Of beta diversity, variance, evenness, and dissimilarity

The amount of variation in species composition among sampling units or beta diversity has become a primary tool for connecting the spatial structure of species assemblages to ecological processes. Many different measures of beta diversity have been developed. Among them, the total variance in the community composition matrix has been proposed as a single-number estimate of beta diversity. In this study, I first show that this measure summarizes the compositional variation among sampling units after nonlinear transformation of species abundances. Therefore, it is not always adequate for estimating beta diversity. Next, I propose an alternative approach for calculating beta diversity in which variance is substituted by a weighted measure of concentration (i.e., an inverse measure of evenness). The relationship between this new measure of beta diversity and so-called multiple-site dissimilarity measures is also discussed

Strong requirements for weak diversities

Weikard et al. (Diversity and Distributions, 12, 215-217) show that the taxonomic diversity measure proposed by Ricotta (2004) violates 'weak species monotonicity'. This condition requires that the addition of a species to a given species set should always increase diversity if abundances change only marginally. They further propose a new taxonomic diversity index that overcomes this drawback. In this paper, some statistical properties of this new diversity index are briefly analysed

A generalized framework for analyzing taxonomic, phylogenetic, and functional community structure based on presence-absence data

Community structure as summarized by presence–absence data is often evaluated via diversity measures by incorporating taxonomic, phylogenetic and functional information on the constituting species. Most commonly, various dissimilarity coefficients are used to express these aspects simultaneously such that the results are not comparable due to the lack of common conceptual basis behind index definitions. A new framework is needed which allows such comparisons, thus facilitating evaluation of the importance of the three sources of extra information in relation to conventional species-based representations. We define taxonomic, phylogenetic and functional beta diversity of species assemblages based on the generalized Jaccard dissimilarity index. This coefficient does not give equal weight to species, because traditional site dissimilarities are lowered by taking into account the taxonomic, phylogenetic or functional similarity of differential species in one site to the species in the other. These, together with the traditional, taxon- (species-) based beta diversity are decomposed into two additive fractions, one due to taxonomic, phylogenetic or functional excess and the other to replacement. In addition to numerical results, taxonomic, phylogenetic and functional community structure is visualized by 2D simplex or ternary plots. Redundancy with respect to taxon-based structure is expressed in terms of centroid distances between point clouds in these diagrams. The approach is illustrated by examples coming from vegetation surveys representing different ecological conditions. We found that beta diversity decreases in the following order: taxon-based, taxonomic (Linnaean), phylogenetic and functional. Therefore, we put forward the beta-redundancy hypothesis suggesting that this ordering may be most often the case in ecological communities, and discuss potential reasons and possible exceptions to this supposed rule. Whereas the pattern of change in diversity may be indicative of fundamental features of the particular community being studied, the effect of the choice of functional traits—a more or less subjective element of the framework—remains to be investigated

Modelling fire occurrence at regional scale. Does vegetation phenology matter?

Through its influence on biomass production, climate controls fuel availability affecting at the same time fuel moisture and flammability, which are the main determinants for fire ignition and propagation. Knowing the role of fuel phenology on fire ignition patterns is hence a key issue for fire prevention, detection, and development of mitigation strategies. The objective of this study is to quantify, at coarse scale, the role of the vegetation seasonal dynamics on fire ignition patterns of the National Park of Cilento, Vallo di Diano and Alburni (southern Italy) during 2000-2013. We applied a habitat suitability model to compare the multitemporal NDVI profiles at the locations of fire occurrence (the used habitat) with the NDVI profiles of the entire study area (the available habitat). Results demonstrated that, from May to October, wildfires occur preferentially at sites where the remotely-sensed NDVI observations have on average lower values than the available habitat. On the other hand, in the period November-April, wildfires tend to occur at sites where the corresponding NDVI observations have higher values than the available habitat. From a practical viewpoint, the proposed method can be implemented using many different ecogeographical variables simultaneously, thus integrating remotely sensed imagery with socioeconomic data, land cover, physiography or any landscape features that are thought to influence fire occurrence in the study area

Bridging the gap between ecological diversity indices and measures of biodiversity with Shannon's entropy: Comment to Izsák and Papp

Most ecological diversity indices summarize the information about the relative abundances of community species without reflecting taxonomic differences between species. Nevertheless, in the environmental conservation practice, data on species abundances are generally unknown. In such cases, to summarize the conservation value of a given site, so-called ‘biological diversity’ measures need to be used. Most of these measures are based on taxonomic relations among species and ignore species relative abundances. In a recent paper, Izsa´k and Papp suggest that the quadratic entropy index (Q) is the only diversity index used to date in the ecological practice that incorporates both speciesrelative abundances and a measure of the pairwise taxonomic differences between species in the analyzed data set. I show here that a number of traditional ecological diversity measures can be generalized to take into account a taxonomic weighting factor. Since these new indices violate part of the mathematical properties that an index should meet to be termed an ecological diversity index, I defined this new family of indices ‘weak diversity indices’

Towards a complex, plural and dynamic approach to diversity: Rejoinder to Myers and Patil, Podani, and Sarkar

This reply paper includes two brief remarks in rejoinder to the commentary papers of Myers and Patil, Podani, and Sarkar. The first observation concerns the fundamental nature of ecological diversity measures, while the second one specifically addresses some interesting mathematical connections between α- and β-diversity

Relating wildfire seasonality to remotely sensed fuel phenology: a tool for a new pyrogeography?

