Evolutionary ecology of a damselfly during range expansion

Abstract

Ranges of manyspecies are shifting polewards in response to contemporary global warming. Rangeshifting species experience different selective forces at the expansion frontcompared with these experienced at the core of their range and this can havelarge impacts on the evolution of phenotypic traits at the expansion front.Determining changes in phenotypic traits during range expansion is important aschanges in traits may influence the rate of further range expansion, abioticinteractions and biotic interactions at the expansion front. Inthis thesis I combined field studies, indoor common garden experiments andoutdoor mesocosm experiments to obtain a multivariate picture on the traitdifferentiation and associated changes in (a)biotic interactions during thepoleward range expansion of the damselfly Coenagrionscitulum. Phenotypicdifferentiation associated with northward range expansion Two importantaspects may induce evolution in phenotypic traits during range expansion.Firstly, populations at the expansion front are assorted by dispersal ability,whereby only the best dispersers colonize the expansion front. Secondly,population densities in newly founded populations at the expansion front arelow which may change selection regimes for life-history traits. Besides effectsof the range expansion process per se, also geographically structured thermalregimes may contribute to changes in phenotypic traits during range expansion.Geographicallystructured thermal regimes in both the larval (length of the growth season) andadult stage (adult flight period temperature) had an impact on different traitsinvestigated. Yet, importantly, these temperature regimes had only a smallcontribution to the phenotypic differentiation between core and edgepopulations. I documented in different studies changes in flight-related traitsthat indicate a better flight ability at the expansion front. I found higherinvestment in flight muscle mass, a lower wing loading and a larger relativethorax length at the range expansion front of C. scitulum. Higher flight ability at the expansion front mayevolve through natural selection whereby individuals that disperse most rapidlyat the expansion front benefit from the low densities of conspecifics at therange edge, and through spatial sorting in dispersal ability. Linked to theevolution of higher dispersal ability, adults at the expansion front had ahigher immune response (encapsulation response). Selection for a highinvestment in immune function to lower parasite load may occur through anegative effect of high parasite load on dispersal ability. Theory predictsselection for a higher population growth rate at the expansion front, which isfavoured at the low population densities in newly colonized habitats. At theexpansion front of C. scitulum Idocumented faster larval growth and development, while fecundity did not differbetween core and edge females. I documented for the first time, an increase ofactivity level in the non-dispersive larval stage at the expansion front; whileno higher activity in adults at the expansion front was detected. My resultsindicated that the higher larval activity at the expansion front evolved tomeet a higher energy demand in edge populations that is allocated to fastergrowth and higher investment in flight-related traits at the expansion front. Abiotic andbiotic interactions at the expansion frontLow foodconditions and high competition had a negative effect on growth rate of larvae;additionally low food conditions in the larval stage decreased investment inflight muscle mass hence likely dispersal ability. Furthermore, my results suggesta higher susceptibility of edge larvae to food limitation, which highlights theimportance of optimal local conditions at the expansion front for animalfitness and further range expansion. In contrast, winter survival was nothigher in edge larvae, indicating that no thermal adaptation for enhancedwinter survival at the expansion front occurred.Thehigher investment in dispersal, faster life history and higher larval activitywas expected to influence biotic interactions at the expansion front. Higherlarval activity and growth at the expansion front may increase visibility tovisual predators, hence was expected to increase predation risk by Anax imperator larvae. Furthermore,given that higher activity and growth rates typically are associated with abetter competitive ability, we predicted larvae at the expansion front to besuperior competitors relative to larvae from core populations. In contrast tothese expectations, we did not find a difference in predator-prey interactionsnor larval competitive ability between core and edge larvae. The relativelysmall changes in larval growth and activity during the range expansion of C. scitulum may be too small to have aprofound impact on the studied biotic interactions. Sexual selection for higherflight duration of males was consistent in core and edge populations. Incontrast, I detected geographical patterns in sexual selection for male bodysize and fat content, however these geographical patterns were rather shaped bydifferences in thermal regimes than by processes associated with rangeexpansion per se.My study not only adds to thegrowing number of studies that document rapid evolutionary changes in dispersaland life-history during range expansion, but also contributes to the scarcestudies investigating abiotic and biotic interactions at the expansion front.Furthermore, my study is the first to document the evolution of a higher adultimmune response and a higher activity level in the non-dispersive larval stageat the expansion front, which may give rise to eco-evolutionary dynamics. 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