Dokument

Summary:
Biodiversity patterns, community composition, and ecological dynamics are linked to species’ responses to climatic conditions, biotic interactions, and dispersal limitations. Climate change has led to shifts in species ranges to higher altitudes and latitudes, changes in species population trends, and changes in species phenology. That makes climate change a fundamental concern for biodiversity conservation and makes understanding the effect of climate change on biodiversity a central theme in ecology. A promising approach to understanding the mechanisms shaping species distribution and community composition is linking physiological processes, functional traits, and the climatic environment. Two ubiquitous features of animals of great functional importance are body size and body colour. For instance, both traits influence the temperature excess of species and thereby have significant consequences on species distributions, abundances, activities, and development. However, thus far, support for the importance of body size and body colour is sparse for insects, particularly at larger spatial and taxonomic scales. In addition, the effects of both traits on community composition and species’ responses to environmental changes remain poorly understood. Whereas it is difficult to monitor species populations across entire ranges, the IUCN established criteria that allow taxon experts to broadly categorize the threat status of species. However, only 1 % of the insects described so far have been assessed, with 26 % of these categorised as data deficient. Especially in tropical regions and for tropical taxa, we lack an understanding of the drivers associated with threatened insect species. The overall objective of this PhD thesis was to investigate the importance of interactions between environmental factors and species’ functional traits across Europe and Africa. My work focused on damsel- and dragonflies (Odonata) because of their ecological importance and exceptional natural history record among insects. With this, I aim to improve our understanding of the mechanistic processes underlying biogeographical patterns and species extinction risk and ultimately improve forecast of the ecological consequences of climate change. In one chapter of this thesis, I quantified the colour lightness and body volume of European Odonates and combined these traits with survey data for local assemblages across Europe. Based on this continent-wide yet spatially explicit dataset, I tested for effects of temperature and precipitation on the colour lightness and body volume of local assemblages and assessed differences in their relative importance and strength between lentic and lotic assemblages. I show that the colour lightness of assemblages of odonates increased, and body size decreased with increasing temperature. My results demonstrate that the mechanisms underlying colour lightness and body size variations scale to local assemblages. Together with previous studies on larger spatial scales, these results underline the general importance of colour- and size-based thermoregulation in insects. Both size- and colour-based thermoregualtion were of similar importance for species preferring lentic and lotic habitats (standing vs. running water), but the higher dispersal ability of lentic species seems to allow them to better track their thermal optimum. In another chapter, I integrated trait-based models with environmental factors to investigate the mechanistic underpinnings of species’ extinction risk for 489 African and European Odonates. Using body size, wing load, and habitat preference, I incorporate current theoretical and empirical support for single effects of environmental variables on species traits into structural ecological models. Specifically, I tested whether species are generally larger in colder environments; whether species adapted to less stable habitats and with lower wing loads have smaller ranges; and finally, the extent to which these trait-environment relationships translate into a higher extinction risk of species. The results of this chapter demonstrate that species adapted to lotic habitats as well as smaller species and species with high wing loads have smaller range sizes. In addition, larger species and those with lower wing loads had more northern distributions and inhabited colder climates. Species with smaller ranges and those occurring in colder and more northern regions had a higher extinction risk. I thereby demonstrate that strong links between intrinsic traits (body size, wing load, and habitat preference) and extrinsic traits (range size, thermal preference, and latitudinal position) can explain a substantial part of the variation in species’ extinction risk. However, in contrast to models of extrinsic traits alone, I emphasize that the mechanisms underpinning species’ extinction risk are important to consider for understanding which species are particularly threatened and why. Thereby, trait-based models have a high potential to forecast and mitigate the negative impacts of environmental changes and other threats to species. In another chapter, I investigated the potential of Odonates as biological control predators of mosquito larvae under almost natural conditions. I found that the widespread dragonfly Bradinopyga strachani is capable of breeding in and naturally colonising water storage containers used in typical rural homes, which are breeding grounds for mosquitoes. My mesocosm experiments show that the presence of B. strachani resulted in a drastic reduction of mosquito larvae density, especially in sunlit containers. My results confirm that dragonfly larvae are effective biological control agents of the disease-causing vector, with great benefits to the livelihood of people. In summary, I demonstrate the importance of the mechanistic links between colour lightness and body size with the temperature regime which shapes the biogeographical patterns of European Odonates using spatially explicit survey data. The consistency of this reiterates the general importance of thermal melanism and Bergmann's rule for ectotherms at the local assemblage scale. However, besides highlighting the essential role of traits involved in thermoregulation in shaping the distribution of Odonates, the greater dispersal ability of lentic species in combination with the climatic history seems to have allowed them to better cope with the historical climatic changes. Furthermore, my results highlight the importance of functional traits in species extinction risk assessments. Body size, habitat preference, and wing load explain why some species are particularly threatened and may thus serve as a red flag for threat assessment in conservation, even for species that lack distribution data. These integrative trait-based analyses are particularly relevant for providing links between ecology and conservation, which are important for completing and predicting species threats. These results underline the ecological importance of Odonates and highlight a great potential for integrating interactions of morphological traits with species phylogenetic data and proxies of dispersal ability, into trait-based models to improve our understanding of biological responses to environmental changes and other potential threats. The importance of the functional traits of species and the generality of their impact on ecological dynamics stress that mobilization of trait data provides an important future avenue to improve baseline predictions and the information basis for large scale conservation of insect diversity.

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