Gradient In Species Richness Research Articles

Multiscale metrics differentiate among tallgrass prairie restorations and remnant ecosystems along a restorative continuum

Most grassland restorations continue to fall short of achieving diversity levels found in reference ecosystems due primarily to establishment methods and species pool limits, and a diversity of metrics may be needed to assess how different restoration methods compare to reference sites. We addressed the extent to which 17 Midwest North American grassland reference systems and 18 restorations represent a compositional gradient, how hierarchical alpha, beta, and gamma diversity vary across this gradient, and how reference ecosystems and restorations differ in functional group diversity. Hierarchical cluster analysis separated reference and restoration sites groups based on alpha and gamma scales. Both were significantly correlated with species richness gradients at nonmetric multidimensional scaling ordination axes and indicator species. Most metrics were significantly correlated, and reference sites had significantly greater diversity than restorations at multiple scales. Reference sites also differed due to the absence of a subset of species from sites with disturbance history and restorations differed in response to restoration methods. Restorations using repeated establishment events from larger species pools achieved reference site alpha diversity and had greater representation of species that characterize undisturbed reference sites. Restorations using a single establishment event resembled disturbed reference sites. However, all restorations represented a subset of reference sites, including functional group diversity. As a result, they fail to conserve rare species that occur among reference sites and may lack temporal stability. Representation of rare species among restorations would help meet regional conservation needs without increasing alpha diversity.

Vertical gradients in species richness and community composition across the twilight zone in the North Pacific Subtropical Gyre

Although metazoan animals in the mesopelagic zone play critical roles in deep pelagic food webs and in the attenuation of carbon in midwaters, the diversity of these assemblages is not fully known. A metabarcoding survey of mesozooplankton diversity across the epipelagic, mesopelagic and upper bathypelagic zones (0-1500m) in the North Pacific Subtropical Gyre revealed far higher estimates of species richness than expected given prior morphology-based studies in the region (4,024 OTUs, 10-fold increase), despite conservative bioinformatic processing. Operational taxonomic unit (OTU) richness of the full assemblage peaked at lower epipelagic-upper mesopelagic depths (100-300m), with slight shoaling of maximal richness at night due to diel vertical migration, in contrast to expectations of a deep mesopelagic diversity maximum as reported for several plankton groups in early systematic and zoogeographic studies. Four distinct depth-stratified species assemblages were identified, with faunal transitions occurring at 100m, 300m and 500m. Highest diversity occurred in the smallest zooplankton size fractions (0.2-0.5mm), which had significantly lower % OTUs classified due to poor representation in reference databases, suggesting a deep reservoir of poorly understood diversity in the smallest metazoan animals. A diverse meroplankton assemblage also was detected (350 OTUs), including larvae of both shallow and deep living benthic species. Our results provide some of the first insights into the hidden diversity present in zooplankton assemblages in midwaters, and a molecular reappraisal of vertical gradients in species richness, depth distributions and community composition for the full zooplankton assemblage across the epipelagic, mesopelagic and upper bathypelagic zones.

Key environmental determinants of global and regional richness and endemism patterns for a wild bee subfamily

Reports of world-wide decline of pollinators, and of bees in particular, raise increasing concerns about maintenance of pollination interactions. While local factors of bee decline are relatively well known and potential mitigation strategies at the landscape scale have been outlined, the regional and continental-scale threats to bee diversity have only been marginally explored. Here we document large-scale spatial patterns for a representative bee subfamily, the determinants of its species richness, and assess major threats to these pollinators. Using a comprehensive global dataset of Colletinae (genera Colletes, also called “polyester” or “cellophane” bees for their underground nests lined with a polyester secretion, and Mourecotelles), a species-rich subfamily whose organismal and physiological ecology is representative of many bees, we measured species richness and endemism on global to continental scales. We explored the relationships between bee species richness and potential environmental stress factors grouped into three categories: contemporary climate, habitat heterogeneity, and anthropogenic pressure. Bees of the subfamily Colletinae demonstrate the reversed latitudinal gradient in species richness and endemism suggested for bees; the highest species richness of Colletinae was found between 30° and 50° latitude in both the northern and southern hemispheres. Centres of endemism largely overlapped with those of species richness. The importance of the Greater Cape Floristic Region, previously identified as a centre of richness and endemism of bees, was confirmed for Colletinae. On the global scale, present-day climate was a significant predictor of species richness as was flowering plant diversity represented by vascular plant species richness and centres of plant diversity. Our main conclusion is that climate change constitutes a potential threat to bee diversity, as does declining diversity of vascular plants. However, a significant overlap between centres of bee richness and plant diversity might increase chances for developing conservation strategies.

