxa common to all host species. Using total parasite biovolume per host individual as a surrogate for community productivity, we tested the relationship between productivity and species richness among assemblages of metazoan parasites in 131 vertebrate host species. Across all host species, we found a linear relationship between total parasite biovolume and parasite species richness, with no trace of a hump-shaped curve. This result remained after corrections for the potential confounding effect of the number of host individuals examined per host species, host body mass, and phylogenetic relationships among host species. Although weaker, the linear relationship remained when the analyses were performed within the five vertebrate groups (fish, amphibians, reptiles, mammals and birds) instead of across all host species. These findings agree with the classic isolationist-interactive continuum of parasite communities that has become widely accepted in parasite ecology. They also suggest that parasite communities are not saturated with species, and that the addition of new species will result in increased total parasite biovolume per host. If the number of parasite species exploiting a host population is not regulated by processes arising from within the parasite community, external factors such as host characteristics may be the main determinants of parasite diversity.     

Multi-extent analysis of the relationship between pteridophyte species richness and climate

Aim To determine the relationship between the distribution of climate, climatic heterogeneity and pteridophyte species richness gradients in Australia, using an approach that does not assume potential relationships are spatially invariant and allows for scale effects (extent of analysis) to be explicitly examined. Location Australia, extending from 10° S to 43° S and 112° E to 153° E. Method Species richness within 50 × 50 km grid cells was determined using point distribution data. Climatic surfaces representing the distribution and availability of water and energy at 1 km and 5 km cell resolutions were obtained. Climate at the 50 km resolution of analysis was represented by their mean and standard deviation in that area. Relationships were assessed using geographically weighted linear regression at a range of spatial bandwidths to investigate scale effects. Results The parameters and the predictive strength of all models varied across space at all extents of analysis. Overall, climatic variables representing water availability were more highly correlated to pteridophyte richness gradients in Australia than those representing energy. Their variance in cells further increased the strength of the relationships in topographically heterogeneous regions. Relationships with water were strong across all extents of analysis, particularly in the tropical and subtropical parts of the continent. Water availability explained less of the variation in richness at higher latitudes. Main conclusions This study brings into question the ability of aspatial and single‐extent models, searching for a unified explanation of macro‐scaled patterns in gradients of diversity, to adequately represent reality. It showed that, across Australia, there is a positive relationship between pteridophyte species richness and water availability but the strength and nature of the relationship varies spatially with scale in a highly complex manner. The spatial variance, or actual complexity, in these relationships could not have been demonstrated had a traditional aspatial global regression approach been used. Regional scale variation in relationships may be at least as important as more general relationships for a true understanding of the distribution of broad‐scale diversity.     

Environmental drivers of ant species richness and composition across the Argentine Pampas grassland

Understanding the underlying mechanisms causing diversity patterns is a fundamental objective in ecology and science‐based conservation biology. Energy and environmental‐heterogeneity hypotheses have been suggested to explain spatial changes in ant diversity. However, the relative roles of each one in determining alpha and beta diversity patterns remain elusive. We investigated the main factors driving spatial changes in ant (Hymenoptera, Formicidae) species richness and composition (including turnover and nestedness components) along a 500 km longitudinal gradient in the Pampean region of Argentina. Ants were sampled using pitfall traps in 12 sample sites during the summer. We performed a model selection approach to analyse responses of ant richness and composition dissimilarity to environmental factors. Then, we computed a dissimilarity partitioning of the contributions of spatial turnover and nestedness to total composition dissimilarity. Temporal habitat heterogeneity and temperature were the primary factors explaining spatial patterns of epigean ant species richness across the Pampas. The distance decay in species composition similarity was best accounted by temperature dissimilarity, and turnover had the greatest contribution to the observed beta diversity pattern. Our findings suggest that both energy and environmental‐heterogeneity‐related variables are key factors shaping richness patterns of ants and niche‐based processes instead of neutral processes appear to be regulating species composition of ant assemblages. The major contribution of turnover to the beta diversity pattern indicated that lands for potential reconversion to grassland should represent the complete environmental gradient of the Pampean region, instead of prioritizing a single site with high species richness.     

