Patterns of species density and productivity at different spatial scales in herbaceous plant communities

A major challenge in evaluating patterns of species richness and productivity involves acquiring data to examine these relationships empirically across a range of ecologically significant spatial scales. In this paper, we use data from herb‐dominated plant communities at six Long‐Term Ecological Research (LTER) sites to examine how the relationship between plant species density and above‐ground net primary productivity (ANPP) differs when the spatial scale of analysis is changed. We quantified this relationship at different spatial scales in which we varied the focus and extent of analysis: (1) among fields within communities, (2) among fields within biomes or biogeographic regions, and (3) among communities within biomes or biogeographic regions. We used species density (D=number of species per m²) as our measure of diversity to have a comparable index across all sites and scales. Although we expected unimodal relationships at all spatial scales, we found that spatial scale influenced the form of the relationship. At the scale of fields within different grassland communities, we detected a significant relationship at only one site (Minnesota old‐fields), and it was negative linear. When we expanded the extent of analyses to biogeographic regions (grasslands or North America), we found significant unimodal relationships in both cases. However, when we combined data to examine patterns among community types within different biogeographic regions (grassland, alpine tundra, arctic tundra, or North America), we did not detect significant relationships between species density and ANPP for any region. The results of our analyses demonstrate that the spatial scale of analysis – how data are aggregated and patterns examined – can influence the form of the relationship between species density and productivity. It also demonstrates the need for data sets from a broad spectrum of sites sampled over a range of scales for examining challenging and controversial ecological hypotheses.     

Convergence of fish communities from the littoral zone of reservoirs

1. Understanding factors that regulate the assembly of communities is a main focus of ecology. Human‐engineered habitats, such as reservoirs, may provide insight into these assembly processes because they represent novel habitats that are subjected to colonization by fishes from the surrounding river basin or transported by humans. By contrasting community similarity within and among reservoirs from different drainage basins to nearby stream communities, we can test the relative constraints of reservoir habitats and regional species pools in determining species composition of reservoirs. 2. We used a large spatial database that included intensive collections from 143 stream and 28 reservoir sites within three major river basins in the Great Plains, U.S.A., to compare patterns of species diversity and community structure between streams and reservoirs and to characterize variation in fish community structure within and among major drainage basins. We expected reservoir fish faunas to reflect the regional species pool, but would be more homogeneous that stream communities because similar species are stocked and thrive in reservoirs (e.g. planktivores and piscivores), and they lack obligate stream organisms that are not shared among regional species pools. 3. We found that fish communities from reservoirs were a subset of fishes collected from streams and dominant taxa had ecological traits that would be favoured in lentic environments. Although there were regional differences in reservoir fish communities, species richness, patterns of rank abundance and community structure in reservoir communities were more homogonous across three major drainage basins than for stream communities. 4. The general pattern of convergence of reservoir fish community structure suggests their assembly is constrained by local factors such as habitat and biotic interactions, and facilitated by the introduction of species among basins. Because there is a reciprocal transfer of biota between reservoirs and streams, understanding factors structuring both habitats is necessary to evaluate the long‐term dynamics of impounded river networks.     

Hyperparasitoid aggregation in response to variation in Aphidius ervi host density at three spatial scales

1. This article investigates the pattern of hyperparasitism of the host Aphidius ervi Haliday (Hymenoptera, Aphidiidae), a primary parasitoid of the pea aphid, Acyrthosiphon pisum (Harris) (Homoptera: Aphididae) at three spatial scales.
2. In the laboratory, the hyperparasitoid Asaphes lucens (Provancher) (Hymenoptera: Pteromalidae) was introduced into cages containing sixteen alfalfa plants with varying numbers of A. ervi mummies (the stage susceptible to hyperparasitism). The pattern of hyperparasitism at the end of the 48-h trials showed no density-dependent hyperparasitoid aggregation, although there was strong density-independent hyperparasitoid aggregation.
3. In the field, the density of A. ervi mummies was manipulated in twelve 2 × 2-m plots containing 1309–1654 alfalfa stems. Variation in hyperparasitism among plots showed no density-dependent aggregation, although there was strong density-independent aggregation.
4. Finally, at the largest scale of the study, the distribution of hyperparasitism was sampled among twelve alfalfa fields within a 5 × 3-km area. At this scale there was both density-dependent and density-independent hyperparasitoid aggregation.
5. The natural variation in A. ervi mummy density is greatest at the larger scales of study. Therefore, density-dependent hyperparasitism occurs only when there is high natural variation in mummy density.     

