Fossil Evidence of Recent Human Impact on the Land Snail Fauna of Madeira

Abstract

The Madeiran islands, located at 33?N in the eastern Atlantic, were colonized early in the 15th century, and perhaps up to a century earlier. The woodland and scrub cover was rapidly reduced and the amount of grassland and dis- turbed ground increased. The effect on the land snail popu- lation has been assessed by analysis of fossil assemblages from a deposit in eastern Madeira. Dates were obtained by measurement of amino acid epimer ratios (D-alloisoleucine/L- isoleucine) of individual shells, calibrated against radiocarbon- dated samples. Because this method allows dating of individual shells, it was possible to reconstruct the chronology of various species from mixed-age assemblages. It also per- mits dating of shells too young for radiocarbon dating (post- AD 1650). Fifteen land snail assemblages were analysed from deposits of post-settlement age (c. 420-50 yr BP). Of the thirty-four species of land snails present in the samples, nine have become extinct over the period. In contrast, only five species became extinct on the island during the previous c. 300,000 years. The majority of the extinctions have occurred within the last two centuries, as a result of habitat destruction. A major decline in the abundance of woodland species took place during the 20th century. The extinction of the endemic Caseolus bowdichianus, abundant in middle Holocene de- posits, occurred about a century after colonization of the island. Theba pisana, a snail of similar size and shape, was introduced around the time of colonization, and is now abun- dant in grassy areas. Two endemic grassland species, Discula polymorpha and Heterostoma paupercula, have also become more abundant and thus apparently benefited from the arrival of man.