Sampling strategies for the assessment of tree species diversity

Abstract. This paper aims at proposing efficient vegetation sampling strategies. It describes how the estimation of species richness and diversity of moist evergreen forest is affected by (1) sampling design (simple random sampling, random cluster sampling, systematic cluster sampling, stratified cluster sampling); (2) choice of species richness estimators (number of observed species vs. non-parametric estimators) and (3) choice of diversity index (Simpson vs. Shannon). Two sites are studied: a 28-ha area situated in the Western Ghats of India and a 25-ha area located at Pasoh in Peninsular Malaysia. The results show that: (1) whatever the sampling strategy, estimates of species richness depend on sample size in these very diverse forest ecosystems which contain many rare species; (2) Simpson's diversity index reaches a stable value at low sample sizes while Shannon's index is affected more by the addition of rare species with increasing sample size; (3) cluster sampling strategies provide a good compromise between cost and statistical efficiency; (4) 300 - 400 sample trees grouped in small clusters (10–50 individuals) are enough to obtain unbiased and precise estimates of Simpson's index; (5) the local topography of the Western Ghats has a major influence on forest composition, the steep slopes being richer and more diverse than the ridges and gentle slopes; (6) stratified cluster sampling is thus an interesting alternative to systematic cluster sampling.     

Effects of different cultivation types on native and alien weed species richness and diversity in Moravia (Czech Republic)

Changes in weed species richness and beta-diversity are partly attributable to different types and intensity of disturbance and partly to broad-scale variation in environmental conditions. We compiled a data set of 434 vegetation plots of weed vegetation in root crop and cereal fields in Moravia (eastern Czech Republic) to compare the effects of environmental conditions and different disturbance regimes on species richness and beta-diversity. To detect changes in species richness, we related the variation in species richness to individual environmental conditions. To assess differences in beta-diversity between the vegetation of cereal and root crop fields, we used Whittaker's measure of beta-diversity. The relative importance of each environmental variable for the variation in species composition was evaluated using canonical correspondence analysis. All analyses were done for all vascular plant species and separately for native species, archaeophytes and neophytes. A comparison of weed vegetation of root crops and cereals showed a distinct dichotomy between these two types of weed vegetation. There was no significant difference in total species richness and native species richness; however, cereal fields were richer in archaeophytes and root crop fields were richer in neophytes. The beta-diversity of weed vegetation was higher in root crops. Environmental factors explained a significant part of the variability in richness of both natives and aliens. The richness of native species increased and beta-diversity decreased with increasing precipitation. The opposite relationship was found for archaeophytes, in both cereals and root crops. These results confirmed the importance of climatic factors and management practices for changes in weed species composition. They also showed a distinct pattern of species richness and beta-diversity of native and alien weed species.     

Interpreting biogeographical boundaries among Afrotropical birds: spatial patterns in richness gradients and species replacement

Aim We analyse the geographical distribution of 1911 Afrotropical bird species using indices of three simple biogeographic patterns. The first index, the frequency of species with range edges (T_e), is formulated to map directly the density of species distribution limits, for comparison with the results of traditional biogeographical classification and ordination procedures, in order to show variations in the strength and breadth of transition zones. The other two indices are formulated to seek to distinguish as directly as possible between two components within these transition-zone patterns: contributions from gradients in species richness (T_g); and contributions from replacements among species (T_r). We test the ability of these indices to discover the same boundaries among Afrotropical bird faunas as one popular procedure for classifying areas (TWINSPAN) and then use them to look for geographical trends in the different kinds of transition zones. Location The analysis is restricted to the sub-Saharan or Afrotropical region, excluding the Arabian Peninsula, Madagascar and all offshore islands. Methods We record the presence of each species in 1961 1°×1° grid cells of the map. To apply the three indices, each (core) grid cell in turn is compared with its neighbouring eight cells in the grid. The range edges index (T_e) counts the number of species with range edges between the core cell and the surrounding cells. The richness gradients index (T_g) counts the largest difference in species richness measured diametrically across the core cell in any direction when there is a consistent trend in richness along this line of three cells. The species replacements index (T_r) counts the number of species pairs recorded within a nine-cell neighbourhood that are not corecorded within any of the cells. Values for each of the 1961 grid cells are calculated and used to produce colour-scale maps of transition zones. Results Large-scale spatial patterns of variation in density of range edges (T_e) are consistent with classifications of the same data and with most previous biogeographical classifications proposed for the region. Variation in richness gradients (T_g) and species replacements (T_r) explain different parts of this pattern, with transition zones around humid forests in the equatorial region being dominated by species replacement, and transition zones around deserts (most extensive in the north and south) being dominated by richness gradients. Main conclusions The three indices distinguish the spatial arrangement and intensity of different kinds of transition zones, thereby providing a first step towards a more rigorous mechanistic understanding of the different processes by which they may have arisen and are maintained. As an example of one such pattern shown by our analyses of Afrotropical birds, there is evidence for a broad latitudinal trend in the nature of transition zones in faunal composition (following the latitudinal distribution of the different kinds of habitat transitions), from being dominated by species replacements near the equator to being dominated by richness gradients further from the equator.     

