Beta diversity, species replacement and nestedness are often examined through pairwise comparisons of sites based on presence-absence data, and the relative importance of these ecological phenomena is evaluated by operations with dissimilarity coefficients. An example is the nestedness resultant dissimilarity (NRD) procedure recently proposed by Baselga (2010, Global Ecology andBiogeography 19: 134–143) to disentangle the nestedness fraction of beta diversity from species replacement. In our view, the component terms in this measure are not scaled uniformly and the nestedness fraction cannot be quantified properly without giving clear definitions for its measurement. We suggest to distinguish among three additive fractions of the species set of two sites: number of species shared (overlap), species replacement (=spatial turnover) and richness difference. Then, absolute beta diversity is obtained as a composite of the second two fractions (known as βWB), while nestedness is derived from the first and the third. To express beta diversity and nestedness in a relativized form, the respective sums are divided by the total number of species. These allow defining a new index to measure the fraction of beta diversity which is shared by nestedness as well, and is calculated as relativized richness difference with the condition that the two sites being compared have at least one species in common. It is called diversity-nestedness intersection coefficient (F). Baselga’s nestedness resultant dissimilarity and the diversity-nestedness intersection coefficient are compared graphically using artificial and actual examples. These functions follow a mathematical relationship for perfectly nested data, otherwise their results are divergent. Discrepancy increases when beta diversity is large, especially if richness differences override species replacement effects in shaping presence-absence data structures. An advantage of F is its compatibility with a general theoretical and methodological framework for revealing pattern in presence-absence data matrices.
Alpha, beta, and gamma diversity are three fundamental biodiversity components in ecology, but most studies focus only on the scale issues of the alpha or gamma diversity component. The beta diversity component, which incorporates both alpha and gamma diversity components, is ideal for studying scale issues of diversity. We explore the scale dependency of beta diversity and scale relationship, both theoretically as well as by application to actual data sets. Our results showed that a power law exists for beta diversity-area (spatial grain or spatial extent) relationships, and that the parameters of the power law are dependent on the grain and extent for which the data are defined. Coarse grain size generates a steeper slope (scaling exponent z) with lower values of intercept (c), while a larger extent results in a reverse trend in both parameters. We also found that, for a given grain (with varying extent) or a given extent (with varying grain) the two parameters are themselves related by power laws. These findings are important because they are the first to simultaneously relate the various components of scale and diversity in a unified manner.
Alpine grasslands harbour species-rich communities of plants and invertebrates. We examined how environmental variables and anthropogenic impact shape species richness and community structure of terrestrial gastropods in alpine grasslands in the Val Müstair (Eastern Alps, Switzerland). Gastropods were sampled using a standardised method at 76 sites spanning an elevation range from 1430 m to 2770 m. A total of 4763 specimens representing 52 species were recorded. Correspondence analysis based on presence/absence data revealed that the grassland gastropod community was structured in a complex way with elevation, wetness, grazing intensity and inclination of the sites as key factors, while abundance-based analysis identified the importance of the elevation and wetness of sites. Generalized linear model showed that species richness decreased with increasing elevation and increased with increasing soil pH. The grassland gastropod communities were characterized by a high beta diversity, as indicated by the SDR-simplex analysis. Species-specific traits of gastropods showed sensitivity to the environmental characters of the sites, as shown by a fourth-corner analysis.
We studied spatial changes in species composition (i.e., beta diversity) of local assemblages of birds along ∼450 km of the Middle Paraná River, an extensive fluvial system of South America. Point counts were used to survey birds at 60 plots located in shrub swamps and marshes of the floodplain within four sites (15 plots per site). Two sites were surrounded by each of the two upland ecoregions. Beta diversity of bird assemblages was high and was more important than alpha diversity in shaping regional diversity (i.e., gamma diversity) of the fluvial system. Compositional changes were related to species turnover among plots, while nestedness dissimilarity was not important for shaping diversity patterns. Variation-partitioning analysis showed that local conditions (i.e., landscape composition within a radius of 200 m from the center of each plot) accounted for more spatial variation in assemblage composition than did location along the fluvial system. Adjacent upland ecoregions did not account for spatial changes in bird composition within the fluvial system. In conclusion, environmental heterogeneity created by flood pulses is an important factor for sustaining regional diversity of birds within the fluvial system through effects on beta diversity.
