Patterns in Biodiversity: Spatial organisation of biodiversity in the Netherlands

A better understanding of biodiversity and its current threats is urgently needed, especially in the Netherlands where high population density, industrialisation, and intensive land-use have radically altered the natural landscape. Often, biodiversity research is seriously hampered by a lack of data. This is not so much a problem in the Netherlands, one of the most intensively inventoried countries of the world. Over the last decades, data on the occurrence of a broad range of species have become digitally available in the form of large geo-referenced databases that can be used for spatial analyses. Data from several of such databases were used in this research, which aims at gaining insight in the spatial organisation of Dutch biodiversity, using a broad array of taxonomic groups: hoverflies (Syrphidae), dragonflies (Odonata), mosses (Bryophyta), grasshoppers and crickets (Orthoptera), and herpetofauna (Reptilia and Amphibia). The objectives of this study are threefold: to assess the potential of Dutch species occurrence databases for biodiversity studies; to describe spatial patterns of species richness and composition for different taxonomic groups and relate them to environmental factors; and to analyse the implications of the results for nature conservation. Patterns of species richness of the different groups showed a fairly positive correlation. In broad outline, in the southeastern part of the country more species are found than in the northwestern part, with the dune area as an important exception. A multiple regression analysis revealed which environmental factors may play a role in the mechanisms underlying these patterns. Heterogeneity of the landscape (e.g. number of habitat types, altitudinal range) explained most of the variation in species richness. Biodiversity is not just about species numbers; an area may be relatively poor in species but can still be of high value when the species occurring there do not occur anywhere else. Based on the distribution of the species from the five studied taxa, the country was divided into five distinct regions, covering 40% of the country. Each region was characterized by unique species that are absent or scarce in other parts of the country. These regions were also characterized by differences in their environmental conditions. A method by means of which quantitative and compositional aspects of biodiversity can be analysed at the same time, the so-called nestedness analysis, was used to investigate biodiversity patterns at a finer scale. The grasshopper and cricket assemblages in 178 semi-natural sites within the Pleistocene sand region of the Netherlands were found to be significantly nested. This means that the species present in species-poor areas constitute a subset of the species from richer locations. It follows that rare species, species with a small distribution area, are predominantly found at species rich locations. The spatial configuration of the hotspots of species richness and areas with an important species composition have been compared with that of the National Ecological Network, one of the most important instruments of Dutch nature conservation. Especially regarding the protection of the areas with a characteristic species composition important improvements can be made.