Beta多样性研究进展

Beta多样性度量时空尺度上物种组成的变化, 是生物多样性的重要组成部分, 与许多生态学和进化生物学问题密切相关, 并且其信息可用于保护区选址和布局规划, 因此在最近10年间成为生物多样性研究的热点问题之一。多年来, 学者们利用各种度量方式和分析方法, 在不同地理区域, 对许多生物类群beta多样性的时空格局和形成机制进行了大量研究。本文主要从beta多样性的度量方法、时空格局、形成机制及其在生物多样性保护中的应用等几个方面, 总结了最近10多年来相关研究的进展。Whittaker(1960)最初提出beta多样性概念时就缺乏严格的定义, 随着概念的不断演化, 度量方法也同样呈现出多样化, 而度量手段的多样化非常不利于不同研究之间的比较。目前应用最普遍的度量方法是采用相似性指数, 如Jaccard和Sørensen指数。最近几年, 新的度量方法还在不断出现, 其中一些方法非常值得注意。Beta多样性具有时空尺度和分类尺度依赖性, 一般随分析粒度(grain)的增加而降低。虽然有些研究表明beta多样性随纬度增加而降低, 但学者们并没有达成共识。山区和生物地理区的交界处beta多样性都比较高, 因而需要在这些地区增加保护区的面积或者数量以囊括物种变化梯度。对时间尺度上beta多样性的研究表明, 气候变化确实导致了物种组成在时间上的变化, 并且物种在不同大陆和地区间的迁移导致了生物同质化。扩散过程和生态位过程共同决定了beta多样性, 只是这两个过程的相对重要性依尺度、地理区域和物种类群的不同而有所差异。综上所述, 我们认为未来beta多样性研究的热点问题是：(1)不同生物类群的进化历史和生物学特征对beta多样性的影响; (2)不同的时空尺度对beta多样性及其维持机制的影响; (3)人类活动对beta多样性的影响。     

北京东灵山辽东栎林植物物种多样性的多尺度分析

多尺度分析物种多样性格局能够为有效保护生物多样性提供重要信息.利用物种多样性的加法分配法则分析了样方-坡位-坡面等级尺度系统辽东栎林植物物种多样性(gamma多样性)的alpha多样性和beta多样性在各尺度上的分配关系.结果表明以物种丰富度为指标的区域物种多样性的最大贡献来自坡面尺度,表明坡面尺度是维持辽东栎林物种多样性的有效尺度;而对Simpson多样性和Shannon多样性的最大贡献则来自样方内,这决定于群落物种优势度和稀有度格局;各尺度间beta多样性组分随尺度的增大而增大可能是环境异质性和扩散作用的综合结果.各尺度间Shannon多样性对总体多样性的贡献大于Simpson多样性的贡献是偶见种在各尺度间分配的结果.物种多样性分配的加法法则为物种多样性格局的多尺度分析提供了理论框架,是检验物种多样性格局形成机制的有效方法.     

哀牢山亚热带中山湿性常绿阔叶林树种beta多样性格局形成的驱动力

Beta多样性通常指群落在时间和空间上物种组成的差异, 包括物种周转组分和物种丰富度差异组分。驱动beta多样性格局形成的生态过程决定了群落的时空动态, 然而关于beta多样性及其两个组分格局形成的驱动力还存在较多争议。以往研究表明, beta多样性的格局存在取样尺度的依赖性, 驱动其形成的生态过程在不同取样尺度下的相对重要性也随之改变。本研究以哀牢山亚热带中山湿性常绿阔叶林20 ha动态监测样地为研究对象, 在不同取样尺度上, 将样方间的Bray-Curtis指数分解为物种周转组分和物种丰富度差异组分, 通过典范冗余分析和方差分解的方法揭示环境过滤和扩散限制对于beta多样性及其两个组分格局形成的相对重要性及其尺度依赖性。结果表明: (1) beta多样性、物种周转组分和物种丰富度差异组分均随取样尺度的增大而减小。在不同取样尺度下, 物种周转组分对于beta多样性的贡献始终占主导地位。(2)随着取样尺度的增大, 环境过滤驱动beta多样性格局形成的相对重要性逐渐增加, 而扩散限制的相对重要性逐渐降低。本研究进一步证实了取样尺度在beta多样性格局形成及其驱动力定量评价中的重要性, 今后的研究需要进一步解析上述尺度效应的形成机制。     