Tree species richness attenuates the positive relationship between mutualistic ant-hemipteran interactions and leaf chewer herbivory.

Interactions across trophic levels influence plant diversity effects on ecosystem functions, but the complexity of these interactions remains poorly explored. For example, the interplay between different interactions (e.g. mutualism, predation) might be an important moderator of biodiversity-ecosystem function relationships. We tested for relationships between trophobioses (facultative ant-hemipteran mutualism) and leaf chewer herbivory in a subtropical forest biodiversity experiment. We analysed trophobiosis and herbivory data of more than 10 000 trees along a tree species richness gradient. Against expectations, chewing damage was higher on trees with trophobioses. However, the net positive relationship between trophobioses and overall herbivory depended on tree species richness, being most pronounced at low richness. Our results point to indirect, positive effects of ant-tended sap suckers on leaf chewers, potentially by altering plant defences. Direct antagonistic relationships of trophobiotic ants and leaf-chewing herbivores-frequently reported to drive community-wide effects of trophobioses in other ecosystems-seemed less relevant. However, antagonistic interactions likely contributed to the attenuating effect of tree species richness, because trophobiotic ant and herbivore communities changed from monocultures to species-rich mixtures. Our findings, therefore, suggest that biodiversity loss might lead to complex changes in higher trophic level effects on ecosystem functions, mediated by both trophic and non-trophic interactions.

Do temperate tree species diversity and identity influence soil microbial community function and composition?

Studies of biodiversity–ecosystem function in treed ecosystems have generally focused on aboveground functions. This study investigates intertrophic links between tree diversity and soil microbial community function and composition. We examined how microbial communities in surface mineral soil responded to experimental gradients of tree species richness (SR), functional diversity (FD), community‐weighted mean trait value (CWM), and tree identity. The site was a 4‐year‐old common garden experiment near Montreal, Canada, consisting of deciduous and evergreen tree species mixtures. Microbial community composition, community‐level physiological profiles, and respiration were evaluated using phospholipid fatty acid (PLFA) analysis and the MicroResp™ system, respectively. The relationship between tree species richness and glucose‐induced respiration (GIR), basal respiration (BR), metabolic quotient (qCO 2) followed a positive but saturating shape. Microbial communities associated with species mixtures were more active (basal respiration [BR]), with higher biomass (glucose‐induced respiration [GIR]), and used a greater number of carbon sources than monocultures. Communities associated with deciduous tree species used a greater number of carbon sources than those associated with evergreen species, suggesting a greater soil carbon storage capacity. There were no differences in microbial composition (PLFA) between monocultures and SR mixtures. The FD and the CWM of several functional traits affected both BR and GIR. In general, the CWM of traits had stronger effects than did FD, suggesting that certain traits of dominant species have more effect on ecosystem processes than does FD. Both the functions of GIR and BR were positively related to aboveground tree community productivity. Both tree diversity (SR) and identity (species and functional identity—leaf habit) affected soil microbial community respiration, biomass, and composition. For the first time, we identified functional traits related to life‐history strategy, as well as root traits that influence another trophic level, soil microbial community function, via effects on BR and GIR.

