Understanding parasite dispersal and distribution in island-colonising birds

Parasites that cause infectious disease can drastically alter host population dynamics. Population impacts may be exacerbated by climate change as host and parasite species respond by shifting their ranges as they track their respective ecological niches, leading to novel interactions and disease dynamics. The ecological and evolutionary factors underpinning parasite dispersal and distribution remain poorly understood despite their critical importance for understanding emerging infectious diseases, protecting biodiversity and maintaining ecosystem health.

Birds are increasingly recognised as important parasite dispersers. Yet, most research on parasite dispersal focusses on blood parasites, such as avian malaria, which rely on arthropod vectors for transmission and typically exhibit specialist host relationships. Parasites associated with the digestive tract ('gut parasites') infect future hosts when their transmission stages are released with faeces and consumed, potentially enabling the infection of a wide range of hosts. The contrasting transmission strategies suggest there may be variation in diversities and infection patterns of blood and gut parasites in birds.

Host migration and dispersal are key to parasite distribution. Both seasonal migration and long-distance dispersal of hosts can result in exposure to multiple parasite faunas, such movements also have
high energetic costs that can weaken the immune system. Therefore, migratory birds may transport a high diversity of parasites and carry high parasite loads. Similarly, long-distance dispersers may escape, introduce or gain parasites. Long-distance dispersal can lead to colonisation of islands. During island colonisation, parasite diversity may follow island biogeography principles related to island
isolation and area in parallel with their hosts.

An excellent system to simultaneously study the impact of annual migration, long-distance dispersal, and island biogeography on parasite diversity and distribution is presented by the silvereye bird (Zosterops lateralis), a passerine distributed in Australia and numerous south-west Pacific Islands. They include partial migrant, dispersive and totally sedentary populations, and they have colonised islands via dispersal over a range of timescales (hundreds to hundreds of thousands of years, allowing changes in parasite diversity to be calibrated.

For my PhD, I will collaborate with the Natural History Museum in London and Nature Metrics (potentially as a CASE partnership) to study parasite dispersal and distributions in island-dwelling terrestrial birds. I will employ cutting edge metabarcoding and High-Throughput Sequencing (HTS) methods to characterise whole gut parasite communities from faeces and multiple parasite infections from blood (all non-destructively sampled). Additionally, I will mine raw Whole Genome Sequences (WGS) already available from silvereye blood samples across their distribution to extract parasite reads. Multiple parasite infections are ubiquitous in nature, yet most research focuses on single parasite systems. My study will provide a rare example of a 'parasite community' approach and improve our understanding of changing parasite distributions and dispersal patterns.