Beta多样性分解:方法、应用与展望

Beta多样性是指不同群落间物种组成的差异,由物种周转(或物种替换)和嵌套(或丰富度差异)这两种过程决定。Beta多样性分解是将这两种过程对总体beta多样性的作用进行拆分,然后分别探讨这两种过程对群落间物种组成差异的影响。2010年之后,人们提出了beta多样性分解的方法,其中占据主导地位的是由Andrés Baselga于2010年提出的BAS法(总体beta多样性分解为物种周转和嵌套组分)和由János Podani和Dénes Schmera于2011年以及JoséC.Carvalho等于2012年提出的POD法(总体beta多样性分解为物种替换和丰富度差异组分)。这两种分解方法引起了持续的争论,促进了该领域的快速发展。作者归纳分析了2010年后有关beta多样性分解的文献后发现,使用BAS法的论文无论在发表量和引用次数上都多于POD法(75%vs.20%)。Beta多样性分解的研究主要集中在欧洲(45%),研究类群则以动物(64%)为主。本文在回顾beta多样性分解方法的提出及其发展过程的基础上,从时空维度(纬度梯度、海拔梯度、生境片断化过程以及季节和年际动态)、多样性的不同方面(物种、功能和谱系多样性)和不同生物类群之间的比较等研究角度出发,进一步阐述了beta多样性分解方法在探讨生物多样性分布格局以及形成机制中的应用。对于beta多样性分解的研究,我们认为需要深入探讨的问题有:(1)beta多样性分解方法的比较分析和整合;(2)结合物种多度信息探讨beta多样性及其组分的分布格局;(3)对大尺度下beta多样性分解的结果进行普适性验证。     

亚热带不同纬度森林群落物种空间分布格局

为探讨我国亚热带森林群落中物种空间分布格局特点及其成因,揭示其共性规律和个性差异,为相关区域的生物多样性保护和监测工作提供科学依据。选取我国亚热带不同纬度的6个1 hm²典型森林群落中所有胸径（DBH,Diameter at Breast Height）≥ 1 cm的物种为研究对象;采用L （t）方程统计所有监测物种的空间分布格局类型,为满足L （t）方程的统计精度,将各群落中多度≥10株的植物以物种为单位统计,多度<10株的低多度物种和单个体物种则以种组为单位进行空间点格局分析;并对物种多度、胸径与空间聚集程度的度量指标L₁₀进行相关性分析。结果表明：（1）亚热带不同纬度群落中多度≥10株物种的空间分布格局表现出相似的尺度依赖性规律：显著聚集分布比例随尺度增大而降低,不同纬度群落的变化幅度存在差异,纬度较低的群落稳定性更强。（2）各群落中低多度种组空间分布格局表现出小尺度显著聚集分布,大尺度随机分布的尺度效应,但尺度效应在不同纬度群落间存在差异;各群落的单个体种组空间分布格局均以随机分布为主。（3）不同纬度群落物种多度与空间聚集程度之间均存在负相关关系,物种多度对空间聚集程度的影响随纬度从高到低逐渐减弱。（4）较高纬度群落物种胸径与空间聚集程度之间存在负相关关系;随纬度从高到低,物种胸径对空间聚集程度影响水平逐渐降低。对亚热带不同纬度森林群落物种空间分布格局形成原因的进一步探讨,认为由纬度差异引起的生境异质性是影响群落物种空间分布格局产生过程的主要因素,此外,林型、扩散限制、密度制约和随机作用是影响各群落物种空间格局成因及多样性维持的次要因素。     

