The distribution of introduced mollusc species in Southern Africa
Artificial distribution patterns of animals and plants may be defined as those caused or fostered deliberately or accidentally by man. Consequently we can distinguish two types of artificial distribution, viz., that of animals accidentally distributed by man (rats, mice, insects, slugs, etc.) and that of animals deliberately transported by man (domestic and pet animals; species involved in transmission of game animals and biological control of insects and plants, etc.). A considerable proportion of accidentally transported animals is made up of non-marine molluscs and it appears that there are few countries on the seaboard that have not yet experienced the immigration of some foreign pulmonate. Accidental transport, invasion and establishment can only be successful in species which have a wide ecological tolerance; not only must they be able to survive the voyage, but in their new country they usually have to adapt themselves to a different climate, soil and vegetation, while at the same time having to compete with the indigenous fauna for available ecological niches. All immigrant animal species are subject to these factors, which have been proven to be of prime importance in South Africa by the (abundant) survival and proliferation of adaptable bird species such as Passer domesticus, Sturnus vulgaris and Acridotheres tristis, as opposed to the disappearance of less adaptable species such as Turdus ericetorum and T. merula. Among all countries in Southern Africa (the subcontinent south of the Zambezi and Cunene Rivers), South Africa proper has had more than its share of these migrations. Various authors have dealt with immigrant molluscs (e.g., BARNARD, 1948; BIGALKE, 1937; CONNOLLY, 1916, 1939; DÜRR, 1946; FORCART, 1963; JOUBERT & WALTERS, 1951; WALDÉN, 1961) but more information has been obtained recently. At the present the list stands at 24 species belonging to eleven families of the subclass Gastropoda Pulmonata (dates of introduction or first discovery added between brackets): 1. Lymnaea columella Say, 1817 (1944), 2. Physastra dispar (Sowerby, 1873) (so far in aquarium tanks only, 1944), 3. Vallonia pulchella (Müller, 1774) (1846), 4. Arion hortensis Férussac, 1819 (before 1939), 5. Arion intermedius Normand, 1852 (1898), 6. Vitrea cristallina (Müller, 1774) (1890), 7. Oxychilus alliarius (Miller, 1822) (± 1894), 8. Oxychilus cellarius (Müller, 1774) (1846), 9. Oxychilus draparnaudi (Beck, 1837) (± 1908), 10. Zonitoides arboreus (Say, 1816) (before 1912), 11. Milax gagates (Draparnaud, 1801)¹) (1873, or even before 1848), 12. Limax flavus Linnaeus, 1758 (before 1900), 13. Limax maximus Linnaeus, 1758 (1900), 14. Limax nyctelius Bourguignat, 1861 (before 1939), 15. Limax valentianus Férussac, 1823 (1961), 16. Deroceras caruanae (Pollonera, 1891) (1963), 17. Deroceras laevis (Müller, 1774) (before 1898), 18. Deroceras reticulatus (Müller, 1774) (before 1898), 19. Subulina octona (Bruguière, 1792) (1905), 20. Testacella maugei (Férussac, 1819) (before 1893), 21. Bradybaena similaris (Férussac, 1821) (± 1860), 22. Cochlicella ventricosa (Draparnaud, 1801) (1909), 23. Theba pisana (Müller, 1774) (1881), 24. Helix aspersa Müller, 1774 (± 1854).
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van Bruggen, A.C. (1964). The distribution of introduced mollusc species in Southern Africa. Beaufortia, 11(144), 161–169.