Biodiversity patterns and the processes regulating them along elevation gradients in the Swedish mountains

Sammanfattning: Biodiversity describes the total variation of life and includes the taxonomic, genetic, and phenotypic differences among organisms. Variations of biodiversity in space and time may be driven by ecological, evolutionary, or neutral processes. The topography in mountains gives rise to substantial gradients in environmental conditions over short geographical distances. This makes them suitable for studies of how environmental conditions influence spatial variation in biodiversity. Additionally, climate change is stronger in high latitude and high elevation environments compared to the global average, which makes mountain environments particularly relevant systems for evaluating the biodiversity consequences of a changing climate. In this thesis I have assumed a declining primary productivity with increasing elevation and tested if this has led to related monotonic declines in biodiversity and a stronger environmental regulation of biodiversity at higher elevations. I pursued the following specific questions; (i) what are the patterns of biodiversity along elevation gradients in the Swedish mountains? (ii) do such patterns vary among organism groups at different trophic levels? (iii) what processes regulate biodiversity along elevation gradients in the Swedish mountains? To address these questions, I quantified patterns of alpha diversity, beta diversity, community composition and community structuring of vascular plants, spiders, insects, and springtails along elevation gradients distributed along the Swedish mountains (Chapter I-III). I also quantified the relative importance of abiotic and biotic environmental conditions for spider diversity (IV), and finally I evaluated if elevational variation in phylogenetic and phenotypic dispersion within vascular plant and spider communities corresponded with an increased environmental regulation at higher elevations (V). Alpha diversity of all organism groups generally declined with elevation. However, while there were geographic differences in these patterns for vascular plants (I-II), there were mainly taxonomic differences in the observed patterns among arthropod groups (III). Taxonomic beta diversity of vascular plants did not show any uniform pattern with elevation but differed both among sites and spatial scales (I-II). The structuring of vascular plant, spider and insect communities were all modular along elevation gradients, but this modularity was less prominent for springtails. Spider and insect communities were also nested along the elevation gradients (III). Vegetation and climate conditions had the largest effects on spider diversity, but the relative effects of different environmental conditions varied both among biodiversity dimensions and spatial scales (IV). Vascular plant and spider communities were both phenotypically and phylogenetically under-dispersed, suggesting that communities were regulated by environmental filtering. However, for vascular plants the phylogenetic dispersion increased while the phenotypic dispersion was constant with elevation, whereas for spiders both phenotypic and phylogenetic dispersion decreased with elevation (V). My results suggest that site-specific and scale-dependent processes may partly override the effects of elevational declines in primary productivity on biodiversity. My results also suggest that biodiversity regulation along gradients can vary among different taxonomic groups and highlight the need to quantify multiple diversity dimensions.