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Journal of Natural History Vol. 42, Nos. 5–8, February 2008, 467–490
Biogeography, land snails and incomplete data sets: the case of three island groups in the Aegean Sea Kostas A. Triantisa,b*, Katerina Vardinoyannisb and Moisis Mylonasa,b a Department of Biology, University of Crete, Vassilika Vouton, Crete, Greece; bNatural History Museum of Crete, Crete, Greece
This research aimed to study the land snails of three island groups (Skyros, Astypalaia, Kalymnos) in the Aegean Sea and the biogeographical patterns arising, with reference to the other Aegean islands. A total of 91 species was found on the 37 islands surveyed, with 89 being extant. For the majority of the islands the malacofauna is reported for the first time. Similarity indices, chorotypes and nestedness were studied for each of the island groups. The islands belonging to the same island group were clustered together, exhibiting a quite delimited malacofauna. The Mediterranean and Palaearctic elements dominate, with the Aegean element increasing in the Astypalaia group. The three island groups, despite their different geological histories and the different level of the present-day geographical isolation, have been formed quite recently in geological time. This ‘‘recent’’ formation of the groups results in islands, especially the small ones, which still behave as parts of a continuous land mass. In the context of the new results from the present work, the completeness of the knowledge of the terrestrial malacofauna of the Aegean islands is discussed, and the caution needed for drawing general biogeographical conclusions based on the available data is emphasized. Keywords: Aegean Sea; land snails; faunal element; similarity; nestedness; biogeography; incomplete data sets
Introduction The biogeography of the Aegean islands has long been studied in order to highlight the major ecological and historical factors that are responsible for the biodiversity patterns observed (e.g. Watson 1964; Mylonas 1982; Vardinoyannis 1994; Sfenthourakis 1996; Foufopoulos and Ives 1999; Welter-Schultes and Williams 1999; Sfenthourakis and Legakis 2001; Fattorini 2002; Bergmeier and Dimopoulos 2003; Dennis et al. 2000; Hausdorf and Hennig 2005; Triantis et al. 2005a, 2005b; Panitsa et al. 2006). The land snails of the Aegean Sea and their biogeography have long attracted scientists (see Heller 1976; Mylonas 1982; Mylonas and Vardinoyannis 1989; Vardinoyannis 1994; Botsaris 1996; Welter-Schultes and Williams 1999; Cameron et al. 2000; Hausdorf and Hennig 2005, 2006; Triantis et al. 2005a, 2005b). The main characteristics of land snails as effective biogeographical tools are: the low dispersal ability, their presence in a great variety of biotopes, the high number of species, and their relatively well established taxonomy. Despite the fact that almost one third of the islands in the Aegean have a total area ranging from 0.001 to 1 km2, very few studies have concentrated on the land *Corresponding author. Email:
[email protected] ISSN 0022-2933 print/ISSN 1464-5262 online # 2008 Taylor & Francis DOI: 10.1080/00222930701835431 http://www.informaworld.com
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468 K.A. Triantis et al. snails in islands of this scale (see Schultes and Wiese 1990, 1992; Botsaris 1996; Welter-Schultes and Wiese 1997a, 1997b; Triantis et al. 2004, 2005a). Furthermore, the majority of biogeographic studies in the Aegean have been concentrated on the whole area, or large parts of it (Mylonas 1982; Vardinoyannis 1994; Welter-Schultes and Williams 1999; Hausdorf and Henning 2005, 2006) and very few of them have dealt with well-defined island groups (e.g. Triantis et al. 2005a, 2005b). In the present paper the malacofauna of three island groups (37 islands in total) of the Aegean Sea is presented, and the biogeographical patterns observed are discussed in relation to faunal similarity and nestedness. Further, the effect of using incomplete data sets for biogeographical studies in Aegean is discussed. Materials and methods Study areas The three island groups studied have quite distinct geographical and palaeogeographical characteristics. The island group of Skyros lies in the north part of the Aegean Sea (Figure 1). The largest island of the group is Skyros (208 km2) with an altitude of 792 m (Kochylas mt.) and is situated about 30 km east of Evvoia Island. About 20 islets are situated around the island of Skyros, all within the isobath of 200 m. The geological substratum of the Skyros is mainly composed of limestone (66%), schist (22%) and Neogene formations (7%) (Melentis 1973). The islets of the
Figure 1. Map of the island group of Skyros, with islands and sites studied.
