The biogeography and filtering of woody plant functional diversity in North and South America

Aim In recent years evidence has accumulated that plant species are differentially sorted from regional assemblages into local assemblages along local‐scale environmental gradients on the basis of their function and abiotic filtering. The favourability hypothesis in biogeography proposes that in climatically difficult regions abiotic filtering should produce a regional assemblage that is less functionally diverse than that expected given the species richness and the global pool of traits. Thus it seems likely that differential filtering of plant traits along local‐scale gradients may scale up to explain the distribution, diversity and filtering of plant traits in regional‐scale assemblages across continents. The present work aims to address this prediction. Location North and South America. Methods We combine a dataset comprising over 5.5 million georeferenced plant occurrence records with several large plant functional trait databases in order to: (1) quantify how several critical traits associated with plant performance and ecology vary across environmental gradients; and (2) provide the first test of whether the woody plants found within 1° and 5° map grid cells are more or less functionally diverse than expected, given their species richness, across broad gradients. Results The results show that, for many of the traits studied, the overall distribution of functional traits in tropical regions often exceeds the expectations of random sampling given the species richness. Conversely, temperate regions often had narrower functional trait distributions than their smaller species pools would suggest. Main conclusion The results show that the overall distribution of function does increase towards the equator, but the functional diversity within regional‐scale tropical assemblages is higher than that expected given their species richness. These results are consistent with the hypothesis that abiotic filtering constrains the overall distribution of function in temperate assemblages, but tropical assemblages are not as tightly constrained.     

The tangled link between β‐ and γ‐diversity: a Narcissus effect weakens statistical inferences in null model analyses of diversity patterns

Understanding the structure of and spatial variability in the species composition of ecological communities is at the heart of biogeography. In particular, there has been recent controversy about possible latitudinal trends in compositional heterogeneity across localities (β‐diversity). A gradient in the size of the regional species pool alone can be expected to impose a parallel gradient on β‐diversity, but whether β‐diversity also varies independently of the size of the species pool remains unclear. A recently suggested methodological approach to correct latitudinal β‐diversity gradients for the species pool effect is based on randomization null models that remove the effects of gradients in α‐ and γ‐diversity on β‐diversity. However, the randomization process imposes constraints on the variability of α‐diversity, which in turn force γ‐ and β‐diversity to become interdependent, such that any change in one is mirrored in the other. We argue that simple null model approaches are inadequate to discern whether correlations between α‐, β‐ and γ‐diversity reflect processes of ecological interest or merely differences in the size of the species pool among localities. We demonstrate that this kind of Narcissus effect may also apply to other metrics of spatial or phylogenetic species distribution. We highlight that Narcissus effects may lead to artificially high rejection rates for the focal pattern (Type II errors) and caution that these errors have not received sufficient attention in the ecological literature.     

A latitudinal gradient of beta diversity for exotic vascular plant species in North America

Determining relationships between the ranges of introduced species and geographical and environmental factors is an important step in understanding the mechanisms and processes of the spread of introduced species. In this study, I examined the beta diversity and latitude relationship for all naturalized exotic species of vascular plants in North America at a continental scale. Beta diversity was calculated as the absolute value of the slope of the relationship between the natural logarithm of the Simpson index of similarity (lnS) and spatial distance between pairs of state‐level exotic floras within four latitudinal zones examined. Relative contributions of spatial distance and environmental difference to species turnover between exotic floras were examined. I found that beta diversity decreased monotonically from low to high latitudes: beta diversity for the southernmost zone was shallower than that for the northernmost zone by a factor of 2.6. Regression models of lnS in relation to spatial distance and environmental (climatic and topographical) difference for each latitudinal zone demonstrated that the explanatory power of these variables diminishes monotonically with latitude: the explained variance in lnS is 70.4%, 62.1%, 53.9%, and 33.9%, respectively, for the four latitudinal zones from south to north. For the southernmost zone, 58.3% of the variance in lnS is explained by climate variables and topography, and spatial distance explains only 2.3% of the variance. In contrast, for the northernmost zone, more than half the amount (22.5%) of the explained variance in lnS is attributable to spatial distance, and the remaining (18.9%) of the explained variance is attributable to climate variables and topography.     

