Vascular plant species richness at the landscape scale: Patterns and processes

The heterogeneous geographic distribution of species richness has fascinated and challenged biologists over centuries. As a result, a large number of possible hypotheses that aim at explaining these patterns have been proposed. Currently, the consensus is that multiple processes simultaneously affect species richness patterns, with the influence of each process differing between taxa, landscapes and periods in time. Generally, environmental conditions are assumed to be the main drivers underlying species richness patterns at large scales. However, as many landscapes and thus species’ habitats are strongly influenced by humans, particularly in Central Europe, the effect of human land use on richness patterns cannot be neglected. The severe anthropogenic changes in land cover, increasing landscape fragmentation, degradation and intensification have already led to large losses in (semi-)natural vegetation, biodiversity and ecosystem functions and are expected to cause even more losses in the future. Protected areas play a major role in the alleviation of anthropogenic pressures on species and their habitats. However, as site selection for many of these areas has not been based on knowledge on species occurrences, their effectiveness might be limited. The present thesis, conducted in the federal states of Lower Saxony and Bremen in northern Germany, therefore aims at 1. ) identifying the richness patterns of vascular plant species in the two states; 2. ) testing the effectiveness of the protected area network for the conservation of threatened species in the lowland region of the study area and 3. ) analysing the relationship between species richness patterns and their underlying processes with special regard to the human influence. In order to address these aims, analyses were carried out on grid-based floristic data obtained from the vascular plant survey of Lower Saxony (Niedersachsisches Pﬂanzenartenerfassungsprogramm, NLWKN 1982-2003) that were combined with high-resolution data on land cover and information on environmental conditions and the location and extent of protected areas. In doing so, the thesis represents the first comprehensive analysis of the extensive floristic data base of Lower Saxony. To analyse the patterns of vascular plant species richness across the study region, the overall species richness as well as the richness of five sub-groups of species aggregated using information on residence and threat status of the species inside each landscape unit (1,762 grid cells of c. 30 km²) was assessed. Richness of all species groups was found to be heterogeneously distributed across the region showing an increase in richness from north to south and – to some extent – also from west to east. The centres of high plant species richness of the different groups largely correlated with each other but also showed some interesting divergences. The effectiveness of the protected area network in the lowland region of the state was shown to be generally high: 'Hotspots' of threatened species were found to be largely covered by nature reserves (German: Naturschutzgebiete) and Natura 2000 sites with the latter areas expediently complementing the former. It remained unclear, however, if the effective 'territorial' protection is reﬂected in an effective species protection. The anthropogenic inﬂuence on the landscape of the study region was assessed using two separate analyses. These two studies focussed on the relationship between species richness and the degree of landscape fragmentation on one hand and landscape modiﬁcation on the other hand. The degree of landscape fragmentation was assessed by applying the Effective Mesh Size Index (meff). The relationship between meff and species richness of the total number of species as well as ﬁve sub-groups of vascular plants aggregated using their residence and threat status was assessed using a variation partitioning approach that separated the effects of landscape fragmentation, environmental conditions and spatial autocorrelation. Landscape fragmentation was found to signiﬁcantly affect richness of all species groups. However, the strength of the effect varied between species, being largest for neophytes and smallest for threatened species. A clustering analysis on landscape metrics describing landscape composition and conﬁguration revealed a six-level gradient of landscape modiﬁcation for the study region. The gradient spanned from highly fragmented urban landscapes to little fragmented landscapes with a high proportion of forest. Species richness of the total number of species as well as seven sub-groups of vascular plants aggregated using the residence and threat status as well as habitat preferences of the species were found to signiﬁcantly differ along the gradient. For the entire and native species pool as well as for forest, low-nutrient indicating and threatened plants, species numbers peaked at the opposite ends of the modiﬁcation gradient. For neophytes, urban plants and high-nutrient indicators richness decreased along the gradient from high species numbers in urbanised to low numbers in less modiﬁed landscapes. The results obtained from the investigations in this thesis provide a basis for future landscape and conservation planning approaches in the study region by facilitating the identiﬁcation of landscapes of high conservation value and/or concern. Provided that appropriate, high quality data like those used in the present thesis are available, the methodologies applied and developed in context of the thesis can be transferred to other regions to accordingly support conservation and landscape planning.