FOSSIL EVIDENCE OF RECENT HUMAN IMPACT ON THE LAND SNAIL FAUNA OF MADEIRA

The Madeiran islands, located at 33 degrees 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 amo unt of grassland and disturbed ground increased. The effect on the lan d snail population 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 indivi dual shells, calibrated against radiocarbon-dated samples. Because thi s method allows dating of individual shells, it was possible to recons truct the chronology of various species from mixed-age assemblages. It also permits dating of shells too young for radiocarbon dating (post- AD 1650). Fifteen land snail assemblages were analysed from deposits o f post-settlement age (c. 420-50 yr sp). 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 d uring the previous c. 300,000 years. The majority of the extinctions h ave occurred within the last two centuries, as a result of habitat des truction. A major decline in the abundance of woodland species took pl ace during the 20th century. The extinction of the endemic Caseolus bo wdichianus, abundant in middle Holocene deposits, occurred about a cen tury after colonization of the island. Theba pisana, a snail of simila r size and shape, was introduced around the time of colonization, and is now abundant in grassy areas. Two endemic grassland species, Discul a polymorpha and Heterostoma paupercula, have also become more abundan t and thus apparently benefited from the arrival of man.