Reducing dispersal limitation via seed addition increases species richness but not above‐ground biomass

Seed dispersal limitation, which can be exacerbated by a number of anthropogenic causes, can result in local communities having fewer species than they might potentially support, representing a potential diversity deficit. The link between processes that shape natural variation in diversity, such as dispersal limitation, and the consequent effects on productivity is less well known. Here, we synthesised data from 12 seed addition experiments in grassland communities to examine the influence of reducing seed dispersal limitation (from 1 to 60 species added across experiments) on species richness and productivity. For every 10 species of seed added, we found that species richness increased by about two species. However, the increase in species richness by overcoming seed limitation did not lead to a concomitant increase in above‐ground biomass production. This highlights the need to consider the relationship between biodiversity and ecosystem functioning in a pluralistic way that considers both the processes that shape diversity and productivity simultaneously in naturally assembled communities.     

Scale dependent drivers of wild bee diversity in tropical heterogeneous agricultural landscapes

Factors associated with agricultural intensification, for example, loss of seminatural vegetation and pesticide use has been shown to adversely affect the bee community. These factors may impact the bee community differently at different landscape scales. The scale dependency is expected to be more pronounced in heterogeneous landscapes. However, the scale‐dependent response of the bee community to drivers of its decline is relatively understudied, especially in the tropics where the agricultural landscape is often heterogeneous. This study looked at effects of agricultural intensification on bee diversity at patch and landscape scales in a tropical agricultural landscape. Wild bees were sampled using 12 permanent pan trap stations. Patch and landscape characteristics were measured within a 100 m (patch scale) and a 500 m (landscape scale) radius of pan trap stations. Information on pesticide input was obtained from farmer surveys. Data on vegetation cover, productivity, and percentage of agricultural and fallow land (FL) were collected using satellite imagery. Intensive areas in a bee‐site network were less specialized in terms of resources to attract rare bee species while the less intensive areas, which supported more rare species, were more vulnerable to disturbance. A combination of patch quality and diversity as well as pesticide use regulates species diversity at the landscape scale (500 m), whereas pesticide quantity drove diversity at the patch scale (100 m). At the landscape scale, specialization of each site in terms of resources for bees increased with increasing patch diversity and FL while at the patch scale specialization declined with increased pesticide use. Bee functional groups responded differentially to landscape characteristics as well as pesticide use. Wood nesting bees were negatively affected by the number of pesticides used but other bee functional groups were not sensitive to pesticides. Synthesis and Applications: Different factors affect wild bee diversity at the scale of landscape and patch in heterogeneous tropical agricultural systems. The differential response of bee functional groups to agricultural intensification underpins the need for guild‐specific management strategies for wild bee conservation. Less intensively farmed areas support more rare species and are vulnerable to disturbance; consequently, these areas should be prioritized for conservation to maintain heterogeneity in the landscape. It is important to conserve and restore seminatural habitats to maintain complexity in the landscapes through participatory processes and to regulate synthetic chemical pesticides in farm operations to conserve the species and functional diversity of wild bees.     

Determinants of avian species richness at different spatial scales

ABSTRACT. Studies of factors influencing avian biodiversity yield very different results depending on the spatial scale at which species richness is calculated. Ecological studies at small spatial scales (plot size 0.0025–0.4 km²) emphasize the importance of habitat diversity, whereas biogeographical studies at large spatial scales (quadrat size 400–50,000 km²) emphasize variables related to available energy such as temperature. In order to bridge the gap between those two approaches the bird atlas data set of Lake Constance was used to study factors determining avian species diversity at the intermediate spatial scales of landscapes (quadrat size 4–36 km²). At these spatial scales bird species richness was influenced by habitat diversity and not by variables related to available energy probably because, at the landscape scale, variation in available energy is small. Changing quadrat size between 4 and 36 km², but keeping the geographical extension of the study constant resulted in profound changes in the degree to which the amount of different habitat types was correlated with species richness. This suggests that high species diversity is achieved by different management regimes depending on the spatial scale at which species richness is calculated. However, generally, avian species diversity seems to be determined by spatial heterogeneity at the corresponding spatial scale. Thus, protecting the diversity of landscapes and ecosystems appears to ensure also high levels of species diversity.     

