Modelling the niche space of desert annuals needs to include positive interactions

The niche is a necessary consideration when estimating habitable area and geographic range of a species. Modellers often examine the fundamental niche and the environmental requirements for plant species, ignoring interactions among species. In deserts, positive plant interactions are important drivers of biodiversity and structure communities through many mechanistic pathways including modifying environmental conditions. Thus, we tested the hypothesis that desert shrubs increase the geographical extent of some annual species because, through modifying the microclimate, they match the niche requirements of beneficiary species. We used the database of the Global Biodiversity Information Facility to construct MaxEnt species distribution models (SDM) with and without reported benefactor species within the Mojave Desert in California. We chose 20 annual species to be modeled including 10 species that had been previously reported in the literature as being facilitated (beneficiary) and 10 that had no record of being facilitated (unreported). Beneficiary annuals co‐occurred significantly more with benefactor shrubs than the unreported annual species. The inclusion of shrubs into SDMs significantly improved model predictability and geographic range for all the beneficiary annual species, but not for the unreported annual species. Thus, positive interactions are species specific and it is possible to determine annual species dependency on benefactor shrubs at the regional scale. The co‐occurrence of benefactor shrubs and annual species can be used as a proxy for facilitation and recent developments in SDM techniques encourage the inclusion of biotic interactions. Species distribution models should include estimates of facilitation because biotic interactions determine the niche of species and can have implications with a changing climate.     

Ecological causes for the evolution of sexual dimorphism: a review of the evidence

Can sexual dimorphism evolve because of ecological differences between the sexes? Although several examples of this phenomenon are well known from studies on birds, the idea has often been dismissed as lacking general applicability. This dismissal does not stem from contradictory data so much as from the difficulties inherent in testing the hypothesis, and its apparent lack of parsimony, in comparison to the alternative explanation of sexual selection. The only unequivocal evidence for the evolution of sexual dimorphism through intersexual niche partitioning would be disproportionate dimorphism in trophic structures (e.g., mouthparts). This criterion offers a minimum estimate of the importance of ecological causes for dimorphism, because it may fail to identify most cases. A review of published literature reveals examples of sexually dimorphic trophic structures in most animal phyla. Many of these examples seem to be attributable to sexual selection, but others reflect adaptations for niche divergence between the sexes. For example, dwarf non-feeding males without functional mouthparts have evolved independently in many taxa. In other cases, males and females differ in trophic structures apparently because of differences in diets. Such divergence may often reflect specific nutritional requirements for reproduction in females, or extreme (sexually selected?) differences between males and females in habitats or body sizes. Ecological competition between the sexes may be responsible for intersexual niche divergence in some cases, but the independent evolution of foraging specializations by each sex may be of more general importance. If ecological causation for dimorphism can be demonstrated in so many cases, despite the inadequacies of the available criteria, the degree of sexual size dimorphism in many other animal species may well also have been influenced by ecological factors. Hence, it may be premature to dismiss this hypothesis, despite the difficulty of testing it.     

植物群落构建机制研究进展

群落构建研究对于解释物种共存和物种多样性的维持是至关重要的,因此一直是生态学研究的中心论题。尽管近年来关于生态位和中性理论的验证研究已经取得了显著的成果,但对于局域群落构建机制的认识仍存在很大争议。随着统计和理论上的进步使得用功能性状和群落谱系结构解释群落构建机制变为可能,主要是通过验证共存物种的性状和谱系距离分布模式来实现。然而,谱系和功能性状不能相互替代,多种生物和非生物因子同时控制着群落构建,基于中性理论的扩散限制、基于生态位的环境过滤和竞争排斥等多个过程可能同时影响着群落的构建。所以,综合考虑多种方法和影响因素探讨植物群落的构建机制,对于预测和解释植被对干扰的响应,理解生物多样性维持机制有重要意义。试图在简要回顾群落构建理论及研究方法发展的基础上,梳理其最新研究进展,并探讨整合功能性状及群落谱系结构的研究方法,解释群落构建和物种多样性维持机制的可能途径。在结合功能性状和谱系结构研究群落构建时,除了考虑空间尺度、环境因子、植被类型外,还应该关注时间尺度、选择性状的种类和数量、性状的种内变异、以及人为干扰等因素对群落构建的影响。     