大别山南坡森林植物群落物种多样性及其与海拔因子的关系

大别山地区植物资源丰富,区系组成复杂且起源古老,为连接华东、华北和华中三大植物区系的纽带,也是我国重要的生物多样性保护和水源涵养生态功能区。采用样方法,在大别山南坡的多枝尖、庵基坪和麒麟沟3个地区,沿着不同海拔高度选取了具有代表性的森林植物群落进行研究,从不同植物群落类型和层次的物种多样性、均匀度和丰富度及其与海拔因子的关系等方面,对大别山南坡的森林植物群落物种多样性进行综合分析。结果表明:1. 共记录有植物108科270属449种,划分为20个森林植物群落类型;2. 森林植物群落各层次物种丰富度表现为草本>乔木>灌木;Shannon-Wiener多样性指数和Simpson多样性指数呈现出乔木>灌木>草本;Pielou均匀度指数变化较为复杂;3. 森林植物群落各层次的物种丰富度随海拔升高而下降;Shannon-Wiener指数和Simpson指数也表现为随着海拔升高而下降,但草本层在1400m之后有上升的趋势。Pielou指数在乔木层中表现为随着海拔的升高而下降,在草本层中表现为先下降后出现上升,在灌木层中则随着海拔的升高而上升,但其波动更为剧烈。本研究对大别山南坡森林植被大范围的采样观测研究,能够全面的展现大别山南坡森林植物的种类分布、空间组成等整体概况及其与海拔因子的关系,能为以后大别山南坡生物多样性的保护提供较为全面真实的数据,从而为大别山地区生物多样性的保护和可持续利用提供理论依据和实践意义。     

氮添加对内蒙古退化草地植物群落多样性和生物量的影响

人为干扰及气候变化导致内蒙古草地发生了大面积退化, 氮添加是促进退化草地生产力恢复的一项重要措施。该文基于2011年建立的氮肥添加实验平台, 以3个不同退化程度(中度退化、重度退化、极度退化)草地群落为研究对象, 设置对照、10、20、30、40和50 g·m ^-2·a ^-1 6种氮添加处理, 分析氮添加对退化草地恢复过程中群落多样性和生物量的影响。结果表明: (1)氮添加降低了中度、重度退化草地恢复进程中物种丰富度和多样性, 对极度退化草地恢复进程中物种丰富度和多样性无明显影响。(2)氮添加促进了3个不同退化程度草地恢复进程中群落地上生物量的增加。(3)氮添加显著增加了群落中禾草的地上生物量及其在群落地上生物量中所占的比例, 降低了杂类草在群落地上生物量中的比例, 但对杂类草地上生物量无显著影响。研究表明在利用施肥措施治理退化草地的过程中, 需要充分考虑草地退化程度以及由氮添加引起的群落多样性和生产力的改变对草地生态系统功能的影响。     

Latitudinal gradients of associations between beta and gamma diversity of trees in forest communities in the New World

Aims We analyze two continental data sets of forest communities from across the New World to examine the latitudinal gradients of beta diversity after accounting for gamma diversity and the latitudinal gradient of gamma diversity after accounting for beta diversity.Methods Correlation and regression analyses were used to relate beta and gamma diversity to latitude along two latitudinal gradients in the New World (one including 72 forest sites located south of the equator and the other including 79 forest sites located north of the equator).Important findings Beta diversity and gamma diversity were negatively correlated with latitude. Beta diversity was strongly and positively correlated with gamma diversity (Pearson's correlation coefficient: 0.783 for New World North and 0.848 for New World South). When beta diversity was regressed on latitude and gamma diversity, 69.8 and 85.7% of the variation in beta diversity were explained, respectively, for New World North and New World South. When gamma diversity was regressed on latitude and beta diversity, 81.8 and 84.3% of the variation in gamma diversity were explained, respectively, for New World North and New World South. After statistically removing the relationship between beta and gamma diversity, latitude has weak or no relationships with beta and gamma diversity. However, strong positive correlations between beta and gamma diversity may not be considered as evidence of one driving the other along a latitudinal gradient.     

