环境和扩散对草地群落构建的影响

植物群落构建机制是生态学研究的热点之一.长久以来这个难题并没有得到很好的解释,且争议较多.生态位理论或中性理论,或是二者的共同作用,这样的结论在不同的研究中都有印证.以黄土高原子午岭地区的草地群落为例,对3种不同的草地群落(5a的弃耕地、阴坡和阳坡的草地)进行了野外群落学调查,采用Mantel test和主轴邻距法(PCNM)分析方法,研究了空间地理距离和环境资源差异对于草本植物群落分布的影响,结果表明:地理距离和环境差异共同解释了群落组成相似性的79.3％,剔除环境因子的影响,地理距离解释了群落组成相似性的33.8％;而剔除地理距离的影响,环境因子解释了群落组成相似性的14.2％.无论是生态位理论还是中性理论,其在黄土高原草本群落构建过程中都有作用,但中性理论扮演了更为重要的角色.     

Climatic history and dispersal ability explain the relative importance of turnover and nestedness components of beta diversity

Aim We tested whether the geographic variation in the proportion of beta diversity attributed to nestedness or turnover components was explained by the effect of past glaciation events. Specifically, we tested the hypothesis that most of the beta diversity in regions retaining ice until recent periods was due to nestedness. Additionally, we tested whether the variation was influenced by thermal tolerance and the dispersal ability of species. Location This study analysed data from the New World. Methods We used presence/absence data for amphibians, birds and mammals of the New World. We calculated beta diversity among each 1°× 1° cell and the adjacent cells using the Sorensen dissimilarity index that expresses the total beta diversity. Furthermore, we partitioned it into turnover and nestedness components. The relative importance of the two latter components was expressed as the proportion of total beta diversity explained by nestedness (β_ratio). We calculated the correlation between β_ratio and the time each cell was free of ice since the last glaciation (cell age). To control the effects of spatial autocorrelation, we calculated geographically effective degrees of freedom. Results The proportion of beta diversity attributed to nestedness was negatively correlated with cell age. Moreover, this effect was stronger for amphibians than mammals, and stronger for mammals than birds. Main conclusions Our results are in accordance with the hypothesis that the nestedness component of beta diversity is more important in areas affected by glaciations until recent time. The beta diversity in high latitudes is the result of past extinctions and recent recolonization, which result in higher levels of nestedness. This process is more evident for vertebrates with lower dispersal ability and lower temperature tolerance.     

Disentangling spatial,environmental and historical effects on tropical forest tree species turnover

空间、环境和历史因素对热带森林树种周转的影响本文通过回答以下问题来分析空间距离、当前和过去的环境差异及这几个因素对树木群落分类和系统发育转换的影响：(i)树木群落分类和系统发育转换是否与空间距离通过环境差异产生的间接影响相关？(ii)树木群落分类和系统发育转换是否随古气候(末次盛冰期和中全新世)的变化而变化？本文对巴西的14个大西洋雨林样点(采样面积20.4 ha) 83个科615个种进行了研究,获得了当前、中全新世和末次盛冰 期的地块地理坐标、土壤变量和样点生物气候变量。我们使用基于距离的结构方程模型(SEM)来(i)检测空间距离和环境差异的直接影响,以及(ii)检测空间距离通过环境差异对分类(Bray-Curtis距离)和系统发育转换(Comdist和Comdistnt距离)的间接影响。研究结果表明,空间距离通过环境差异对分类和系统发育转换的间接影响较弱。基于生态位中性的历史过程带来的直接影响驱动了树木群落的转换。因此,我们推断古气候(历史过程)促进了产生当前植物区系分支的选择,而空间距离(中性的过程)限制了物种从区域种库向外扩散的范围,环境条件(基于生态位的过程)则在本地选择能够持续生存的分类群。     

The impact of ecological differentiation and dispersal limitation on species turnover and phylogenetic structure of inselberg's plant communities

