Changes in dewpond numbers and amphibian diversity over 20 years on chalk Downland in Sussex, England

The fate of dewponds, the only substantive wetland habitat on the chalk Downs of southern England, was monitored over an area of 150 km(2) between 1977 and 1996 together with changes in use by the five species of amphibians that inhabit them. Despite a proactive pond restoration programme there was an overall net loss of seven pools (21% of the initial number of 33) by 1996, although a higher proportion and absolute number of surviving ponds were in good condition in 1996 (58%, 15 ponds) compared with 1977 (24%, eight ponds). Anurans (common frogs Rana temporaria and toads Bufo bufo) were more widespread on the Downs in 1996 compared with 1977, probably because they colonised new ponds effectively, whereas urodeles (smooth newts Triturus vulgaris, palmate newts T. helveticus and crested newts T. cristatus) all declined over the same period. T. cristatus, a species ceded maximum protection under the European Union Habitats Directive, occurred in nine sites in 1977 but in just three by 1996. Total destruction of ponds was the most common cause of crested newt extinction, but at two sites the species disappeared following the establishment of fish populations. Various indices of amphibian diversity in dewponds are also compared. (C) 1997 Published by Elsevier Science Ltd.