The additive partitioning of species diversity: recent revival of an old idea

Ecologists have traditionally viewed the total species diversity within a set of communities as the product of the average diversity within a community (alpha) and the diversity among the communities (beta). This multiplicative concept of species diversity contrasts with the lesser known idea that α- and β-diversities sum to give the total diversity. This additive partitioning of species diversity is nearly as old as the multiplicative concept, yet ecologists are just now beginning to use additive partitioning to examine patterns of species diversity. In this review we discuss why additive partitioning remained "hidden" until just a few years ago. The rediscovery of additive partitioning has expanded the way in which ecologists define and measure β-diversity. Beta diversity is no longer relegated to describing change only along an environmental gradient. Through additive partitioning, β-diversity is explicitly an average amount of diversity just as is α-diversity. We believe that the additive partitioning of diversity into α and β components will continue to become more widely used because it allows for a direct comparison of α- and β-diversities. It also has particular relevance for testing ecological theory concerned with the determinants of species diversity at multiple spatial scales and potential applications in conservation biology.     

Species diversity in neutral metacommunities: a network approach

Biologists seek an understanding of the processes underlying spatial biodiversity patterns. Neutral theory links those patterns to dispersal, speciation and community drift. Here, we advance the spatially explicit neutral model by representing the metacommunity as a network of smaller communities. Analytic theory is presented for a set of equilibrium diversity patterns in networks of communities, facilitating the exploration of parameter space not accessible by simulation. We use this theory to evaluate how the basic properties of a metacommunity – connectivity, size, and speciation rate – determine overall metacommunity γ -diversity, and how that is partitioned into α - and β -components. We find spatial structure can increase γ -diversity relative to a well-mixed model, even when θ is held constant. The magnitude of deviations from the well-mixed model and the partitioning into α - and β -diversity is related to the ratio of migration and speciation rates. γ -diversity scales linearly with metacommunity size even as α - and β -diversity scale nonlinearly with size.     

Additive partitioning of rarefaction curves and species-area relationships: unifying alpha-, beta- and gamma-diversity with sample size and habitat area

Additive partitioning of species diversity is widely applicable to different kinds of sampling regimes at multiple spatial and temporal scales. In additive partitioning, the diversity within and among samples ( α and β ) is expressed in the same units of species richness, thus allowing direct comparison of α and β . Despite its broad applicability, there are few demonstrated linkages between additive partitioning and other approaches to analysing diversity. Here, we establish several connections between diversity partitions and patterns of habitat occupancy, rarefaction, and species–area relationships. We show that observed partitions of species richness are equivalent to sample-based rarefaction curves, and expected partitions from randomization tests are approximately equivalent to individual-based rarefaction. Additive partitions can also be applied to species–area relationships to determine the relative contributions of factors influencing the β -diversity among habitat fragments.     

Additive partition of parametric information and its associated beta-diversity measure

A desirable property of a diversity index is strict concavity. This implies that the pooled diversity of a given community sample is greater than or equal to but not less than the weighted mean of the diversity values of the constituting plots. For a strict concave diversity index, such as species richness S, Shannon's entropy H or Simpson's index 1-D, the pooled diversity of a given community sample can be partitioned into two non-negative, additive components: average within-plot diversity and between-plot diversity. As a result, species diversity can be summarized at various scales measuring all diversity components in the same units. Conversely, violation of strict concavity would imply the non-interpretable result of a negative diversity among community plots. In this paper, I apply this additive partition model generally adopted for traditional diversity measures to Aczél and Daróczy's generalized entropy of type . In this way, a parametric measure of -diversity is derived as the ratio between the pooled sample diversity and the average within-plot diversity that represents the parametric analogue of Whittaker's -diversity for data on species relative abundances.     

Stream–floodplain connectivity and fish assemblage diversity in the Champlain Valley, Vermont, U.S.A.