A multi‐scale analysis of fragmentation effects on remnant plant species richness in Phoenix,Arizona

Aim Understanding complex ecological phenomena, such as the determinants of species richness, is best achieved by investigating their properties at different spatial scales. Factors significantly affecting the number of species occurring at one scale may not impact on richness at other scales. While this scale dependence has become increasingly recognized, there still remains a need to elucidate exactly how richness is structured across scales, and which mechanisms are influential for determining this important community property. This study explores how woody plant species richness varies in a fragmented system at multiple scales, and which factors are primarily responsible for these patterns. Location The study area is located in the Sonoran Desert within the bounds of metropolitan Phoenix, Arizona, which is the locus of the Central Arizona–Phoenix Long‐Term Ecological Research (CAP‐LTER) site. Methods Estimates of local and fragment plant species richness were generated from field data collected from 22 sites. Independent variables describing fragment sites were also calculated, including area, habitat heterogeneity, density of individuals, mean elevation, and extent of isolation. Structural equation modelling, multiple regression, and analysis of covariance were used to assess the contribution of independent variables to richness at the fragment and local scales. Results Fragment species richness was significantly influenced by area, though not isolation, habitat heterogeneity, mean elevation, or density of individuals. Local richness was not significantly related to fragment area, but was positively related to fragment richness, plant density, and elevation. Main conclusions The fragment species–area effect resulted from larger remnants supporting higher numbers of individuals at comparable densities, increasing richness through either passive sampling of progressively less common species and/or lower extinction rates among larger populations. Without using multi‐temporal data it is not possible to disentangle these mechanisms. We found that patterns evident at one scale are not necessarily apparent at other scales, as elevation and density of individuals significantly affected richness at the local scale but not at the fragment scale. These results lend support to the concept that mechanisms influencing the species richness of natural communities may be operable only within certain domains and that relevant scales should be specified.     

Influence of crop type,heterogeneity and woody structure on avian biodiversity in agricultural landscapes

Agriculture is a primary factor underlying world-wide declines in biodiversity. However, different agricultural systems vary in their effects depending on their resemblance to the natural ecosystem, coverage across the landscape, and operational intensity. We combined data from the North American Breeding Bird Survey with remotely sensed measures of crop type and linear woody feature (LWF) density to study how agricultural type, woody structure and crop heterogeneity influenced the avian community at landscape scales across a broad agricultural region of eastern Canada. Specifically, we examined whether 1) avian diversity and abundance differed between arable crop agriculture (e.g., corn, soy) and forage (e.g., hay) and pastoral agriculture, 2) whether increasing the density of LWF enhances avian diversity and abundance, and 3) whether increasing the heterogeneity of arable crop types can reduce negative effects of arable crop amount. Avian diversity was lower in landscapes dominated by arable crop compared to forage agriculture likely due to a stronger negative correlation between arable cropping and the amount of natural land cover. In contrast, total avian abundance did not decline with either agricultural type, suggesting that species tolerant to agriculture are compensating numerically for the loss of non-tolerant species. This indicates that bird diversity may be a more sensitive response than bird abundance to crop cover type in agricultural landscapes. Higher LWF densities had positive effects on the diversity of forest and shrub bird communities as predicted. Higher crop heterogeneity did not reduce the negative effects of high crop amount as expected except for wetland bird abundance. In contrast, greater crop heterogeneity actually strengthened the negative effects of high crop amount on forest bird abundance, shrub-forest edge bird diversity and total bird diversity. We speculate that this was due to negative correlations between crop heterogeneity and the amount of shrub and forest habitat patches in crop-dominated landscapes in our study region. The variable response to crop heterogeneity across guilds suggests that policies aimed at crop diversification may not enhance avian diversity on their own and that management efforts aimed at the retention of natural forest and shrub patches, riparian corridors, and hedge-rows would be more directly beneficial.     