Plant species richness and diversity along an altitudinal gradient in the Sierra Nevada, Mexico

This article presents an analysis of plant species richness and diversity and its association with climatic and soil variables along a 1300‐m elevation gradient on the Cerro Tláloc Mountain in the northern Sierra Nevada in Mexico. Two 1000‐m² tree sampling plots were created at each of 21 selected sampling sites, as well as two 250‐m² plots for shrubs and six 9‐m² plots for herbaceous plants. Species richness and diversity were estimated for each plant life form, and beta diversity between sites was estimated along the gradient. The relationship between species richness and diversity and environmental variables was modelled using simple linear correlation and regression trees. Species richness and diversity showed a unimodal pattern with a bias towards high values in the lower half of the elevation gradient under study. This response was consistent for all three life forms. Beta diversity increased steadily along the elevation gradient, being lower between contiguous sites at intermediate elevations and high – the species replacement rate was nearly 100%– between sites at the extremes of the gradient. Few species were adapted to the full spectrum of environmental variation along the elevation gradient studied. The regression tree suggests that differences in species richness are mainly influenced by elevation (temperature and humidity) and soil variables, namely A₂ permanent wilting point, organic matter and horizon field capacity and A₁ horizon Mg²⁺.     

Interactive effects of species richness and species traits on functional diversity and redundancy

The importance of species diversity for ecosystem function has emerged as a key question for conservation biology. Recently, there has been a shift from examining the role of species richness in isolation towards understanding how species interact to effect ecosystem function. Here, we briefly review theoretical predictions regarding species contributions to functional diversity and redundancy and further use simulated data to test combined effects of species richness, number of functional traits, and species differences within these traits on unique species contributions to functional diversity and redundancy, as well as on the overall functional diversity and redundancy within species assemblages. Our results highlighted that species richness and species functional attributes interact in their effects on functional diversity. Moreover, our simulations suggested that functional differences among species have limited effects on the proportion of redundancy of species contributions as well as on the overall redundancy within species assemblages, but that redundancy rather was determined by number of traits and species richness. Our simulations finally indicated scale dependence in the relative effects of species richness and functional attributes, which suggest that the relative influence of these factors may affect individual contributions differently compared to the overall ecosystem function of species assemblages. We suggest that studies on the relationship between biological diversity and ecosystem function will benefit from focusing on multiple processes and ecological interactions, and that the relative functional attributes of species will have pivotal roles for the ecosystem function of a given species assembly.     

Plant and insect diversity along a pollution gradient: understanding species richness across trophic levels

We analysed species richness of plants and true bugs (Insecta, Heteroptera) along a pollution gradient in Scots pine stands in Central Germany. As a consequence of particulate deposition, pH-values of soils increased in the vicinity of the emission source. Therefore, emission increased productivity. Species richness of plants increased with decreasing distance from emission source, and thus with increasing productivity. Similarly, species richness of herbivorous Heteroptera increased with decreasing distance from emission source, whereas, surprisingly, abundance decreased. The proportion of specialised herbivorous bug species is largest in the vicinity of the emission source. Thus, the diversity pattern of herbivores may be explained by the specialisation hypothesis and not the consumer rarity hypothesis. Species richness and abundance of carnivorous Heteroptera showed no significant trend along the gradient. Overall our data favour the bottom-up control of species diversity in the analysed system.     

