Calatayud et al. analyzed the climate that 26,000 species of tetrapods experience in their home range, which led to the identification of 16 climate regions. Climates were analyzed in terms of water level (measured as annual rainfall) and energy level (estimated amount of evaporation that occurs each month). Plotting the species richness (y-axis; left), energy (x-axis) and water levels (y-axis; right) in each of these regions reveals that areas with mid-levels of energy and high rainfall have the greatest species richness (left side graph). Calatayud et al. found that similar climate regions tended to have similar tetrapod populations, even if they were thousands of miles apart. However, this was not the case for arid regions such as deserts (shown in orange on the map): for example, bearded lizards that have colonized the Sahara are not found in the Eremaean desert region in Australia, despite both having similar climates.