Community assembly, species richness and nestedness of arbuscular mycorrhizal fungi in agricultural soils

Understanding how communities assemble is a central goal of ecology. This is particularly relevant for communities of arbuscular mycorrhizal fungi (AMF), because the community composition of these beneficial plant symbionts influences important ecosystem processes. Moreover, AMF may be used as sensitive indicators of ecological soil quality if they respond to environmental variation in a predictable way. Here, we use a molecular profiling technique (T-RFLP of 25S rRNA gene fragments) to test which factors determine AM fungal community composition in 40 agricultural soils in the Netherlands. In particular, we test whether species richness, dominance structure and community nestedness are influenced by management type (in pairs of organically and conventionally farmed fields), and we examine the contribution of crop species (maize vs. potato), soil type (sand vs. clay-textured soils) and habitat (plant root vs. bulk soil) on AMF community characteristics. AMF richness varied from 1 to 11 taxa per field. Communities from species-poor fields were found to be subsets of those in richer fields, indicating nestedness and a progressive 'loss' from the species pool. AMF taxa richness and occurrence in soil and plant roots were highly correlated, and richness was related to management intensity (phosphate availability and grass-cropping history together explained 32% and 50% of richness in roots and soils). Soil type together with soil chemical parameters explained only 17% of variance in AMF community structure. We synthesize these results by discussing the potential contribution of a 'bottleneck effect' on AMF communities through increased stochastic effects under environmental stress.     

The influence of agricultural land-use intensity on bryophyte species richness

This study is a quantitative approach to the estimation of bryophyte species richness in relation to land-use intensity at three spatial scales in highly cultivated areas. A total of 460 randomly selected habitats and their various substrates within 29 study sites were investigated with regard to their land-use intensity and their bryophyte species richness in an agricultural region of eastern Austria. On bare soils (substrate-scale), low but regular disturbance increases bryophyte diversity in comparison to lower land-use intensity. However, more frequent disturbance (e.g. ploughing more than two times a year) dramatically reduces species richness at these sites, with more than 50% of these sites showing no bryophytes. The production of reproductive units (sporophytes and vegetative units) is highest at an intermediate disturbance regime. On the habitat, as well as on the landscape-scale, there is a significant increase in total bryophyte species number as well as in the number of threatened species with decreasing land-use intensity. This is mainly due to habitat and structural diversity, which increases with decreasing land-use intensity. There are significant correlations between landuse intensity, structural diversity and species richness at the habitat as well as on the landscape scale.     

Spatial patterns of invertebrate species richness in a river: the relationship between riffles and microhabitats

This study describes the pattern of invertebrate species richness in a river reach with large differences in habitat complexity at two, hierarchically nested, spatial scales. The aim was to determine whether the mass effect was likely to be increasing invertebrate species richness in epilithic microhabitats in this river. The mass effect is the process by which the species richness of a patch is increased when it acts as a ‘sink’ for species generated by ‘source’ patches. Microhabitat patch types in Mountain River, Tasmania, were distinguished on the basis of physical structure and orientation on the river bed. They were nested within two types of riffle with contrasting structural complexity: bedrock and boulder-cobble riffles. It was hypothesized that microhabitats with high species richness would act as source patches, contributing species to other microhabitats (sinks) and thereby increasing their species richness. Microhabitat sampling was carried out in four consecutive seasons and rarefaction was used to estimate riffle-scale species richness. Analysis of variance ( ANOVA ) was used to compare the identical microhabitats present in the contrasting riffle types, to detect evidence of the mass effect in either riffle type. The more structurally complex boulder-cobble riffles had higher species richness than did bedrock riffles. Amongst the microhabitats, the spaces beneath the cobbles had the most species. Microhabitats accounted for a higher percentage of the variation in species richness than did differences between riffles of the same type. No evidence was found for the operation of the mass effect in either riffle type. The majority of species found only in boulder-cobble riffles were unique to the beneath-cobble microhabitat and appeared to be unable to colonize other microhabitats, even as transients. In Mountain River, small-scale habitat characteristics appeared to be more important than larger-scale effects in determining microhabitat species richness.     

