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In Southern Norway, there are strong indications that acidic precipitation is the main reason for the extinction of 94% of all known populations (n = 47) of the vulnerable pearl mussel Margaritifera margaritifera (L). An example is the famous pearl mussel river, the River Audna, where the death of both young and old mussels took place in the period 1930 - 50. When the pH is in the range 5.0 - 5.5 or lower for prolonged periods of time, the mussel can no longer tolerate the acidity. The only three known localities remaining in this part of the country, with ageing populations of the pearl mussel, are all lowland streams that are only very little acidified. The main threat there is eutrophication.

The common frog Rana temporaria L. was absent from a poorly buffered, anthropogenically acidified, inland area (Solhomfjell) of Southern Norway where the pH values of ponds and lakes ranged from 4.3 to 4.8 and the Ala concentration was usually below 150 μg/L. Closer to the coast, the calcium concentration was higher and R. temporaria was present, although not common. Successful spawning and larval development were recorded at pH values down to 4.6 and an Ala concentration of 100-300 μg/L. In a reference area (Høylandet) in Central Norway with similar altitude, geology and flora, and a pH in the range of 4.6 to 6.8, R. temporaria was very common at all pH levels. The Ala concentration was usually below 100 μg/L. The comparison between Solhomfjell and Høylandet strongly suggests that acidification explains the absence of R. temporaria at Solhomfjell. This was also demonstrated indirectly by the use of multivariate analyses, although chloride (NaCl) was shown to be the most important explanatory factor for the presence of R. temporaria in the acidified area. Also at Høylandet, where pH was not directly limiting, the lack of salts, and also increased humus content in the water, to some degree restricted the distribution of the frog. In both areas, a pH of approximately 4.6 may be critical for R. temporaria populations in small, poorly buffered, boggy, water bodies like those we have investigated.