Statutory protected areas and avian species richness in Britain

An effective portfolio of protected areas should, all else being equal, give rise to positive relationships between the amount of protected land in a region and the numbers of species present. Tests of this prediction are, however, extremely scarce, and most do not control for the potentially confounding effects of environmental factors that influence broad geographic trends in biodiversity. Here, we document the form of the relationship between species richness and coverage by protected areas using the British avifauna as a case study. We contrast relationships that arise for breeding and wintering assemblages, considering both all species collectively and threatened species only. We use spatially explicit multiple regression analyses that take into account environmental factors previously shown to exert a marked influence on avian species richness in Britain (temperature and altitude). Avian species richness and the amount of protected land are consistently positively correlated with each other, and the slopes of these relationships do not differ between assemblages (breeding/wintering and all species/threatened species). Explanatory power is, however, very weak which may be indicative of the ability of conservation measures in the wider landscape to maintain avian species richness, reducing any distinctive influence of protected areas. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Gopher tortoise herbivory increases plant species richness and diversity

Plant Ecology - Mammalian herbivores often alter plant species richness and diversity, but such impacts have not been much investigated in reptiles. This study examined the effects of gopher...     

Plant species richness and productivity determine the diversity of soil fungal guilds in temperate coniferous forest and bog habitats

Fungi have important roles as decomposers, mycorrhizal root symbionts and pathogens in forest ecosystems, but there is limited information about their diversity and composition at the landscape scale. This work aimed to disentangle the factors underlying fungal richness and composition along the landscape‐scale moisture, organic matter and productivity gradients. Using high‐throughput sequencing, we identified soil fungi from 54 low‐productivity Pinus sylvestris‐dominated plots across three study areas in Estonia and determined the main predictors of fungal richness based on edaphic, floristic and spatial variables. Fungal richness displayed unimodal relationship with organic matter and deduced soil moisture. Plant richness and productivity constituted the key predictors for taxonomic richness of functional guilds. Composition of fungi and the main ectomycorrhizal fungal lineages and hyphal exploration types was segregated by moisture availability and soil nitrogen. We conclude that plant productivity and diversity determine the richness and proportion of most functional groups of soil fungi in low‐productive pine forests on a landscape scale. Adjacent stands of pine forest may differ greatly in the dominance of functional guilds that have marked effects on soil carbon and nitrogen cycling in these forest ecosystems.     

Plant species richness as the main driver of moth metacommunities

1. Ecologists have recognised several factors that may explain the distribution of species in a metacommunity. These factors may be related to the dispersal of individuals among the patches and environmental conditions. 2. Here, we attempted to determine which of the four different metacommunity frameworks (patch dynamics, mass effect, neutral processes, and species sorting) explain the distribution of Arctiinae moths in Brazilian savanna areas with different tree species richness. 3. The Arctiinae moths were categorised as habitat specialists or generalists, common or rare, and belonging to the tribes Arctiini and Lithosiini. We hypothesized that environmental variables best explain the abundance and occurrence of habitat specialist species, common species, and members of Lithosiini; whereas spatial processes are more closely related to habitat generalists, rare species, and members of Arctiini. 4. Contrary to our expectations, we found that the species sorting (mainly dictated by the species richness of trees) best explained the variation in abundance and occurrence of the majority of species groups. Spatial processes (more related to patch dynamics, mass effect, and neutral), although they were significantly related to some species groups, were not strong enough to explain the distribution of these species in the study area. 5. The plant species richness was the most important environmental condition, related to moth species niches. Therefore, species sorting best explained the distribution of the species of Arctiinae in the Brazilian savanna.     

Application of species richness estimators for the assessment of fungal diversity

Species richness and distribution patterns of wood-inhabiting fungi and mycetozoans (slime moulds) were investigated in the canopy of a Central European temperate mixed deciduous forest. Species richness was described with diversity indices and species-accumulation curves. Nonmetrical multidimensional scaling was used to assess fungal species composition on different tree species. Different species richness estimators were used to extrapolate species richness beyond our own data. The reliability of the abundance-based coverage estimator, Chao, Jackknife and other estimators of species richness was evaluated for mycological surveys. While the species-accumulation curve of mycetozoans came close to saturation, that of wood-inhabiting fungi was continuously rising. The Chao 2 richness estimator was considered most appropriate to predict the number of species at the investigation site if sampling were continued. Gray's predictor of species richness should be used if statements of the number of species in larger areas are required. Multivariate analysis revealed the importance of different tree species for the conservation and maintenance of fungal diversity within forests, because each tree species possessed a characteristic fungal community. The described mathematical approaches of estimating species richness possess great potential to address fungal diversity on a regional, national, and global scale.     

