Distributions of anuran populations in central Ontario relative to habitat acidity

Adult anuran densities in 20 ponds in central Ontario were determined by using daytime visual censuses and by counting males calling during the breeding season. The pH of ponds varied from 4.55 to 6.37 and alkalinity from ?1.9 mg L^?1 to 23.2 CaCO₃. Bullfrog (Rana catesbeiana), small (2 to 3 em snout-vent length) green frog (R. clamitans), and spring peeper (Hyla crucifer) densities were lower in more acidic ponds even though pH levels were not low enough to cause acute toxicity to eggs or larvae. The wood frog (R. sylvatica) was the only species whose density increased in more acidic ponds. In a census of another 40 ponds for wood frog egg masses (an estimate of the number of females breeding in the pond), egg mass density was reduced as pond pH and alkalinity decreased even though the pH levels were not low enough to be acutely toxic to eggs or larvae (pH ranged from 4.44 to 6.63). These results suggest that long before the H⁺ concentration in pond habitats increases to levels that cause acute toxicity, chronic sublethal effects in conjunction with indirect ecological effects of pond acidification (eg. changes in habitat structure, food resources, predator-prey relationships) may be affecting anuran populations.