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469
group are mainly of limestone (Melentis 1973; Harder et al. 1983), while the islet of Exo Diavatis is the only one mainly composed of semi-metamorphic clastic formations (mainly gneiss) (personal field observations). According to Dermitzakis (1990) the isolation of the landmass, the fragmentation of which led to the formation of the archipelago of Skyros, occurred 4.5 mya ago, but there is uncertainty regarding the geological history of the island of Skyros itself (Melentis 1973; Dermitzakis 1990). Nevertheless, the fragmentation of that landmass and the formation of the archipelago are supposed to have occurred by the early Holocene (Van Andel and Shackleton 1982; Perissoratis and Conispoliatis 2003). The island group of Astypalaia is one of the most isolated groups of the Aegean Sea, lying in the middle of the Aegean Sea, comprising a transitional zone between the Cyclades and Dodecanese islands. The largest island is Astypalaia with a total area of 95.87 km2 and altitude reaching 482 m. About 20 small islets are found within the isobath of 200 m around Astypalaia, with the exception of Ofidousa islet. The geological history of the area remains unclear and exact patterns of island isolation are uncertain (Dermitzakis and Papanikolaou 1981; Dermitzakis 1990; Lambeck 1996). For instance, Dermitzakis (1990) claims that the area was submerged about 4.5–3.5 mya ago. As far as the formation of the island group is concerned, Ofidousa and Pontikousa islets were the first to be isolated either at 22,000 years ago (Perissoratis and Conispoliatis 2003) or 14,000 (Lambeck 1996), while the rest of the islets were formed during the Holocene (Van Andel and Shackleton 1982; Lambeck 1996; Perissoratis and Conispoliatis 2003). The geology of Astypalaia island is quite complex. The two extreme parts of the island consist mainly of limestone, while the central part is flysch and alluvial. The majority of the islets are mainly limestone with the exception of Fokionisi M, Fokionisi S and Diapori (Leontaris 1973 and personal field observations). The island group of Kalymnos is situated in the southeast part of the Aegean between the islands Leros and Kos (Figure 2). Kalymnos island has a total area of 110 km2. The island is very rugged, with a significant number of hills exceeding 300 m with Profitis Ilias being the highest peak (669 m). The second largest island of the group is Pserimos (14.6 km2) situated between Kalymnos and Kos. The formation of the group took place quite recently. The area was connected with Asia Minor, at least until 21,000 years ago. Later, around 11,000–9000 years ago, it was disconnected from the coasts of the Asia Minor but remained part of an elongated landmass which included the island of Leros and the island group of Arkoi (Van Andel and Shackleton 1982; Lambeck 1996; Perissoratis and Conispoliatis 2003). Hence, the formation of the island group is quite recent, no older than 5000 years, while some of the islands were formed during historical times, such as Telendos and Kalavros (Papageorgiou 2001). The island group of Kalymnos is dominated by limestone, with scarce presence of schist (Triantafyllis and Karfakis 1994). Today, besides the cultivated areas and scattered forest remnants, maquis and phrygana constitute the dominant vegetation types in the Aegean area. The two vegetation types frequently intermingle in small-scale mosaics determined mainly by soil properties and moisture regime, and there are intergradations between them. Among the three island groups, only in the northwest part of Skyros are there extensive forested areas with Pinus halepensis, while on Astypalaia and Kalymnos, maquis and phrygana dominate.
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470 K.A. Triantis et al.
Figure 2. Map of the island group of Astypalaia, with islands and sites studied. 13: Diapori island.
Sampling The island group of Skyros was visited three times, during late June 2001, and in early January and late April 2002. Surveys of the island group of Astypalaia were made during late April 2004 and late February 2005 and in the island group of Kalymnos during late February 2004, late March and early April 2005. The land snails of the Aegean exhibit varying biological cycles (Mylonas 1982), and thus the two different sampling periods provide relatively complete knowledge of habitat preference of each species and increase the completeness of the dataset. In total land snails were collected from 37 islands; 12 in Skyros, 12 in Kalymnos and 13 in Astypalaia. Of the sites surveyed on each of the three large islands, there were 23 on Skyros, 13 on Astypalaia and 14 on Kalymnos, spread throughout the islands, aimed at sampling all different biotopes present (Figures 1, 2, 3). On the smaller islands, land snails were sampled from the whole area of the island. Each locality was sampled by at least two experts until no new species were found. Samples of litter and soil were obtained from each site, collected from patches likely to yield small species. Species identifications were done by K.A. Triantis, M. Mylonas, and K. Vardinoyannis. Prof. A. Wiktor (University of Wrocław) identified the slug species from Skyros and Dr A. Parmakelis (University of Athens) the Mastus species. All collected material has been deposited in the Natural History Museum of Crete.
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Journal of Natural History
Figure 3. Map of the island group of Kalymnos, with islands and sites studied.
