Anika Neu

• Species range margins are highly dynamic, changing as a result of complex eco-evolutionary processes. Determining both the abiotic and biotic drivers and the species-specific traits underlying successful range expansions will be vital for predicting future distribution patterns, especially in the current era of anthropogenic environmental change. In pursuit of this knowledge, I used the rapid range expansion of the Mediterranean butterfly species Pieris mannii (MAYER, 1851) into Central Europe as a case study. Specifically, I was interested in studying the relative importance of niche tracking versus niche evolution and the contributions of genetic trait changes and (evolved) phenotypic plasticity within the context of niche evolution. Towards this end, I adopted a multi-methodological approach, including correlative species distribution models (to test for the role of niche tracking in response to climate change; Chapter 1) and common garden experiments. With the latter, I tested for niche evolution in host plant use behaviour, thermal tolerance and life-history traits by comparing replicated populations from P. mannii’s historical and newly established range (Chapter 2). I additionally used the tropical butterfly species Bicyclus anynana (Butler, 1879) to assess how reproductive behaviour is affected by the interaction of temperature and host plant quality, both prone to variations across the range of herbivorous insects (Chapter 3). The results from species distribution modelling revealed that niche tracking, driven by climate change, can be largely ruled out as the primary driver of P. mannii’s range expansion. There was, however, evidence for rapid niche evolution in various phenotypic and life history traits, including female host plant use behaviour. Females from the expanding populations showed increased flexibility in their acceptance and preference for different host plant species, potentially increasing the availability of favourable habitats as connecting “stepping stones” in space and time. The experiments with B. anynana yielded, moreover, interesting insights into temperature-dependent variations in reproductive output on different host plants (reduced fecundity on low-quality host plants under stressfully high-temperature conditions) and a potential trade-off between high selectivity and fecundity. The question of whether these patterns are prevalent in range-expanding species in natural settings warrants further in-depth examinations. Despite shifts in host use behaviour, expanding populations of P. mannii exhibited an enhanced cold tolerance, a trait considered essential for terrestrial ectotherms undergoing successful poleward range expansions in temperate regions. Heat tolerance, however, showed no significant variation across populations, which might indicate a relaxation of selection pressure on upper thermal tolerance limits at higher latitudes. Compared to populations of the ancestral distribution, individuals from range edge populations also exhibited an increased fecundity, frequently characterised as a key trait underlying successful range expansions. As mechanisms underlying trait changes, I identified both (increased and decreased) phenotypic plasticity, demonstrated by increased cold tolerance and accelerated larval development (in individuals of the expanding populations, developing under cool temperature conditions; Genotype x Environment interaction), and genetic trait changes, underlying shifts in host preference and fecundity in reproducing females of range edge populations. With the results of the present thesis, I demonstrate that successful range expansions can be driven by rapid niche evolution in multiple traits, which are associated with abiotic and biotic environmental characteristics. Thus, I highlight the importance of eco-evolutionary dynamics underlying the expression of species-specific "Expansive Phenotypes". Several open questions are discussed and the need for further research, especially concerning potential trade-offs and costs associated with trait expression during range expansion, is underlined.