取样尺度效应对滇西北地区种子植物物种多样性纬度分布格局的影响

滇西北地区是全球25个生物多样性保护热点地区之一, 是验证生物多样性理论的理想场所。为探索取样尺度效应对植物物种多样性纬度分布格局的影响, 我们探讨了不同取样尺度下滇西北地区种子植物物种多样性的纬度分布格局及其影响因子。我们利用野外考察数据和文献资料建立了群落尺度下的源数据集和区域尺度(县域尺度)下的源数据集, 共建立、收集了68个植物群落样方和26个县域的种子植物物种丰富度; 采用二元相关性和多元逐步回归分析植物物种多样性纬度分布格局与气候、地理因子间的关系。结果表明, 从南到北, 物种多样性在群落尺度下呈单调递减格局, 在区域尺度下反而呈线性递增趋势; 在群落尺度下受到热量因子的显著影响, 在区域尺度下主要受单位面积海拔高差的影响。这一结果在一定程度上表明了取样的尺度效应对物种多样性纬度分布格局的显著影响。了解滇西北地区植物多样性的热点区域, 应该基于不同取样尺度下的分析, 以消除或减少植物多样性保护的盲点。在今后的相关研究中, 应关注不同的取样尺度下多样性的纬度分布格局可能的表现形式及其内在机制, 这或许可以减少或消除相关研究中的争议或不一致。     

吉林灌木群落物种多样性与气候和局域环境因子的关系

为了研究气候和局域环境因子对物种多样性的相对作用大小,以及验证两种均匀度地理格局的假说在半湿润地区次生灌丛的适用性,对吉林东、南部地区的灌木群落进行了研究。共调查森林破坏后形成的次生灌丛样方45个,结合气候数据和局域环境因子数据,研究了气候、局域环境因子对群落、灌木层、草本层的物种丰富度、均匀度的影响,以及对不同水分生态型(旱生、旱中生、湿中生)灌木影响的差异。结果表明:1)吉林次生灌丛的群落、草本层物种丰富度,以及草本层均匀度,随纬度增加而显著上升。2)对物种多样性和气候、局域环境因子的分析表明,群落、草本层物种数主要受局域环境因子而不是气候的影响;其物种丰富度与纬度的反常关系,是由于灌木层盖度随降水增加而上升,从而导致物种数下降。灌木层物种数与纬度、气候因子的相关性不显著,则是由于不同水分生态型对气候梯度的响应不一致,反映出功能群对多样性格局的影响。3)群落、灌木层均匀度主要受气候因子的影响;而草本层均匀度主要受局域环境因子的影响,降水同样通过对灌木层盖度的影响间接作用于草本均匀度。但群落、灌木和草本层的结果,都支持均匀度随着环境条件改善而增加的假说,而不支持随着生产力增加、竞争加剧,从而导致均匀度下降的假说。结果表明,物种丰富度和均匀度的影响机制存在很大差异,但二者都受到局域环境因子的强烈影响。气候通过局域生物因素(如盖度、生活型)间接作用于多样性格局,是气候对多样性影响的一个重要方面,但尚未得到应有的重视。由于局域生物因素也随气候而变化,仅研究多样性和气候的表面关系,将无法准确预测气候变化对多样性的影响。     

A taxonomic and phylogenetic perspective on plant community assembly along an elevational gradient in subtropical forests

亚热带森林植物群落沿海拔梯度的分类与系统发育研究 生物多样性沿海拔梯度的分布格局已受到广泛关注。然而,生物多样性格局沿海拔梯度的变异及其潜在机制尚不清楚。整合生物多样性的多维度信息为理解群落构建机制提供了新思路。本研究在我国东部亚热带森林沿海拔270–1470 m的梯度上设置了17个木本植物固定样地,分析了沿海拔梯度植物群落 构建的生态和进化驱动力。基于样地内物种出现(0–1数据)和多度信息,计算群落内被子植物的物种和系统发育alpha和beta多样性、系统发育结构等,并量化多样性指标与微气候和地形之间的关系。研究发现,不论多度加权与否,物种alpha多样性均沿海拔升高而增加,物种和系统发育的相似性随海拔距离的增加而呈衰减趋势。然而,多度加权与否会形成不同的系统发育alpha多样性格局。对于系统发育结构而言,沿海拔增加并无明显趋势。地形和微气候是多样性格局和系统发育结构的主要驱动力。与未考虑物种多度的多样性指标相比,多度加权的指标与坡度和胸高断面积相关性更高。这些结果表明,由局域物种多度介导的确定性过程对沿海拔梯度的植物群落构建具有一定影响。     