Functional trait dissimilarity drives both species complementarity and competitive disparity

Abstract Niche complementarity and competitive disparity are driving mechanisms behind plant community assembly and productivity. Consequently, there is great interest in predicting species complementarity and their competitive differences from their functional traits as dissimilar species may compete less and result in more complete use of resources. Here we assessed the role of trait dissimilarities for species complementarity and competitive disparities within an experimental gradient of plant species richness and functional trait dissimilarity. Communities were assembled using three pools of grass and forb species based on a priori knowledge of traits related to (1) above‐ and below‐ground spatial differences in resource acquisition, (2) phenological differences or (3) both. Complementarity and competitive disparities were assessed by partitioning the overyielding in mixed species communities into species complementarity and dominance effects. Community overyielding and the underlying complementarity and competitive dominance varied strongly among the three plant species pools. Overyielding and complementarity were greatest among species that were assembled based on their variation in both spatial and phenological traits. Competitive dominance was greatest when species were assembled based on spatial resource acquisition traits alone. In communities that were assembled based on species variation in only spatial or phenological traits, greater competitive dominance was predicted by greater differences in SLA and flowering initiation respectively, while greater complementarity was predicted by greater dissimilarity in leaf area and flowering senescence respectively. Greater differences in leaf area could also be linked to greater species complementarity in communities assembled based on variation in both phenological and spatial traits, but trait dissimilarity was unrelated to competitive dominance in these communities. Our results indicate that complementarity and competitive disparity among species are both driven by trait dissimilarities. However, the identity of the traits that drives the complementarity and competitive disparity depends on the trait variation among species that comprise the community. Moreover, we demonstrate that communities assembled with the greater variation in both spatial and phenological traits show the greatest complementarity among species. A plain language summary is available for this article.

Integrating data-cleaning with data analysis to enhance usability of biodiversity big-data

Biodiversity big-data (BBD) has the potential to provide answers to some unresolved questions – at spatial and taxonomic swathes that were previously inaccessible. However, BBDs contain serious error and bias. Therefore, any study that uses BBD should ask whether data quality is sufficient to provide a reliable answer to the research question. We propose that the question of data quality and the research question could be addressed simultaneously, by binding data-cleaning to data analysis. The change in signal between the pre- and post-cleaning phases, in addition to the signal itself, can be used to evaluate the findings, their implications, and their robustness. This approach includes five steps: Downloading raw occurrence data from a BBD. Data analysis, statistical and / or simulation modeling in order to answer the research question, using the raw data after the necessary basic cleaning. This part is similar to the common practice. Comprehensive data-cleaning. Repeated data analysis using the cleaned data. Comparing the results of steps 2 and 4 (i.e., before- and after data-cleaning). This comparison will address the issue of data quality, as well as answer the research question itself. The results of step 2 alone may be misleading, due to the error and bias in the data. Even the results of step 4 may not be trustworthy, since data-cleaning is never complete, and some of the error and much bias remain in the data. However, the changes in the results before- and after cleaning are important keys to answer the research question. If cleaned data reveal a stronger and clearer signal than raw data, then the signal is most likely trustworthy, and the respective hypothesis is confirmed. Conversely, if the cleaned data show a weaker signal than obtained from the raw data, then the respective hypothesis, even if confirmed by original data, needs to be rejected. Lastly, if there is a mixed trend, whereby in some cases the signal is stronger and in others it is weaker – the data is probably inadequate and findings cannot be considered conclusive. Thus, we propose that data-cleaning and data analysis should be conducted jointly. We present a case study on the effects of environmental factors on species distribution, using GBIF data of all Australian mammals. We used the performance of a species distribution model (SDM) as a proxy for the strength of environmental factors in determining gradients of species richness. We implemented three different SDM algorithms for 190 species in several different grid cells, that vary in their species richness. We examined the correlations between species richness and 10 different SDM performance indices. Species-environment affinity was weaker in species-rich areas, across all SDM algorithms. The results support the notion that the impact of environmental factors on species distribution at a continental scale decreases with increasing species richness. Seemingly, the results also support the continuum hypothesis, namely that in species-poor areas, species have strong affinities to particular niches, but this structure breaks in species-rich communities. Furthermore, a much stronger signal was revealed after data-cleaning. Thus, a joint study of a research question and data-cleaning provides a more reliable means for using BBDs.