Modelling species richness and diversity in grassland communities of the Central Caucasus

In this study I used small squares (4 cm×4 cm) as a sampling technique within plots (128 cm×128 cm) of different elevation, aspect and slope angle in grassland communities (20 plots examined). Then I used a rectangular hyperbole equation (the Michaelis-Menten model) to describe species richness and the Inverse of Simpson Concentration (ISC) as functions of sample size. I checked robustness and precision of the model both by interpolation and extrapolation. Interpolation was similarly good in both cases, while extrapolation produced reliable predictions of ISC but underestimated species richness. Dominance analysis indicated that the underestimation of richness depends on the proportion of bottom species, and that the predicted values of richness roughly coincide with the numbers of dominant species found in plots. Therefore, the model may be used to assess number of dominant species when precision is less important than saving time during a survey. However, the rectangular hyperbole equation appears to be precise and robust in the prediction of ISC, at least in grassland communities. This property may also be employed for extrapolation of diversity indices with a limited sampling effort.     

Plant species richness in the mallee region of Western Australia

Sixteen sites (area 1000 m²) within the mallee region of southern Western Australia were sampled for vascular plant species richness. Species richness ranged from 17 species per 1000m² in a Halosarcia syncarpa salt-complex site and a Eucalyptus occidentalis tree mallee site, up to 48 species per 1000 m² in a Eucalyptus angulosa-Eucalyptus tetragona shrub mallee site. Woodland, woodland/mallee and mallee sites consisted mainly of perennial species while shrubland sites and salt-complex sites had a higher percentage of ephemeral species. Sites with the highest species richness occurred on soils with the lowest nutrient content. Sites with lowest species numbers were those with severe habitat conditions or where better nutrient conditions may have provided the dominants with a competitive advantage to suppress associated species.     

Pyramids of species richness: the determinants and distribution of species diversity across trophic levels

How species richness is distributed across trophic levels determines several dimensions of ecosystem functioning, including herbivory, predation, and decomposition rates. We perform a meta‐analysis of 72 large published food webs to investigate their trophic diversity structure and possible endogenous, exogenous, and methodological causal variables. Consistent with classic theory, we found that published food webs can generally be described as ‘pyramids of species richness’. The food webs were more predator‐poor, prey‐rich and hierarchical than is expected by chance or by the niche or cascade models. The trophic species richness distribution also depended on centrality, latitude, ecosystem‐type and methodological bias. Although trophic diversity structure is generally pyramidal, under many conditions the structure is consistently uniform or inverse‐pyramidal. Our meta‐analysis adds nuance to classic assumptions about food web structure: diversity decreases with trophic level, but not under all conditions, and the decrease may be scale‐dependent. Synthesis The distribution of species richness across trophic levels has not been evaluated in recent decades, despite improvement in food web resolution and the relevance of biodiversity distribution to ecosystem function. Our meta‐analysis of 72 large, recent food webs, illustrates that published food webs can generally be described as basal‐rich, top‐poor ‘pyramids of species richness’, consistent with classic theory. Although trophic diversity structure is generally pyramidal, under some environmental and ecological conditions the structure is uniform or inverse‐pyramidal. Our meta‐analysis confirms classic theory about food web structure, while adding nuance by describing conditions under which classic pyramid structure is not observed.     

Protected areas and regional avian species richness in South Africa

Protected areas are generally regarded as essential for the long-term maintenance of biodiversity. Evidence for their effectiveness in this regard is, however, somewhat equivocal. Here, we document the relationship between the proportion of protected land and species richness in a region, both with and without taking spatial variation in environmental energy availability into account. Using the South African avifauna as a case study, we find that total and threatened species richness exhibit modest increases with the proportion of protected land. While the protected area network should be expanded, it is essential that conservation efforts also focus on maintaining biodiversity in the wider unprotected landscape that supports high species richness.     

