Populations crossing habitat boundaries in the face of environmental change

Almost all ecosystems on Earth are facing the impacts of rapid environmental change. As the human population continues to increase and its activities expand, threats on biodiversity intensify. This thesis contributes to the theoretical foundation necessary to understand the effects of rapid environmental change on species that connect different habitats through its life cycle, e.g. Atlantic salmon (freshwater and oceanic habitats). It shows that the effects of multiple threats to natural populations (chapter 3) do not behave simply as the addition of single threats (chapter 2) and therefore that predicting the consequences of multiple threats in a cumulative way requires gaining further insight into the mechanisms causing these non-linear effects. In addition to the study of the ecological consequences of a changing environment, this thesis investigates its evolutionary consequences and demonstrates that considering both ecological and evolutionary responses together yields novel outcomes. Specifically, chapter 4 contradicts classical theory on evolution of life history that is based only on individual optimization. Chapter 5 and 6 shows that evolutionary changes can affect the resilience of ecosystems, in contrast to traditional theory in ecology that assume that only external conditions affect ecosystem resilience. This thesis characterizes mechanisms whereby populations respond to a changing environment. If we are to predict ecological consequences of environmental change and thus to adopt conservation measures accordingly, this mechanistic understanding of how life cycles and population dynamics interact in response to environmental change is certainly required.