Phylogenetic signals in the climatic niches of the world's amphibians

The question of whether closely related species share similar ecological requirements has attracted increasing attention, because of its importance for understanding global diversity gradients and the impacts of climate change on species distributions. In fact, the assumption that related species are also ecologically similar has often been made, although the prevalence of such a phylogenetic signal in ecological niches remains heavily debated. Here, we provide a global analysis of phylogenetic niche relatedness for the world's amphibians. In particular, we assess which proportion of the variance in the realised climatic niches is explained on higher taxonomic levels, and whether the climatic niches of species within a given taxonomic group are more similar than between taxonomic groups. We found evidence for phylogenetic signals in realised climatic niches although the strength of the signal varied among amphibian orders and across biogeographical regions. To our knowledge, this is the first study providing a comprehensive analysis of the phylogenetic signal in species climatic niches for an entire clade across the world. Even though our results do not provide a strong test of the niche conservatism hypothesis, they question the alternative hypothesis that niches evolve independently of phylogenetic influences.     

Biodiversity and the productivity and stability of ecosystems

Attempts to unveil the relationships between the taxonomic diversity, productivity and stability of ecosystems continue to generate inconclusive, contradictory and controversial conclusions. New insights from recent studies support the hypothesis that species diversity enhances productivity and stability in some ecosystems, but not in others. Appreciation is growing for the ways that particular ecosystem features, such as environmental variability and nutrient stress, can influence biotic interactions. Alternatives to the diversity-stability hypothesis have been proposed, and experimental approaches are starting to evolve to test these hypotheses and to elucidate the mechanisms underlying the functional role of species diversity.     

Niche similarities among white‐eared opossums (Mammalia,Didelphidae): Is ecological niche modelling relevant to setting species limits?

A complex of white‐eared opossums (Didelphis spp.) is distributed across three distinct areas of South America, but recent taxonomic treatments have disagreed regarding species limits in the group. We used ecological niche modelling to test whether ecological niches have been conserved or have diverged among the three forms in this group. Differences in combinations of niche and range were clear; however, when hypotheses of accessible areas for each species were considered, coarse‐grained niche dimensions (i.e. climatic dimensions) were seen not to differ across the complex. We discuss implications of these results for taxonomic recognition of species based on geographic and ecological characteristics and the implications of using ENM approaches to setting species limits. We suggest that ENM should be used to explore speciation mechanisms, rather than being applied to questions of setting species limits.     

Spatio-ecological niche segregation of two sympatric species of<Emphasis Type="Italic">Clidemia (Melastomataceae)</Emphasis> in Western Amazonian non-flooded rainforests

The frequent occurrence of sympatric series of closely related plant species in tropical rainforests has evoked claims for and against the application of the competitive exclusion principle in these ecosystems. Narrow niche limits defined by biotic as well as abiotic specialization have been reported for sympatric species of the same genus or family. In Amazonian lowland rainforests this question deserves renewed attention because: (1) the existence of edaphically defined community types has recently been well established, and (2) spatio-ecological niche segregation of congeneric species may help explain not only the maintenance of the high Amazonian alpha-diversity, but also its origin through sympatric ecological speciation. In this study, the morphology, ecology, and distribution patterns of two species,Clidemia epiphytica andC. longifolia (Melastomataceae), from western Amazonia, were analyzed. The aims were to find out whether they really are two distinct taxonomic species and if so, whether they also can be considered biological species; if the species are sympatric; and if they are ecologically specialized. The results showed that the morphological variation of the species seems continuous, but that they exhibit opposite morphological responses to variation in soil cation concentration, which suggests that they also are separate biological species. Furthermore, the species occur sympatrically but in different habitats. It is suggested that a part of the enigma of sympatric congeners in rainforests may be explainable by spatial segregation stemming from ecological specialization in relation to subtle environmental variation. It is hypothesized that the studied species are a good candidate case of sympatric speciation driven by ecological specialization.     