Phylogenetic structure analysis is a novel way to address the relative importance of stochastic and deterministic processes governing species assemblages. Here we investigate the phylogenetic structure of the vegetation of inselbergs located in the African rain forest. Inselbergs combine strong ecological gradients at the local scale due to soil depth variation and insular properties at the regional scale. They are therefore ideal models to assess the influence of ecological sorting and dispersal limitation on the phylogenetic structure of plant communities. On 21 inselbergs separated by up to 200 km where five microhabitat-types were recognized, 311 vegetation plots were inventoried. We found that floristic similarity between plots depended on both microhabitat differentiation and spatial distance, while phylogenetic clustering (i.e. excess of phylogenetic similarity between species from a same plot) only appeared between plots from differentiated microhabitats and increased with ecological distance. Within a microhabitat-type, the absence of phylogenetic structure between inselbergs indicates that species turnover is probably due to dispersal limitation rather than to regional-scale variations in environmental factors. Hence, phylogenetic structure analysis can help disentangle the effects of ecological sorting and dispersal limitation on species assemblages. To estimate the time-scale of the processes generating the phylogenetic structure, we investigated how lineage similarity changes with increasing age in the phylogenetic tree. High lineage similarity levels between ecologically very differentiated plots were only reached at the proximity of the root of the phylogenetic tree. This was observed even when considering only plots sharing no species and indicates that phylogenetic niche conservatism has been important for generating the observed phylogenetic structure. Hence, ancient diversification exerts an impact on the assembly of current plant communities.     

Effects of geographic distance and climatic dissimilarity on species turnover in alpine meadow communities across a broad spatial extent on the Tibetan Plateau

Understanding the underlying mechanisms that generate species turnover or beta diversity among biological communities is a central theme in ecology. Here, we distinguish the effects of geographic distance and climatic dissimilarity on species turnover of vascular plants in alpine meadow communities on the Tibetan Plateau in China. We calculated species turnover between each pair of 17 sites, using the Jaccard??s and Simpson??s indices. We selected six variables to quantify climate at each site, and subjected values of the climatic variables to a principal component analysis. We applied a variance partitioning approach to disentangle the effects of geographic distance and climatic dissimilarity on species turnover in alpine meadow communities. We also examined the effect of elevation variation on species turnover. Geographic distance and climate dissimilarity together explained 49.1?% of the variation in compositional difference between alpine meadow communities; the amount of the variation explained purely by geographic distance and purely by climatic dissimilarity was 6.8?% and 2.8?%, respectively. When geographic distance, climate dissimilarity, and elevation difference were included in an analysis, they together explained 55?% of the variation in compositional difference between alpine meadow communities; the pure effect of each of the three sets of explanatory variables was 4.8, 4.3, and 3.5?%, respectively. The fact that the vast majority of the variation explained by geographic distance and climatic dissimilarity cannot be independently attributed to either factor suggests that the two factors operate together in determining regional patterns of species composition in alpine meadows on the Tibetan Plateau.     

Segregation,nestedness and homogenisation in plant communities dominated by native and alien species

ABSTRACT

Background: Highly modified landscapes offer the opportunity to assess how environmental factors influence the integration of alien plant species into native vegetation communities and determine the vulnerability of different communities to invasion.

Aims: To examine the importance of biotic and abiotic drivers in determining whether alien plant species segregate spatially from native plant communities or become integrated and lead to biotic homogenisation.

Methods: Ordination and classification of a floristic survey of over 1200 systematically located 6 m × 6 m plots were used to examine how plant community segregation, nestedness and homogenisation varied in relation to climate, environmental and human-related factors across Banks Peninsula, New Zealand.

Results: The analyses of community structure indicated that native and alien plant communities were spatially and ecologically segregated due to different responses primarily to an anthropogenic impact gradient and secondly to environmental factors along an elevation gradient. Human-land use appeared most strongly linked to the distribution of alien species and was associated with increased vegetation homogenisation. However, despite spatial segregation of alien and native plant communities, biotic homogenisation not only occurred in highly managed grasslands but also in relatively less managed shrublands and forest.