Vascular plant diversity structures bryophyte colonization in experimental grassland

AbstractQuestionsNumerous grassland biodiversity experiments have explored how plant diversity influences colonization by vascular plants, but no such studies have examined how sown vascular plant diversity structures colonization by bryophytes, which can contribute greatly to grassland diversity.LocationGrassland biodiversity experiment (Jena Experiment), Germany.MethodsWe studied bryophyte composition in experimental grasslands encompassing a gradient of vascular plant species richness (1, 2, 4, 8, 16 and 60 species), functional group richness and composition (one to four; grasses, legumes, tall herbs, small herbs) as well as in additional bare ground and spontaneously colonized plots and semi‐natural control meadows.ResultsIncreasing vascular plant species richness and functional group richness as well as the presence of legume species decreased bryophyte species richness and increased the spatial variability of this richness. Bryophyte species richness and cover responded positively to grass presence, which also decreased the spatial variability of both. The proportion of acrocarpous species was largest in bare ground plots and decreased with increasing vascular plant species richness and with grass and legume presence. Non‐metric multidimensional scaling revealed that bryophyte species composition followed the gradient in sown vascular plant species richness and was dependent on grass and legume presence. The effect of plant diversity on bryophytes was only partly attributable to increased vascular plant species cover. Bryophyte habitat indicator values suggested that conditions in communities of higher vascular plant diversity that included grasses were more shady, moist and nutrient‐rich than in communities with lower vascular plant diversity and without grasses, whereas bryophyte assemblages in communities with legumes indicated well‐lit nutrient‐rich conditions. Even after 6 yr, bryophyte species richness and cover was considerably lower in experimental grasslands than in semi‐natural meadows, suggesting a role for dispersal limitation or habitat filtering.ConclusionsOur study shows that bryophyte colonization is not random, rather it is structured by vascular plant species diversity and composition. Not all components of a plant community respond positively to increased vascular plant diversity and advocating increased richness of one taxonomic group over another in grassland may reduce net species diversity.

Consistent increase in herbivory along two experimental plant diversity gradients over multiple years

AbstractResearch on the functional importance of biodiversity, motivated by global species loss, has documented that plant species richness affects many plant‐related ecosystem functions. Less is known about the effects of plant species richness on functions related to higher trophic levels, such as the consumption of biomass by animals, that is, herbivory. Previous studies have shown positive, neutral, or negative effects of plant species richness on herbivory. In the framework of a grassland biodiversity experiment (the Jena Experiment), we investigated herbivory (the proportion of leaf area damaged and the amount of leaf biomass consumed by arthropod herbivores) along two experimental gradients of plant species richness ranging from 1 to 60 species (Main Experiment) and from 1 to 8 species (Trait‐Based Experiment) biannually for five and three years, respectively. Additionally, plant functional diversity, based on traits related to plant growth, was manipulated as the number of functional groups in a community (Main Experiment) or a gradient of functional trait dissimilarity (Trait‐Based Experiment). Herbivory at the level of plant communities ranged from 0% to 31% (0 and 33.8 g/m2) in the Main Experiment and 0% to 8% (0 and 13.7 g/m2) in the Trait‐Based Experiment, and it was on average higher in summer than in spring. For both experimental gradients and all years investigated, we found a consistent increase in damaged leaf area and consumed biomass with increasing plant species richness. As mechanistic explanations for effects of plant species richness, we propose changes in plant quality and herbivore communities. The presence of specific plant functional groups significantly affected herbivory, likely related to traits affecting plant defense and nutritional value, but we found little evidence for effects of plant functional diversity. The general positive relationship between plant species richness and herbivory might contribute to effects of plant species richness on other ecosystem functions such as productivity and nutrient mineralization and can cascade up the food web also affecting higher trophic levels.