In the Mediterranean region, notwithstanding the high human pressure, fire activity is essentially drought-driven, such that fuel moisture represents the main determinant for sustaining fire ignition and spread. Seasonal variations in remotely sensed vegetation indices, such as the Normalized Difference Vegetation Index (NDVI), proved to be indicative of variations in fuel amount and moisture content and associated fire risk. This study aims to propose a general method to represent the combined pattern of remotely sensed vegetation indices and fire ignitions throughout the year, as potential tool to classify terrestrial ecosystems at local to global scale. To jointly visualize the temporal dynamics of remotely sensed vegetation indices and annual fire density, we propose to log-transform the ‘annual fire density’ (AFD) values, thus expressing the temporal dynamics of fire ignitions in orders of magnitude and producing a pyrophenological diagram in which both quantities vary approximately in the same range. The methodological approach proposed in this study proved to be independent of the local characteristics and applicable with any available remotely sensed vegetation index. The combined NDVI-fire diagrams may contribute to the global pyrophenological classification and mapping of terrestrial ecosystems based on the integrated monitoring of remotely sensed vegetation phenology and fuel seasonality

Plant invasion as an emerging challenge for the conservation of heritage sites. The spread of ornamental trees on ancient monuments in Rome, Italy

Cultural heritage sites such as historical or sacred areas provide suitable habitats for plants and play an important role in nature conservation, particularly in human-modified contexts such as urban environments. However, such sites also provide opportunities for the spread of invasive species, whose impact on monuments has been raising growing concerns. The aim of this study was to investigate the patterns of distribution and spread of invasive plants in heritage areas, taking the city of Rome as an example. We focused on woody species as they pose the greatest threat to the conservation of monuments, owing to the detrimental effects of their root system. We analysed changes in the diversity and traits of native and non-native flora growing on the walls of 26 ancient sites that have been surveyed repeatedly since the 1940s. We found that the diversity of the native flora has steadily decreased, while there has been an increase in non-native, larger and more damaging species. The introduced species that have expanded most are ornamental wind- or bird-dispersed trees, which represent a major management problem as their propagules can reach the upper sections of the monuments, where they become more difficult to control. The most widespread and damaging of such species is Ailanthus altissima, which has recently been included among the invasive species of European Union concern (EU Regulation 2019/1262). Our findings show that plant invasion is an emerging challenge for the conservation of heritage sites and needs to be prioritized for management to prevent future expansion

Functional and phylogenetic similarity among communities

Summary Ecological studies often rely on coefficients of intercommunity (dis)similarity to decipher effects of ecological, evolutionary and human-driven mechanisms on the composition of communities. Yet, two main criticisms have been levelled at (dis)similarity coefficients. First, few developments include information on species' abundances, and either phylogeny or functional traits. Secondly, some (dis)similarity coefficients fail to always provide maximum dissimilarity between two completely distinct communities, that is, communities without common species and with zero similarities among their species. Here, we introduce a new family of similarity coefficients responding to these criticisms. Within this family, we concentrate on four coefficients and compare them to Rao's dissimilarity on macroinvertebrate communities, and simulated data. Our new coefficients correctly treat maximally dissimilar communities: similarities are always zero between two completely distinct communities. The originality of these new coefficients is even more profound as the existence of maximally dissimilar communities was not a requirement for the new coefficients to behave differently than Rao's dissimilarity coefficient. Our new family of similarity coefficients relies on the abundances or occurrences of species within communities and on phylogenetic, taxonomic or functional similarities among species. We demonstrate that this new family embeds many of the recent developments in both functional and phylogenetic diversity. It provides a unique framework for comparing traditional compositional turnover with functional or phylogenetic similarities among communities

Beware of contagion!

Landscape ecology starts from the assumption that diversity and spatial arrangement of ecosystem mosaics has ecological implications and tries to understand the interactions between diversity and structure of large spatially heterogeneous areas and its ecological functions. This approach implies effective use of earth observation techniques and geographic information systems, enabling a global view of the landscape mosaics. Consequently, a large number of indices has been used to quantify the structure of categorical maps as a surrogate of actual landscapes and correlate them to ecological processes. In particular, the entropy-based contagion index has been extensively used to summarize the amount of clumping or fragmentation of patches on raster categorical maps. However, despite its widespread application, the contagion index is very dependent on pixel resolution. This effect may render it inadequate as a meaningful measure of landscape structure. To overcome this major shortcoming, in this short note we propose to quantify pixel adjacency with a bivariate summary statistics that is not adversely influenced by pixel resolution.L'articolo è disponibile sul sito dell'editore www.elsevier.co