Niche properties of Central European spiders: shading, moisture and the evolution of the habitat niche

Aim  Niche theory emphasizes the importance of environmental conditions for the distribution and abundance of species. Using a macroecological approach our study aimed at identifying the important environmental gradients for spiders. We generated numerical values of niche position and niche width. We also investigated relationships between these niche properties as well as the degree of phylogenetic conservatism in order to draw conclusions about the evolution of the habitat niche.
Location  Central Europe: lowlands of Austria, Belgium, Germany, Luxembourg, the Netherlands and Switzerland.
Methods  We analysed 244 published spider communities from 70 habitat types by correspondence analysis. The resulting community scores were used to test for correlations with habitat characteristics. Species scores were used to derive niche position (mean scores) and niche width (standard deviation of scores). To test for niche conservatism we estimated variance components across the taxonomic hierarchy.
Results  The first two axes of the correspondence analysis were correlated with shading and moisture, respectively. Niche width had a hump-shaped relationship to both environmental gradients. β-diversity was strikingly higher in open habitats than in forests. Habitat niche conservatism was lower than phylogenetic conservatism in body size.
Main conclusions  Environmental factors are important drivers for the β-diversity of spiders, especially across open habitats. This underlines the importance of preserving the whole range of moisture conditions in open habitats. Narrow niches of species occurring at the ends of both environmental gradients indicate that adaptations to extreme habitats lead to constraints in ecological flexibility. Nevertheless, the habitat niche of species seems to evolve much faster than morphological or physiological traits.     

Evolutionarily stable range limits set by interspecific competition

A combination of abiotic and biotic factors probably restricts the range of many species. Recent evolutionary models and tests of those models have asked how a gradual change in environmental conditions can set the range limit, with a prominent idea being that gene flow disrupts local adaptation. We investigate how biotic factors, explicitly competition for limited resources, result in evolutionarily stable range limits even in the absence of the disruptive effect of gene flow. We model two competing species occupying different segments of the resource spectrum. If one segment of the resource spectrum declines across space, a species that specializes on that segment can be driven to extinction, even though in the absence of competition it would evolve to exploit other abundant resources and so be saved. The result is that a species range limit is set in both evolutionary and ecological time, as the resources associated with its niche decline. Factors promoting this outcome include: (i) inherent gaps in the resource distribution, (ii) relatively high fitness of the species when in its own niche, and low fitness in the alternative niche, even when resource abundances are similar in each niche, (iii) strong interspecific competition, and (iv) asymmetric interspecific competition. We suggest that these features are likely to be common in multispecies communities, thereby setting evolutionarily stable range limits.     

Niche differentiation in rainforest ant communities across three continents

A central prediction of niche theory is that biotic communities are structured by niche differentiation arising from competition. To date, there have been numerous studies of niche differentiation in local ant communities, but little attention has been given to the macroecology of niche differentiation, including the extent to which particular biomes show distinctive patterns of niche structure across their global ranges. We investigated patterns of niche differentiation and competition in ant communities in tropical rainforests, using different baits reflecting the natural food spectrum. We examined the extent of temporal and dietary niche differentiation and spatial segregation of ant communities at five rainforest sites in the neotropics, paleotropics, and tropical Australia. Despite high niche overlap, we found significant dietary and temporal niche differentiation in every site. However, there was no spatial segregation among foraging ants at the community level, despite strong competition for preferred food resources. Although sucrose, melezitose, and dead insects attracted most ants, some species preferentially foraged on seeds, living insects, or bird feces. Moreover, most sites harbored more diurnal than nocturnal species. Overall niche differentiation was strongest in the least diverse site, possibly due to its lower number of rare species. Both temporal and dietary differentiation thus had strong effects on the ant assemblages, but their relative importance varied markedly among sites. Our analyses show that patterns of niche differentiation in ant communities are highly idiosyncratic even within a biome, such that a mechanistic understanding of the drivers of niche structure in ant communities remains elusive.     