Bird species richness patterns of northern Taiwan: primary productivity, human population density, and habitat heterogeneity

Energy, climate, habitat heterogeneity, and human activity are important correlates of spatial variation in species richness. We examined the correlation between species richness and these variables using the birds that breed in northern Taiwan. We conducted general linear models (GLMs) and spatial correlation models to examine the relationship between bird species richness (BSR) and environmental variables. We found that normalized difference vegetation index (NDVI) was the most important predictor of BSR. We suggest productivity is the primary process of BSR. Additionally, we hypothesized that scale dependency might exist in the relationship between BSR and NDVI in Taiwan. Human population density, the second most important factor, was inversely correlated with BSR. The factor and BSR did not have similar response to NDVI, which contradicted observations in most of the previous studies on human population vs. species richness. We proposed that the human population density had an effect on NDVI, which in turn had an effect on BSR. Moreover, we hypothesized that the contradiction between our study and the previous studies might arise from a higher level of human disturbance in Taiwan than in other areas. The necessity of conserving native species in intensively developed lowlands of Taiwan cannot be overemphasized. Number of land cover type was another significant predictor of BSR. Habitat heterogeneity may have an effect on BSR in Taiwan.     

Plant species and growth form richness along altitudinal gradients in the southwest Ethiopian highlands

Questions: Do growth forms and vascular plant richness follow similar patterns along an altitudinal gradient? What are the driving mechanisms that structure richness patterns at the landscape scale? Location: Southwest Ethiopian highlands. Methods: Floristic and environmental data were collected from 74 plots, each covering 400 m². The plots were distributed along altitudinal gradients. Boosted regression trees were used to derive the patterns of richness distribution along altitudinal gradients. Results: Total vascular plant richness did not show any strong response to altitude. Contrasting patterns of richness were observed for several growth forms. Woody, graminoid and climber species richness showed a unimodal structure. However, each of these morphological groups had a peak of richness at different altitudes: graminoid species attained maximum importance at a lower elevations, followed by climbers and finally woody species at higher elevations. Fern species richness increased monotonically towards higher altitudes, but herbaceous richness had a dented structure at mid‐altitudes. Soil sand fraction, silt, slope and organic matter were found to contribute a considerable amount of the predicted variance of richness for total vascular plants and growth forms. Main Conclusions: Hump‐shaped species richness patterns were observed for several growth forms. A mid‐altitudinal richness peak was the result of a combination of climate‐related water–energy dynamics, species–area relationships and local environmental factors, which have direct effects on plant physiological performance. However, altitude represents the composite gradient of several environmental variables that were interrelated. Thus, considering multiple gradients would provide a better picture of richness and the potential mechanisms responsible for the distribution of biodiversity in high‐mountain regions of the tropics.     

Anthropogenic impacts upon plant species richness and net primary productivity in California

We assess the importance of anthropogenic land‐use, altered productivity, and species invasions for observed productivity–richness relationships in California. To this end, we model net primary productivity (NPP) c. 1750 AD and at present (1982–1999) and map native and exotic vascular plant richness for 230 subecoregions. NPP has increased up to 105% in semi‐arid areas and decreased up to 48% in coastal urbanized areas. Exotic invasions have increased local species diversity up to 15%. Human activities have reinforced historical gradients in species richness but reduced the spatial heterogeneity of NPP. Structural equation modelling suggests that, prior to European settlement, NPP and richness were primarily controlled by precipitation and other abiotic variables, with NPP mediating richness. Abiotic variables remain the strongest predictors of present NPP and richness, but intermodel comparisons indicate a significant anthropogenic impact upon statewide distributions of NPP and richness. Exotic and native species each positively correlate to NPP after controlling for other variables, which may help explain recent reports of positively associated native and exotic richness.     