Analyses Zoogeographical composition In order to approach the chorotypes (faunal element) represented in the species of the three island groups, the species were classified according to their major distribution in the following categories: Palaearctic, Mediterranean, Balkan, Anatolian, Aegean and endemic (group or single island endemic). In the Aegean element, the species distributed among the islands of the Aegean as well as the coasts of continental Greece and Asia Minor were included. The assignment of each species to a chorotype was based on the following sources: Mylonas (1982), Vardinoyannis (1994), Kerney et al (1983), Schu¨tt (2001), Heller (1993), Cossignani and Cossignani (1995), Dedov (1998). Faunal similarity A matrix of binary data (absence50; presence51) was compiled for the species occurring on each island. In order to evaluate the similarities of the malacofaunas found on the islands of the three groups, the data were studied via Q-mode, which measures the relationship among objects (islands) based on descriptors (species). A large number of coefficients that use binary data to measure association between objects are available (Shi 1993). A favourable characteristic of similarity coefficients for biogeograpical purposes is their ability to exclude double-zeros, otherwise regions with low species richness would be grouped on the basis of shared absences
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472 K.A. Triantis et al. rather than presences (Legendre and Legendre 1983). On the basis of Shi’s (1993) review, and availability in NTSYS software (Rohlf 1993), Dice’s, Jaccard’s, Yule’s and Kulczynski’s #2 coefficients were utilized in this study. Formulae are provided in Legendre and Legendre (1983) and Rohlf (1993). Nestedness For the exploration of nestedness patterns the Nestedness program (Ulrich 2006) was used. Higher temperatures are indicative of lower community nestedness, while ‘‘colder’’ matrices are more nested. The free software developed by Ulrich (2006) calculates a temperature value (T) for each community that is similar to the widely applied Nestedness Temperature Calculator (Atmar and Patterson 1993). Although, the Nestedness Temperature Calculator method has been widely used in the literature and has become a familiar tool to many researchers, it has some major limitations (Wright et al. 1998; Fischer and Lindenmayer 2002; Rodrigue´z-Girone´s and Santamarı´a 2006; Ulrich 2006). Thus, the Nestedness method was applied; it is based on the same principles as the Nestedness Temperature Calculator but overcomes some of the major shortcomings of the method (for details, see Ulrich 2006). The number of randomizations to compute the null model means and standard deviations, as well as the upper and lower 95% confidence limits, was set to 1000. Results Faunal account: distributional and taxonomical remarks Previous data for the distribution of land snails on the three island groups under study were quite scarce. In any case, the information was available only for the larger islands of the island groups (Skyros, Skyropoula, Sarakino, Kalymnos, Telendos and Pserimos) and no data (or very few) were available for Astypalaia and the smaller islets. The malacofauna of Kalymnos Island has been studied in the past by ˝ rstan et al. (2005). Bank and Reischu¨tz (1986), Bank and Neuteboom (1988), and O ˝ rstan et al. (2005) have also studied the island of Pserimos. Neuteboom (1988) and O For the island group of Skyros the only complete survey was that of Liebegott (1986), who studied many of the Sporades islands. In Triantis et al. (2005a, 2005b) the total malacofauna of the group was presented but not analytically by island. Lastly, for the island group of Astypalaia, there was no up-to-date complete survey, with the exception of the work of Hausdorf and Hennig (2005), who reported the malacofauna of the island (16 species) using mainly data from museum collections and unpublished references. In total, 89 species of land snails were found on the islands of the three groups, belonging to 54 genera; 42 species were found on the island group of Skyros, 34 in the Astypalaia group and 47 in the Kalymnos group. For the three large islands under study new records account for 16% of the Kalymnos species, 58% for Astypalaia and 50% for Skyros (see Tables 1–3). Based on the characteristics of their genitalia and morphology, Helicella sp. from Skyros (Kochylas mt.) and Helicopsis sp. from Ofidousa, Pontikousa and Kounoupoi islands (Astypalaia group) are considered to be new species to science and will be described in the near future (Vardinoyannis et al. in preparation).