东北阔叶红松林群落分类、排序及物种多样性比较

植物群落是植物与环境相互作用的产物,探讨森林群落的分布格局、多样性变化规律及其与环境因子的关系,有助于该区森林群落稳定性的维持和生物多样性保护。在东北阔叶红松林区,沿纬度梯度在典型区域选择未受干扰的原始阔叶红松林进行群落调查,并运用多元回归树(MRT)、物种多样性指数比较和典范对应分析(CCA)方法,对森林群落进行分类、比较和排序。结果表明:东北阔叶红松林森林群落可分为4个类型,不同群落类型间物种组成及多样性差异显著。物种丰富度及多样性指数均为低纬度区的千金榆-枫桦-红松林显著高于高纬度地区的冷杉-红松林群落。CCA排序结果较好地反映了各群落类型的分布范围及其与环境因子的关系,其变化格局主要受温度和降水的影响,其次是土壤养分。该结果为气候变化下阔叶红松林的管理和保护提供了相应的理论依据。     

稀有种和常见种对黄土高原辽东栎群落物种多样性贡献的多尺度分析

明确物种多样性的多尺度格局及其形成过程是进行物种多样性保护的重要前提。通过加性分配的方法,在α1(小样方内)、β1(小样方间)、β2(样方间)、β3(样地间) 4个空间尺度上,研究了黄土高原子午岭辽东栎(Quercus wutaishanica)群落植物多样性的多尺度变化,确定了稀有种(频度5%)和常见种(频度25%)对物种多样性格局的相对贡献及其尺度依赖性,旨在阐明黄土高原辽东栎群落物种多样性的多尺度格局及其形成过程,为该区物种多样性的保护提供理论依据。结果表明:(1)乔、灌、草3层呈现了相似的物种多样性格局。物种丰富度基本上呈现了随尺度的增大而增加的变化趋势,小样方尺度上的α多样性(α1)对群落物种丰富度的贡献最小,样地尺度上的β多样性(β3)对各层物种丰富度的贡献最大(β3 50%)。(2) Shannon多样性呈现了与物种丰富度截然相反的格局,小样方尺度上的α多样性成分(α1)是Shannon多样性的主要贡献者(α145%),尔后随尺度的增大,Shannon多样性逐渐减小。(3)去除常见种引起的物种丰富度减小效应表现为随尺度的增加,影响逐渐减小,其效应主要集中在小样方内(α1尺度)以及小样方间(β1尺度) 2个尺度上;而去除稀有种引起的物种丰富度减小效应表现为随尺度的增加,影响逐渐增大,其效应主要集中在样方间(β2)以及样地间(β3) 2个尺度上。(4)去除稀有种对Shannon多样性的影响在每一尺度上均不明显,去除前后的曲线几乎重合,而去除常见种对Shannon多样性的影响在每一尺度上均十分明显。其中,去除常见种,草本层Shannon多样性在每一尺度都相应增大,而对于灌木层和乔木层,Shannon多样性则表现为在α1尺度上减小,而在其他尺度上增大。本研究揭示了空间尺度在评价稀有种和常见种对植物群落物种多样性相对贡献中的重要性。稀有种主要在较大尺度(β2和β3)上影响物种丰富度格局,而常见种不仅在小尺度(α1和β1)上影响物种丰富度,而且在较大尺度(β2和β3)上影响物种均匀度。同时,物种多样性的加性分配也是在多尺度上揭示物种多样性变化格局以及变异来源的有效且比较简单的方法。     

Patterns of functional diversity across an extensive environmental gradient: vertebrate consumers, hidden treatments and latitudinal trends

