Soil ingestion, nutrition and the seasonality of anthrax in herbivores of Etosha National Park

The arcsine square root transformation has long been standard procedure when analyzing proportional data in ecology, with applications in data sets containing binomial and non-binomial response variables. Here, we argue that the arcsine transform should not be used in either circumstance. For binomial data, logistic regression has greater interpretability and higher power than analyses of transformed data. However, it is important to check the data for additional unexplained variation, i.e., overdispersion, and to account for it via the inclusion of random effects in the model if found. For non-binomial data, the arcsine transform is undesirable on the grounds of interpretability, and because it can produce nonsensical predictions. The logit transformation is proposed as an alternative approach to address these issues. Examples are presented in both cases to illustrate these advantages, comparing various methods of analyzing proportions including untransformed, arcsine- and logit-transformed linear models and logistic regression (with or without random effects). Simulations demonstrate that logistic regression usually provides a gain in power over other methods.

The global distribution of anthrax is largely determined by soils with high calcium levels and a pH above 6.1, which foster spore survival. It is speculated that the spore exosporium probably plays a key part by restricting dispersal and thereby increasing the probability of a grazing animal acquiring a lethal dose. 'Anthrax Seasons' are characterized by hot-dry weather which stresses animals and reduces their innate resistance to infection allowing low doses of spores to be infective. Necrophagic flies act as case-multipliers and haemophagic flies as space-multipliers; the latter are aided by climatic factors which play a key part in whether epidemics occur. Host death is a function of species sensitivity to the toxins. The major function of scavengers is to open the carcass, spill fluids, and thereby aid bacilli dispersal and initiate sporulation. In the context of landscape ecology viable spore distribution is a function of mean annual temperature, annual precipitation, elevation, mean NDVI, annual NDVI amplitude, soil moisture content, and soil pH.