Peninsula effect and species richness gradient in terrestrial mammals on the Korean Peninsula and other peninsulas

Abstract Although the original concept of the peninsula effect was that fewer mammal species inhabited peninsulas than were found on the adjacent mainland, biogeographers have focused their attention on species richness gradients that bisect the long axes of peninsulas, instead of on the original concept. We evaluated the evidence for the peninsula effect, by comparing the richness of terrestrial mammal species on the Korean Peninsula with richness on mainland areas, and by investigating species richness gradients on 14 other peninsulas distributed globally. Our objectives were to: (1) demonstrate the existence of the peninsula effect, by comparing species richness on the Korean Peninsula with that on continental mainland areas of the same size and shape; and (2) test whether a declining gradient in species richness towards the tip represents a universal tendency associated with peninsulas, by investigating the species richness of 14 global peninsulas. We used the International Union for Conservation of Nature's spatial datasets of terrestrial mammals and the WorldClim climate dataset to create 1000 Korean Peninsula‐shaped polygons, and randomly placed them in continental areas. We recorded the number of mammalian species and mean annual temperature and precipitation for each polygon, and used these as variables in a linear regression model. We counted the number of mammal species along the central line of 14 peninsulas over 50000 km2 in size. Although the entire Korean Peninsula did not exhibit a significant peninsula effect (P = 0.1), South Korea did (P < 0.05) after controlling for temperature and precipitation. Fewer mammal species were found in South Korea than on similar continental areas. We modelled the general pattern of decline in species richness of mammals along 14 peninsulas (β = ‐3.63, P < 0.001). In 10 peninsulas, a negative relationship existed between species richness and the distance from the mainland boundary; in four peninsulas, this relationship was positive. Isolated peninsulas without ‘stepping stone’ islands exhibited the strongest declines in mammalian species richness. Our results on the Korean Peninsula and declining patterns of species richness in 14 global peninsulas illustrate that the peninsula effect can be revealed with careful analyses, and may indicate effects on biogeography and biodiversity.

Species‐specific environmental preferences associated with a hump‐shaped diversity/temperature relationship across tropical marine fish assemblages

AbstractAimTo identify key environmental factors associated with local fish species richness across a large tropical marine region.LocationWider Caribbean region.MethodsSpecies richness estimates were based on a sightings database covering the wider Caribbean region. Environmental variables considered were distance to key habitats, habitat area, temperature, depth, salinity, nutrient concentration, as well as natural and anthropogenic disturbance. We test the significance of associations between these factors and species richness, establish the shape of these relationships and use spatial cross‐validation to test the generality of these results. Species‐specific environmental associations within these relationships were then tested.ResultsFive environmental variables showed significant associations with species richness, but only two, temperature and depth, proved robust to spatial cross‐validation. Temperature was the best performing environmental predictor, showing a unimodal relationship with species richness and optimum temperatures consistent across analytical choices and data sets. Relationships between species richness and other environmental factors were also typically unimodal. Variation in species‐specific temperature associations was significant within species richness gradients; with a higher number of species associated with optimal temperatures.Main conclusionsTemperature represents the dominant environmental predictor of fish species richness identified by our study, showing a hump‐shaped curve, peaking at around 27.4°C. This meant that the warmest sites within our study were not necessarily the most species rich. Variation in species thermal niches may play a key role in driving species richness gradients, with significant general positive trend for fish species to occur in locations exhibiting optimum sea surface temperatures.

Separating effects of species identity and species richness on predation, pathogen dissemination and resistance to invasive species in tropical ant communities

Abstract Ants are abundant in natural and managed tropical ecosystems and can have an impact on herbivorous arthropods, as well as plant pathogens. Although it has been shown for plants that the diversity of communities can result in improved ecosystem functioning, it remains uncertain how the species richness of ants affects multiple ecosystem services and disservices. In the present study, we used experimentally enhanced natural gradients in ant species richness on 100 cacao trees in a plantation aiming to analyze the effect of ant species identity and species richness on predation pressure and the incidence of cacao pod borer (CPB), as well as the spread of black pod disease (BPD). Ant species richness did not significantly improve predation of experimentally exposed insects, and was not associated with a reduction in the incidence of CPB. However, the incidence of BPD was higher in ant species rich trees, presumably because more ant species were pathogen vectors. The identity of the dominant ant species affected the incidence of CPB and BPD, as well as predation pressure. Although both ant species richness and identity affected ecosystem services and disservices delivered by the ant community, the results of the present study suggest that the identity of dominant ants is the main driver for ecosystem services in these systems.

