Species richness in Madeiran land snails, and its causes

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L. M. Cook Figure 1 The Madeiran archipelago. The main early author of Madeiran mollusc taxonomy was Lowe (1854 and earlier), and there has been a thorough review by Waldén (1983). Cameron et al. (2006) refer to other recent taxonomic work, which pre-dates the revisions of Bank et al. (2002). The taxa recognized are morphospecies, usually based on shell characters, sometimes with anatomical descriptions. The 19th century descriptions have been supported by later work, and the species-level systematics is relatively stable. Wollaston (1878) was the first to discuss distribution on the islands in detail, and this was later examined by Waldén (1984). We recorded 117 extant endemic and 36 non-endemic species in our surveys. On Madeira, 24 endemics were restricted to the west, south and east coasts, 23 to upland forest. On Porto Santo, nine endemic species were found only in the west, 15 only in the east, and 20 were distributed more broadly. There are 25 endemic species on the Desertas, 10 restricted to that group and 11 on only one of the islands. Species encountered only as fossils bring the totals to 141 endemics and 37 non-endemics, 178 in total (see Appendix S1 in Supplementary Material). Almost all non-endemic species are European, introduced since human colonization in the 15th century, and mostly on Madeira. The number of species collected in different parts of the archipelago is shown in Table 1. Small islets all have reduced faunas. Numbers of endemics in other sections vary from 12 in the sandy northern part of Porto Santo to 27 in Madeiran section D, where forest comes down to the coast and upland species feed into the fauna coming from coastal regions to east and west. More homogeneous sections have between 14 and 23 species. Section B is the eastern peninsula. It is small in area, quite low-lying and dry, but with some forest species, probably relicts from an earlier, damper phase (Goodfriend et al., 1996). It is impossible to know whether any rare species have been overlooked, but the surveys have been thorough enough to provide substantially correct lists for the different sections. Despite the high count for the whole island group, the average number of species in individual regions is not large compared Table 1 Numbers of land snail species in identified subsections of the Madeiran islands. Location Total species Mean per sample Total endemics Madeira A 32 18.8 19 5 B 22 12.3 14 3 I 38 13.7 19 11 H 39 15.1 26 9 C 30 13.1 17 9 D 41 25.7 27 3 E disturbed wood 36 12.2 21 12 F wood > 600 m 29 14.3 19 8 G wood > 900 m 35 10.1 23 14 Deserta Islands Ilhéu Chão 9 8.2 9 4 Deserta Grande 25 7.2 14 10 Bugio 22 8.0 11 4 Porto Santo Ilhéu de Ferro 15 13.0 13 2 Ilhéu de Baixo 15 12.5 13 2 West 32 10.1 28 15 North 16 8.3 12 7 East 40 13.1 32 34 Ilhéu de Cima 15 11.3 13 3 Ilhéu das Cenouras 9 9.0 7 1 Ilhéu de Fora 9 9.0 8 1 No. samples Madeira: letters refer to divisions in Fig. 3 and tables in Cameron & Cook, 2001;. Desertas: Cameron & Cook (1999a) 19th century totals. Porto Santo: Cameron et al. (1996b). More samples are available for many of these areas; those shown were collected under comparable conditions. with other islands or continental areas, including western Europe (Solem, 1984; De Winter & Gittenberger, 1998; Cameron et al., 2003; Nekola, 2005). The faunas of the island groups are distinct. In our surveys, Madeira (with 62) and Porto Santo (with 47) have only three species in common. Vertiginidae and Vitrinidae are today only on Madeira. Three helicid genera are only on Porto Santo. Leiostyla (Pupillidae) has 20 species on Madeira, seven on Porto Santo and two on the Desertas. In the helicid genus Discula, there are two species on Madeira, 15 (including extinct) on Porto Santo, and three on the Desertas. Among fossil endemics, Craspedopoma mucronatum (Mencke), Heterostoma paupercula (Lowe) and Caseolus punctulatus (Sowerby) are common to all three groups. Craspedopoma mucronatum is now limited to Madeira. Caseolus punctulatus is no longer on Madeira, where H. paupercula is largely restricted to the eastern parts in conditions like those widespread on Porto Santo. The high forests in Madeira are the centres for Vertiginidae, Vitrinidae and Pupillidae. On Porto Santo, there are eastern and western faunal groupings that show up when data from sample sites are clustered (Fig. 2). On Madeira, clustering indicates at least four coastal regions with different faunal compositions (Fig. 3), some species showing 648 Journal of Biogeography 35, 647–653 ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd
(a) (b) Faunal similarity Endemic species Figure 2 (a) Porto Santo, showing regional localization of taxa in two subgenera of Discula. The central part of the island is lower and sandier than the east, which has a series of peaks, or the west, which also rises but to a lower altitude. (b) Faunal similarity, based on the determinant of presence–absence data for each pairwise comparison using endemic species only. Data from Cameron et al. (1996b). Divisions: W, western higher land; N, sandy north-central; WIs, western islets; EIs, eastern islets; and north-east, south-east and central sections of the eastern part of the island. geographical replacement. There have been species radiations on the islands – can they be said to be adaptive? Adaptive and non-adaptive radiation A radiation is the differentiation of a single ancestor into an array of species. It may be non-adaptive, occurring simply because the habitat is fragmented and without different adaptive peaks in the localities the new species come to occupy (Gittenberger, 1991). It is adaptive when the new species occur in different environments and differ in traits used to exploit them (Schluter, 2000). In that case, the speciation process may be mediated by competition, resulting in disruptive selection. This is not necessarily so, however, and we should look critically for evidence of competition among the molluscs. (a) (b) Faunal similarity Endemic species Madeiran land snails Figure 3 (a) Madeira, showing regions found to have differing faunas. Regions E, F and G are endemic laurel forest at successively higher altitudes. (b) Faunal similarity, based on the determinant of presence–absence data for each pairwise comparison using endemic species only. Modified from Cameron & Cook (2001). Direct evidence of competition is not easy to establish, but we have tried two indirect routes. First, the Madeiran endemic fauna is often associated with non-endemics introduced after human colonization. On Porto Santo, endemic species are much more abundant on rocky volcanic sites than on sandy ones, while the reverse is true for introduced species (Cameron et al., 1996b). It is likely that, having evolved in southern Europe, the introduced species are pre-adapted to sandy conditions that the local fauna has yet to penetrate. At drier sites from Madeira and the Desertas, introduced species add to, rather than replace, the endemic species, so as to produce richer associations (Cook, 1984; Cameron & Cook, 2001). In the Madeiran forest, numbers of introduced species are positively correlated with numbers of endemics (Cameron & Cook, 1997). This correlation could indicate that good sites favour non-endemics and endemics alike, or that high species number is connected with greater microhabitat heterogeneity; it does not indicate replacement. The most convincing evidence for replacement is loss of the fossil Caseolus bowdichianus (Férussac) at about the time of introduction of Theba pisana (Müller), a species similar in size and shape (Goodfriend et al., 1996). Even here, however, the period of human colonization was one of massive ecological change that could have exterminated the one and independently favoured the other. Journal of Biogeography 35, 647–653 649 ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd
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