Over the last two decades, although much has been learned regarding the multifaceted nature of biodiversity, relatively little is known regarding spatial variation in constituents other than species richness. This is particularly true along extensive environmental gradients such as latitude. Herein, we describe latitudinal gradients in the functional diversity of New World bat communities. Bat species from each of 32 communities were assigned to one of seven functional groups. Latitudinal gradients existed for the richness, diversity and scaled‐dominance of functional groups. No significant patterns were observed for evenness of functional groups. Measures of functional diversity were different in magnitude and increased towards the equator at a faster rate than expected given the underlying spatial variation in species richness. Thus, latitudinal gradient in species richness alone do not cause the latitudinal gradient in functional diversity. When variation in species composition of the regional fauna of each community was incorporated into analyses, many differences between observed and simulated patterns of functional diversity were not significant. This suggests that those processes that determine the composition of regional faunas strongly influence the latitudinal gradient in functional diversity at the local level. Nonetheless, functional diversity was lower than expected across observed sites. Community‐wide responses to variation in the quantity and quality of resources at the local level probably contribute to differences in functional diversity at local and regional scales and enhance beta diversity.     

Biomass compensation: Behind the diversity gradients of tidepool fishes

Species richness is unevenly distributed on the Earth, with biodiversity gradients of various spatial scales supposedly being affected by abiotic as well as biotic factors including community traits such as body size spectra and relative abundance patterns. To explore large-scale spatial variation in species diversity and their processes, tidepool fish communities were investigated through an intensive field work conducted on 55 shore sites in south-western Japan. Multiple ecological measures were taken into account to assess changes in local community structures with changes in the number of species. Biomass (total fish wet weight) per unit area showed no systematic change with latitude, while taxa richness and number of individuals tended to increase toward lower latitudes. In addition, median fish body weight scaled positively with latitude, which was more conspicuous in Blenniidae than in Gobiidae. The latitudinal gradient of diversity in tidepool fish assemblages appears to be characterized by partitioning of total biomass that tends to stay constant across latitudes, suggesting the phenomenon of “biomass compensation” whereby body size and abundance/diversity change in opposite directions with latitude. Our study highlights that biomass compensation can be part of processes involved in generating gradients of species richness even without an apparent energy/resource gradient.     

Latitudinal gradients in the phenetic diversity of New World bat communities

Although the examination of latitudinal gradients of species richness is common, little attention has been devoted to other components of biodiversity such as phenetic diversity. Because the phenotype reflects aspects of an organism's environment, ecological relationships and evolutionary history, measures of phenetic diversity likely provide complimentary information to that of species richness, and may provide unique insights for understanding the mechanistic basis to patterns of biodiversity. Herein, we evaluate latitudinal gradients in the phenetic diversity of 32 New World bat communities. Seven morphological characters were used to estimate phenotypic variation among bat species within local communities. Principal components analysis decomposed this variation into axes of size and shape. Three measures of phenetic diversity were calculated separately for size and for shape axes. The range of species scores on a particular axis described the amount of phenetic variation encompassed by species in a community. The standard deviation of minimum spanning‐tree segment lengths described uniformity of species. Average nearest‐neighbor distances described local packing. We separately regressed these six measures on local species richness and latitude separately. Variation in species richness accounted for a significant amount of variation in each measure of phenetic diversity. Latitude also accounted for significant variation in phenetic diversity except for the standard deviation of minimum‐spanning tree segment lengths and the average nearest‐neighbor distance on the shape axis. More importantly, gradients in phenetic diversity were significantly different than would be expected as a consequence of latitudinal gradients in species richness. Nonetheless, when variation among communities regarding the richness and composition of their regional faunas was taken into consideration, differences between empirical and simulated gradients were nonsignificant. Thus, factors that determine the composition of regional faunas have a great impact on the phenetic diversity of communities and ultimately the latitudinal gradient in biodiversity.     

