Decline of the natterjack toad Bufo calamita in Britain: Palaeoecological, documentary and experimental evidence for breeding site acidification

Abstract

Natterjack toads Bufo calamita have declined sharply at heathland sites in Britain during the 20th century. A significant feature of these habitats is the predominance of oligotrophic and dystrophic ponds on substrates with low buffering capacities. Acid ponds at one site, Woolmer Forest, were classified into two groups: shallow pools dominated by Sphagnum and high concentrations of organic solutes, and larger bodies of water in which pH was primarily influenced by inorganic anions (especially SO 4). Pond pHs responded transiently to episodes of very acid ( pH < 4) rain, but acidity in the larger ponds was probably modulated mainly by mobilisation of sulphur accumulated in sedimentary peat over many decades. Rain was significantly acidified (on average about threefold) after passage through pine canopies, but this effect was not observed after percolation through birch foliage. Two large ponds previously used but deserted by natterjacks within the last 50 years were too acid ( pH < 4.5) in the 1980s to support embryonic and larval development. Evidence from diatom, macrophyte, heavy metal and soot particle analyses of sediment cores from these ponds indicated that recent acidification has occurred as a consequence of atmospheric pollution. The pH of one acidified pond rose rapidly after removal of recent sedimentary peat. The implications of these observations for heathland conservation are discussed.