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Journal of Natural History
473
The records of Xerolenta obvia are the first from the Skyros island group. The determination of the species was based on the proximal genital system and the shell characteristics of the individuals studied. In general, the determination of the genus in the family Hygromiidae is based on the study of the genital system, while shell characteristics are important for the determination of the species. Liebegott (1986) reports Xeropicta krynickii from Skyros Island. This species has a shell similar to that of Xerolenta obvia. Since no specimens of X. krynickii were found, it is possible that the individuals Liebegott (1986) determined as X. krynickii belong to the species X. obvia; thus, X. krynickii was not included in the species list (Table 1). Riedel (1992) assigned all the individuals of Vitrea contracta he studied from Astypalaia to V. aff. clessini. Studying the specimens collected from all the islands of the group and comparing them with the individuals that Riedel determined as V. contracta aff. clessini, it is concluded that V. contracta is not present on the island group, and all the specimens were assigned to V. clessini. A few subfossil shells of Pomatias elegans were found on the island of Skyros, with no live individuals being present; thus, P. elegans is considered to have become extinct from the island group. Helix figulina was reported from Skyros island group by Martens (1889), Fuchs and Ka¨ufel (1936) and Liebegott (1986). Though during the three surveys all the areas from which the species was reported in the past (Liebegott 1986) and also all the potential places where species might occur were searched intensively, no live specimens were collected, but a vast number of empty shells were found. This is believed to indicate a recent extinction of the species in the island group and it is not included in the extant species list (see also Triantis et al. 2005a). In the case of Kalymnos island, H. figulina was reported by two previous studies (Reischu¨tz 1986; ˝ rstan et al. 2005). It is Bank and Neuteboom 1988) but not from the most recent (O important to note here that the species is absent from the nearby Bodrum peninsula ˝ rstan et al. 2005). Moreover, Hausdorf and Henning (2005), studying among other (O animal groups, land snails from 34 islands of the central Aegean Sea, did not report H. figulina either from Kalymnos or Pserimos. No live specimens or shells of H. figulina were found at any of the sites on Kalymnos or the islands of the group. On the contrary, a large number of H. aff. pachya were found on five islands of the group (see Table 4), which exhibit great shell similarity to H. figulina; thus, H. figulina is not included in the species list. The cases of Helix nucula, H. valentini and H. cincta on the island group of Kalymnos are quite similar. Reischu¨tz (1986) reported H. cincta from a single site of Kalymnos (Myrties) and the species has also been reported by Knipper (1939). In ˝ rstan et al. (2005) reported contrast, neither Bank and Neuteboom (1988) nor O ˝ the species either from Kalymnos or Pserimos. Orstan et al. (2005) reported H. nucula from Pserimos but not from Kalymnos, while Hausdorf and Henning (2005) report the species from both islands. Finally, the distributional information for H. valentini is even more complex. The species was described by Kobelt (1891), with the island of Kalymnos as its locus typicus. Fuchs and Ka¨ufel (1936) reported the species from Pserimos. Nevertheless, in the studies of Reischu¨tz (1986) and Bank and Neuteboom (1988), the species was not located on either of the two islands, although ˝ rstan et al. (2005) found the species on Pserimos. As shown in Table 3, the recently, O species was found on seven out of the 11 islands of the group and in quite dense populations. Thus, having in mind all the above information and the anatomical
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Species
Myosotella myosotis (Draparnaud, 1805) Truncatella subcylindrica (Linnaeus, 1767) Lauria cylindracea (Da Costa, 1778) Orculella critica (Pfeiffer, 1856) Pagodulina pagodula (Des Moulins, 1830) Pyramidula chorismenostoma (Blanc, 1879) Granopupa granum (Draparnaud, 1801) Rupestrella philippii (Cantraine, 1840) Rupestrella rhodia (Roth, 1839) Truncatellina callicratis (Scacchi, 1833) Truncatellina cylindrica (Fe´russac, 1807) Mastus carneolus (Mousson, 1863) Mastus giuricus (Westerlund, 1902) Albinaria cristatella O. Boettger, 1892 Idyla bicristata (Rossma¨ssler, 1839) Cecilioides acicula (O.F. Mu¨ller, 1774) Vitrea sporadica Pinter, 1978 Daudebardia rufa (Draparnaud, 1805) Oxychilus hydatinus (Rossma¨ssler, 1838) Oxychilus mylonasi Riedel, 1983 Oxychilus glaber (Rossma¨ssler, 1835) Eopolita protensa (Fe´russac, 1823) Tandonia sowerbyi (Fe´russac, 1823) Limax flavus Linnaeus, 1758 Lehmannia valentiana (Fe´russac, 1823) Deroceras parnasium Wiktor, 1984 D. panormitanum (Lessona et Pollonera, 1882)
Chorotype
Skyros Skyropoula
Valaxa
Sarakino
Platia
Erinia
Koulouri
Mediterranean
N
Mediterranean
N
N
Palaearctic Aegean Palaearctic
x x N
N x
N N
x x
N N
N N
N N N
Aegean
N
N
N
N
N
N
N
Mediterranean
x
N
N
x
N
N
N
N
N
N
N
N
N
Lakkonisi
N
M. Diavatis
Ag. Fokas Thalia E. Diavatis
N
N
Mediterranean
N
Mediterranean Palaearctic
N N
Palaearctic
N
N
Aegean Aegean Aegean
x x x
N x
N N N
Balkan Palaearctic
x N
x N
Endemic Palaearctic
x N
N
Mediterranean
x
x
Aegean Palaearctic
x N
Mediterranean Palaearctic Mediterranean Mediterranean
x N N N
x
N
Balkan Palaearctic
N N
N
N
N N
x
N
N
x
N
N
N N
N
N
N
N
N
x
N
N
N
N
N
N
x
x
N
N
N
N N
N
N
N N
N N
474 K.A. Triantis et al.
Table 1. List of land snail species in the island group of Skyros. x: presence, N: New reference.
Species
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Table 1. (Continued.)