To evaluate the influence of main channel–floodplain connectivity on fish assemblage diversity in floodplains associated with streams and small rivers, fish assemblages and habitat characteristics were surveyed at 24 stream reaches in the Champlain Valley of Vermont, U.S.A. Fish assemblages differed markedly between the main channel and the floodplain. Fish assemblage diversity was greatest at reaches that exhibited high floodplain connectivity. Whereas certain species inhabited only main channels or floodplains, others utilized both main channel and floodplain habitats. Both floodplain fish α-diversity and γ-diversity of the entire stream corridor were positively correlated with connectivity between the main channel and its floodplain. Consistent with these results, species turnover (as measured by β-diversity) was negatively correlated with floodplain connectivity. Floodplains with waterbodies characterized by a wide range of water depths and turbidity levels exhibited high fish diversity. The results suggest that by separating rivers from their floodplains, incision and subsequent channel widening will have detrimental effects on multiple aspects of fish assemblage diversity across the stream–floodplain ecosystem.     

Species diversity in vertical, horizontal, and temporal dimensions of a fruit-feeding butterfly community in an Ecuadorian rainforest

To test the hypotheses that fruit-feeding nymphalid butterflies are randomly distributed in space and time, a community of fruit-feeding nymphalid butterflies was sampled at monthly intervals for one year by trapping 6690 individuals of 130 species in the canopy and understory of four forest habitats: primary, higraded, secondary, and edge. The overall species abundance distribution was well described by a lognormal distribution. Total species diversity (γ-diversity) was partitioned into additive components within and among community subdivisions (α-diversity and β-diversity) in vertical, horizontal and temporal dimensions. Although community subdivisions showed high similarity (1 —β-diversity/γ-diversity), significant β-diversity existed in each dimension. Individual abundance and observed species richness was lower in the canopy than in the understory. However, rarefaction analysis and species accumulation curves revealed that canopy had higher species richness than understory. Observed species richness was roughly equal in all habitats, but individual abundance was much greater in edge, largely due to a single, specialist species. Rarefaction analysis and species accumulation curves showed that edge had significantly lower species richness than all other habitats. Samples from a single habitat, height and time contained only a small fraction of the total community species richness. This study demonstrates the feasibility, and necessity, of large-scale, long-term sampling in multiple dimensions for accurately measuring species richness and diversity in tropical forest communities. We discuss the importance of such studies in conservation biology.     

Are communities saturated? On the relationship between α, β and γ diversity

A popular way to suggest a regional influence on local species diversity has been to plot local versus regional diversity. The form of these curves has been interpreted as evidence for or against "community saturation" due to species interactions. This interpretation, however, is unwarranted. Using the concepts of α, β and γ diversity, I show that local–regional richness curves are determined by the way total diversity is partitioned between its α and β components, which itself is a matter of scale. Changing the scale of the local community amounts to changing the scale at which the heterogeneity of the interactions between organisms and their environment manifests itself, and hence the balance between α and β diversity. Community saturation may occur because of physical limitations, but there are no theoretical grounds for the belief that species interactions set an absolute upper limit to diversity at any scale. A distinction between different meanings of the concept of "saturation" is proposed to clarify this issue. I argue that the challenge now is to understand the relationship between α and β diversity at multiple scales, and the processes that determine it.     

Assessment of species diversity from species abundance distributions at different localities

We show how the spatial structure of species diversity can be analyzed using the correlation between the log abundances of the species in the communities, assuming that two communities at different localities can be described by a bivariate lognormal species abundance distribution. A useful property of this approach is that the log abundances of the species at two localities can be considered as samples from a bivariate normal distribution defined by only five parameters. The variances and the correlation can be estimated by maximum likelihood methods even if there is no information about the sampling intensity and the number of unobserved species. This method also enables estimation of over-dispersion in the sampling relative to a Poisson distribution that allows sampling adjustment of the estimate of β-diversity. Furthermore, we also obtain a partitioning of species diversity into additive components of α-, β- and γ-diversity. For instance, if the correlation between the log abundances of the species is close to one, the same species will be common and rare in the two communities and the β-diversity will be low. We illustrate this approach by analysing similarities of communities of rare and endangered species of oak-living beetles in south-eastern Norway. The number of recorded species was estimated to be only 48.1% of the total number of species actually present in these communities. The correlations among communities dropped rather quickly with distance with a scaling of order 200 km. This illustrates large spatial heterogeneity in species composition, which should be accounted for in the design of schemes of such devices for assessing species diversity in these habitat-types.     

