Yanjie Xu

A well-connected network of habitat sites is essential for bird migration. Under rapid habitat loss caused by climate and land use changes, the connectivity of these networks is decreasing, which impacts the bird populations that depend on these networks. Moreover, this also affects other ecological processes that dependent on bird movements, such as disease transmission of avian influenza. Conserving these bird migration networks is crucial for efficient and effective maintenance or restoration of its ecological functions. Although this has been pointed out by scientists 20 years ago, this specific aspect has not yet been included in international conservation efforts for migratory bird species. Conservation can be improved by a better understanding of the causes and consequences of decreasing connectivity in bird migration networks. However, this knowledge remains limited.

Therefore, this thesis advances the understanding of the extent to which environmental changes breakdown bird migration networks, and its potential consequences, such as its effect on population declines and disease transmission. Furthermore, to conserve the connectivity of bird migration networks, it is important to prioritize conservation efforts of sites that have a crucial contribution to the connectivity of the entire network. I propose a quantitative methodological framework for monitoring and understanding of the dynamics of bird migration networks from a local to a flyway scale, which can also assist policy makers when planning conservation actions.

Taking advantage of remote sensing datasets, I quantified habitat loss, fragmentation, and isolation from 1992 to 2012 for eight migratory waterfowl species in the East Asian-Australasian Flyway (Chapter 2). The patterns of habitat degradation differed spatially across the flyway, i.e., wetlands sites degraded more in the southwest of the flyway while wetland availability improved in the northwest. I analysed these differences in habitat degradation over the eight studied species, and found that habitat degradation affected species with different migration extents to a different degree. Migratory species with longer and narrower migration corridors were more affected by habitat degradation, because they had fewer alternative stopover sites at similar latitudes. The species with shorter and broader migration corridors could take advantage of improved habitat conditions in the west of their migration pathway. These findings improve the understanding of the variation in effects of environmental changes on different migratory species.

Next to these spatial patterns in habitat degradation, I monitored the changes in functional connectivity of migration networks of the eight waterfowl species in the East Asian-Australasian Flyway between 1992-2015 (Chapter 3). This enabled me to test the degree to which the breakdown of migration networks impacted the population sizes of migratory bird species. I found that the loss of functional connectivity in migration networks significantly predicted population declines in the studied migratory species, while no previously considered species traits could explain these declines. Thus, the decreasing connectivity in migration networks as a consequence of habitat loss and degradation, could negatively and crucially impact population sizes of migratory birds. This novel predictor for population declines of migratory birds provides new insights of the underlying mechanisms that affect population trends of migratory birds under environmental changes.

Thus, conservation of network connectivity is an essential step for safeguarding migratory bird populations, which can be obtained by setting conservation priorities for sites that are more important than others in their contribution to the connectivity of the network. To facilitate the calculation of the importance of a site from a network perspective, I investigated to what extent different network metrics (degree, betweenness, and node resistance) and site-specific habitat loss patterns contributed to the breakdown of migration networks (Chapter 5). By simulating site loss from migration routes of four bird species in the Asian flyways under different scenarios (i.e., in order of degree, betweenness, node centrality, and degree of habitat loss), I observed that site loss in order of the highest to the lowest betweenness value generated a significantly more rapid decrease in connectivity of bird migration networks. According to this conclusion, I identified keystone sites in a network by first selecting sites with a relatively high betweenness value, and then identifying which of those sites was experiencing habitat loss, indicating a higher risk of being removed from the network. I found that 42% of these keystone sites for the four studied species were not designated as protected areas. These results point out the urgency and importance of prioritizing conservation efforts for keystone sites from a network perspective.

A better understanding of the dynamics of these migration networks and early-warning signals for network collapse can be obtained from quantifying how environmental factors predict site quality for migratory birds. Using greater white-fronted geese (Anser albifrons) as an indicator species, I tested the extent to which environmental factors (i.e., both anthropogenic and ecological factors) influence their movement patterns at stopover sites in the East Asian-Australasian Flyway (Chapter 4). I found that environmental factors accurately predicted movement patterns (i.e., median movement distance and percentage of flying time) of the geese at stopover sites. Notably, farming activities negatively affected habitat quality for geese, as indicated by their higher frequency and longer distance of local movements in landscapes with a higher proportion of farmlands. These results provide insights into the probability that a site may be lost from a migration network, and can thereby serve as a quantitative tool for monitoring and understanding the dynamics of migration networks of waterfowl species.

Lastly, besides impacts on bird population, I investigated whether the spread of bird-borne infectious diseases was accelerated by changing structures of migration networks under global changes (Chapter 6). I constructed migration networks for 47 bird species (all Anatidae) in the northern hemisphere, and monitored their structural changes from 1950-2019. This enabled me to map the changes in intra-species and inter-species crossroads between flyways where different populations mix and viruses can change hosts. I observed a northward shift of North American migratory birds, and an eastward shift in Eurasian birds. This may be responsible for an increased intra-species crossing intensity in North America, and an increased inter-species crossing intensity in Eurasia. Thus, the changes in movement networks of migratory hosts driven for example by climate warming and land use changes may contribute to increasing risks of long-distance transmissions and broad-scale outbreaks of infectious diseases.

Combining the evidence and techniques presented in this thesis, I synthesized the various consequences of weakening network connectivity, and proposed a quantitative framework that can enable scientists and policy makers to prioritize site conservation in movement networks (Chapter 7). Decreasing connectivity in migration networks can not only trigger population declines, but also impact multiple functional processes including information diffusion, seed dispersal, predator-prey interactions, pathogen transmission, and gene flow. Thus, maintaining or restoring the connectivity of bird movement networks safeguards not only bird populations, but also the ecosystem services provided by their movements. I identified gaps between policy and science, i.e., a minority of criteria in international policies for defining site importance accounted for the connectivity of bird movements, all of which remain qualitative. Thus, I recommend to improve conservation policy making from a network perspective. The proposed methodological framework in this thesis facilitates systematic conservation decisions for movement networks.