Increased tropospheric ozone levels enhance pathogen infection levels of amphibians.

As a result of anthropogenic activities, changes to the chemistry of Earth's atmosphere pose a threat to ecosystem health and biodiversity. One such change is the increase in tropospheric ozone (O3), which is particularly severe in the Mediterranean basin area, where the levels of this pollutant are chronically high during spring and summer time. Within this region, Mediterranean mountain ecosystems are hot spots for biodiversity which may be especially vulnerable to changes in O3 levels. Declines in montane amphibian populations have been recorded worldwide, including the Mediterranean basin. A significant driver of these declines is the emerging infection disease, chytridiomycosis, caused by the aquatic fungus Batrachochytrium dendrobatidis (Bd). Chytridiomycosis has negatively affected populations of several amphibian species in the Spanish Central Range, including in the Sierra Guadarrama, and interactions with other biotic and abiotic factors are an important part of these declines. However, there is little evidence or knowledge of whether tropospheric O3 levels may be another factor in the outbreaks of this disease. To test the hypothesis that O3 levels are another interactive driver of Bd infection dynamics, two different approaches were followed: 1) an experimental study in open top chambers was used to quantify the aspects of how Bd infection progressed throughout the metamorphic process under four different O3 levels; and 2) a field epidemiological study was used to analyse the relationship between the Bd infection load in the Sierra de Guadarrama and tropospheric O3 levels during a 9 year period. Our results suggest that high O3 levels significantly delayed the rate of development of tadpoles and increased Bd infection, providing empirical evidence of two new separate ways that may explain population declines of montane amphibians.