Anatomies and lifestyles,morphometrics and niche metrics: Tools for studying primate evolution

In recent years, comparative observations of primate anatomies have been extended using measurements and computer analyses: morphometrics. A question then follows. Can we extend field observations of animal lifestyles using measurement and computer analysis, using, in other words, a new «niche metrics», for comparison with the morphometrics of animal anatomies? This has been attempted (Oxnard, Crompton & Lieberman, 1990) by «measuring» 17 lifestyle variables (on locomotion, environment and diet) for 28 prosimian primate species. These data have been analysed by multivariate statistical methods entirely similar to those used in morphometrics. The results define groups of prosimian species that seem not to be related directly with taxonomy and evolution, but with functional adaptation to lifestyle. Comparisons between the morphometric and niche metric results reveal a degree of similarity of species grouping in each that is apparently related to adaptation and remarkably detailed. But further study of both the morphometric and niche metric results suggests that questions pertaining to evolution may also be asked. Thus, both morphometric investigations seeking clusterings of anatomical variables and niche metric studies seeking clustering of higher taxonomic groups of species produce results that seem to be of developmental and phylogenetic rather than directly adaptive import. Through the combination of these two approaches, it may eventually be possible to partition both evolutionary and adaptive components, even though, of course, there must always be overlap between them.     

Taxonomic scale-dependence of habitat niche partitioning and biotic neighbourhood on survival of tropical tree seedlings

In order to differentiate between mechanisms of species coexistence, we examined the relative importance of local biotic neighbourhood, abiotic habitat factors and species differences as factors influencing the survival of 2330 spatially mapped tropical tree seedlings of 15 species of Myristicaceae in two separate analyses in which individuals were identified first to species and then to genus. Using likelihood methods, we selected the most parsimonious candidate models as predictors of 3 year seedling survival in both sets of analyses. We found evidence for differential effects of abiotic niche and neighbourhood processes on individual survival between analyses at the genus and species levels. Niche partitioning (defined as an interaction of taxonomic identity and abiotic neighbourhood) was significant in analyses at the genus level, but did not differentiate among species in models of individual seedling survival. By contrast, conspecific and congeneric seedling and adult density were retained in the minimum adequate models of seedling survival at species and genus levels, respectively. We conclude that abiotic niche effects express differences in seedling survival among genera but not among species, and that, within genera, community and/or local variation in adult and seedling abundance drives variation in seedling survival. These data suggest that different mechanisms of coexistence among tropical tree taxa may function at different taxonomic or phylogenetic scales. This perspective helps to reconcile perceived differences of importance in the various non-mutually exclusive mechanisms of species coexistence in hyper-diverse tropical forests.     

What governs the functional diversity patterns of fishes in the headwater streams of the humid forest enclaves: environmental conditions,taxonomic diversity or biotic interactions?

The relationship between functional and taxonomic diversity is a major issue in ecology. Biodiversity in aquatic environments is strongly influenced by environmental gradients that act as dispersion and niche barriers. Environmental conditions act as filters to select functional traits, but biotic interactions also play a role in assemblage structure. In headwater streams, the relationship between functional and taxonomic diversity remains unclear. In this study we investigated how environmental conditions, taxonomic diversity and biotic interactions influence the spatial distribution of traits and functional diversity in stream fish species. Standardized sampling of fish species was carried out in 50 m sections of 16 streams located in rainforest enclaves in a semiarid region of Brazil (Caatinga biome). The functional diversity indices displayed different responses to the predictor variables used. Functional richness was mainly influenced by environmental conditions, while functional evenness was mostly determined by taxonomic diversity. On the other hand, functional dispersion was explained by a combination of environmental conditions and taxonomic diversity. The spatial distribution of fish species with the same functional traits was random, indicating that biotic interactions are not a strong predictor in these ecosystems. Channel width, pH and substrate were the most important variables in the spatial distribution of the functional traits of the fish species. Our results suggest that the functional structure of fish assemblages in headwater streams depends mainly on environmental conditions and taxonomic diversity.     

Dominant species and dispersal limitation regulate tree species distributions in a 20‐ha plot in Xishuangbanna,southwest China