Balkan Mediterranean
Skyros Skyropoula x x
Valaxa
Sarakino
Platia
Erinia
Koulouri
Lakkonisi
M. Diavatis
Ag. Fokas Thalia E. Diavatis
x
N
x
N
N
N
N
N
N
x
N
x
N
N
N
Aegean
x
Mediterranean Mediterranean
x N
Mediterranean Mediterranean Endemic Aegean
N N N x
N
N
x
Aegean
x
x
N
x
N
N
N
Mediterranean Mediterranean
x x
x
N
x
N
N
N
Mediterranean
x
Mediterranean Mediterranean
x N X X
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N N
N
N
N
New references
21
11
21
3
15
14
14
7
10
7
6
4
Total
44
20
21
16
15
14
14
7
10
7
6
4
Journal of Natural History
Lindholmiola lens (Fe´russac, 1821) Cochlicella acuta (O.F. Mu¨ller, 1774) Monacha parumcincta (L. Pfeiffer, 1841) Xerocrassa cretica (Fe´russac, 1821) Xerotricha conspurcata (Draparnaud, 1801) Xerolenta obvia Menke, 1828 Cernuella ionica (Mousson, 1854) Helicella sp. Metafruticicola giurica (O. Boettger, 1892) Helicigona cyclolabris (Deshayes, 1820) Theba pisana (O.F. Mu¨ller, 1774) Eobania vermiculata (O.F. Mu¨ller, 1774) Cantareus aspersus (O.F. Mu¨ller, 1774) Helix aperta Born, 1778 Helix cincta O.F. Mu¨ller, 1929 Helix figulina Rossma¨ssler, 1839 Pomatias elegans (O.F. Mu¨ller, 1774)
Chorotype
475
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Species Myosotella myosotis (Draparnaud, 1801) Truncatella subcylindrica (Linnaeus, 1767) Lauria cylindracea (Da Costa, 1778) Orculella critica (L. Pfeiffer, 1856) Pyramidula chorismenostoma (Westerlund and Blanc, 1879) Granopupa granum (Draparnaud, 1801) Rupestrella rhodia (Roth, 1839) Truncatellina cylindrica (Reinhardt, 1916) Helix godetiana Kobelt, 1878 Mastus rossmaessleri (L. Pfeiffer, 1847) Albinaria brevicollis (L. Pfeiffer, 1850) Cecilioides acicula (O.F. Mu¨ller, 1774) Rumina decollata (Linnaeus, 1758) Vitrea clessini (Hesse, 1982) Oxychilus hydatinus (Rossma¨ssler, 1838) Eopolita protensa (A. Fe´russac, 1832) Milax aegaeicus Wiktor and Mylonas, 1986 Zonites pergranulatus Kobelt, 1878 Deroceras astypalaeensis Wiktor and Mylonas, 1992 Caracollina lenticula (Michaud, 1831) Cochlicella acuta (O.F. Mu¨ller, 1774) Monacha pseudorothii Hausdorf, 2003 Xerocrassa cretica (A. Fe´russac, 1821)
Chorotype
Astypalaia Ofidousa
Kounoupoi
Mediterranean
N
N
Mediterranean
N
N
Palaearctic
N
Aegean
N
N
N
Aegean
N
Mediterranean
N
Mediterranean Palaearctic
N N
Aegean Aegean
Pontikousa Fokionisi M. Koutsomytis
N
Chondros
Ag. Kyriaki
Lianos
Fokionisi F.
Tigani
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N X
N x
N N
N N
N N
N
N
N N
N
N
Aegean
X
N
N
N
N
N
N
N
N
N
Palaearctic
N
N
N
N
N
N
N
Mediterranean
X
x
N
N
N
N
N
N
Aegean Mediterranean
X X
N N
N N
N N
N N
N N
N N
N N
N
N
N
N
N
N
N
N
N
N N
Fteno
Diapori
N
N
N
N
N N N
N N
N
Mediterranean
X
Aegean
X
Aegean
X
Endemic
X
Mediterranean
N
Mediterranean
X
Aegean
X
x
N
N
N
N
N
N
N
N
N
N
N
Mediterranean
X
x
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N N
N
N N N
N
476 K.A. Triantis et al.
Table 2. List of land snail species in the island group of Astypalaia.
Species
Chorotype
Astypalaia Ofidousa
Kounoupoi
Pontikousa Fokionisi M. Koutsomytis
Chondros
Ag. Kyriaki
Lianos
Fokionisi F.
Tigani
Fteno
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Aegean
N
N
N
N
Aegean
N
N
N
N
Mediterranean
N
N
N
Aegean Endemic Aegean
N N x
N
N N
Aegean
N N
N
N
N
N
N
X
Aegean
X
Mediterranean
N
Mediterranean
X
Mediterranean
X
N N
N
Diapori
N
N
N
x
N
N N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
New references
17
12
25
17
9
20
20
16
19
11
13
9
8
Total
33
18
25
17
9
20
20
16
19
11
13
9
8
Note: x: presence, N: New reference.