农业景观非农生境植物多样性及其影响因素

农业景观是人类生活所需资料的最主要来源地和生物多样性保护的重要区域之一。黄河中下游地区是我国最重要的农业区之一,在区域农业景观中,非农生境中各群落植物物种多样性组成特征有何差异?在不同尺度上,景观异质性特征与生境特征对植物物种多样性特征有何影响?在黄河中下游典型区域布点,对区内主要非农生境(次生林、次生灌丛、人工林和农田边缘等)中的植物群落物种多样性进行调查。结果显示:(1)次生灌丛中的物种丰富度和多样性显著高于其他生境,而农田边缘也保育有较多的物种;(2)从物种组成来看,景观尺度上样点间物种多样性差异(β_2)是所有生境中物种丰富度的最主要组成部分,而次生灌丛与次生林两类自然生境中小尺度(α和β1)物种丰富度的贡献相对较高。(3)景观异质性特征指标对物种多样性的影响主要体现在样方尺度上(α_(样方)),而β和γ多样性与之的关系并不甚密切。表征生境特征的群落高度和盖度指标能更好的预测物种多样性的特征。研究发现,常用的表征景观异质性的指数在各尺度上对植物群落物种多样性特征的影响也并不显著,局地群落特征的影响更为直接和重要。在探讨景观异质性特征与生物多样性关系时,常用的多样性指数(Shannon多样性指数、均匀度指数和Simpson指数等)并不合适,而拆分后的物种丰富度会更有效。但景观异质性对生物多样性的影响也不容忽视,它的改变会是影响群落物种多样性及其组成结构的重要潜在因素,在更大尺度上景观异质性会通过对景观组成要素特征(生境组成和构型)的影响进而影响到区域植物物种丰富度的变化。     

Dragonfly species richness on man-made ponds: effects of pond size and pond age on newly established assemblages

We studied the abundance and species richness of adult dragonflies in 11 artificial ponds which were recently established (within 2years). We found that the adult dragonfly assemblage patterns were influenced by pond size as well as pond age. The species richness was positively correlated with the pond size, which was because the distributional patterns of species were significantly nested according to pond area. The species richness was highly correlated with pond age in association with the vegetation cover within ponds. It was suggested that the species richness was enhanced by the increasing immigration rate of species which favor well-vegetated ponds.     

Plant functional group composition and large‐scale species richness in European agricultural landscapes

Question: Which are the plant functional groups responding most clearly to agricultural disturbances? Which are the relative roles of habitat availability, landscape configuration and agricultural land use intensity in affecting the functional composition and diversity of vascular plants in agricultural landscapes? Location: 25 agricultural landscape areas in seven European countries. Methods: We examined the plant species richness and abundance in 4 km × 4 km landscape study sites. The plant functional group classification was derived from the BIOLFLOR database. Factorial decomposition of functional groups was applied. Results: Natural habitat availability and low land use intensity supported the abundance and richness of perennials, sedges, pteridophytes and high nature quality indicator species. The abundance of clonal species, C and S strategists was also correlated with habitat area. An increasing density of field edges explained a decrease in richness of high nature quality species and an increase in richness of annual graminoids. Intensive agriculture enhanced the richness of annuals and low nature quality species. Conclusions: Habitat patch availability and habitat quality are the main drivers of functional group composition and plant species richness in European agricultural landscapes. Linear elements do not compensate for the loss of habitats, as they mostly support disturbance tolerant generalist species. In order to conserve vascular plant species diversity in agricultural landscapes, the protection and enlargement of existing patches of (semi‐) natural habitats appears to be more effective than relying on the rescue effect of linear elements. This should be done in combination with appropriate agricultural management techniques to limit the effect of agrochemicals to the fields.     

Trade-offs in community ecology: linking spatial scales and species coexistence

Trade‐offs in species performances of different ecological functions is one of the most common explanations for coexistence in communities. Despite the potential for species coexistence occurring at local or regional spatial scales, trade‐offs are typically approached at a single scale. In recent years, ecologists have increasingly provided evidence for the importance of community processes at both local and regional spatial scales. This review summarizes the theoretical predictions for the traits associated with trade‐offs under different conditions and at different spatial scales. We provide a spatial framework for understanding trade‐offs, coexistence and the supportive empirical evidence. Predictions are presented that link the patterns of diversity observed to the patterns of trade‐offs that lead to coexistence at different spatial scales. Recent evidence for the evolution of trade‐offs under different conditions is provided which explores both laboratory microcosm studies and phylogenetic tests. Examining trade‐offs within a spatial framework can provide a strong approach to understanding community structure and dynamics, while explaining patterns of species diversity.     

