PhD Thesis
Migratory species undertake some of the most extreme feats of endurance known in the animal kingdom. Despite many species migrating across continents, oceans and hemispheres, most cannot survive everywhere. Migrants are highly dependent on strategically located breeding and feeding sites for survival and reproduction. Indeed, many species are constrained in time and space by seasonal resource availability, thus forming migratory bottlenecks. Threats operating in such bottlenecks can impact the population as a whole, and can impact survival and reproduction at later migratory stages. In fact, migratory species worldwide are declining at greater rates than non-migratory species. Many migratory species are at risk of extinction if no conservation action is taken.
Pinpointing where and when threats occur, and understanding how they impact population dynamics of migratory species is complex. Few tools are available for diagnosing declines, and even fewer for prioritising conservation actions in migratory species. As a result, our understanding of how to conserve migratory species is remarkably poor. In my thesis, I tackle these fundamental gaps in our knowledge by 1) developing and testing a method to distinguish local from remote drivers of population growth rate in migratory species, 2) mapping critical habitat, and 3) prioritising conservation actions at local and international scales. I use the migratory shorebirds of the East-Asian Australasian Flyway as a case study. The East-Asian Australasian Flyway supports 60 species migrating across 23 political jurisdictions, many of which are in significant decline.
In Chapter 1, I introduce my thesis by outlining key topics surrounding the conservation of migrants, linking them to my case study. In Chapter 2, I distinguish between local and remote drivers of population growth in migrants, using (i) count data from a single site within a migratory flyway, and (ii) a list of potential stopover sites. Indeed, for migratory species with large distributions, count data, mark-recapture studies or tagging records from across the entire geographic distribution are rare. Monitoring data are typically only available for one or a few sites. Newly available remote sensing data offer an opportunity to investigate how conditions in other parts of migratory cycle affect population growth rate at a monitored site. Analysing count data from Moreton Bay, Australia, I show that it is possible to identify effects of climatic conditions throughout the flyway on the population growth rate of migratory shorebirds as measured at a non-breeding site. My results also show that declines are occurring consistently across all study species, but that there are some clear differences in temperature and rainfall impacts on population growth rate.
Information about the distribution and status of habitat is crucial when devising conservation plans for migratory species, yet very little is known about the distribution, extent and protection of the intertidal habitats used by migratory shorebirds. In Chapter 3, using freely available satellite imagery, I produce the first map of intertidal habitats in Australia. I find that levels of protection vary greatly between states, with some states primary under terrestrial protection, others primarily under marine protection, and some under both. Overall, 39% of intertidal habitats are protected in Australia. Shorebirds are declining despite high levels of protection in Australia, suggesting that better management within protected areas could be important.
Management of disturbance in the intertidal zone is one of the key conservation actions that can be taken for shorebirds in Australia. Active management only occurs within protected areas, where managers must decide where and when to carry out enforcement given limited budgets. In Chapter 4, I develop a novel method of prioritising enforcement for wildlife management at the local scale, which accounts for diminishing returns on investment from repeatedly enforcing at the same site and show that robust management decisions can be made despite limited data on effectiveness of management.
Protected areas are one of the most widely used conservation tools. However, setting conservation priorities at the international scale can be complex, particularly given limited data on migratory connectivity. Currently, conservation priorities for migrants are usually based on the number of birds using a site, with little consideration for migratory connectivity due to limited data. Chapter 5, I develop a multi-species prioritisation for 250 sites using tracking data to estimate migratory connectivity empirically, discovering that sites that are highly connected are more critical for maintaining migratory populations than sites simply supporting large numbers of birds.
I synthesise my thesis in Chapter 6, by placing my research in the broader context of conservation planning for migratory species, acknowledging the limitations of my methods, and suggesting improvements and future directions. Ultimately, my PhD has delivered practical solutions for the management of migrants, and theoretical advances in conservation planning in dynamic migratory networks. Limited data is often cited as a primary barrier to conserving migratory species; however, conservation decisions can and should be made despite uncertainty, if we are to prevent one of the most spectacular phenomena on earth from disappearing.