Modelling Chlamydia - koala interactions: coexistence, population dynamics and conservation implications
David J Augustine
Department of Fisheries and Wildlife, University of Minnesota, 1980 Folwell Ave, St Paul, Minnesota, 55108, USA
1. Considerable research has been conducted on the koala Phascolarctos cinereus population dynamics and epidemiology of Chlamydia psittaci infection in koalas, but the impact of Chlamydia on koala populations has been difficult to asses.
2. I developed a model of koala and Chlamydia population dynamics to exmaine interactions between Chlamydia transmissions and pathogenicity, koala mating behaviour and demography, and koala population persistence.
3. Simulations based on sexual and parent-offspring parasite trasmission demonstrate that stable Chlamydia-koala coexistence is possible in a small population for a broad range of demographic, behavioural, pathogenicity and transmission parameter estimations. Koala populations persistence was most sensitive to reduced annual survivorship of adults (4-10-year-old males and 2-12-year old females), highlighting the need for accurate field estimates of adult survivorship in order to asses Chlamydia's impact on specific populations.
4. If koalas become less resistant to disease in fragmented, high-stress habitats (i.e. experience increased Chlamydia-induced mortality and sterility rates), Chlamydia is not predicted to cause koala extinctions under most conditions. Extinctions are only predicted if Chlamydia transmission rates also increase (e.g. due to new transmission pathways or increased mating frequency), or other non-disease factors change birth and mortality rates to reduce koala population's instrinsic rates of increase below 0.1.
5. The most important predicted effect of habitat fragmentation and other forms of human disturbance on this unique host-parasite relationship is the extinction of Chlamydia in populations where koala resistence to disease decreases.