植物群落密度调控研究进展

植物群落密度调控是生态学领域的研究热点。从Yoda提出-3/2自疏法则以来,围绕植物群落密度调控规律从自疏线可变性、环境因素对密度调控影响的规律等进行了大量研究。尤其是基于代谢生态学理论建立的WBE自疏规律提出后,密度调控规律的争论和研究更为活跃,其焦点主要是自疏线斜率是否存在相对恒定的α值和值的大小。根据国内外文献,综述了-3/2自疏法则、-4/3自疏法则、质量-密度的等速比例关系和环境对密度调控规律的影响等方面的研究,指出密度调控的机制以及胁迫条件下的密度调控及其应用是未来研究的趋势。     

The distribution of the macrobenthos of the White Sea littoral in different spatial scales

Spatial distribution of macrobenthos of middle intertidal zone was studied in scale from centimetres to 30 kilometres along the coastline. The community structure and distribution of the 5 most abundant species (Hydrobia ulvae, Mya arenaria, Macoma baltica. Peloscolex benedeni, Arenicola marina) were considered. Spatial heterogeneity of macrobenthos, estimated as mean dissimilarity between samples, kept constant in scale of centimetres--meters, but increased significantly when enlarged area is considered. Patterns of many species changed with scale from random mosaic to more or less pronounced patchiness, whereas the density of H. ulvae and the structure of the whole community demonstrated fractal (self-similar) patch pattern in wide range of scale from dozens of meters to several kilometres. Spatial correlations between species (the composition of assemblages) and between species and environmental factors were also scale dependent. Some possible effects of scale on the observed spatial distribution of benthos are discussed, and multiscaled analysis of biotic heterogeneity is concluded to be very fruitful.     

东湖沿岸带底栖藻类群落的时空变化

武汉东湖是一个浅水富营养湖泊,其沿岸带底栖藻类群落的季节分布特征明显,夏季的优势种群是一些体积很小的硅藻类曲壳藻(Achnanthes spp.),其他大部分季节是绿藻门的团集刚毛藻(Cladophora glomerata)和硅藻门的变异直链藻(Melosira varians)。不同样点间的底栖藻类现存量差异不显著,但优势种类不同,以团集刚毛藻为优势的绿藻在处于中富营养区域的磨山样点比例最高,以变异直链藻为优势种的硅藻在严重富营养化的水果湖样点中比例最高。底栖藻类月平均现存量(以叶绿素a计)的时间变化格局为夏季最低(41.5μg/cm2),逐渐增加到秋、冬季节的最高峰值(55.3μg/cm2)。造成东湖夏季底栖藻类现存量低的原因是雨季水体环境的不稳定性和水位升高造成的低光照。不同样点间的chlb、chlc及chlb/a、chlc/a值具有显著差异,分析这种差异显示:在东湖的富营养化条件下,过渡的富营养化能增加底栖藻类群落中硅藻的比例,却降低了绿藻的比例;不同环境条件下的底栖藻类群落结构具有特异性,分析底栖藻类群落结构可以用来评价水环境健康状况。东湖底栖藻类现存量的季节变化规律与浮游藻类不同,它和水温成显著的负相关,但温度和浮游藻类并不是影响东湖沿岸带底栖藻类生物量变化的直接因子。团集刚毛藻等底栖丝状藻类给各种底栖动物提供了食物和栖息环境,在东湖生态系统中具有重要的作用和功能。     

Structure and dynamics of zooplankton communities in the littoral zone of some North Carolina lakes

Species composition and seasonal, dynamics of zooplankton in the littoral zone were studied in three piedmont North Carolina lakes for one year. Nygaard's compound index indicated oligotrophic conditions in Belews Lake and eutrophic conditions in Salem Lake and Lake 150. A total of 32 species of zooplankton were found in the samples. Significantly fewer species of truely littoral zooplankton were present in the oligotrophic lake regardless of season (P < 0.05). Eutrophication appears to favor increased diversity in zooplankton of the littoral zone. Habitat heterogeneity, provided by a well developed littoral zone containing aquatic macrophytes, may be the major factor contributing to the maintenance of more taxa of zooplankton in eutrophic systems.Distinct taxa of Cladocera and Copepoda are present in the littoral zone, exhibiting seasonal dynamics that are largely different from the limnetic fauna. Such dynamics may result from predation by characteristically limnetic species of cyclopoid copepods which seek prey in the littoral zone. The numerical and seasonal abundance of predators and prey are inversely related and the true littoral taxa, represented in the present study by the genera Alona, Chydorus, Pleuroxus, Sida, Simocephalus, and Eucyclops, contained no predaceous species. The seasonal dynamics of these genera are very similar even in widely separated geographical regions, indicating that the factor(s) responsible operate independently of climatic and chemical variables. Intrazooplankton predation appears to be a more plausable explanation than segregation along depth which, although consistent with data from studies of limnetic waters, cannot be related to populations of zooplankton in shallow littoral areas.     

