Peter Søgaard Jørgensen:
Human induced changes in the earth system, such as anthropogenic climate change, cause loss of biodiversity that feed back as food, health and environmental challenges for human society. Climate change is one of the main threats to biodiversity and human society due to its global manifestation, long interval from reduction in green house emission to cessation of warming, and the uncertain capacity of the natural systems to buffer greenhouse gas emissions. This thesis explores current challenges in our understanding of how climate change will affect biodiversity and how consequent challenges for human society can be minimized. It comprises eight chapters and two appendices.
Chapter I presents an integrated framework and synthesis for how the study of species’ responses to changing environments can be improved through a combination of existing methods and by explicitly accounting for their strengths and weaknesses when measuring organismal processes and environmental impacts over space and time.
Chapter II provides an example of such integration. Genetic material from wild populations serves as a resource to reconstruct past changes in numbers, but the accuracy of reconstructions is often uncertain. This chapter presents an example of combining existing methods by comparing genetically inferred numbers to historical records. The comparison confirms predictions about how accuracy should change with the magnitude and point in time of historical changes. This indicates the utility of population genetics in improving our understanding of strong population declines due to human impact dating back to before any reliable observation accounts existed.
Two of the most comprehensive wildlife monitoring programs that have been running since the middle of the 20th century is The North American and European breeding bird surveys. The next four chapters in the thesis (chapters III-VI) are based on these programs. The chapters seek to answer questions about the continental-scale responses of biodiversity to climate change through investigation of population dynamics since 1980. Chapter III presents an investigation of land-use and climate change effects on common European farmland birds. It shows that suspected impacts of land-use changes are most evident over decadal time-scales whereas climate change impacts can be detected both at the decadal and annual time-scale through increases in more equatorially distributed species and divergent responses of migratory strategies to seasonal weather.
Chapter IV presents the first of three comparative studies of the North American and European avifauna. Since the mid-20th century, birds that migrate to the tropics have exhibited decreases in population size with climate change being proposed as one of the causes. The chapter investigates the evidence for recent increases in tropical precipitation and primary productivity to cause a recovery in migrant populations. It presents novel evidence for two dichotomies in the effect of such “re-greening”. Over yearly time spans “re-greening” leads to increased population size of migrants present in seasonal habitats through increases in both survival and reproduction. Migrants in more seasonally stable habitats are less affected by short-term re-greening and mainly through effects on reproduction. The second dichotomy is detected on the time span of decades: as a likely result of re-greening, migrants present in tropical areas closer to the breeding area have partially recovered from past declines, while migrants that have to travel further to reach their destinations are not responding to long-term re-greening and many are still in decline.
Movement of species toward the poles and associated increases in poleward populations is one of the most generally documented biodiversity effects of climate change. Chapter V investigates the generality of this response in European and North American breeding birds. I also investigate the presence of continental gradients in seven metrics of population dynamics that may be indicative of current and future vulnerability to climate change. This study provides evidence for poleward shifts in abundance in three out of four investigated regions and shows that equatorial populations have the least stable population dynamics. It also shows that geographical gradients in population dynamics are most frequent in regions with presence of cool summer climates.
Abrupt irreversible changes in geophysical and biological systems are particular concern as a potential of climate change. Until now most evidence for such effects comes from destabilization of large-scale geophysical systems or local ecosystems. Chapter VI provides evidence that stability of continental-scale breeding bird communities in Europe and North America has declined over the past 30 years in concert with pronounced global warming. The connection of these findings to global warming warrant further investigation, but has the potential to change our perspective of climate change as a destabilizing factor in wildlife communities at large spatial scales.
The potential for cold- and warm-adapted species to coexist during climate warming is of much interest, but poorly understood in most taxa. Chapter VII investigates this in two species-rich groups of insects - moths and beetles. It provides some of the first evidence in animal communities that divergent phenological responses by cold- and warm-adapted species may serve as a mechanism for co-existence, minimizing local extinctions of coldadapted species during global warming.
Effects on biodiversity of human-mediated environmental change feed back to human society by challenging food production, human health and environmental management. These challenges are caused by declines in species that human societies depend on and due to rapid evolution in species that are considered pests and pathogens. Chapter VIII takes a step back and provides a multidisciplinary review of how biological knowledge of environmental change effects can be turned into solutions to minimize current global challenges for human society in areas of food production, health and environmental protection. This kind of solution-oriented contributions in the ecological sciences will be important for the protection of biodiversity and human well-being during this century - that will be marked by climate change. Overall the results of the thesis point towards a diverse se of impact of climate change on the common breeding birds of Europe and North America. Such diverse responses challenge our ability to predict future outcomes of biodiversity responses to climate change and the need for a multi-facetted approach to conservation of species during climate change as well as to the study of climate change effects themselves.