From individual migrations to biomass flows: Identifying and understanding patterns, determinants, and implications of long-distance movements

Abstract

Animal migrations span the globe, involving trillions of individuals from a wide range of taxa that transport nutrients, energy, and other organisms throughout their journeys. Through a variety of transport and trophic effects, migrants can uniquely alter nutrient and energy flow, the structure and dynamics of (meta-)communities and represent a powerful yet underappreciated dimension of biodiversity. These transport and trophic effects also constitute a multitude of services and disservices that are relevant for human agriculture, economy, and health - with benefits or costs in the order of billions of dollars annually. In this habilitation, I set up the conceptual framework for the role of migratory animals in linking community structure and ecosystem functioning world-wide, make the link from individual migration behaviour to biomass flows and explore the implications these may have for the transport of parasites, the conservation of migrant populations and ultimately, biodiversity. I also show how monitoring, understanding and forecasting migrations is relevant to human agriculture, economy and health. This thesis consists of two parts. In the first, I review current developments, ways and examples of identifying routes and timing of individual bird movements and their environmental determinants but also how the sensor network of European weather radars can be used to provide a complementary view on the large-scale biomass flows of bird migrations. As the timing of migration determine the fate of individuals and migratory populations, I review the environmental cues and factors that migrants across the animal kingdom use in making these timing decisions. Finally, I introduce the most relevant threats that migratory populations currently face. In the second part, I elaborate on major implications of long-distance movements for the structure and function of communities and ecosystems, taking parasite dispersal as one major example. As the loss, alteration and fragmentation of habitats, climatic changes, expansion of human structures and activities have led to massive declines in migratory populations, their conservation has become an internationally shared priority. However, conservation and management of highly mobile organisms is particularly challenging as they use multiple sites throughout their annual cycles that span migratory networks. I will show that efficient conservation is only possible by considering the interconnected nature of site use and that the resilience of migratory populations crucially depends on the structure and composition of their networks. Furthermore, I demonstrate the potential of continental weather radar networks in monitoring the biomass flows of aerial migrants and providing information to a variety of stakeholders. To tap this potential, however, meteorological data policies and infrastructures need to be adjusted for data use beyond meteorology. Based on these two parts, I synthesize avenues for future research and set personal priorities in tackling the outstanding challenges in the realm of migration ecology.