Habitat heterogeneity and dispersal limitation are widely considered to be the two major mechanisms in determining tree species distributions. However, few studies have quantified the relative importance of these two mechanisms at different life stages of trees. Moreover, rigorous quantification of the effects of dominant tree species in determining species distributions has seldom been explored. In the present study, we tested the hypothesis that the distribution of tree species is regulated by different mechanisms at different life history stages. In particular, we hypothesised that dispersal limitation regulates the distribution of trees at early life stages and that environmental factors control the distribution of trees as they grow, because of niche differentiation resulting from environmental filtering. To test this, trees in 400‐m² quadrats in a 20‐ha plot in Xishuangbanna, southwest China were grouped into four classes on the basis of the diameter at breast height (DBH) that roughly represent different stages in the life history of trees. A neighbourhood index was computed to represent a neutral spatial autocorrelation effect. We used both biotic (dominant species) and abiotic (topography and soil) predictor variables to model the distribution of each target species while controlling for spatial autocorrelation within each of the DBH classes. To determine which factors played the largest role in regulating target species distribution, the simulated annealing method was used in model selection based on Akaike information criterion (AIC) values. The results showed that the relative importance of neutral and niche processes in regulating species distribution varied across life stages. The neutral neighbourhood index played the most important role in determining the distributions of small trees (1 cm ≤ DBH ≤ 10 cm), and dominant species, as biotic environmental predictor variables, were the next most important regulators for trees of this size. Environmental predictor variables played the most important role in determining the distributions of large trees (10 cm ≤ DBH). This finding builds on previous research into the relative importance of neutral and niche processes in determining species distributions regardless of life stages or DBH classes.     

ISLAND BIOGEOGRAPHY AND PALAEOBIOLOGY: IN SEARCH FOR EVOLUTIONARY EQUILIBRIA

1. The concept of evolutionary equilibrium has been derived from the theory of island biogeography via an ecological rationale for increase in species extinction rate and decrease in speciation rate with increasing diversity of the system.
2. This concept is theoretically plausible at the species level and at a regional scale but, in spite of several empirical tests in the fossil record, it has thus far remained unsupported by empirical evidence. In order to test it conclusively, one has to analyze not only the pattern of species number through time but also its relationship to speciation and species extinction rates; independent evidence for perturbations must also be available.
3. The concept of evolutionary equilibrium at the global scale must be extrapolated over higher levels of taxonomic hierarchy, for reliable species-level data are unavailable at this scale. A theoretical justification for this concept cannot, then, be derived from the theory of island biogeography.
4. The rates of family extinction and origination in the Phanerozoic show no evidence for diversity-dependence, which undermines most quantitative models of biotic diversification based on the concept of global evolutionary equilibrium. Rigorous testing of these models cannot be done at the present state of knowledge because of the uncertainty about the empirical pattern (sampling and taxonomic biases, absolute time scale).     

Is There Any Evidence for Rapid,Genetically-Based,Climatic Niche Expansion in the Invasive Common Ragweed?

Climatic niche shifts have been documented in a number of invasive species by comparing the native and adventive climatic ranges in which they occur. However, these shifts likely represent changes in the realized climatic niches of invasive species, and may not necessarily be driven by genetic changes in climatic affinities. Until now the role of rapid niche evolution in the spread of invasive species remains a challenging issue with conflicting results. Here, we document a likely genetically-based climatic niche expansion of an annual plant invader, the common ragweed (Ambrosia artemisiifolia L.), a highly allergenic invasive species causing substantial public health issues. To do so, we looked for recent evolutionary change at the upward migration front of its adventive range in the French Alps. Based on species climatic niche models estimated at both global and regional scales we stratified our sampling design to adequately capture the species niche, and localized populations suspected of niche expansion. Using a combination of species niche modeling, landscape genetics models and common garden measurements, we then related the species genetic structure and its phenotypic architecture across the climatic niche. Our results strongly suggest that the common ragweed is rapidly adapting to local climatic conditions at its invasion front and that it currently expands its niche toward colder and formerly unsuitable climates in the French Alps (i.e. in sites where niche models would not predict its occurrence). Such results, showing that species climatic niches can evolve on very short time scales, have important implications for predictive models of biological invasions that do not account for evolutionary processes.     

Does taxonomic homogenization imply functional homogenization in temperate forest herb layer communities?