Journal of Natural History
Xerocrassa didyma Westerlund, 1879 Candidula syrensis (L. Pfeiffer, 1846) Xeromunda candiota (Mousson 1854) Cernuella ionica-virgata Helicopsis? sp. Metafruticicola coartata Fuchs and Ka¨ufel, 1936 Metafruticicola pellita (Fe´russac, 1832) Helicigona fuchsiana (Knipper, 1939) Theba pisana (O.F. Mu¨ller, 1774) Eobania vermiculata (O.F. Mu¨ller, 1774) Cantareus apertus (Born, 1778)
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Table 2. (Continued.)
477
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Species
Myosotella myosotis (Draparnaud, 1801) Truncatella subcylindrica (Linnaeus, 1767) Lauria cylindracea (Da Costa, 1778) Orculella ignorata Hausdorf, 1996 Sphyradium doliolum (Bruguie`e, 1792) Pyramidula chorismenostoma (Westerlund and Blanc, 1879) Pleurodiscus balmei (Potiez and Michaud, 1838) Rupestrella rhodia (Roth, 1839) Rupestrella philippii (Cantraine, 1840) Truncatellina rothi (Reinhardt, 1916) Truncatellina cylindrica (Reinhardt, 1916) Granopupa granum (Draparnaud, 1801) Jaminia loewii (Philippi, 1844) Zebrina cosensis (Reeve, 1849) Mastus rossmaessleri (L. Pfeiffer, 1847) Albinaria lerosiensis (L. Pfeiffer 1841) Bulgarica erberi (Frauenfeld, 1867) Cecilioides acicula (O.F. Mu¨ller, 1774) Cecilioides tumulorum (Bourguignat, 1856) Rumina decollata (Linnaeus, 1758) Vitrea contracta (Westerlund, 1871) Vitrea riedeli Damjanov and Pinter, 1969 Vitrea sossellai Pinter, 1978 Daudebardia brevipes (Draparnaud, 1805) Oxychilus cyprius (L.Pfeiffer, 1847) Oxychilus samius (Martens, 1889) Oxychilus hydatinus (Rossma¨ssler, 1838) Eopolita protensa (A. Fe´russac, 1832) Milax aegaeicus Wiktor and Mylonas, 1986 Deroceras samium Ra¨hle, 1983 Lindholmiola lens (Fe´russac, 1821) Caracollina lenticula (Michaud, 1831) Cochlicella acuta (O.F. Mu¨ller, 1774) Monacha claustralis (Menke, 1828) Monacha syriaca (Ehrenberg, 1831) Trochoidea pyramidata (Draparnaud, 1805) Xerocrassa cretica (A. Fe´russac, 1821) Xeropicta krynickii (Krynicki, 1833) Xerotricha apicina (Lamarck 1822) Cernuella virgata (da Costa, 1778)
Chorotype
Kalymnos
Mediterranean Mediterranean Palaearctic Aegean Palaearctic Aegean
N x x x N x
N x N N
N N N N
Mediterranean Mediterranean Mediterranean Balkan Palaearctic Mediterranean Anatolian Aegean Aegean Aegean Aegean Palaearctic Balkan Mediterranean Palaearctic Balkan Aegean Palaearctic Mediterranean Aegean Mediterranean Mediterranean Aegean Aegean Balkan Mediterranean Mediterranean Balkan Mediterranean Mediterranean Mediterranean Mediterranean Meditteranean Palaearctic
x x x x
x N x B
N
x x N x x
x x
x x x x x x B x B x x N N x x
Pserimos
x x x x
Telendos
Plati Nera Kalavros
N N N
N N N N
N
Ag. Kyriaki
Safonidi
Sari
N N
Ag. Andreas
Nekrothiki Krevatti
N
N
N N
N N N N N N
N N
N N N N
N
N N
N N
N
N
N
N
N
x
N
N
x N
N N
N N
N N N N
N
N
N N N
N
N
N N N
N
N N
N
N
N
N
N x x x B N N x x
N N
N N
N N N
N
x B x N x
x
N N
x
x
N
x
N N
N
N
N
N
N N N
N N N N
N
N
N
N
N N
N
N N
N
N N
N N N N
N N N
478 K.A. Triantis et al.
Table 3. List of land snail species in the island group of Kalymnos.
Species
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Table 3. (Continued.)