Determining the relative roles of species replacement and species richness differences in generating beta‐diversity patterns

Aim To determine the relative contribution of species replacement and species richness differences to the emergence of beta‐diversity patterns. Innovation A novel method that disentangles all compositional differences (β_cc, overall beta diversity) in its two components, species replacement (β_‐3) and species richness differences (β_rich) is proposed. The performance of the method was studied with ternary plots, which allow visualization of the influence of the relative proportions of shared and unique species of two sites over each metric. The method was also tested in different hypothetical gradients and with real datasets. The novel method was compared with a previous proposal based on the partitioning of overall compositional differences (β_sor) in replacement (β_sim) and nestedness (β_nes). The linear response of β_cc contrasts with the curvilinear response of β_sor to linear gradients of dissimilarity. When two sites did not share any species, β_sim was always 1 and β_‐3 only reached 1 when the number of exclusive species of both sites was equal. β_‐3 remained constant along gradients of richness differences with constant replacement, while β_sim decreased. β_rich had a linear response to a linear gradient of richness differences with constant species replacement, whereas β_nes exhibited a hump‐shaped response. Moreover, β_sim > β_nes when clearly almost all species of one site were lost, whereas β_‐3 < β_rich in the same circumstances. Main conclusions The behaviour of the partition of β_cc into β_‐3 and β_rich is consistent with the variation of replacement and richness differences. The partitioning of β_sor into β_sim and β_nes overestimates the replacement component and underestimates richness differences. The novel methodology allows the discrimination of different causes of beta‐diversity patterns along latitudinal, biogeographic or ecological gradients, by estimating correctly the relative contributions of replacement and richness differences.     

High tropical net diversification drives the New World latitudinal gradient in palm (Arecaceae) species richness

Aim Species richness exhibits striking geographical variation, but the processes that drive this variation are unresolved. We investigated the relative importance of two hypothesized evolutionary causes for the variation in palm species richness across the New World: time for diversification and evolutionary (net diversification) rate. Palms have a long history in the region, with the major clades diversifying during the Tertiary (65–2 Ma). Location Tropical and subtropical America (34° N–34° S; 33–120° W). Methods Using range maps, palm species richness was estimated in a 1° × 1° grid. Mean lineage net diversification was estimated by the mean phylogenetic root distance (MRD), the average number of nodes separating a species from the base of the palm phylogeny for the species in each grid cell. If evolutionary rate limits richness, then richness should increase with MRD. If time limits richness, then old, relict species (with low root distance) should predominantly occur in long‐inhabited and therefore species‐rich areas. Hence, richness should decrease with MRD. To determine the influence of net diversification across different time frames, MRD was computed for subtribe, genus and species levels within the phylogeny, and supplemented with the purely tip‐level measure, mean number of species per genus (MS/G). Correlations and regressions, in combination with eigenvector‐based spatial filtering, were used to assess the relationship between species richness, the net diversification measures, and potential environmental and geographical drivers. Results Species richness increased with all net diversification measures. The regression models showed that richness and the net diversification measures increased with decreasing (absolute) latitude and, less strongly, with increasing energy/temperature and water availability. These patterns therefore reflect net diversification at both deep and shallow levels in the phylogeny. Richness also increased with range in elevation, but this was only reflected in the MS/G pattern and therefore reflects recent diversification. Main conclusions The geographical patterns in palm species richness appear to be predominantly the result of elevated net diversification rates towards the equator and in warm, wet climates, sustained throughout most of the Tertiary. Late‐Tertiary orogeny has caused localized increases in net diversification rates that have also made a mark on the richness pattern.     

Resource enrichment combined with biomass removal maintains plant diversity and community stability in a long-term grazed grassland

长期重度放牧通过加剧水氮限制减少植物多样性与生态系统功能。相反,缺乏放牧干扰(生物量移除)会通过增加光竞争减少物种多样性,从而弱化生态系统稳定性,而这一过程会由于资源富集而加剧。因此,如何维持物种多样性和群落稳定性仍然是世界草原可持续管理的一大挑战。在本研究中,我们在中国内蒙古3个放牧强度的长期实验样地设置了为期4年的控制实验,来探究结合资源添加与生物量移除(同时增加水、氮和光的有效性)对物种丰富度与群落稳定性的影响。研究结果表明,在4年资源添加结合生物量移除处理下,群落初级 生产力增加,同时物种丰富度与群落稳定性能够维持。在物种与功能群水平上,地上生产力能够保持时间稳定性。物种丰富度维持的主要 原因是：在资源富集的情况下,生物量移除能够增加光的有效性。物种异步性与功能群稳定性有助于群落稳定性维持。我们的研究结果表 明,资源添加与生物量移除结合的管理方式,不仅能够增加草地初级生产力,而且能维持其物种多样性、异步性与群落稳定性。在世界范 围内,长期放牧导致的草原退化与资源富集引起的其生物多样性丧失仍将是人类面临的主要生态问题,我们的研究结果对于半干旱草原以 及其它草原的适应性管理具有重要的意义。     