Variation in coral-associated cryptofaunal communities across spatial scales and environmental gradients

Coral Reefs - Most of the diversity on coral reefs is in the cryptofauna, the hidden organisms that inhabit the interstitial spaces of corals and other habitat-forming benthos. However, little is...     

Temporal variation in juvenile fish communities of Kaunas reservoir littoral zone,Lithuania

The aim of this study was to assess juvenile fish communities in terms of species composition, fish diversity and density in the littoral zone of the Kaunas reservoir before (in 1989–1990, period I) and after (in 1999–2000, period II, and in 2006–2007, period III) launching the Kruonis hydroelectric pumped plant (Kruonis HPP). During the whole research period, 20 fish species were caught. According to the frequency of occurrence, the three-spined stickleback Gasterosteus aculeatus, European perch Perca fluviatilis and roach Rutilus rutilus were regarded as constant species in all investigated periods. Significant differences were established in juvenile fish community density between period I and periods II and III, whereas species richness (S) and species diversity indices (H′, J′) did not change significantly. The density of the shoreline community in period III was more than two times lower than in period I, probably due to higher fluctuations in water level of the reservoir, resulting from the Kruonis HPP operation.     

尺度与密度:测定不同尺度下的种群密度

群落中的种群密度由于空间尺度的变化而存在着一定差异,那么,某一种群的密度随着空间尺度的变化会发生怎样的变化?抑或某一物种相对于另一物种而言,随着空间尺度的变化其密度会怎样变化?这是与尺度有关的种群密度问题,当属生态学的基本问题.该文提出这样的问题,并把不同尺度下的种群密度称之为尺度密度(scale density).O-Ring函数的实质是计算不同尺度下的种群密度.因此,在研究实例中,应用O-Ring函数计算了典型草原处于不同恢复阶段的羊草(Leymuschinensis)种群、米氏冰草(Agropyron michnoi)种群,以及米氏冰草种群相对于羊草种群在不同尺度下的种群密度,结果发现:羊草和米氏冰草2个种群的尺度密度,在小尺度范围内严重退化群落均高于恢复演替群落,这一结果验证了“胁迫梯度假说”,同时表明该结果是放牧胁迫下正相互作用所致;通过比较羊草种群与米氏冰草相对于羊草的尺度密度发现,在严重退化的群落中,羊草与米氏冰草的种间关联为负联结,这种负联结是由正相互作用引起的,而在恢复8年和恢复21年群落中,二者之间是正联结,当为竞争所致.该实例说明分析种群密度随尺度变化的规律对于深入认识生态学问题可能会有很大帮助.     

深圳湾潮间带1995-2010年大型底栖动物群落的时空变化

本文根据1995-2010年在深圳湾潮间带5条断面采集的大型底栖动物数据,分析了群落的物种数、栖息密度、多样性指数(H)、丰度指数(d)和多变量海洋底栖生物指数(M-AMBI)的时空变化.结果表明,物种数、多样性指数、丰度指数和多变量海洋底栖生物指数存在明显的空间差异,即距离深圳河口越近,大型底栖动物物种数越少,多样性指数、丰度指数和多变量海洋底栖生物指数越低.空间差异还体现在越靠近深圳河口的取样断面,大个体物种,如羽须鳃沙蚕(Dendronereis pinnaticirris)和腺带刺沙蚕(Neanthes glandicincta)的栖息密度越低,而小个体、生命周期短的小头虫(Capitella capitata)和寡毛类动物栖息密度所占的比例越高,丰度生物量比较(abundance biomass comparison,简称ABC)法证实了深圳湾潮间带大型底栖动物群落的空间差异.造成这种差异的原因推测是由于深圳市人口的增加和工、农业的发展,排入深圳河的污水量增加,导致距离深圳河越近,有机质含量越高.大型底栖动物群落的物种数、栖息密度、多样性指数、丰度指数、多变量海洋底栖生物指数的年和季节变化各不相同,没有明显的变化趋势.     

Multiple spatial scales affect direct and indirect interactions between a non-native and a native species

Plant Ecology - Plant–plant interactions influence community assembly and species responses to environmental change. However, species interactions are complex phenomena influenced by context...     

