COMPARING CLIMATE AND VEGETATION AS LIMITING FACTORS FOR SPECIES RANGES OF AFRICAN TICKS

Abstract

The broad-scale factors that limit the species ranges of ticks have not been definitively established. I used environmental data covering the whole of mainland Africa and a database of 34 063 published locality records for African ticks to assess the importance of different environmental variables as limiting factors for tick species ranges. The abilities of categorical and continuous variables (including rainfall, temperature, vegetation type, a 6 × 6 degree grid, political regions, and Normalized Difference Vegetation Index [NDVI]) to predict tick distributions were established using a new set of methods where logistic regression equations based on different variables for each of 50 tick species are compared. The results show that, on average, climatic variables are better predictors of tick distributions than vegetation-related variables. Given also that most tick distributions are not limited by those of their host species, it can be inferred that the primary factor preventing the expansion of tick species ranges is the direct effect of climate. Minimum temperature, maximum temperature, and rainfall are of similar predictive ability; when considered together, their predictive ability increases substantially. I conclude that the key to describing tick distributions is the covariance of temperature and rainfall. Attempts at tick eradication are unlikely to be successful unless suitably broad areas are targeted, but the close relationship between tick distributions and climate should make the prediction of tick-borne disease outbreaks relatively easy.