A routine was established for sampling pH in aquatic environments which permitted assessment of an “average site pH”±0.5 units by measuring that of a single sample at any time of year. Requisites for accuracy were that samples should be taken in clear water away from weed, and at least three days after significant rainfall.

pH measurements were taken in a number of ponds and compared with the use of these waters for breeding purposes by Common toads (Bufo bufo) and frogs (Rana lemporaria). Toads were found breeding in water slightly more acid than those tolerated by frogs. Similar measurements of pond pH in dune and heath habitats frequented by Natterjack toads (Bufo calamita) were compared with breeding site choices by these anurans. A marked avoidance of acid waters was evident in the predominantly acid heathland habitats. This was probably not directly related to the concentrations of inorganic ions in the pools.

Development of Rana lemporaria, Bufo bufo and B. calamita spawn under various pH conditions was examined. Survival rates were optimal in the pH ranges found to be chosen in the wild, and no differences were observed between Bufo calamita spawn taken from dune or heathland populations. Developmental abnormalities in the tadpoles were more frequent under acid conditions.

A preliminary study of ponds used by the three species of anurans native to Britain was carried out by measuring pH with a tested field procedure. Differences between Bufo bufo and Rana temporaria breeding sites were not significant on the basis of pH in the areas studied, but B. calamita populations on heathland were found to avoid the acid pools. The field data correlated well with the pH tolerance of the anuran larvae reared in captivity. The significance of these findings is discussed.

The authors wish to thank Dr A. Cooke (Institute of Terrestrial Ecology) and Dr D. Sutcliffe (Freshwater Biological Association) for analyses of selected water samples; Mrs M. Beebee for statistical treatment of the data; and the Carnegie (U.K.) Trust for financial support.