Differentiation between responses of primary productivity and phosphorus exploitation to species richness

Several studies have shown that ecosystem functioning increases with increasing species richness. Most of these studies examined the effects of species richness on primary productivity. The underlying mechanism that explains this pattern is usually the selection effect. The higher the diversity in plant communities the higher the chance in including a very productive species that dominates the community, or a legume species that brings N into the soil. Less attention has been given so far to the effects of species richness on phosphorus exploitation. The aim of this work was to investigate the effect of species richness on aboveground primary productivity and P accumulation in a plant diversity experiment. For this reason, 14 grassland plant species were grown in containers as monocultures and in mixtures of 2-, 3-, 4-, 8-, 11- and 14-species combinations. Results show that the aboveground phytomass and total P increased with increasing species richness. Complementarity effects, probably through partitioning of resources, were most apparent in the highest levels of species richness, and were observed to be greater for total P in comparison to phytomass. Selection effects generally were greater for phytomass than for total P; they were significantly positive at the 2- to 8-species combinations but close to 0 or negative in the highest levels of species richness. The increases in phytomass and total P at the highest levels of species richness appeared to be caused by the increased performance of intermediate-productive species. Responsible Editor: Tibor Kalapos.     

Mixed effects of habitat fragmentation on species richness and community structure in a microarthropod microecosystem

Abstract.  1. Theory is unclear about the optimal degree of isolation of habitat fragments where the aim is to maximise species richness. In a field-based microecosystem of Collembola and predatory and non-predatory mites, moss patches of the same total area were fragmented to varying degrees. The habitat was left for several months to allow the communities to approach a new state of equilibrium.
2. The species richness (in particular of predatory mites) of a given area of habitat was greater when it was part of a large mainland area than part of an island, in agreement with theory.
3. Conversely, species richness and abundance were largely unaffected by fragmentation of a fixed area of island habitat. In this case, it is suggested here that the advantages of several small patches (e.g. reduced impact of environmental stochasticity, wider range of habitats overall) were equally balanced by the advantages of a single large patch (e.g. reduced effect of demographic stochasticity, wider range of habitats within a single patch, reduced edge effect), or that both effects were small.
4. The shapes of rank–abundance curves were similar among the levels of fragmentation of a fixed area of island habitat, implying that fragmentation had little impact on community structure. Conversely, the species composition of non-predatory mites varied weakly, but significantly, by fragmentation.     

Unravelling the correlates of species richness and ecological uniqueness in a metacommunity of urban pond insects

City ponds have the potential to harbour a rich biodiversity of aquatic insects despite being located in an urban landscape. However, our current knowledge on the correlates of pond biodiversity is limited and even less is known about the factors that influence the ecological uniqueness of urban ponds. The multiple environmental gradients, at different spatial scales, that may affect biodiversity and ecological uniqueness of urban ponds can thus be seen both as an opportunity and as a challenge for a study. In this study, we aimed to fill this gap by focusing on aquatic insect assemblages in 51 ponds in the Swedish city of Stockholm, using a metacommunity perspective. We found that species richness was primarily determined by the density of aquatic insects, water depth and proportion of buildings around the pond. The uniqueness of ponds was estimated as local contributions to beta diversity (LCBD), and it was primarily related to the proportion of arable land and industry around the ponds. With regard to the metacommunity we found two interesting patterns. First, there was a negative relationship between richness and LCBD. Second, biodiversity was spatially independent, suggesting that spatially-patterned dispersal did not structure species richness or LCBD. These last two patterns are important when considering conservation efforts of biodiversity in city ponds. We hence suggest that the conservation of insect biodiversity in urban pond should consider the surroundings of the ponds, and that high-richness ponds are not necessarily those that require most attention because they are not ecologically the most unique.     