Conclusions: The role played by anthropogenic factors in shaping alien and native plant species community structure should not be ignored and, even along a marked environmental gradient, if the recipient sites have a long history of human-related disturbance, biotic homogenisation is often strong.     

Effects of environmental synchrony and density-dependent dispersal on temporal and spatial slopes of Taylor's law

Taylor's law (TL) is an empirical rule that describes an approximate relationship between the variance and mean of population density: log₁₀(variance) ≈ log₁₀(a) + b × log₁₀(mean). Population synchrony is another prevailing feature observed in empirical populations. This study investigated the effects of environmental synchrony and density-dependent dispersal on the temporal (b_T) and spatial (b_S) slopes of TL, using an empirical dataset of gray-sided vole populations and simulation analyses based on the second-order autoregressive (AR) model. Eighty-five empirical populations satisfied the temporal and spatial TLs with b_T = 1.943 (±SE 0.143) and b_S = 1.579 (±SE 0.136). The pairwise synchrony of population was 0.377 ± 0.199 (mean ± SD). Most simulated populations that obeyed the AR model satisfied the form of the temporal and spatial TLs without being affected by the environmental synchrony and density-dependent dispersal; however, those simulated slopes were too steep. The incorporation of environmental synchrony resulted in reduced simulated slopes, but those slopes, too, were still unrealistically steep. By incorporating density-dependent dispersal, simulated slopes decreased and fell within a realistic range. However, the simulated populations without environmental synchrony did not exhibit an adequate degree of density synchrony. In simulations that included both environmental synchrony and density-dependent dispersal, 92.7% of the simulated datasets provided realistic values for b_T, b_S and population synchrony. Because the two slopes were more sensitive to the variation of density-dependent dispersal than that of environmental synchrony, density-dependent dispersal may be the key to the determination of b_T and b_S.     

Distinguishing the importance between habitat specialization and dispersal limitation on species turnover

Understanding what governs community assembly and the maintenance of biodiversity is a central issue in ecology, but has been a continuing debate. A key question is the relative importance of habitat specialization (niche assembly) and dispersal limitation (dispersal assembly). In the middle of the Loess Plateau, northwestern China, we examined how species turnover in Liaodong oak (Quercus wutaishanica) forests differed between observed and randomized assemblies, and how this difference was affected by habitat specialization and dispersal limitation using variation partitioning. Results showed that expected species turnover based on individual randomization was significantly lower than the observed value (P < 0.01). The turnover deviation significantly depended on the environmental and geographical distances (P < 0.05). Environmental and spatial variables significantly explained approximately 40% of the species composition variation at all the three layers (P < 0.05). However, their contributions varied among forest layers; the herb and shrub layers were dominated by environmental factors, whereas the canopy layer was dominated by spatial factors. Our results underscore the importance of synthetic models that integrate effects of both dispersal and niche assembly for understanding the community assembly. However, habitat specialization (niche assembly) may not always be the dominant process in community assembly, even under harsh environments. Community assembly may be in a trait‐dependent manner (e.g., forest layers in this study). Thus, taking more species traits into account would strengthen our confidence in the inferred assembly mechanisms.     

Partitioning the turnover and nestedness components of beta diversity

Aim  Beta diversity (variation of the species composition of assemblages) may reflect two different phenomena, spatial species turnover and nestedness of assemblages, which result from two antithetic processes, namely species replacement and species loss, respectively. The aim of this paper is to provide a unified framework for the assessment of beta diversity, disentangling the contribution of spatial turnover and nestedness to beta-diversity patterns.
Innovation  I derive an additive partitioning of beta diversity that provides the two separate components of spatial turnover and nestedness underlying the total amount of beta diversity. I propose two families of measures of beta diversity for pairwise and multiple-site situations. Each family comprises one measure accounting for all aspects of beta diversity, which is additively decomposed into two measures accounting for the pure spatial turnover and nestedness components, respectively. Finally, I provide a case study using European longhorn beetles to exemplify the relevance of disentangling spatial turnover and nestedness patterns.
Main conclusion  Assigning the different beta-diversity patterns to their respective biological phenomena is essential for analysing the causality of the processes underlying biodiversity. Thus, the differentiation of the spatial turnover and nestedness components of beta diversity is crucial for our understanding of central biogeographic, ecological and conservation issues.     

