Late Quaternary land snail faunas of the Desertas Islands (Madeira): high diversity and endemism followed by recent impoverishment and extinction

Abstract

ABSTRACT Late Quaternary deposits containing fossil snail shells were sampled from nine sites on Deserta Grande and three on Bugio, two of the three Desertas Islands in the oceanic Madeiran archipelago. In total, 3,432 shells representing 45 species or subspecies were retrieved. Calibrated amino acid racemization yielded ages ranging from >45 to 6 ka, with variation by site often confounded by reworking of mostly colluvial deposits. The assemblages found are compared with the extant fauna, itself a subject of intensive recent study. Of the 45 taxa, 18 are apparently extinct, and a further 5 no longer survive on the Desertas. A similar pattern emerges when each island is considered separately. The level of local extinction (c. 50%) is greater than seen on the larger, nearby islands of Madeira and Porto Santo. There are few endemic species in the extant fauna not represented in the fossil assemblages, but there are a number of possibly adventitious species, some with wide distributions in Europe. There is little evidence of faunal change within the limits of the age range available, and extinction seems to be concentrated in the later Holocene. It is attributed to the massive habitat destruction that occurred subsequent to human occupation from the 15th century CE onwards. The study emphasizes the depauperate nature of many extant island faunas subject to human disturbance.