Macroinvertebrate diversity in headwater streams: a review

1. Headwater streams are ubiquitous in the landscape and are important sources of water, sediments and biota for downstream reaches. They are critical sites for organic matter processing and nutrient cycling, and may be vital for maintaining the 'health' of whole river networks.
2. Macroinvertebrates are an important component of biodiversity in stream ecosystems and studies of macroinvertebrate diversity in headwater streams have mostly viewed stream systems as linear reaches rather than as networks, although the latter may be more appropriate to the study of diversity patterns in headwater systems.
3. Studies of macroinvertebrate diversity in headwater streams from around the world illustrated that taxonomic richness is highly variable among continents and regions, and studies addressing longitudinal changes in taxonomic richness of macroinvertebrates generally found highest richness in mid-order streams.
4. When stream systems are viewed as networks at the landscape-scale, α-diversity may be low in individual headwater streams but high β-diversity among headwater streams within catchments and among catchments may generate high γ-diversity.
5. Differing ability and opportunity for dispersal of macroinvertebrates, great physical habitat heterogeneity in headwater streams, and a wide range in local environmental conditions may all contribute to high β-diversity among headwater streams both within and among catchments.
6. Moving beyond linear conceptual models of stream ecosystems to consider the role that spatial structure of river networks might play in determining diversity patterns at the landscape scale is a promising avenue for future research.     

Testing the higher-taxon approach: a case study of aquatic marcophytes in China’s northwest arid zone and its implications for conservation

Assessments of biodiversity are time-consuming and require a high level of expert knowledge. A reduced set of taxonomic ranks other than species has been proved to be useful for rapid and cost-effective assessment of biodiversity. However, few studies have examined how well this method performs for aquatic plant group that of enormous ecological importance. We studied the aquatic plant flora in the arid zone of China and examined whether the distribution of species α- and β-diversity could be predicted well from genus-, and family-levels. Analyses of 3 years field data showed that significant and positive relations exist between α-diversity of species and α-diversity of genera and family in both entire arid zone and five sub-zones. In contrast, β-diversity at species level is difficult to predict from β-diversity indexes at higher taxonomic level. The results indicate that higher-taxon α-diversity, especially at the generic level in our research, can be useful surrogates of species α-diversity for aquatic plants conservation.     

Patterns of bryophyte and vascular plant richness in European subalpine springs

The diversity of spring habitats can be determined not only by local environmental conditions, but also by large-scale biogeographical effects. The effects can differ across various groups of organisms. We compared α-, β- and γ-diversity patterns of bryophytes and vascular plants of (sub)alpine springs in three contrasting mountain ranges: Alps (Switzerland), Balkans (Bulgaria), Western Carpathians (Slovakia, Poland). We used univariate and multivariate statistics to test for the effects of pH, conductivity, altitude, slope, mean annual temperature and annual precipitation on diversity patterns of both taxonomic groups and compared diversity patterns among the regions for particular pH and conductivity classes. We identified acidophyte and basiphyte, calcifuge and calcicole species using species response modelling. All regions displayed significant relationship between conductivity and α-diversity of vascular plants. Bulgaria showed the highest α-diversity of vascular plants for the middle part of the conductivity gradient. For both taxonomic groups, the β-diversity in the middle part of gradient was highest in Swiss Alps. The total species pool was lowest in Bulgaria. The percentage of basiphyte and calcicole species was highest in the Alps. In (sub)alpine springs, mineral richness was a better determinant of vascular plant α-diversity than pH, and the extent of the alpine area did not coincide with α-diversity. Observed inter-regional differences in diversity patterns could be explained by the different proportion of limestone bedrock and different biogeographic history. The differences in α-diversity between both taxonomic groups are presumably result of the different rates of adaptation processes.     

