Temporal, Spatial and Inter-Specific Variability in Levels of Chytridiomycosis in Frogs of Southeast Queensland

Abstract

Chytridiomycosis is an emerging infectious disease of amphibians associated with mass mortalities and population declines worldwide. Previous chytridiomycosis field studies have yielded limited information on the ecology of the disease because survey methodologies were generally limited to opportunistic sampling across a broad range of species, age classes, altitudes, habitat types, seasons, and years. Thus, it has been difficult to disentangle or quantify the effects of these individual parameters. We conducted chytridiomycosis surveys in Southeast Queensland to determine 1) the effects of altitude on the prevalence and severity of the disease in Litoria pearsoniana, L. wilcoxii, and L. chloris; 2) the magnitude of seasonal variation in disease levels in a single population of Litoria wilcoxii; and 3) the difference in disease prevalence between frog species with various breeding habitat requirements. The overall prevalence of chytridiomycosis in the frogs sampled was 32.5% (n = 821), but the disease was not evenly distributed across altitudes, breeding habitats, or seasons. There was a strong effect of altitude for four of six altitudinal transects, with high-altitude frogs significantly more likely to carry infections than their lowland counterparts. Frogs that bred in permanent streams or stream-side pools were often infected with chytridiomycosis, whereas no frogs that bred in ponds or ephemeral waterbodies tested positive for the disease. A strong seasonal effect of the disease was evident, with prevalence in a single population of L. wilcoxii peaking at approximately 50% in early spring, and dropping to 0% by late summer. These results support the hypothesis that chytridiomycosis is responsible for many of the precipitous declines of high-altitude, permanentstream breeding amphibians in recent decades. Furthermore, they emphasize the importance of including temporal, spatial, and inter-specific variability in the experimental design of disease surveys of wild amphibians.