Physiologische und historische Bestimmungsfaktoren der Verbreitung und Abundanz von Insekten

Field: Agricultural and Biological Sciences (Life Sciences)

Understanding the consequences of past and future climatic changes on biodiversity has become one of the most important challenges of current ecological research. Due to the fundamental importance of climate for determining the distribution and abundance of species, climatic changes have led to strong shifts of species’ ranges to higher altitudes and latitudes as well as to local changes in the phenology and abundance of species during the last decades. Nevertheless, most organisms are incapable of rapid responses to such changes as they are constrained by, for instance, phylogenetic conservatism in thermal adaptations and dispersal limitations. Therefore, a mechanistic understanding of the variation in functional traits of species is crucial for predicting biological responses to climate change. However, so far, most trait-based inferences focused on endotherm taxa, whereas the physiological processes shaping the diversity patterns of ectothermic organisms, particularly of insects, remain poorly understood. The overall objective of this PhD thesis is to investigate the importance of interactions between environmental factors and species’ functional traits across regions, scales and taxa, to improve forecasts of the ecological consequences of climate change as well as our understanding of the ecological and evolutionary processes that determine biogeographical patterns, the range size and the abundance of insects. Insects, like 99.9 % of the species on our planet, are ectothermic organisms that in contrast to endothermic organisms, mainly depend on thermal energy from their environment for their activity and for maintaining vital physiological processes. Ectotherms therefore evolved adaptations to the temperature regime in which they live. From a physiological perspective, strong arguments exist that biophysical principles link variation in species’ colour lightness and body size to heat gain and loss in endothermic animals. Larger species retain body heat more efficiently than smaller species owing to their lower surface-area-to-volume ratio, and darker coloured species heat up faster than lighter coloured species because they absorb more solar radiation. Other functions include enhanced immunocompetence of larger species and enhanced pathogen resistance (Gloger’s rule) as well as UV protection of darker species. Mechanistic links between these two morphological traits, species’ physiology and climate are hence probably important determinants of variation in the distribution and abundance of ectotherm organisms, but the limited availability of distributional and morphological data has so far hampered a large-scale perspective on the physiological processes that shape biogeographical patterns in insects. Constraints to the evolution of species’ morphological traits and dispersal abilities can limit the colonization of regions characterized by new climates or habitats and thereby influence geographical patterns in the phylogenetic diversity or geographical rarity of taxa. On the one hand, spatial concentrations of rare species are important conservation targets, because they indicate the distribution of species that are both particularly vulnerable to extinction in the future and unique elements of biodiversity. On the other hand, overall patterns of these facets of diversity provide information about past dispersal events and the ecological processes that shaped contemporary patterns of biodiversity. In six chapters of my thesis I investigate whether biogeographical patterns of insect assemblages are driven by variation in the colour lightness and the body size. I show that melanin-based thermoregulation, pathogen resistance and UV protection are important mechanisms that influence the distribution of dragonflies, butterflies and moths at both local and continental scales. In all studies, species assemblages in cooler climates are on average darker coloured than assemblages in warmer climates. Furthermore, in line with the prediction that darker colouration is advantageous in regions with high humidity and in regions with high solar radiation due to the protective functions of melanin, colour lightness generally decreases with increasing precipitation and insolation. Body size clines are less strong and differ considerably among the considered taxa. In addition, I demonstrate that contrasting effects of the benefits and the energetic costs of an investment into body size and melanization on the range size and abundance of butterfly species can offset each other when their interactions with components of the energy budget are not taken into account. Thus, larger and darker butterfly species only have wider distributions and are more abundant if they compensate the costs of an investment into body size and melanization by reducing mobility costs or increasing energy uptake. In three additional chapters, I investigate whether evolutionary constraints on species’ thermal adaptations and dispersal ability influence the composition of insect assemblages and I assess the extent to which diversity patterns of insects are shaped by the contemporary climate and historical climatic changes. Using European dragonflies, I show that both phylogenetic conservatism of thermal adaptations and dispersal limitations constrain the recolonization of previously glaciated areas of Europe, resulting in a decrease of the endemism and phylogenetic diversity of assemblages with decreasing temperature and the increasing proportion of species with a high dispersal ability. In addition, I demonstrate that the climatic changes since the Last Glacial Maximum are consistently major drivers of the endemism and species richness of mammals, birds, amphibians and dragonflies across Africa. However, the results of this study also indicate that the signatures of species’ responses to historical climatic changes differ considerably between the considered taxa and are currently less effectively protected. Finally, using a group of flightless orthopterans endemic to Africa, I exemplify that the diversity of this group, and probability most of the insect diversity today found in the Eastern Arc Mountain biodiversity hotspot, has been generated by the interplay of humid periods that allowed the spread of forest-bound lineages across Africa with aridity-driven fragmentations of forests and their associated faunas. In conclusion, I demonstrate that both body size and colour lightness are major determinants of distribution and abundance of insects, across taxa, regions and scales. Despite the significant contributions of other functions of colour lightness, such as pathogen resistance and UV protection, as well as of the thermoregulatory function of body size, melanin-based thermoregulation is the most important and a strikingly general mechanism that shapes biogeographical patterns of insect. To understand and predict the effects of body size and colour lightness on ecological dynamics of insect species it is, however, crucial to account for their interactions with components of the energy budget, because the contrasting effects of an investment into body size, wing size and melanization on the range size and abundance of species can partly offset each other. Purely correlative approaches that predict spatio-temporal variation in the distribution and abundance of insect species based on easily measured morphological traits are therefore prone to false mechanistic conclusions and likely underestimate the functional importance of morphological traits. Furthermore, phylogenetic conservatism of thermal adaptations and dispersal limitations affect trait-environment relationships and species’ responses to historical climatic changes. Together these results highlight the potential of models that integrate morphological, climatic and phylogenetic data for improving predictions of species’ responses to climate change as well as our understanding of the processes that generated and maintain the remarkable diversity of insects on Earth.