Connectivity of migration landscapes: from suitable wetland habitats to suitable networks for migratory waterbirds

During their seasonal journeys between breeding and wintering areas, migratory waterbirds rely on the availability of high-quality and well-connected wetland stopovers. In case of the Palearctic-Afrotropical flyway, the world’s largest migration system accommodating billions of birds, flight routes are funnelled through bottlenecks in vicinity of major ecological barriers such as the Mediterranean Sea and the Sahara Desert. Wetland chains located in these bottleneck regions may provide crucial stepping stones for the functioning of entire flyways.

However, despite the flexibility in migration routes and strategies, combined effects of anthropogenic and climate-induced wetland habitat changes have caused severe and widespread declines of migratory bird populations. These negative trends increase the need for a better understanding of how habitat composition and configuration combined (= network structure) support migration processes at multiple ecological scales. In this respect, the importance of functional connectivity for long-term population persistence is widely recognised, yet hardly addressed or even completely absent in conservation frameworks. Graph theoretic models, i.e. mathematical representations of landscapes composed of nodes and links, and more specifically Probability of Connectivity (PC) metrics, have been shown to excel at analysing connectivity, and identifying crucial sites within networks. Thus far, the potential of these models has not been exploited at flyway-scale, nor were they adapted to capture the actual role of stopovers in facilitating migration. Therefore, in this thesis, we aimed to develop PC-based connectivity metrics tailored to migratory populations, that can be used to identify wetlands that are important for flyway connectivity as potential targets for prioritising conservation efforts. We focused on Eastern Mediterranean bottlenecks in a nested approach at multiple spatial scales.

PART I of this thesis covers network composition, where our goal was to gain insight into quantifying wetland quality for migratory waterbirds. In CHAPTER 1, we found that food availability was seven times higher in spring than in autumn in a set of protected wetlands of the Ionian flyway, while differences between wetland sites and habitat types were less pronounced. These spatiotemporal patterns of food resources have important implications for assessing the capacity of wetlands for supporting refuelling in different waterbird species. Based on this information, in CHAPTER 2, we simulated the foraging performance of waders with different body plans and foraging strategies in silico, in virtual landscapes with different horizontal and vertical prey distributions. Here, the finer-scale spatial distribution of resources had strong but variable effects on the foraging performance of different wader species.

The extent to which waders could exploit prey was also heavily dependent on species-specific energetic trade-offs of foraging, especially if prey was scarce and spatially patchy. However, foraging opportunities and overall habitat use by waterbirds can to a great extent be impacted by human disturbance, the response to which is highly variable and depends on the local context and the presence of different species, as evidenced in CHAPTER 3. From our results, we proposed that, when evaluating the suitability of wetland stopovers or when developing set-back zones in management plans, the sensitivity of local species assemblages to disturbance must be addressed.

The objectives of PART II were (I) to combine knowledge on network composition and configuration to analyse connectivity of Palearctic-Afrotropical flyways, and (II) to combine properties of mechanistic and landscape models to customise connectivity metrics for migratory populations. In CHAPTER 4, we discovered clear differences in the structure and connectivity of the four major flyways that connect the Western Palearctic and Africa based on PC metrics. More importantly and alarmingly, for populations with both modest and strong flight abilities, a substantial portion of overall connectivity was supported by unprotected wetlands. In CHAPTER 5, to investigate how the inclusion of more realistic constraints to migration would affect the presumed importance of stopovers, we introduced a novel function based on graph theoretic connectivity metrics and features of mechanistic migration models. Compared to more traditional metrics, including directionality and energetic constraints resulted in a drastically different picture of the relative importance of stopovers for network connectivity. By combining knowledge on wetland quality (food availability, disturbance) and the function of spatial organisation of resources in flyways, this project allowed us to gain insight on how wetland stopovers support network connectivity for migratory waterbirds. This thesis demonstrates the utility of graph-based metrics for investigating present and future conservation scenarios. Applications may include the development of a connectivity criterion that can be implemented into conservation planning, as it is currently still lacking. The necessity and incorporation of such a connectivity criterion in policy is explored in CHAPTER 6,along with conceptual and practical issues related to migratory waterbirds.