Metafruticicola redtenbacheri (L. Pfeiffer 1856) Theba pisana (O.F. Mu¨ller, 1774) Levantina spiriplana (Olivier, 1801) Eobania vermiculata (O.F. Mu¨ller, 1774) Cantareus aspersus (O.F. Mu¨ller, 1774) Helix valentini Kobelt 1891 Helix aff. pachya Bourguignat 1860
Chorotype
Kalymnos
Pserimos
Telendos
Aegean Mediterranean Mediterranean Mediterranean Mediterranean Aegean Aegean
x
x
N
Plati Nera Kalavros N
N
N
Ag. Kyriaki
Safonidi
N
N N
Sari
Ag. Andreas
Nekrothiki Krevatti
N N
N N
N
N
N N
N
x x x x N
x
N
N
N
x N
N N
N N
N
New references
7
8
28
23
21
9
18
14
7
10
5
9
Total
42
32
28
23
21
9
18
14
7
10
5
9
N
N
N N
Journal of Natural History
Note: x: presence, N: New reference, B: bibliographic data only.
N N N
479
480 K.A. Triantis et al.
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Table 4. The percentage of each chorotype in the three island groups. Chorotype/Island group Palaearctic Mediterranean Anatolian Balkan Aegean Endemic
Skyros
Kalymnos
Astypalaia
21% 45% – 7% 22% 5%
15% 45% 2% 11% 27% –
9% 41% – – 44% 6%
characters, only H. valentini is listed from the island group, and H. nucula and H. cincta are excluded from the species list. In the area of the eastern Mediterranean, the study of the genus Helix is especially complicated by the lack of a comprehensive review, and a revision is needed in order for a number of taxonomical and distributional issues to be resolved (Vardinoyannis et al. 2004; Vardinoyannis in preparation). Zoogeographical composition The chorotypes to which the species of each island group belong are presented in Table 4. In all the three island groups most of the land snail species belong to the Mediterranean element, with the Palaearctic being the second most important for Skyros, while for the Astypalaia and Kalymnos groups it is the Aegean element. The percentage of the endemic element is very low for the Skyros and Astypalaia groups, while there is no endemic species in the Kalymnos group. The Anatolian element is present only in Kalymnos, with a very low percentage. Faunal similarity All four similarity coefficients produced very similar UPGMA trees, with cophenetic correlation coefficients ranging between 0.749 and 0.920 (0.888 for Kulczynski’s no. 2 index; 0.878 for Dice’s index; 0.749 for Yule’s index; and 0.920 for Jaccard’s index). Therefore, all clusters appeared to represent the relationships equally well. In all the analyses applied the islands of each group were clustered together, exhibiting distinct sets as far as their malacofauna is concerned. The only major difference between the indices was the relationships among the three island groups. Using Dice’s index and Kulczynski’s no. 2 index, the Astypalaia group was connected with the Kalymnos group; when using the Jaccard’s and Yule’s indices Astypalaia was connected with the Skyros group. In order to resolve this problem, seven islands of the Cyclades were added to the analysis (i.e. Naxos, Paros, Amorgos, Andros, Keros, Pano Koufonisi and Dilos; data from Mylonas (1982) and Hausdorf and Henning (2005)). The addition of these seven islands led to a resolution of the above problem, since for all indices the Astypalaia group was connected with the islands of the Cyclades and then to the island group of Kalymnos (see Figure 4). The cophenetic correlation coefficients remained at a high level (i.e. r50.851 for Dice’s and r50.903 for Jaccard’s index). It must be noted here, that the intra-island group topology remains largely the same, even if the similarity coefficients are applied to each group separately.
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Figure 4. UPGMA tree based on the similarity matrix obtained by applying Jaccard’s index to the binary data matrix.
Nestedness The results of the Nestedness method resulted in high nestedness for all three island groups (see Table 5). Among them, the Kalymnos group exhibited the highest system temperature (16.02) while for the two other groups the temperature was very low: 5.43 and 6.93 for the Skyros and the Astypalaia group, respectively. Note that the nestedness values of the particular method were similar with those of the Atmar and Patterson’s method (results not shown). Discussion Biogeographical patterns Most of the Aegean islands used to be parts of larger areas and usually reconnections and re-isolations occurred between them and/or the mainland, mainly due to tectonic and eustatic events, such as the westwards movement of the Anatolian Plate, the Messinian Salinity Crisis and the Pleistocene sea-level fluctuations caused by glaciation effects. Even for islands that remained isolated after their disconnection from the mainland, change of size, uplift or submergence are common events throughout the whole Aegean area (see Dermitzakis and Table 5. Results of nestedness analysis for land snails of the three island groups. Island group Skyros group Astypalaia group Kalymnos group
Temperature
Matrix Fill
Z-score
5.43 5.73 16.85
0.49 0.49 0.38
2.42 2.10 2.35