How are bryophyte alpha and beta diversities distributed across spatial scales along an elevational gradient in an oceanic island? Which mechanisms and drivers operate to shape them? Starting from a multiscale hierarchical sampling approach along an 1000 m elevational transect, we used additive diversity partitioning and null modeling to evaluate the contributions of the alpha and beta diversity components to overall bryophyte diversity in Terceira Island, Azores. Substrate-level diversity patterns were explored by means of the Sørensen Similarity Index and the Lloyd Index of Patchiness. Elevation-level beta diversity was decomposed into its replacement and richness differences components, with several environmental variables being evaluated as diversity predictors. Bryophyte diversity proved to be primarily due to beta diversity between elevation sites, followed by diversity among substrates. Compositional differences between neighboring sites decreased with elevation, being mainly caused by species replacement and correlating with differences in relative humidity and disturbance. At the substrate level, we found a great homogeneity in terms of species composition, coupled with a low substrate specialization rate. We conclude that, in Terceira’s native vegetation patches, regional processes, such as environmental gradients associated with elevation, play a greater role in shaping bryophyte diversity than local processes. Moister and less disturbed areas at mid-high elevation harbor a richer bryoflora, consistently more similar and stable between neighbouring sites. Simultaneously, the different substrates available are somewhat ecologically redundant, supporting few specialized species, pointing to these areas providing optimal habitat conditions for bryophytes. Our findings provide a better understanding of how bryophyte diversity is generated in Terceira Island, indicating that management and conservation measures should focus on island-level approaches, aiming to protect and rehabilitate additional natural vegetation patches at different elevations, especially in the severely disturbed lowlands.
Biological invasion is a crucial problem in the world because of its negative consequences for protected areas. The degradation stage of vegetation might affect the success of invasion. One of the most abundant and threatening invasive species is the common milkweed (Asclepias syriaca L.) which has invaded already 23 countries of Europe and in several habitat types its further spreading is promoted by climate change. Pannonian sand grassland is one of the most threatened habitat by common milkweed invasion. Therefore, invasion in sand grassland vegetation is an important issue. However, the effects of the invasive plant in the open sand grassland are rather controversial. In order to clarify the existing contradictory results, the study was carried out in a strictly protected area, near Fülöpháza (Hungary) in a reserve core area in a UNESCO biosphere reserve. Microcoenological study was applied to determinate the fine-scale community characteristics of non-invaded and invaded stands in natural and seminatural vegetation and data were processed by Juhász-Nagy's information theory models. Shannon diversity of species combinations (compositional diversity) which describes the ways of the coexistence of species, and the number of realized species combinations were used for measuring beta diversity. Differences between stands were analyzed by two-way ANOVA. The maximum compositional diversity of species and main life-forms (annuals, perennials and cryptogams) did not differ significantly between the non-invaded and invaded stands. In contrast, significantly larger characteristic areas of compositional diversity were detected in the invaded stands. Based on these results, it could be concluded that diversity of species combinations did not change but those values have shifted to coarser scales in case of invaded stands. The direction of this change suggests a kind of impoverishment in the presence of Asclepias. Thus, it is worth mentioning from the invasion management point of view that protection of the habitats against disturbance is a more cost-effective and successful way than protection against the establishment or extirpation of invasive species, since disturbance facilitates the invasions throughout the impoverishment of the community.
Ecotones, representing the transition zones between species or communities, have been suggested as focal points for detecting early shifts in vegetation composition due to anthropogenic impact. Here we examined if changes in ecotone location or properties can be used as reliable indicators of hydrological change in temperate wetland communities. We examined 38 woodland-wetland-woodland transitions, distributed across four sites with different anthropogenic disturbance histories and hydrological traits. We tested whether: 1) the ecotones are associated with environmental gradients, and 2) the location or properties of these ecotones change with disturbance history. Well-defined ecotones were associated with steep gradients in soil depth and soil moisture. Most ecotones showed a change in vegetation from hydrophytic to dryland species. There was also some evidence that in highly disturbed sites the link between ecotones and environmental gradients was less apparent. By sampling across boundaries we can better understand the factors controlling the distribution of species. This allows us to make better predictions about the impacts of anthropogenic change in wetland communities. By investigating the transitions between different vegetation communities we were able to highlight key indicators that could be used to monitor these ecologically sensitive and diverse wetland communities.