The potential contribution of vegetation ecology to biodiversity research

The contribution of vegetation ecology to the study of biodiversity depends on better communication between the different research paradigms in ecology. Recent developments in vegetation theory and associated statistical modelling techniques are reviewed for their relevance to biodiversity. Species composition and collective properties such as species richness vary as a continuum in a multi-dimensional environmental space; a concept which needs to be incorporated into biodiversity studies. Different kinds of environmental gradients can be recognised and species responses to them vary. Species response curves of eucalypts to an environmental gradient of mean annual temperature have been shown to exhibit a particular pattern of skewed response curves. Generalised linear modelling (GLM) and generalised additive modelling (GAM) techniques are important tools for biodiversity studies. They have successfully distinguished the contribution of environmental (climatic) and spatial (history and species dispersal ability) variables in determining forest tree composition in New Zealand. Species richness studies are examined at global, regional and local scales. At all scales, direct and resource environmental gradients need to be incorporated into the analysis rather than indirect gradients e.g. latitude which have no direct physiological influence on biota. Evidence indicates that species richness at the regional scale is sensitive to environment, confounding current studies on local/regional species richness relationships. Plant community experiments require designs based on environmental gradients rather than dependent biological properties such as productivity or species richness to avoid confounding the biotic components. Neglect of climatic and other environmental gradients and the concentration on the collective properties of species assemblages has limited recent biodiversity studies. Conservation evaluation could benefit from greater use of the continuum concepts and statistical modelling techniques of vegetation ecology. The future development of ecology will depend on testing the different assumptions of competing research paradigms and a more inclusive synthesis of ecological theory.     

The hierarchical continuum concept

Abstract. Two general models have been proposed to explain the structure of the plant community: the community-unit model of Clements and the continuum model of Whittaker and Curtis, the latter based on Gleason's individualistic distribution of species. It is generally assumed that most ecologists now accept the continuum model. Empirical evidence suggests, however, that the continuum in its current form does not fully describe the observed patterns of vegetation along environmental gradients. In this paper, we introduce the hierarchical continuum as a general concept to represent dynamic community structure along regional spatial gradients. The hierarchical continuum is derived from a combination of the individualistic distribution of species, hierarchical assemblage structure, and the core-satellite species hypothesis. The hierarchical continuum concept predicts that the distribution of species across sites in a region will be polymodal, which reflects hierarchical structure, and that the distribution and abundance of species within and between sites will be spatially and temporally dynamic. Regional distribution of plant species in North American tallgrass prairie, southeastern flood-plain hardwood forests, northern upland hardwood forests, and boreal forests were either bimodal or polymodal as predicted by the hierarchical continuum concept. Species in tallgrass prairie were spatially and temporally dynamic with an average turnover of 8–9 species per 50 m² yr¹. In addition, the hierarchical continuum concept predicts the potential for fractal (self-similar) patterns of community structure, and provides a framework for testable hypotheses concerning species distributions along environmental gradients.     

敦煌西湖荒漠-湿地生态系统优势物种生态位研究

水文情势改变会引起土壤盐分变化,直接影响到荒漠-湿地生态系统植被的分布与演替。基于对57个样地、171个样方植物物种分布影响较大的土壤pH值和土壤电导率两个环境因子,将其划分为6个梯度等级,测度分析了敦煌西湖植被群落中15个主要优势种的生态位特征,了解不同物种利用资源和占据生态空间的能力,对维持和科学保育植物群落的多样性具有重要意义。结果表明：（1）在土壤pH值和电导率梯度两个资源维上,多枝柽柳和芦苇的重要值和生态位宽度均较大,说明这两个物种适应能力强能够较好地利用环境资源,分布范围大且均匀。它们作为敦煌西湖植被群落中的广域种,具有重要的生态地位和作用。其次生态位较宽的疏叶骆驼刺、胡杨和苏枸杞对环境因子也具有较强的适应能力。（2）两个土壤因子梯度下植物种群生态位宽度相似,但也存在差异。如泡泡刺、蒙古沙枣在土壤pH值梯度资源维上的生态位宽度值远大于在土壤电导率梯度资源维上,但尖叶盐爪爪和甘蒙柽柳在土壤电导率梯度资源维上生态位宽度较大,表现出较强的耐盐能力,从而说明这些物种对不同土壤因子的利用能力和适应性不完全相同。（3）在两个资源维上优势物种间的生态位重叠值小于0.5的种对均为61对,占总种对的58.10%,因此生态位重叠值整体保持在较低水平,说明物种在土壤pH值和土壤电导率两个环境梯度上生态位分化明显。（4）敦煌西湖优势物种间总体表现为不显著的负关联,表明物种之间处于竞争关系,但竞争强度不大且群落结构稳定性较弱。     