Short Rotation Coppice (SRC) Plantations Provide Additional Habitats for Vascular Plant Species in Agricultural Mosaic Landscapes

Increasing loss of biodiversity in agricultural landscapes is often debated in the bioenergy context, especially with respect to non-traditional crops that can be grown for energy production in the future. As promising renewable energy source and additional landscape element, the potential role of short rotation coppice (SRC) plantations to biodiversity is of great interest. We studied plant species richness in eight landscapes (225 km²) containing willow and poplar SRC plantations (1,600 m²) in Sweden and Germany, and the related SRC α-diversity to species richness in the landscapes (γ-diversity). Using matrix variables, spatial analyses of SRC plantations and landscapes were performed to explain the contribution of SRC α-diversity to γ-diversity. In accordance with the mosaic concept, multiple regression analyses revealed number of habitat types as a significant predictor for species richness: the higher the habitat type number, the higher the γ-diversity and the lower the proportion of SRC plantation α-diversity to γ-diversity. SRC plantation α-diversity was 6.9 % (±1.7 % SD) of species richness on the landscape scale. The contribution of SRC plantations increased with decreasing γ-diversity. SRC plantations were dominated more by species adapted to frequent disturbances and anthropo-zoogenic impacts than surrounding landscapes. We conclude that by providing habitats for plants with different requirements, SRC α-diversity has a significant share on γ-diversity in rural areas and can promote diversity in landscapes with low habitat heterogeneity and low species pools. However, plant diversity enrichment is mainly due to additional species typically present in disturbed and anthropogenic environments.     

Does the Existence of Bird's Nest Ferns Enhance the Diversity of Oribatid (Acari: Oribatida) Communities in a Subtropical Forest?

We examined the effects of the presence of bird's nest ferns on the species diversity of oribatid mites in the whole forest in terms of the three categories of species diversity (α-, β-, and γ-diversity) in a subtropical forest in south-western Japan. The species diversity (1 − D) of oribatid communities in the ferns was significantly lower than those in bark of trees and the forest-floor litter and soil, and was similar to that in the branches. The oribatid faunas in the litter in and the roots of the fern were more similar to those in both the forest-floor litter and soil than to the faunas in the other arboreal habitats. However, the ferns can be colonized by endemic oribatid species specialized to such environments. The number of oribatid species estimated for a hypothetical stand with no ferns was about 180 species from 80 samples; this value did not differ significantly from that in another hypothetical stand with ferns (ca. 190 species). Thus, the species richness of oribatid communities estimated for the whole forest (the γ-diversity) was not affected by the presence or absence of bird's nest ferns. The α- and β-diversities of oribatid communities on bird's nest ferns were lower than those in other habitats, and they might not dramatically raise the overall γ-diversity of invertebrate communities in the whole forest. The bird's nest ferns, however, can generate a unique habitat for specialized species, and this would help to maintain species diversities of invertebrates at the whole-forest scale in subtropical forests.     

Floristics of the Dunbar Valley serpentinite site, Songimvelo Game Reserve, South Africa

The Dunbar Valley serpentinite outcrop has a flora comprising 254 taxa (species and below) in 172 genera and 63 families. Dunbar Valley has more species than other studied serpentinite sites in the Barberton Greenstone Belt (BGB). The genus Senecio is the most speciose genus in the BGB. The level of species endemism at Dunbar Valley is 2.0%. Most of the serpentinite endemics in the BGB show phytogeographical affinities with the Sudano-Zambezian Region. Six modified-Whittaker plots, three on serpentinite and three on non-serpentinite soils, were sampled. Sørenson's index was 0.312, indicating low similarity in species between serpentinite and non-serpentinite sites (β-diversity) at Dunbar Valley, as at other sites in the BGB. α-diversity (using the Shannon–Wiener index) for the serpentinite was 2.631 ± 0.901 and for the non-serpentinite, 2.886 ± 0.130. However, a Student's t -test showed no significant difference in α-diversity between the two habitats. There was also no significant difference in species richness between serpentinite and non-serpentinite sites. Total species showed negative correlations with total nickel, altitude and serpentinite outcrop size (area) for six sites in the BGB. Number of endemic taxa showed no correlation with environmental variables.  © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society , 2003, 143 , 271–285.     

