MODELING AVIAN MALARIA IN HAWAIIAN BIRDS – DISEASE, CLIMATE, AND GEOGRAPHIC INTERACTIONS

Mathematical modelling of host populations and their interaction with parasites, diseases, and vectors offers a means of assessing the importance of disease agents on wildlife populations and the potential effectiveness of alternative disease management strategies. Models can also summarize what we know about a disease system and identify the relative importance of different parts of the system. Recently, modelling has become an increasingly sophisticated tool for integrating the medical, epidemiological, and ecological approaches for understanding and predicting the dynamics of disease. Avian malaria plays a major role in the population dynamics and conservation of Hawaiian native birds and the persistence and dynamics of avian malaria are influenced by a complexity of interrelated factors that produce unexpected patterns. These factors include endogenous components of the disease system (vector and host abundance) and exogenous components that drive the system (climate), as well as landscape components (elevation and habitat) that influence the rates of biological processes, distribution of species, and abundance of organisms. We developed a model of the dynamics of avian malaria in the Hawaiian forest ecosystem using a Susceptible, Infected, and Recovered (SIR) model, based on Ordinary Differential Equations. Our model includes the dynamics of the four host species, vector, and parasite, as well as the effects of climate and landscape changes on these processes. The model was used to evaluate the potential impact of avian malaria on Hawaiian birds, determine which model variables have the strongest influence on our results, and evaluate several conservation strategies to benefit Hawaiian birds. Our model simulations illustrate several key attributes of the malaria-forest bird system in Hawaii. Malaria infection patterns are characterized by: (1) high levels of transmission in low-elevation forests with little seasonal or annual variation in infection rates; (2) episodic levels of transmission in mid-elevation forests with site-to-site, seasonal, and annual variation depending on mosquito dynamics; and (3) disease refugia in high elevation forests with only slight risk of infection occurring during summer when climatic conditions are briefly favorable to pathogen and mosquito development. These infection patterns are driven by the effects of climate (temperature and rainfall) on mosquito dynamics across an elevational gradient. Overall our model demonstrates that the introduction of avian malaria and a competent mosquito vector can significantly reduce the diversity and abundance of native Hawaiian birds, especially the unique and highly visible honeycreepers in low- and mid-elevation forests. Climate change is likely to have significant impacts on the future of this unique avifauna.