Scales and costs of habitat selection in heterogeneous landscapes

Summary Two scales of habitat selection are likely to influence patterns of animal density in heterogeneous landscapes. At one scale, habitat selection is determined by the differential use of foraging locations within a home range. At a larger scale, habitat selection is determined by dispersal and the ability to relocate the home range. The limits of both scales must be known for accurate assessments of habitat selection and its role in effecting spatial patterns in abundance. Isodars, which specify the relationships between population density in two habitats such that the expected reproductive success of an individual is the same in both, allow us to distinguish the two scales of habitat selection because each scale has different costs. In a two-habitat environment, the cost of rejecting one of the habitats within a home range can be expressed as a devaluation of the other, because, for example, fine-grained foragers must travel through both. At the dispersal scale, the cost of accepting a new home range in a different habitat has the opposite effect of inflating the value of the original habitat to compensate for lost evolutionary potential associated with relocating the home range. These costs produce isodars at the foraging scale with a lower intercept and slope than those at the dispersal scale.Empirical data on deer mice occupying prairie and badland habitats in southern Alberta confirm the ability of isodar analysis to differentiate between foraging and dispersal scales. The data suggest a foraging range of approximately 60 m, and an effective dispersal distance near 140 m. The relatively short dispersal distance implies that recent theories may have over-emphasized the role of habitat selection on local population dynamics. But the exchange of individuals between habitats sharing irregular borders may be substantial. Dispersal distance may thus give a false impression of the inability of habitat selection to help regulate population density.     

Mixed effects of habitat fragmentation on species richness and community structure in a microarthropod microecosystem

Abstract.  1. Theory is unclear about the optimal degree of isolation of habitat fragments where the aim is to maximise species richness. In a field-based microecosystem of Collembola and predatory and non-predatory mites, moss patches of the same total area were fragmented to varying degrees. The habitat was left for several months to allow the communities to approach a new state of equilibrium.
2. The species richness (in particular of predatory mites) of a given area of habitat was greater when it was part of a large mainland area than part of an island, in agreement with theory.
3. Conversely, species richness and abundance were largely unaffected by fragmentation of a fixed area of island habitat. In this case, it is suggested here that the advantages of several small patches (e.g. reduced impact of environmental stochasticity, wider range of habitats overall) were equally balanced by the advantages of a single large patch (e.g. reduced effect of demographic stochasticity, wider range of habitats within a single patch, reduced edge effect), or that both effects were small.
4. The shapes of rank–abundance curves were similar among the levels of fragmentation of a fixed area of island habitat, implying that fragmentation had little impact on community structure. Conversely, the species composition of non-predatory mites varied weakly, but significantly, by fragmentation.     

The Habitat Amount Hypothesis implies negative effects of habitat fragmentation on species richness

The habitat amount hypothesis (HAH) predicts that species richness in a habitat site increases with the amount of habitat in the ‘local landscape’ defined by an appropriate distance around the site, with no distinct effects of the size of the habitat patch in which the site is located. It has been stated that a consequence of the HAH, if supported, would be that it is unnecessary to consider habitat configuration to predict or manage biodiversity patterns, and that conservation strategies should focus on habitat amount regardless of fragmentation. Here, I assume that the HAH holds and apply the HAH predictions to all habitat sites over entire landscapes that have the same amount of habitat but differ in habitat configuration. By doing so, I show that the HAH actually implies clearly negative effects of habitat fragmentation, and of other spatial configuration changes, on species richness in all or many of the habitat sites in the landscape, and that these habitat configuration effects are distinct from those of habitat amount in the landscape. I further show that, contrary to current interpretations, the HAH is compatible with a steeper slope of the species–area relationship for fragmented than for continuous habitat, and with higher species richness for a single large patch than for several small patches with the same total area (SLOSS). This suggests the need to revise the ways in which the HAH has been interpreted and can be actually tested. The misinterpretation of the HAH has arisen from confounding and overlooking the differences in the spatial scales involved: the individual habitat site at which the HAH gives predictions, the local landscape around an individual site and the landscapes or regions (with multiple habitat sites and different local landscapes) that need to be analysed and managed. The HAH has been erroneously viewed as negating or diminishing the relevance of fragmentation effects, while it actually supports the importance of habitat configuration for biodiversity. I conclude that, even in the cases where the HAH holds, habitat fragmentation and configuration are important for understanding and managing species distributions in the landscape.