A review on spatial aggregation methods involving several time scales

This article is a review of spatial aggregation of variables for time continuous models. Two cases are considered. The first case corresponds to a discrete space, i.e. a set of discrete patches connected by migrations, which are assumed to be fast with respect to local interactions. The mathematical model is a set of coupled ordinary differential equations (O.D.E.). The spatial aggregation allows one to derive a global model governing the time variation of the total numbers of individuals of all patches in the long term. The second case considers a continuous space and is a set of partial differential equations (P.D.E.). In that case, we also assume that diffusion is fast in comparison with local interactions. The spatial aggregation allows us again to obtain an O.D.E. governing the total population density, which is obtained by integration all over the spatial domain, at the slow time scale. These aggregations of variables are based on time scales separation methods which have been presented largely elsewhere and we recall the main results. We illustrate the methods by examples in population dynamics and prey–predator models.     

Diversity partitioning of moorland plant communities across hierarchical spatial scales

Understanding of the scaling of diversity is critical to enhance conservation strategies for subalpine moorland ecosystems vulnerable to future environmental changes. However, a paucity of quantitative data strongly limits such attempts. In this study, we used an additive diversity partitioning framework and quantified diversity patterns of moorland plant communities across hierarchical spatial scales, within- and between-sample transects, and between sites (corresponding to α and two levels of β diversity). Moorland sites markedly differed in size (range 1,000–160,000 m²) and were isolated from each other to varying extents within an inhospitable matrix (i.e., forests). We found that β diversity components were consistently higher, whereas the local α diversity component was consistently lower than expected by chance. We observed substantial contribution at the between-site scale to total species richness. By focusing on diversity patterns of moorland plant communities across multiple hierarchical spatial scales, we could thus identify the scale at which regional diversity is maximized. Our results suggest that protection of as many moorland sites as possible, to ensure beta diversity between sites, will effectively conserve total diversity. The use of additive diversity partitioning is a major step forward in providing strategies for the biological conservation of subalpine moorland ecosystems vulnerable to future environmental changes.     

Phytoplankton communities in the littoral zone of lakes: Observations on structure and dynamics in oligotrophic and eutrophic systems

Summary The structure and seasonal dynamics of phytoplankton communities in the littoral zone were compared between oligotrophic and eutrophic lakes in the southeastern United States. Differences in diversity and species composition between lakes could be ascribed to long-term variation in nutrients corresponding to trophic status. However, significant within-lake variation could not be accounted for by microstratification of nutrients or other abiotic variables. Local biotic factors, perhaps dominated by the spawning activities of centrarchid fishes, resuspend periphyton and generate tychoplankton which becomes a persistent and integral part of the phytoplankton community in eutrophic systems. The patchy distribution of these biotic factors and resultant tychoplankton may lead to the observed variation. Grazing by herbivorous zooplankton was considered to be the major factor affecting the relative abundance of phytoplankton in the littoral zone, completely overriding the effects of nutrient concentration and biotic interactions between phytoplankton species during spring and summer.The relative importance of tychoplankton and grazing as regulatory factors operates independently of the trophic status or geographical location of a lake, making comparisons of different studies difficult and perhaps meaningless if traditional analyses based only on nutrients and interactions between species of phytoplankton are used. Limnetic as well as littoral components must be considered in future studies of phytoplankton communities in the littoral zone.     

Food-web structure of seagrass communities across different spatial scales and human impacts

Seagrass beds provide important habitat for a wide range of marine species but are threatened by multiple human impacts in coastal waters. Although seagrass communities have been well-studied in the field, a quantification of their food-web structure and functioning, and how these change across space and human impacts has been lacking. Motivated by extensive field surveys and literature information, we analyzed the structural features of food webs associated with Zostera marina across 16 study sites in 3 provinces in Atlantic Canada. Our goals were to (i) quantify differences in food-web structure across local and regional scales and human impacts, (ii) assess the robustness of seagrass webs to simulated species loss, and (iii) compare food-web structure in temperate Atlantic seagrass beds with those of other aquatic ecosystems. We constructed individual food webs for each study site and cumulative webs for each province and the entire region based on presence/absence of species, and calculated 16 structural properties for each web. Our results indicate that food-web structure was similar among low impact sites across regions. With increasing human impacts associated with eutrophication, however, food-web structure show evidence of degradation as indicated by fewer trophic groups, lower maximum trophic level of the highest top predator, fewer trophic links connecting top to basal species, higher fractions of herbivores and intermediate consumers, and higher number of prey per species. These structural changes translate into functional changes with impacted sites being less robust to simulated species loss. Temperate Atlantic seagrass webs are similar to a tropical seagrass web, yet differed from other aquatic webs, suggesting consistent food-web characteristics across seagrass ecosystems in different regions. Our study illustrates that food-web structure and functioning of seagrass habitats change with human impacts and that the spatial scale of food-web analysis is critical for determining results.