Species turnover in lake littorals: spatial and temporal variation of benthic macroinvertebrate diversity and community composition

Aims Despite wide consensus that ecological patterns and processes should be studied at multiple spatial scales, the temporal component of diversity variation has remained poorly examined. Specifically, rare species may exhibit patterns of diversity variation profoundly different from those of dominant taxa. Location Southern Finland. Methods We used multiplicative partitioning of true diversities (species richness, Shannon diversity) to identify the most important scale(s) of variation of benthic macroinvertebrate communities across several hierarchical scales, from individual samples to multiple littorals, lakes and years. We also assessed the among‐scale variability of benthic macroinvertebrate community composition by using measures of between‐ and within‐group distances at hierarchical scales. Results On average, a single benthic sample contained 23% of the total regional macroinvertebrate species pool. For both species richness and Shannon diversity, beta‐diversity was clearly the major component of regional diversity, with within‐littoral beta‐diversity (β₁) being the largest component of gamma‐diversity. The interannual component of total diversity was small, being almost negligible for Shannon index. Among‐sample (within‐littoral) diversity was related to variation of substratum heterogeneity at the same scale. By contrast, only a small proportion of rare taxa was found in an average benthic sample. Thus, dominant species among lakes and years were about the same, whereas rare species were mostly detected in a few benthic samples in one lake (or year). For rare species, the temporal component of diversity was more important than spatial turnover at most scales. Main conclusions While individual species occurrences and abundances, particularly those of rare taxa, may vary strongly through space and time, patterns of dominance in lake littoral benthic communities are highly predictable. Consequently, many rare species will be missed in temporally restricted samples of lake littorals. In comprehensive biodiversity surveys, interannual sampling of littoral macroinvertebrate communities is therefore needed.     

The variation of tree beta diversity across a global network of forest plots

Aims With the aim of understanding why some of the world's forests exhibit higher tree beta diversity values than others, we asked: (1) what is the contribution of environmentally related variation versus pure spatial and local stochastic variation to tree beta diversity assessed at the forest plot scale; (2) at what resolution are these beta‐diversity components more apparent; and (3) what determines the variation in tree beta diversity observed across regions/continents? Location World‐wide. Methods We compiled an unprecedented data set of 10 large‐scale stem‐mapping forest plots differing in latitude, tree species richness and topographic variability. We assessed the tree beta diversity found within each forest plot separately. The non‐directional variation in tree species composition among cells of the plot was our measure of beta diversity. We compared the beta diversity of each plot with the value expected under a null model. We also apportioned the beta diversity into four components: pure topographic, spatially structured topographic, pure spatial and unexplained. We used linear mixed models to interpret the variation of beta diversity values across the plots. Results Total tree beta diversity within a forest plot decreased with increasing cell size, and increased with tree species richness and the amount of topographic variability of the plot. The topography‐related component of beta diversity was correlated with the amount of topographic variability but was unrelated to its species richness. The unexplained variation was correlated with the beta diversity expected under the null model and with species richness. Main conclusions Because different components of beta diversity have different determinants, comparisons of tree beta diversity across regions should quantify not only overall variation in species composition but also its components. Global‐scale patterns in tree beta diversity are largely coupled with changes in gamma richness due to the relationship between the latter and the variation generated by local stochastic assembly processes.     

小兴安岭阔叶红松林地表甲虫Beta多样性

Beta多样性用来衡量集群内物种组成的变异性,可以被分解为空间物种转换和物种集群镶嵌两个组分,是揭示群落构建机制的重要基础。目前开展了较多的地上生态系统beta多样性研究,然而地下生态系统beta多样性进展缓慢。以小兴安岭凉水和丰林自然保护区为研究地区,于2015年8、10月采用陷阱法对阔叶红松林进行调查,揭示地表甲虫(步甲科、隐翅虫科、葬甲科)的beta多样性。结果表明:(1)凉水共发现39种、856只地表甲虫,丰林共发现43种、1182只地表甲虫。8月凉水明显具有较高的全部甲虫(三个科的总和)物种多样性和丰富度,10月正好相反。(2)凉水和丰林之间地表甲虫beta多样性的差异仅发现于8月的步甲科和葬甲科之间。(3)凉水和丰林地表甲虫的beta多样性主要由空间物种转换组成,物种集群镶嵌对beta多样性的贡献很小,说明地表甲虫物种组成变异主要由本地物种之间较高的转换引起。研究表明小兴安岭阔叶红松林地表甲虫的beta多样性主要由空间物种转换组成,在揭示群落构建机制过程中,其内部物种交换和环境调控不容忽视。     

