Lars Lønsmann Iversen:
Species associated with freshwater ecosystems are currently undergoing severe global declines and freshwater ecosystems are regarded as some of the most endangered ecosystems in the world. These declines are a consequence of decades of human overexploitation, pollution and climate change. If adequate conservation actions are to be implemented, there is an urgent need for improving our understanding of the specific habitat requirements and the ecological niches of species in freshwater ecosystems. This thesis explores current challenges in our understanding of how spatial processes structure and shape the habitat requirements and distribution of one of the most affected groups of freshwater species: aquatic insects. It comprises four chapters each addressing different spatial factors in relation to the occurrence of aquatic insects in Europe.
Chapter I examine two spatial ecological processes (time since glacial disturbance and habitat stability) and question the generality of these processes for the understanding of species richness gradients in European rivers. Using regional distributions of European mayflies, stoneflies, and caddisflies this chapter demonstrates that differences in latitudinal richness patterns between headwater streams and rivers are as great as the differences between river and lake habitats. Thus, the paper questions former spatiotemporal hypotheses stating that the dichotomic diversity patterns between rivers and lakes in Europe are solely created by post-glacial dispersal and differences in species dispersal capacity between the two habitat types. The results instead suggest that integrating effects caused by environmental filtering or other factors would expand our understanding of the mechanisms causing the current diversity patterns within these habitats.
Chapters II exemplifies that national changes in land use can influence the perception of a species’ realized niche across different landscapes. By observing the environmental niche of a threatened European dragonfly in landscapes with different land use history it is shown that if a species’ realized niche is derived from local distribution patterns, without incorporating landscape history it can lead to an erroneous niche definition.
Chapter III provides some of the first evidence for differences in dispersal phenology related to flight potential in aquatic insects. The chapter highlights seasonality and resource availability as factors leading to differences in flight potential within a group of aquatic beetles. Among the study species the documented differences in dispersal scale to the dependency of spatial connectivity of freshwater habitats when colonizing newly created ponds Chapter IV documents the species composition of diving beetles across a spatial gradient from the core to the edge of one of the most threatened European aquatic habitats: Bog pools. The chapter identifies the key environmental factors structuring communities along the core-edge gradient and highlights that when undergoing environmental change, species communities associated with open bog pool habitats are affected by a multitude of factors. From this finding it is concluded that bog pool habitats are affected by the same core-edge gradients and face the same conservation challenges as other isolated unique habitats. The chapter highlights rewetting schemes followed by tree removal as potential restoration measures for pristine bog pool communities.