The role of regional and local scale predictors for plant species richness in Mediterranean forests

Abstract

Both local and regional predictors play a role in determining plant community structure and composition. Climate, soil features as well as different local history and management affect forest understorey and tree species composition, but to date their specific role is relatively unknown. Few studies have addressed the importance of these predictors, especially in the Mediterranean area, where environmental conditions and human impacts have generated heterogeneous forest communities. In this study, the relationships between environmental variables and species richness of different groups of vascular plants (vascular species, woody species and open habitat species) and bryophytes were investigated in Tuscan forests. A total of 37 environmental variables were used by generalised linear model fitting in order to find parsimonious sub-sets of environmental factors (predictors) that are able to explain species diversity patterns at the local scale. Moreover, the role of regional and local variable groups on species richness of the considered plant groups was estimated by using the variance partitioning approach. We found that local variables, such as forest management and structure, explained more variance than regional variables for total species richness, open habitat species richness and bryophyte species richness. On the other hand, regional variables (such as elevation) played a central role for woody species richness.     

Plant species richness and community productivity: why the mechanism that promotes coexistence matters

This paper stresses that the mechanism of coexistence is the key to understanding the relationship between species richness and community productivity. Using model plant communities, we explored two general kinds of mechanisms based on resource heterogeneity and recruitment limitation, with and without any trade-off between reproductive and competitive abilities. We generated different levels of species richness by changing model parameters, in particular the number of species in the regional pool, the degree of recruitment limitation, and the level of heterogeneity. Different diversity–productivity patterns are obtained with different coexistence mechanisms, indicating there is no reason to expect any general relationship between species richness and productivity. We discuss these results in the context of the within-site and across-site aspects of the relationship between species richness and productivity. Furthermore, we extend these results to hypothesize the relationship between species richness and productivity for other coexistence mechanisms not explicitly considered here.     

Geographic variation in the relationship between large-scale environmental determinants and bat species richness

Several studies have already shown the close relationship between geographic gradients of biodiversity and distinct environmental determinants such as energy, environmental heterogeneity and seasonality. Nevertheless, whether and how such relationships vary around the globe remains poorly understood. Here we used spatial models to answer whether the bat species richness-environment relationship on a global scale are constant across geographic space. We also partitioned the contribution of the different environmental determinants on bat species richness at different regions of the globe. We found that the relationship between bat species richness and environment is not constant across geographic space and that the shared contributions of environmental determinants are more important than their unique contributions. We conclude that understanding geographic gradients of biodiversity and its environmental determinants, particularly for bats, is more complex than previously thought because the relationship between species richness and environment varies considerably across geographic space.     

Relationships between alpha diversity of plant species in bloom and climatic variables across an elevation gradient

This study analyzes a 20-year record of flowering observations collected near Tucson, Arizona, USA. In contrast to traditional phenological records, this dataset is a record of all species observed in bloom collected in five segments of approximately 1 mile (1.61 km) in length across a 4,158-ft (1,200-m) elevation gradient. The data showed differing seasonal and interannual patterns, demonstrating the influence of climatic factors and elevation on flowering. Miles at higher elevations showed bloom peaks in summer, consistent with temperate and montane communities. Conversely, lower miles demonstrated two distinct flowering seasons, typical of the surrounding Sonoran Desert. Interannual fluctuations in total species observed in bloom were not consistent across the 5 miles (c. 8 km), suggesting that these communities respond to different flowering cues. Consistent with documented flowering triggers in semi-arid systems, the alpha diversity of species in bloom at lower elevations in this study was strongly influenced by precipitation. Upper elevation bloom numbers were heavily influenced by temperature, correspondent with bloom triggers in temperate and montane systems. In general, different life forms exhibited similar bloom triggers within the study miles, believed to be a function of shallow soils. Multivariate community analyses showed that anomalous climate conditions yielded unique seasonal bloom compositions. Over the course of the study, average summer temperature showed an upward trend; the number of species in bloom in summer (July–October) in the highest mile (1,940–2,210 m) demonstrated a concurrent increasing trend. Community analysis suggested a gradual shift in the composition of species in bloom in this mile over the study period.