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It is examined whether caddisfly assemblages collected by light trapping are influenced by altitude. From twenty-nine sites, a total of 29 season's catches were obtained in 1995. The total number of caddisfly species collected was 53, and the number of individuals caught was 11,128. As a result of non-metric multidimensional scaling based on quantitative data, two groups of caddisfly assemblages could be distinguished: those collected in lowland (sites under 150 m a.s.l.) and the others from highland habitats (sites above 150 m a.s.l.). The indicator value method found assemblages and species most characteristic of the two kinds of habitats. Ecnomus tenellus and Neureclipsis bimaculata were the two significant indicator species of the lowland habitat, and Stenophylax permistus was the significant indicator of the highland habitat.
Although overlap of communities is a key issue in studies ranging from community ecology to biogeography, a clear definition of community overlap and related terms hinder the development of the field. The absence of a unified terminology is remarkable even when the overlap of a pair or multiple communities is characterized. As a remedy, I suggest a definition of community overlap and two measures of it (number of overlapping species and total overlap size). Although both measures quantify different aspects of community overlap, in studying pairs of communities they yield identical results. The present findings demonstrate the need for a unified terminology in research on community overlap as well as for pairwise and multiple measures for quantifying the phenomenon.
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.
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.
Although our knowledge of the spatial distribution of stream organisms has been increasing rapidly in the last decades, there is still little consensus about trait-based variability of macroinvertebrate communities within and between catchments in near-pristine systems. Our aim was to examine the taxonomic and trait based stability vs. variability of stream macroinvertebrates in three high-latitude catchments in Finland. The collected taxa were assigned to unique trait combinations (UTCs) using biological traits. We found that only a single or a highly limited number of taxa formed a single UTC, suggesting a low degree of redundancy. Our analyses revealed significant differences in the environmental conditions of the streams among the three catchments. Linear models, rarefaction curves and beta-diversity measures showed that the catchments differed in both alpha and beta diversity. Taxon- and trait-based multivariate analyses also indicated that the three catchments were significantly different in terms of macroinvertebrate communities. All these findings suggest that habitat filtering, i.e., environmental differences among catchments, determines the variability of macroinvertebrate communities, thereby contributing to the significant biological differences among the catchments. The main implications of our study is that the sensitivity of trait-based analyses to natural environmental variation should be carefully incorporated in the assessment of environmental degradation, and that further studies are needed for a deeper understanding of trait-based community patterns across near-pristine streams.
A new approach to the measurement of functional diversity based on two-state nominal traits is developed from the florula diversity concept of P. Juhász-Nagy. For evaluating functional diversity of an assemblage, first a traits by species matrix is compiled. Various information theory functions are used to examine structural properties in this matrix, including the frequency distribution of trait combinations. The method is illustrated by actual examples, the first from plant communities prone to fire in Spain, and the second from running water invertebrate assemblages in Hungary. The results suggest that of the various functions used the standardized joint entropy, termed combinatorial functional evenness supplies most meaningful results. In plant communities, high fire recurrence decreased combinatorial functional evenness, while this measure for freshwater assemblages was uncorrelated with stream width and negatively correlated with the degree of human impact. Stream width is negatively correlated with the number of manifested functional combinations. In both case studies, combinatorial functional evenness has an inverse relationship to species richness — i.e., fewer species have a larger chance to produce equiprobable functional combinations.