A comprehensive framework for the study of species co‐occurrences,nestedness and turnover

Binary presence–absence matrices (rows = species, columns = sites) are often used to quantify patterns of species co‐occurrence, and to infer possible biotic interactions from these patterns. Previous classifications of co‐occurrence patterns as nested, segregated, or modular have led to contradictory results and conclusions. These analyses usually do not incorporate the functional traits of the species or the environmental characteristics of the sites, even though the outcomes of species interactions often depend on trait expression and site quality. Here we address this shortcoming by developing a method that incorporates realized functional and environmental niches, and relates them to species co‐occurrence patterns. These niches are defined from n‐dimensional ellipsoids, and calculated from the n eigenvectors and eigenvalues of the variance–covariance matrix of measured environmental or trait variables. Average niche overlap among species and the spatial distribution of niches define a triangle plot with vertices of species segregation (low niche overlap), nestedness (high niche overlap), and modular co‐occurrence (clusters of overlapping niches). Applying this framework to temperate understorey plant communities in southwest Poland, we found a consistent modular structure of species occurrences, a pattern not detected by conventional presence–absence analysis. These results suggest that, in our case study, habitat filtering is the most important process structuring understorey plant communities. Furthermore, they demonstrate how incorporating trait and environmental data into co‐occurrence analysis improves pattern detection and provides a stronger theoretical framework for understanding community structure.     

环境筛选和扩散限制在地表和地下螨群落物种共存中的调控作用

识别扩散限制和环境筛选在群落物种共存中的相对作用,是土壤动物群落物种共存机制研究的重要内容,然而少有针对地表和地下土壤动物群落的探讨。在三江平原农田生态系统,设置一个50 m×50 m的空间尺度,探讨环境筛选和扩散限制对地表和地下土壤螨群落物种共存的调控作用。基于Moran特征向量图(MEMs)和变差分解的方法来区分环境筛选和扩散限制的调控作用;采用偏Mantel检验进一步分析环境距离和空间距离的相对贡献;使用RDA分析环境因子对螨群落物种组成的解释能力。变差分解结果表明,空间变量对地表、地下和地表-地下土壤螨群落具有较大的显著方差解释量,而环境变量和空间环境结构的解释量相对较小且不显著;偏Mantel检验没有发现环境距离或空间距离的显著贡献;RDA分析表明土壤p H值、大豆株高和土壤含水量对土壤螨群落具有显著的解释能力,说明环境变量对螨群落物种组成的重要作用。研究表明,在三江平原农田生态系统,地表和地下土壤螨群落物种共存主要受到扩散限制的调控作用,同时环境筛选的调控作用也不容忽视。     

Effects of environmental and spatial gradients on Quercus-dominated Mountain forest communities in the Hindu-Kush ranges of Pakistan

Quercus-dominated forests are among the most important broad-leaved evergreen forests of the Hindu Kush ranges and are currently prone to drastic anthropogenic and climatic changes. The aim of this study was to provide basic data for the development of a regional oak forest ecosystem framework for ecological restoration and management plan development to maintain local peoples’ livelihoods. Hence, we analyzed distribution patterns and environmental factors that affect regional oak forests’ species composition and diversity. Ward’s Agglomerative clustering divided oak-dominated forest communities into three groups: i.e., Group I, dominated by Quercus baloot had an importance value index (IVI) of 89.87 ± 4.31, Group II, dominated by Quercus dilatata had an IVI of 32.16 ± 15.01, and Group III, dominated by Quercus oblongata had an IVI of 83.14 ± 4.67, respectively. The environmental factors which vary significantly within these communities were latitude, elevation, clay content and bulk density of the soil. Wilting point, saturation point, and electrical conductivity were also considered as ecosystem structural variables. Canonical correspondence analysis (CCA) indicated that community structure was affected by various environmental factors including precipitation, slope angle, elevation, clay content, and relative humidity.     