Effects of foundation species genotypic diversity on subordinate species richness in an assembling community

Foundation (dominant or matrix) species play a key role in structuring plant communities, influencing processes from population to ecosystem scales. However, the effects of genotypic diversity of foundation species on these processes have not been thoroughly assessed in the context of assembling plant communities. We modified the classical filter model of community assembly to include genotypic diversity as part of the biotic filter. We hypothesized that the proportion of fit genotypes (i.e. competitively superior and dominant) affects niche space availability for subordinate species to establish with consequence for species diversity. To test this hypothesis, we used an individual‐based simulation model where a foundation species of varying genotypic diversity (number of genotypes and variability among genotypes) competes for space with subordinate species on a spatially heterogeneous lattice. Our model addresses a real and practical problem in restoration ecology: choosing the level of genetic diversity of re‐introduced foundation and subordinate species. Genotypic diversity of foundation species significantly affected equilibrium community diversity, measured as species richness, either positively or negatively, depending upon environmental heterogeneity. Increases in genotypic diversity gave the foundation species a wider niche breadth. Under conditions of high environmental heterogeneity, this wider niche breadth decreased niche space for other species, lowering species richness with increased genotypic diversity until the genotypes of the foundation species saturated the landscape. With a low level of environmental heterogeneity, increasing genotypic diversity caused the foundation species niche breadth to be overdispersed, resulting in a weak positive relationship with species richness. Under these conditions, some genotypes are maladapted to the environment lowering fitness of the foundation species. These effects of genotypic diversity were secondary to the larger effects of overall foundation species fitness and environmental heterogeneity. The novel aspect of incorporating genotype diversity in combination with environmental heterogeneity in community assembly models include predictions of either positive or negative relationships between species diversity and genotypic diversity depending on environmental heterogeneity, and the conditions under which these factors are potentially relevant. Mechanistically, differential niche availability is imposed by the foundation species.     

Functional diversity (FD), species richness and community composition

Functional diversity is an important component of biodiversity, yet in comparison to taxonomic diversity, methods of quantifying functional diversity are less well developed. Here, we propose a means for quantifying functional diversity that may be particularly useful for determining how functional diversity is related to ecosystem functioning. This measure of functional diversity “FD” is defined as the total branch length of a functional dendrogram. Various characteristics of FD make it preferable to other measures of functional diversity, such as the number of functional groups in a community. Simulating species' trait values illustrates how the relative importance of richness and composition for FD depends on the effective dimensionality of the trait space in which species separate. Fewer dimensions increase the importance of community composition and functional redundancy. More dimensions increase the importance of species richness and decreases functional redundancy. Clumping of species in trait space increases the relative importance of community composition. Five natural communities show remarkably similar relationships between FD and species richness.     

Plant species richness in mountain forests of the Bavarian Alps

Abstract

Based on a stratified random sample of 93 vegetation plots (144 m²) from montane and subalpine climax forests in a representative section through the Bavarian Alps, spatial pattern and environmental correlates of species density of trees, vascular understorey and epigeic bryophytes were analysed. Detecting landscape scale patterns in beta- and gamma-diversity based on interpretation of rarefaction curves proved to be difficult in a sample that had been stratified by ecological criteria. In 144 m² plots tree species density (5 ± 2.0, max. 10) declined with elevation and increased with stand age (multiple R ² = 0.557). The latter effect can be attributed to the secular history of game management and browsing pressure, which has hindered the regeneration of species-rich tree stands since ca. 150 yr. Species density of the forest undergrowth reached remarkably high levels for vascular plants (42 ± 12.8, max. 69) and bryophytes (14 ± 6.0, max. 30) and strongly depended on cover of the respective layer in a unimodal pattern, suggesting to separate direct and indirect effects, mediated through the mass effect, in the subsequent construction of regression models. Multiple regression (R ² = 0.47) revealed that vascular species density is limited chiefly through low plant cover, which in turn decreases with tree cover, elevation and soil quality, and secondly by species pools that contain larger numbers of species requiring high pH and ample light. Cover and direct effects had roughly equal weight in controlling bryophyte species density (R ² = 0.57). Biomass depended on the proportion of conifers in the tree layer and on site quality, less fertile sites tending to have higher bryophyte cover. The increase of bryophyte species density with elevation was interpreted as an effect of a pool of largely boreal-subalpine species. The increase of species density with stand age suggests dispersal limitation and deserves further study.