Woody Encroachment Removal from Midwestern Oak Savannas Alters Understory Diversity across Space and Time

Recovering biodiversity is a common goal during restoration; however, for many ecosystems, it is not well understood how restoration influences species diversity across space and time. I examined understory species diversity and composition after woody encroachment removal in a large-scale savanna restoration experiment in central Iowa, United States. Over a 4-year time series, restoration had profound effects across space and time, increasing richness at local and site-level scales. Restoration sites had increased α (within sample) Simpson's diversity and α and γ (site level) species richness relative to control sites, although γ and β (among sample) Simpson's diversity, β richness, and α species evenness were not affected. Changes in richness were driven by graminoids at the α and γ scales and woody species (and some evidence for forbs) at the α scale. Interestingly, indicator species analysis revealed that at least some species from all functional groups were promoted by restoration, although no species were significant indicators of pre-treatment or control sites. Both savanna and nonsavanna species were indicators of restored sites. Restoration promoted exotic species at both scales, although species with spring phenologies were unaffected. Woody encroachment removal may be a means to promote species establishment in savannas; however, in this study, it resulted in establishment and proliferation of native and exotic and savanna and nonsavanna species. Future work might consider reintroduction of key savanna species to supplement those that have established. Work like this demonstrates the utility of restoration experiments for conducting research on large- and multiscale processes, such as species diversity.     

Community assembly along a species pool gradient: implications for multiple-scale patterns of species diversity

Various ecological processes influence patterns of species diversity at multiple spatial scales. One process that is potentially important but rarely considered is community assembly. I assembled model communities using species pools of differing size to examine how the history of community assembly may affect multi-scale diversity patterns. The model contained three scales at which diversity could be measured: local community, metacommunity, and species pool. Local species saturation occurred, as expected from the competition and predation built in the model. However, local communities did not become resistant to invasions except when the species pool was very small. Depending on dispersal rate and trophic level, the larger the species pool, the harder it was to predict which species invades which local community at a given time. Consequently, local-community dissimilarity maintained by assembly history increased linearly with pool size, even though local diversity was decoupled from pool size. These results have two implications for multi-scale diversity patterns. First, assembly history may provide an explanation for scale-dependent relationships between local and regional diversity: assembly causes the relationship to be curvilinear at one scale (local community), while linear at another (metacommunity). Second, assembly history influences how -diversity is partitioned into - and -diversity: assembly causes the relative contribution of to increase with pool size. Overall, this study suggests that community assembly history interacts with species pool size to regulate multi-scale patterns of species diversity.     

Microenvironmental variability and species diversity in treefall gaps in a sub-tropical broadleaved forest

Microenvironmental variability and species diversity in gaps and forest understorey were studied to assess the role of treefall gaps in maintaining composition and patchy distribution in a broad-leaved sub-tropical climax forest, Mawphlang, Meghalaya, India. Photon flux density was higher in gaps than in the surrounding understorey. Relative humidity was low and the litter layer was relatively thin in gaps throughout the year. Soil moisture and photon flux density in the gaps significantly varied between seasons and gaps of different sizes. Relative humidity significantly varied between seasons but difference among gaps was insignificant. Among-gap and among-season variations in soil and air temperature were insignificant.The number of tree species in the gaps was positively correlated with gap area, and tree species abundance showed higher equitability in larger than in smaller gaps. In gaps, -diversity was highest for herbs and lowest for shrubs. -diversity was highest for shrubs and lowest for tree seedlings. -diversity of tree seedlings was higher in the gaps than in the forest understorey. Conversely, -diversity was higher in the understorey than in the gaps. Low species similarity for tree seedlings among the gaps could be an effect of patchy distribution of parent tree species in the forest. Thus a significant change in light and moisture regimes along the gap size gradient played an important role in influencing the composition and abundance of shade tolerant and intolerant tree species in gaps on one hand, and affected the overall species diversity of the forest, on the other.     

Habitat isolation changes the beta diversity of the vascular epiphyte community in lower montane forest,Veracruz, Mexico

Habitat isolation is one of the most important factors endangering the biodiversity, but little research has been done with vascular epiphytes. In order to understand the effect of isolation on the epiphyte community, we studied epiphyte diversity on three plots in a forest fragment, two riparian forest fragments, and in isolated pastureland trees. We found 118 vascular epiphyte species. On forest plots, both epiphyte richness per tree (S_tree) and species turnover rate within trees (β_tree) registered the highest values, although the lowest S_tree diversity was also found there; additionally inside the forest were host species with clearly different epiphyte community. S_tree and β_tree diversities of riparian fragments behaved similarly to those of the forest. Isolated trees had the second highest S_tree diversity, although their β_tree diversity was the lowest. In the forest plots were both, the highest and lowest expected accumulated richness (α diversity); on riparian fragments it was intermediate, and the second lowest α diversity was registered for isolated trees. Species turnover rate among plots (β) was high and was associated with both, isolation and a distance gradient from permanent water sources. The epiphyte community on isolated trees was clearly different to the other habitats. Results suggest that deforestation eliminated dry areas and specific hosts that were important for the maintenance of epiphyte species richness. In pastureland trees the epiphyte β_tree diversity diminished, suggesting a simplification of the environment for epiphytes and causing a low α diversity.     