Temporal population variability in local forest communities has mixed effects on tree species richness across a latitudinal gradient

Among the local processes that determine species diversity in ecological communities, fluctuation‐dependent mechanisms that are mediated by temporal variability in the abundances of species populations have received significant attention. Higher temporal variability in the abundances of species populations can increase the strength of temporal niche partitioning but can also increase the risk of species extinctions, such that the net effect on species coexistence is not clear. We quantified this temporal population variability for tree species in 21 large forest plots and found much greater variability for higher latitude plots with fewer tree species. A fitted mechanistic model showed that among the forest plots, the net effect of temporal population variability on tree species coexistence was usually negative, but sometimes positive or negligible. Therefore, our results suggest that temporal variability in the abundances of species populations has no clear negative or positive contribution to the latitudinal gradient in tree species richness.     

Latitudinal gradients in diversity: real patterns and random models

Mid-domain models have been argued lo provide a default explanation for the best known spatial pattern in biodiversity, namely the latitudinal gradient in species richness. These models assume no environmental gradients, but merely a random latitudinal association between the size and placement of the geographic ranges of species. A mid-domain peak in richness is generated because when the latitudinal extents of species in a given taxonomic group are bounded to north and south, perhaps by a physical constraint such as a continental edge or perhaps by a climatic constraint such as a critical temperature or precipitation threshold, then the number of ways in which ranges can be distributed changes systematically between the bounds. In addition, such models make predictions about latitudinal variation in the latitudinal extents of the distributions of species, and in beta diversity (the spatial turnover in species identities). Here we test how well five mid-domain models predict observed latitudinal patterns of species richness, latitudinal extent and beta diversity in two groups of birds, parrots and woodpeckers, across the New World. Whilst both groups exhibit clear gradients in richness and beta diversity and the general trend in species richness is acceptably predicted (but not accurately, unless substantial empirical information is assumed), the fit of these models is uniformly poor for beta diversity and latitudinal range extent. This suggests either that, at least for these data, as presently formulated mid-domain models are too simplistic, or that in practice the mid-domain effect is not significant in determining geographical variation in diversity.     

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1. Understanding the role of environmental filtering and spatial processes along environmental gradients in assembling and maintaining aquatic communities in rare habitats is crucial for land management and biological conservation. We investigated the relative roles of environmental and spatial factors influencing beta (β) diversity of aquatic macroinvertebrate assemblages in 36 interdunal wetlands from five freshwater sand dune areas across two ecoregions spanning the latitudinal gradient of Lake Michigan. We hypothesised that aquatic macroinvertebrate diversity and composition would vary along the coastline because of an underlying environmental gradient. We predicted high species replacement correlated with environmental (local and regional) conditions and increasing species diversity with decreasing latitude.
2. We calculated sample completeness, obtained diversity estimates based on Hill numbers and used abundance-based β partitioning and multivariate analysis to examine β diversity, and its replacement and nestedness components in relation to local and regional predictors.
3. Despite a short latitudinal gradient, we detected a significant increase in species richness with decreasing latitude, underpinned by a strong temperature and precipitation gradient. Species replacement (balanced variation in abundance) was high at all spatial scales (wetland, dune area, ecoregion and coastline), and correlated with local and regional environmental variables.
4. Community dissimilarity showed no marked increase with spatial extent, which suggests a system where local-scale environmental filtering prevents dispersal driven homogenisation. Both local and bioclimatic factors were correlated with aquatic macroinvertebrate dissimilarity, but local factors played a larger role.
5. This study simultaneously examined the response of alpha and β diversity to geographical and environmental variables. Our results indicate that macroinvertebrates respond to abiotic factors by tracking suitable environmental conditions among locally variable interdunal wetlands. Thus, high dispersal along the coastline conveys resiliency to a hydrologically dynamic wetland system, which allows aquatic macroinvertebrates to contribute an integral portion of sand dune biodiversity within the Laurentian Great Lakes region. The high species turnover found suggests that conservation strategies should consider coastline connectivity among dune areas to maintain freshwater biodiversity.