Prior empirical research shows that climatic factors, such as El Niño Southern Oscillation events (ENSO) and climate change, and infectious disease can interact to have profound effects on host population dynamics. The Galápagos sea lion, Zalophus wollebaeki, is vulnerable to both disease and climate perturbations, and we were interested in how a novel disease, canine distemper virus (CDV), is spread by introduced dogs on the Galápagos Islands. Using a spatially explicit, two-host model, we predicted the individual and interactive effects of CDV and ENSO on Galápagos sea lion populations. We used a susceptible-exposed-infected-recovered model influenced by temperature-dependent parameters and a unique spatial structure with dogs on the interior and sea lions on the periphery of each island.
We found that when model parameters were manipulated by temperature, there were complex interactions between the two extrinsic sources of mortality, disease and climate. When ENSO depletes the production of newborn pups, disease incidence is reduced, but not always enough to compensate for the ENSO-caused mortality. This model delineates conditions where ENSO mediates the effects of disease and where it enhances it by characterizing disease induced and climate induced mortality. Using the predictions from the model, we evaluated management strategies for years where ENSO is present and for when it is absent. These management recommendations are important to taking steps to protect an endangered, endemic species.