Response of reed community to the environment gradient-water depth in the Yellow River Delta, China

We investigated and monitored a reed community in the fields. Data on the bio-ecological characteristics and β-diversity of reed communities in different environmental gradients (mainly based on water depth) of the Yellow River Delta were collected through multianalysis, extremum analysis and β-diversity index analysis. In accordance with the square sum of deviations (Ward) cluster analysis, 10 sampling plots were divided into six types with the dominant plants in different plots varying according to the change in environmental gradients. The dominant plants in these plots varied from aquatic plants to xerophytes and salt tolerant plants as water depth decreased. The average height and diameter of the reeds at breast level were significantly correlated with the average water depth. The fitness curves of average density and coverage with average water depth were nonlinear. When the average water depth was 0.3 m, the average density and coverage of reeds reached the apex value, while the height and diameter of the reeds at breast level increased with the water depth. There were obvious changes to the environmental gradient in the Yellow River Delta. The transitional communities were also found to exist in the Yellow River Delta by β-diversity analysis. Vicarious species appeared with the change in water depth. The occurrence of substitute species is determined by the function of common species between adjacent belts. The different functions of common species led to differences in community structure and function and differences in dominant plants. The result reflects the variations of species present in different habitats and directly reflects environmental heterogeneity. The values of β-diversity indices of adjacent plots were higher than those of nonadjacent plots. There are transition zones between the xerophytes and aquatic plants in the Yellow River Delta. In an aquatic environment, the similarity of reed community is higher than that of xeromorphic plants. The β-diversity index can reflect plant succession trends caused by the change in environmental gradients in the Yellow River Delta. The β-diversity index reveals plant responses to changes in environmental gradient and is helpful in observing changes in patterns of species diversity in relation to environmental gradient change and evolving trends in the future, which in turn plays a prominent role when environmental water requirements of wetland are discussed. __________ Translated from Acta Ecologica Sinica, 2006, 26(5): 1533–1541 [译自: 生态学报]     

Role of G Protein βγ Subunits in Muscarinic Receptor-Induced Stimulation and Inhibition of Adenylyl Cyclase Activity in Rat Olfactory Bulb

Abstract: In the olfactory bulb, muscarinic receptors exert a bimodal control on cyclic AMP, enhancing basal and G_s-stimulated adenylyl cyclase activities and inhibiting the Ca²⁺/calmodulin- and forskolin-stimulated enzyme activities. In the present study, we investigated the involvement of G protein βγ subunits by examining whether the muscarinic responses were reproduced by the addition of βγ subunits of transducin (βγ_t) and blocked by putative βγ scavengers. Membrane incubation with βγ_t caused a stimulation of basal adenylyl cyclase activity that was not additive with that produced by carbachol. Like carbachol, βγ_t potentiated the enzyme stimulations elicited by vasoactive intestinal peptide and corticotropin-releasing hormone. RT-PCR analysis revealed the expression of mRNAs encoding both type II and type IV adenylyl cyclase, two isoforms stimulated by βγ synergistically with activated G_s. In addition, βγ_t inhibited the Ca²⁺/calmodulin- and forskolin-stimulated enzyme activities, and this effect was not additive with that elicited by carbachol. Membrane incubation with either one of two βγ scavengers, the GDP-bound form of the α subunit of transducin and the QEHA fragment of type II adenylyl cyclase, reduced both the stimulatory and inhibitory effects of carbachol. These data provide evidence that in rat olfactory bulb the dual regulation of cyclic AMP by muscarinic receptors is mediated by βγ subunits likely acting on distinct isoforms of adenylyl cyclase.