Distinguishing between turnover and nestedness in the quantification of biotic homogenization

Compositional changes through local extinction and colonization are inherent to natural communities, but human activities are increasingly influencing the rate and nature of the species being lost and gained. Biotic homogenization refers to the process by which the compositional similarity of communities increases over time through a non-random reshuffling of species. Despite the extensive conceptual development of the homogenization framework, approaches to quantify patterns of homogenization are scarcely developed. Most studies have used classical dissimilarity indices that actually quantify two components of compositional variation: turnover and nestedness. Here we demonstrate that a method that partitions those two components reveals patterns of homogenization that are otherwise obscured using traditional techniques. The forest understorey vegetation of an unmanaged reserve was recorded in permanent plots in 1979 and 2009. In only thirty years, the local species richness significantly decreased and the variation in the species composition from site to site shifted towards a structure with reduced true species turnover and increased dissimilarity due to nestedness. A classic analysis masked those patterns. In summary, we illustrated the need to move beyond the simple quantification of homogenization using classical indices and advocate integration of the multitude of ways to quantify community similarity into the homogenization framework.     

Leaf litter species identity alters the structure of pond communities

The input of leaf litter resources is a major driver of ecosystem processes in terrestrial and freshwater habitats. Although variation exists in the quantity and composition of litter inputs due to natural and anthropogenic causes, few studies have examined how such variation influences the structure and composition of aquatic food webs. Using outdoor mesocosms, we examined the bottom–up effects of 10 chemically distinct tree litter species on microbial, algal, invertebrate and vertebrate fauna found in temperate ponds. We hypothesized that individual litter species, which differ in their traits, would differentially and predictably affect abiotic and biotic elements of pond communities. We further hypothesized that the presence of leaf litter, regardless of species, would elevate resource supply and increase the biomass of community members. Finally, we hypothesized that a mixture of litter species would have non‐additive effects on community responses. We followed the system for > 4 months and measured > 30 abiotic and biotic responses related to primary and secondary production. The different species of leaf litter had major effects on abiotic and biotic responses, including phytoplankton, periphyton, zooplankton, snails, amphipods and tadpoles. Most biological responses were negatively associated with soluble carbon content of litter, or litter decay rate. Other litter traits, including phenolic concentrations and litter C:N were of secondary importance but did exhibit both positive and negative associations with several responses. The absence of litter had pervasive effects on abiotic attributes, but did not promote substantial changes in organism biomass. Most responses to the litter mixture were additive. Our results suggest that changes in temperate forest composition can strongly affect pond communities.     

Effects of different abiotic and biotic factors on spatial primary seed dispersal in the semachorous species Scrophularia canina

Seed dispersal is one of the most important steps in the plant life cycle. However, there is, generally, a lack of fieldworks focused on wind dispersal and especially on semachorous dispersal (seeds spread when the fruits are shaken by wind and other vectors, such as animals), including boleochorous dispersal. Therefore, we aimed to determine how different types of wind and animals affected seed dispersal under natural conditions in the widespread species Scrophularia canina. We evaluated the effects of wind gusts (simulating them using a leaf blower) and wild animals (using differently sized dogs) on seed dispersal in a population located in south-western Europe. We found that S. canina is a semachorous species, and its spatial seed dispersal was affected by wind gust speed and direction, plant structure and vector type. The results also revealed the presence of xerochasy, individual anisotropy with strong winds, and primary short-distance dispersal associated with successional processes independent of the vector. Additionally, there was a masking effect of plant structure on the seed shadow outline. It is essential to conduct fieldworks to reveal what actually happens in nature, taking into account the characteristics determining seed dispersal. In addition, in these works it is important to find out what factors affect seed distributions of anemochorous and semachorous species.