Multifaceted biodiversity hotspots of marine mammals for conservation priorities

AbstractAimIdentifying the multifaceted biodiversity hotspots for marine mammals and their spatial overlap with human threats at the global scale.LocationWorld‐wide.MethodsWe compiled a functional trait database for 121 species of marine mammals characterized by 14 functional traits grouped into five categories. We estimated marine mammal species richness (SR) as well as functional (FD) and phylogenetic diversity (PD) per grid cell (1° × 1°) using the FRic index (a measure of trait diversity as the volume of functional space occupied by the species present in an assemblage) and the PD index (the amount of evolutionary history represented by a set of species), respectively. Finally, we assessed the spatial congruence of these three facets of biodiversity hotspots (defined as 2.5% and 5% of the highest values of SR, FD and PD) with human threats at the global scale.ResultsWe showed that the FRic index was weakly correlated with both SR and the PD index. Specifically, SR and FRic displayed a triangular relationship, that is, increasing variability in FRic along the species richness gradient. We also observed a striking lack of spatial congruence (<0.1%) between current human threats and the distribution of the multiple facets of biodiversity hotspots.Main ConclusionsWe highlighted that functional diversity calculated using the FRic index is weakly associated with the species richness of marine mammals world‐wide. This is one of the most endangered vertebrate groups playing a key ecological role in marine ecosystems. This finding calls for caution when using only species richness as a benchmark for defining marine mammal biodiversity hotspots. The very low level of spatial congruence between hotspots of current threats and those of the multiple facets of marine mammal biodiversity suggests that current biodiversity patterns for this group have already been greatly affected by their history of exploitation.

Directed diversity manipulations within natural phytoplankton communities

AbstractThere is general concern about the rapid decline in biodiversity. The consequences of biodiversity declines are expected to impact ecosystem functioning. Primary production is one of the ecosystem—functions where existing data already support such expectations. However, for a mechanistic understanding of these processes more experimental investigations are needed. In terrestrial primary producer communities a directed removal of species is one important tool in the experimental portfolio to study the consequences of a decline in biodiversity. However, this is notoriously difficult in suspended aquatic primary producer communities consisting of mostly unicellular phytoplankton. As phytoplankton are responsible for about 50% of global primary production, detailed knowledge of whether and how diversity declines will impact the functioning of natural phytoplankton communities is of crucial importance. Here, we tested two different methods, dilution and disturbance—to manipulate diversity of natural freshwater plankton communities along an experimental gradient in a directed way. Dilution will alter community composition by affecting mainly rare species whereas disturbance will affect mainly stress sensitive species. Both methods resulted in distinct and replicable species richness gradients. Such gradients enable the mechanistic study of general responses to biodiversity loss in natural phytoplankton communities with shared evolutionary history in an ecological meaningful way.

Floristic diversity, richness and distribution of Trentepohliales (Chlorophyta) in Neotropical ecosystems

Trentepohliales is one of the most diverse and abundant algal group of the terrestrial microflora in tropical regions; however, information about composition, richness and distribution of these algae in Neotropical regions are scarce. We described the flora and evaluated species richness and composition of Trentepohliales in four Brazilian different phytophysiognomies and analyzed the influence of environmental and spatial factors on species richness and distribution. Specimens of Trentepohliales were gathered from six natural areas in Brazil, and they were studied according to their vegetative and reproductive morphological traits. We applied tests to evaluate the richness and similarity in species composition in the different environments: rarefaction curves to compare richness; dendrogram with Jaccard’s index to evaluate the similarity of species composition; Mantel’s test to detect the influence of the spatial geographic distance on the assemblages; and partial linear regression to analyze the influence of the spatial and environmental factors on species richness. Thirty-three morphospecies were recorded; most of them in one location and none was observed occurring in all sampled areas. Rainforest areas showed the highest species richness and the species composition was variable among the areas. Partial linear regression revealed that the spatial features plus environmental features drove the Trentepohliales’ species richness gradient. The composition of species of communities presented distance decay of similarity pattern, since differences in composition of such assemblages were positively related to the distance among the areas. In this way, the dispersion jointly with the variation of the environmental conditions may determine the Trentepohliales species composition in each ecosystem.