Differences in species diversity of Monogenea between the Pacific and Atlantic Oceans

Data from five extensive surveys each in the Pacific and Atlantic Oceans show that relative species diversity (number of parasite species per host species) of gill Monogenea of coastal marine fishes is greater in the northern and southwestern Pacific than in the northeastern and central- and southwestern Atlantic. Relative species diversity is markedly lower in the cold northeastern Atlantic than in the warmer parts of the Atlantic examined, and in the northern Pacific than in the warm southwestern Pacific. The difference between the northern Pacific and Atlantic is entirely or almost entirely due to a much greater number of species of Gyrodactylidae in the northern Pacific. A species-area relationship cannot explain the difference, because the area of the northern Pacific is not larger than that of the northern Atlantic and because Gyrodactylidae are cold-water forms which cannot have immigrated from warmer seas. The difference is tentatively explained by an evolutionary time hypothesis: more species of Gyrodactylidae have accumulated in the much older Pacific than in the Atlantic Ocean. Alternatively, an ecological time hypothesis may explain the difference: ice sheets during the last glaciation covered much more of the continental shelf in the northern Atlantic than in the northern Pacific, possibly extinguishing more Monogenea in the former than in the latter Ocean.     

Seasonality regulates the effects of resource addition on plant diversity and ecosystem functioning in semi-arid grassland

季节性调控资源添加对半干旱草原物种多样性与生态系统功能的影响在资源富集的条件下,物种丰富度、群落地上生产力以及群落稳定性的季节变化通常被忽视。本研究致力于探究在干旱区草原,资源添加如何在生长季的不同月份影响物种丰富度、群落地上生产力及其稳定性。我们在内蒙古草原设置了为期3年的资源添加(氮添加-N、水添加-W以及水、氮共同添加-NW)实验,利用季节性取样的方法,去检验资源添加(水、 氮)对物种丰富度、群落地上生产力及其稳定性的影响,并通过构建结构方程模型分析资源添加调控不同月份与整个生长季群落稳定性的内在机制及其相对重要性。研究结果表明,资源添加在整体上未改变5与6月的群落地上生产力,而氮与水氮共同添加显著提高了7与8月的群落地上生产力。资源添加在整体上未改变物种丰富度、物种异步性与群落稳定性。氮添加与水氮共同添加提高了7与8月的群落地上生产力,主要源于其增加了多年生丛生禾草的地上生产力。结构方程模型分析表明：在生长季前期与整个生长季,物种异步性是决定群落稳定性的主要机制;在生长季后期,多年生丛生禾草的稳定性是驱动群落稳定性的主要因子。我们的研究证明：在半干旱草原,季节与资源的有效性可以交互影响群落地上生产力及其稳定性。这些发现对于半干旱草原的季节性可持续管理具有重要意义,以期减轻土地利用与全球变化带来的影响。     

长期封育演替下典型草原植物群落生产力与多样性关系

封育是退化草地的重要恢复措施, 理解长期封育过程中草地群落生产力和植物多样性变化特征及两者间关系, 有助于草地植被的恢复管理与利用。该研究依托宁夏云雾山国家级自然保护区典型草原长期封育演替梯度, 选择持续放牧、封育9年、26年和34年的草地群落作为研究对象, 分析其地上生产力、物种多样性和功能多样性的变化特征及内在联系。结果表明, 封育显著提高典型草原植物群落的地上生产力、凋落物生物量、功能丰富度和功能离散度, 未改变草地群落的物种丰富度、Shannon-Wiener指数和功能均匀度, 但Simpson优势度指数和Pielou均匀度指数在长期封育(34年)后显著下降。此外, 封育对不同植物群落加权平均功能性状的影响存在差异。随机森林模型和方差分解结果显示, 群落加权平均功能性状对封育草地群落地上生产力变异的解释度高达70.70%, 其中植株高度是最主要的解释因子; 功能多样性的解释度为36.86%, 主要由功能丰富度贡献; 而物种多样性的解释度仅为14.72%。由此可见, 植物功能性状和功能多样性对草地群落地上生产力的贡献远高于物种多样性, 建议将其纳入植物群落恢复演替动态研究, 以便全面了解植物多样性与生态系统功能的关系, 为更好地实现生态恢复目标奠定基础。