Biotic homogenization, the decrease in beta diversity among formerly distinct species assemblages, has been recognized as an important form of biotic impoverishment for more than a decade. Although researchers have stressed the importance of the functional dimension to understand its potential ecological consequences, biotic homogenization has mostly been studied at a taxonomic level. Here, we explore the relationship between taxonomic and functional homogenization using data on temperate forest herb layer communities in NW Germany, for which taxonomic homogenization has recently been demonstrated. We quantified beta diversity by partitioning Rao’s quadratic entropy. We found a general positive relationship between changes in taxonomic and functional beta diversity. This relationship was stronger if multiple functional traits were taken into account. Averaged across sites, however, taxonomic homogenization was not consistently accompanied by functional homogenization. Depending on the traits considered, taxonomic homogenization occurred also together with functional differentiation or no change in functional beta diversity. The species shifts responsible for changes in beta diversity differed substantially between taxonomic and functional beta diversity measures and also among functional beta diversity measures based on different traits. We discuss likely environmental drivers for species shifts. Our study demonstrates that functional homogenization must be explicitly studied as an independent phenomenon that cannot be inferred from taxonomic homogenization.     

The interplay of stress and mowing disturbance for the intensity and importance of plant interactions in dry calcareous grasslands

Background and Aims There is still debate regarding the direction and strength of plant interactions under intermediate to high levels of stress. Furthermore, little is known on how disturbance may interact with physical stress in unproductive environments, although recent theory and models have shown that this interplay may induce a collapse of plant interactions and diversity. The few studies assessing such questions have considered the intensity of biotic interactions but not their importance, although this latter concept has been shown to be very useful for understanding the role of interactions in plant communities. The objective of this study was to assess the interplay between stress and disturbance for plant interactions in dry calcareous grasslands. Methods A field experiment was set up in the Dordogne, southern France, where the importance and intensity of biotic interactions undergone by four species were measured along a water stress gradient, and with and without mowing disturbance. Key Results The importance and intensity of interactions varied in a very similar way along treatments. Under undisturbed conditions, plant interactions switched from competition to neutral with increasing water stress for three of the four species, whereas the fourth species was not subject to any significant biotic interaction along the gradient. Responses to disturbance were more species-specific; for two species, competition disappeared with mowing in the wettest conditions, whereas for the two other species, competition switched to facilitation with mowing. Finally, there were no significant interactions for any species in the disturbed and driest conditions. Conclusions At very high levels of stress, plant performances become too weak to allow either competition or facilitation and disturbance may accelerate the collapse of interactions in dry conditions. The results suggest that the importance and direction of interactions are more likely to be positively related in stressful environments.     

The niche, biogeography and species interactions

In this paper, I review the relevance of the niche to biogeography, and what biogeography may tell us about the niche. The niche is defined as the combination of abiotic and biotic conditions where a species can persist. I argue that most biogeographic patterns are created by niche differences over space, and that even ‘geographic barriers’ must have an ecological basis. However, we know little about specific ecological factors underlying most biogeographic patterns. Some evidence supports the importance of abiotic factors, whereas few examples exist of large-scale patterns created by biotic interactions. I also show how incorporating biogeography may offer new perspectives on resource-related niches and species interactions. Several examples demonstrate that even after a major evolutionary radiation within a region, the region can still be invaded by ecologically similar species from another clade, countering the long-standing idea that communities and regions are generally ‘saturated’ with species. I also describe the somewhat paradoxical situation where competition seems to limit trait evolution in a group, but does not prevent co-occurrence of species with similar values for that trait (called here the ‘competition–divergence–co-occurrence conundrum’). In general, the interface of biogeography and ecology could be a major area for research in both fields.     

Local‐scale biotic interactions embedded in macroscale climate drivers suggest Eltonian noise hypothesis distribution patterns for an invasive grass

A hierarchical view of niche relations reconciles the scale‐dependent effects of abiotic and biotic processes on species distribution patterns and underlies most current approaches to distribution modeling. A key prediction of this framework is that the effects of biotic interactions will be averaged out at macroscales – an idea termed the Eltonian noise hypothesis (ENH). We test this prediction by quantifying regional variation in local abiotic and biotic niche relations and assess the role of macroclimate in structuring biotic interactions, using a non‐native invasive grass, Microstegium vimineum, in its introduced range. Consistent with hierarchical niche relations and the ENH, macroclimate structures local biotic interactions, while local abiotic relations are regionally conserved. Biotic interactions suppress M. vimineum in drier climates but have little effect in wetter climates. A similar approach could be used to identify the macroclimatic conditions under which biotic interactions affect the accuracy of local predictions of species distributions.