Compartmentalized organization of ecological niche occupation in insular invertebrate communities

Understanding the mechanisms of species distribution within ecosystems is a fundamental question of ecological research. The current worldwide changes and loss of habitats associated with a decline in species richness render this topic a key element for developing mitigation strategies. Ecological niche theory is a widely accepted concept to describe species distribution along environmental gradients where each taxon occupies its own distinct set of environmental parameters, that is, its niche. Niche occupation has been described in empirical studies for different closely related taxa, like ant, ungulate, or skink species, just to name a few. However, how species assemblages of whole ecosystems across multiple taxa are structured and organized has not been investigated thoroughly, although considering all taxa of a community would be essential when analyzing realized niches. Here, we investigated the organization of niche occupation and species distribution for the whole ground‐associated invertebrate community of small tropical insular ecosystems. By correlating environmental conditions with species occurrences using partial canonical correspondence analysis (pCCA), we demonstrated that the ground‐associated invertebrate community does not spread evenly across the overall niche space, but instead is compartmentalized in four distinct clusters: crustacean and gastropod taxa occurred in one cluster, attributable to the beach habitat, whereas hexapods and spider taxa occurred in three distinct inland clusters, attributable to distinct inland habitats, that is, grassland, open forest, and dense forest. Within the clusters, co‐occurrence pattern analysis suggested only a few negative interactions between the different taxa. By studying ground‐associated insular invertebrate communities, we have shown that species distribution and niche occupation can be, similar to food webs, organized in a compartmentalized way. The compartmentalization of the niche space might thereby be a mechanism to increase ecosystem resilience, as disturbances cascade more slowly throughout the ecosystem.     

Environmental heterogeneity affects the location of modelled communities along the niche–neutrality continuum

The continuum hypothesis has been proposed as a means to reconcile the contradiction between the niche and neutral theories. While past research has shown that species richness affects the location of communities along the niche–neutrality continuum, there may be extrinsic forces at play as well. We used a spatially explicit continuum model to quantify the effects of environmental heterogeneity, comprising abundance distribution and spatial configuration of resources, on the degree of community neutrality. We found that both components of heterogeneity affect the degree of community neutrality and that species'' dispersal characteristics affect the neutrality–heterogeneity relationship. Narrower resource abundance distributions decrease neutrality, while spatial configuration, which is manifested by spatial aggregation of resources, decreases neutrality at higher aggregation levels. In general, the degree of community neutrality was affected by complex interactions among spatial configuration of resources, their abundance distributions and the dispersal characteristics of species in the community. Our results highlight the important yet overlooked role of the environment in dictating the location of communities along the hypothesized niche–neutrality continuum.     

Towards a trait-based quantification of species niche

Aims Although the niche concept is of prime importance in ecology, the quantification of plant species' niches remains difficult. Here we propose that plant functional traits, as determinants of species performance, may be useful tools for quantifying species niche parameters over environmental gradients.Important findings Under this framework, the mean trait values of a species determine its niche position along gradients, and intraspecific trait variability determines its niche breadth. This trait-based approach can provide an operational assessment of niche for a potentially large number of species, making it possible to understand and predict species niche shifts under environmental changes. We further advocate a promising method that recently appeared in the literature, which partitions trait diversity into among- and within-community components as a way to quantify the species niche in units of traits instead of environmental parameters. This approach allows the switch of the focus from ecological niches to trait niches, facilitating the examination of species coexistence along undefined environmental gradients. Altogether, the trait-based approach provides a promising toolkit for quantifying the species ecological niche and for understanding the evolution of species niche and traits.     

Among‐species overlap in rodent body size distributions predicts species richness along a temperature gradient

Temperature is widely regarded as a major driver of species richness, but the mechanisms are debated. Niche theory suggests temperature may affect richness by filtering traits and species in colder habitats while promoting specialization in warmer ones. However, tests of this theory are rare because niche dimensions are challenging to quantify along broad thermal gradients. Here, we use individual‐level trait data from a long‐term monitoring network spanning a large geographic extent to test niche‐based theory of community assembly in small mammals. We examined variation in body size among 23 communities of North American rodents sampled across the National Ecological Observatory Network (NEON), ranging from northern hardwood forests to subtropical deserts. We quantified body size similarity among species using a metric of overlap that accounts for individual variation, and fit a structural equation model to disentangle the relationships between temperature, productivity, body size overlap, and species richness. We document a latitudinal gradient of declining similarity in body size among species towards the tropics and overall increase in the dimensions of community‐wide trait space in warmer habitats. Neither environmental temperature nor net primary productivity directly affect rodent species richness. Instead, temperature determines the community‐wide niche space that species can occupy, which in turn alters richness. We suggest a latitudinal gradient of trait space expansion towards the tropics may be widespread and underlie gradients in species diversity.     