A role for the sampling effect in invaded ecosystems

Species loss and invasion of exotic species are two components of global biodiversity change that are expected to influence ecosystem functioning. Yet how they interact in natural settings remains unclear. Experiments have revealed two major mechanisms for the observed increase in primary productivity with plant species richness. Plant productivity may rise with species richness due to the increased amount of resources used by more diverse communities (niche complementarity) or through the increased probability of including a highly productive, dominant species in the community (sampling effect). Current evidence suggests that niche complementarity is the most relevant mechanism, whereas the sampling effect would only play a minor and transient role in natural systems. In turn, exotic species can invade by using untapped resources or because they possess a fitness advantage over resident species allowing them to dominate the community. We argue that the sampling effect can be a significant biodiversity mechanism in ecosystems invaded by dominant exotic species, and that the effect can be persistent even after decades of succession. We illustrate this idea by analyzing tree species richness–productivity relationships in a subtropical montane forest (NW Argentina) heavily invaded by Ligustrum lucidum, an evergreen tree from Asia. We found that the forest biomass increased along a natural gradient of tree species richness whether invaded by L. lucidum or not. Consistent with the sampling effect, L. lucidum invasion tripled total tree biomass irrespective of species richness, and monocultures of L. lucidum were more productive than any of the most species‐rich, uninvaded communities. Hence, the sampling effect may not be restricted to randomly assembled, synthetic communities. We emphasize that studying invaded ecosystems may provide novel insights on the mechanisms underlying the effect of biodiversity on ecosystem function.

Differential response to abiotic stress controls species distributions at biogeographic transition zones

Understanding range limits is critical to predicting species responses to climate change. Subtropical environments, where many species overlap at their range margins, are cooler, more light‐limited and variable than tropical environments. It is thus likely that species respond variably to these multi‐stressor regimes and that factors other than mean climatic conditions drive biodiversity patterns. Here, we tested these hypotheses for scleractinian corals at their high‐latitude range limits in eastern Australia and investigated the role of mean climatic conditions and of parameters linked to abiotic stress in explaining the distribution and abundance of different groups of species. We found that environmental drivers varied among taxa and were predominantly linked to abiotic stress. The distribution and abundance of tropical species and gradients in species richness (alpha diversity) and turnover (beta diversity) were best explained by light limitation, whereas minimum temperatures and temperature fluctuations best explained gradients in subtropical species, species nestedness and functional diversity. Variation in community structure (considering species composition and abundance) was most closely linked to the combined thermal and light regime. Our study demonstrates the role of abiotic stress in controlling the distribution of species towards their high‐latitude range limits and suggests that, at biogeographic transition zones, robust predictions of the impacts of climate change require approaches that account for various aspects of physiological stress and for species abundances and characteristics. These findings support the hypothesis that abiotic stress controls high‐latitude range limits and caution that projections solely based on mean temperature could underestimate species’ vulnerabilities to climate change.

A latitudinal phylogeographic diversity gradient in birds.

High tropical species diversity is often attributed to evolutionary dynamics over long timescales. It is possible, however, that latitudinal variation in diversification begins when divergence occurs within species. Phylogeographic data capture this initial stage of diversification in which populations become geographically isolated and begin to differentiate genetically. There is limited understanding of the broader implications of intraspecific diversification because comparative analyses have focused on species inhabiting and evolving in restricted regions and environments. Here, we scale comparative phylogeography up to the hemisphere level and examine whether the processes driving latitudinal differences in species diversity are also evident within species. We collected genetic data for 210 New World bird species distributed across a broad latitudinal gradient and estimated a suite of metrics characterizing phylogeographic history. We found that lower latitude species had, on average, greater phylogeographic diversity than higher latitude species and that intraspecific diversity showed evidence of greater persistence in the tropics. Factors associated with species ecologies, life histories, and habitats explained little of the variation in phylogeographic structure across the latitudinal gradient. Our results suggest that the latitudinal gradient in species richness originates, at least partly, from population-level processes within species and are consistent with hypotheses implicating age and environmental stability in the formation of diversity gradients. Comparative phylogeographic analyses scaled up to large geographic regions and hundreds of species can show connections between population-level processes and broad-scale species-richness patterns.