Reconciling niche and neutrality: the continuum hypothesis

In this study, we ask if instead of being fundamentally opposed, niche and neutral theories could simply be located at the extremes of a continuum. First, we present a model of recruitment probabilities that combines both niche and neutral processes. From this model, we predict and test whether the relative importance of niche vs. neutral processes in controlling community dynamics will vary depending on community species richness, niche overlap and dispersal capabilities of species (both local and long distance). Results demonstrate that niche and neutrality form ends of a continuum from competitive to stochastic exclusion. In the absence of immigration, competitive exclusion tends to create a regular spacing of niches. However, immigration prevents the establishment of a limiting similarity. The equilibrium community consists of a set of complementary and redundant species, with their abundance determined, respectively, by the distribution of environmental conditions and the amount of immigration.     

干旱荒漠区植物生态位对水盐的响应

全球气候变化对生态系统功能的影响决定着种群地位和群落演替,由于干旱区的特殊性成为研究生物响应环境变化的热点地区。为探索艾比湖流域荒漠区植物生态位对土壤水盐梯度的响应,在研究水、盐含量和植物生态位特征的基础上,应用变系数模型,分析了土壤水盐梯度下的群落组成和生态位响应趋势。结果显示:(1)随土壤水盐梯度的降低,艾比湖流域群落的物种组成呈倒"V"型分布,说明土壤水盐交互作用影响着植被分布和群落类型。(2)高水盐环境下,胡杨(Populus euphratica)、琵琶柴(Reaumuria soongorica)和梭梭(Haloxylon ammodendron)等生态位宽度值(B_i)较大,柽柳(Tamarix ramosissima)、骆驼刺(Alhagi sparsifolia)和甘草(Glycyrrhiza uralensis)等B_i较小;中水盐环境下,白刺(Nitraria schoberi)和琵琶柴的B_i较大,铃铛刺(Halimodendron halodendron)、甘草和盐节木(Halocnemum strobilaceum)的B_i较小;低水盐环境下,梭梭、花花柴(Karelinia caspica)和琵琶柴等B_i较大,盐节木的B_i较小;说明土壤水盐环境和物种生态学特性是影响B_i的重要因素。(3)物种水平上,B_i较大的种群与其他物种的生态位重叠(O_(ij))一般较大,但B_i较小的物种间的O_(ij)不一定小(例如:高水盐土壤环境下柽柳和骆驼刺的O_(ij)为1);群落水平上,B_i与O_(ij)具有一定的正相关性;说明由于资源纬度的相似性增大了低B_i物种间的O_(ij),并且B_i和O_(ij)由物种向群落尺度转换时,两者之间的关系存在冗余。(4)土壤水盐梯度与群落生态幅呈现一种非线性相关特征,高水盐格局对群落B_i有一定的促进效应,而低水盐模式对B_i具有一定的限制作用,表明土壤水盐的协同效应影响着物种在群落的地位,并在一定程度上决定了群落向正负两极演替的方向。     

Processes of community assembly in an environmentally heterogeneous,high biodiversity region

Despite decades of study, the relative importance of niche‐based versus neutral processes in community assembly remains largely ambiguous. Recent work suggests niche‐based processes are more easily detectable at coarser spatial scales, while neutrality dominates at finer scales. Analyses of functional traits with multi‐year multi‐site biodiversity inventories may provide deeper insights into assembly processes and the effects of spatial scale. We examined associations between community composition, species functional traits, and environmental conditions for plant communities in the Kouga‐Baviaanskloof region, an area within South Africa's Cape Floristic Region (CFR) containing high α and β diversity. This region contains strong climatic gradients and topographic heterogeneity, and is comprised of distinct vegetation classes with varying fire histories, making it an ideal location to assess the role of niche‐based environmental filtering on community composition by examining how traits vary with environment. We combined functional trait measurements for over 300 species with observations from vegetation surveys carried out in 1991/1992 and repeated in 2011/2012. We applied redundancy analysis, quantile regression, and null model tests to examine trends in species turnover and functional traits along environmental gradients in space and through time. Functional trait values were weakly associated with most spatial environmental gradients and only showed trends with respect to vegetation class and time since fire. However, survey plots showed greater compositional and functional stability through time than expected based on null models. Taken together, we found clear evidence for functional distinctions between vegetation classes, suggesting strong environmental filtering at this scale, most likely driven by fire dynamics. In contrast, there was little evidence of filtering effects along environmental gradients within vegetation classes, suggesting that assembly processes are largely neutral at this scale, likely the result of very high functional redundancy among species in the regional species pool.