Pattern or process? Evaluating the peninsula effect as a determinant of species richness in coastal dune forests.

The peninsula effect predicts that the number of species should decline from the base of a peninsula to the tip. However, evidence for the peninsula effect is ambiguous, as different analytical methods, study taxa, and variations in local habitat or regional climatic conditions influence conclusions on its presence. We address this uncertainty by using two analytical methods to investigate the peninsula effect in three taxa that occupy different trophic levels: trees, millipedes, and birds. We surveyed 81 tree quadrants, 102 millipede transects, and 152 bird points within 150 km of coastal dune forest that resemble a habitat peninsula along the northeast coast of South Africa. We then used spatial (trend surface analyses) and non-spatial regressions (generalized linear mixed models) to test for the presence of the peninsula effect in each of the three taxa. We also used linear mixed models to test if climate (temperature and precipitation) and/or local habitat conditions (water availability associated with topography and landscape structural variables) could explain gradients in species richness. Non-spatial models suggest that the peninsula effect was present in all three taxa. However, spatial models indicated that only bird species richness declined from the peninsula base to the peninsula tip. Millipede species richness increased near the centre of the peninsula, while tree species richness increased near the tip. Local habitat conditions explained species richness patterns of birds and trees, but not of millipedes, regardless of model type. Our study highlights the idiosyncrasies associated with the peninsula effect—conclusions on the presence of the peninsula effect depend on the analytical methods used and the taxon studied. The peninsula effect might therefore be better suited to describe a species richness pattern where the number of species decline from a broader habitat base to a narrow tip, rather than a process that drives species richness.

Papua New Guinea terrestrial‐vertebrate richness: elevation matters most for all except reptiles

AbstractAimsTo examine species richness patterns in Papua New Guinea's terrestrial vertebrates and test for geographical congruence between the four classes, and between lizard and snake subgroups. To assess the environmental correlates of Papua New Guinean terrestrial‐vertebrate richness, and contrast effects of varying analytical resolution and correction for spatial autocorrelation. We predict congruence in the bird, mammal and to a lesser extent amphibian richness, with weak congruence or incongruence between reptiles and the other taxonomic groups. We further predict these patterns will stem from relative or in the case of reptiles dissimilar, correlative trends with environmental predictors such as elevation and temperature.LocationPapua New Guinea.MethodsHaving created and updated distribution maps for reptiles, we compare them with known ranges of amphibians, birds and mammals and generate species richness grids at quarter‐, half‐ and one‐ degree spatial resolutions. We examine congruence in species richness between vertebrate groups and between reptile subgroups. We employed spreading‐dye models to simulate species richness according to eight environmental predictors and one random model. We accounted for spatial autocorrelation in all analyses.ResultsPapua New Guinean amphibian, bird and mammal species richness are spatially congruent, a trend which strengthens with decreasing spatial resolution. Reptiles and the lizard and snake subgroups reveal remarkably different spatial‐richness trends. Elevational predictors, particularly elevational range at coarse resolutions, provide the strongest correlates of species richness. Terrestrial‐vertebrate richness increases with elevation, whereas reptile richness decreases.Main conclusionsCongruent species richness gradients in Papua New Guinea are observed in most terrestrial vertebrates, except reptiles. Topographic heterogeneity and associated climatic clines promote diversity in most terrestrial vertebrates but appear to strongly constrain reptile diversity. The topographical complexity and climatic stratification of tropical mountains clearly present a wealth of opportunities for diversification in most terrestrial vertebrate groups. As reptiles are strongly constrained by temperature, tropical mountains present more of a diversification barrier for them.