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482 K.A. Triantis et al. Papanikolaou 1981; Van Andel and Shackleton 1982; Dermitzakis 1990; Lambeck 1996; Perissoratis and Conispoliatis 2003). The three island groups under study, despite their different geological histories and the different degree of present day geographical isolation, exhibit a significant common characteristic; they have been formed quite recently in the geological time. This ‘‘recent’’ formation of the island groups results in islands, especially the small ones, that still behave as parts of a continuous land mass, and consequently, the reduction of area has not yet led to a significant loss of species (Terborgh and Winter 1980; Karr 1982), with small islands exhibiting high numbers of species (see also Triantis et al. 2005a, 2005b). Moreover, species such as Helix cincta and H. valentini are abundant on these small islets whereas they are commonly absent from small islands. Characteristic is the case of Mesa Diavatis (Skyros group), which holds both Helix cincta and Cantareus aspersus. The short distances separating the islands and the long presence of man contribute towards this direction, as the probability of new immigrants reaching the islands is high, despite the low dispersal ability of terrestrial land snails. Furthermore, as has been shown by the analysis of the faunal element, the species present in all three island groups have a broad distributional range (Mediterranean and Palaearctic elements), which may be considered as an indication of the species being relatively effective dispersers. This acts as an additional reason for the reduction of the ‘‘island character’’ of the islands under study for the land snails. Finally, the reduced ‘‘island character’’ feature of the three island groups is also validated by the quite low (,0.2) values of the slopes of the species-area relationships for all the three island groups (Triantis et al. 2005a, 2005b; Triantis 2006). With islands that, for the taxon under study, behave as parts of the mainland, environmental heterogeneity and ‘‘passive sampling’’ of individuals become the major determinants of species diversity (Rosenzweig 1995; Triantis et al. 2006). In such cases, extinction and immigration are not as important as has been described for insular environments (MacArthur and Wilson 1967; Lomolino et al. 2005). As it was concluded by Triantis et al. (2005a, 2005b) environmental heterogeneity is one of the most important factors influencing species richness for the land snails in the Skyros group, and especially for the small islands of the group. The islands belonging to the same island group were clustered together regardless of the similarity index used, exhibiting a quite restricted malacofauna. This signifies that for each group the malacofauna, even on the smaller islands, exhibits a distinct composition and structure, still without the presence of significant endemism. However, the inter-group clustering exhibits quite low values of similarity. The inclusion of seven islands of the Cyclades offered a better-resolved phenogram regarding the intra-group relationships, resulting in a closer relationship of the Astypalaia group with the islands of the Cyclades. This cluster is then connected with the Kalymnos group, while the Skyros group exhibits the lowest similarity with the other two groups (Figure 4). The biogeographical position of Astypalaia in relation to the Cyclades and the eastern islands has been debated over the years (for a discussion, see Sfenthourakis (1996)). The present data confirm that although Astypalaia is under the influence of both the Cyclades and the eastern Aegean Islands, the cycladic element is stronger as far as the land snails are concerned. This is in agreement with the palaeogeographical data, according to which Astypalaia was connected with the Cyclades during the upper Meiocene (see Creutzburg 1963; Dermitzakis and Papanikolaou 1981; Dermitzakis 1990).
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In a number of works studying the land snails of the Aegean (see Mylonas 1982; Vardinoyannis 1994; Botsaris 1996; Triantis et al. 2004) it has been established that the Mediterranean element dominates the majority of the faunas and that, moving from the islands close to the mainland (either Greece or Turkey) towards the central part of the Aegean, mainly the Cyclades, the Aegean element increases. That is clearly evident in the present work as well. In all the island groups studied, the Mediterranean element was the dominant one, and in the Astypalaia group, which lies in the central part of the Aegean, the Aegean element was significantly more represented in the malacofauna, compared to the two other groups. Due to the geological evolution of the area, the islands situated in the middle of the Aegean exhibit a more distinct malacofauna, due to the reduced effects of the nearby mainland (Mylonas 1982; Mylonas et al. 2004). The endemism of all three groups was very low, both at the island group level and at the single island level. Although the endemism of land snails in the Aegean area is very high, 50% of the 400–500 species (Mylonas et al. 2004), the endemic species are not restricted to single islands and small island groups; rather the majority of them are endemic to larger parts of the Aegean region or to the whole area of the Aegean. The exception to this pattern is the island of Crete with its satellite islets, where the endemism is significantly high (see Vardinoyannis 1994; Triantis et al. 2004). The structured character of the malacofauna of the three groups is also reflected in their nestedness values. For all the island groups the temperature calculated is quite low, especially for the Skyros and Astypalaia groups (Table 5), indicating highly nested faunas. On the other hand, the Kalymnos group exhibited the highest temperature (T516.02), indicating a higher disorder in species presences and absences compared to the two other groups. Nevertheless, this value is lower than that estimated for the central Aegean islands (T535.19; Sfenthourakis et al. 1999). The island mainly contributing to the increase in ‘‘disorder’’ due to unexpected presences and absences for the malacofauna of Kalymnos island, is Krevatti (temperature of the island, T535.66). This islet, which is the smallest island of the group studied, besides exhibiting a high number of species in relation to its size, also exhibits three unexpected occurrences: Cantareus aspersus which is found only on Kalymnos and Ag. Kyriaki; Cochlicella acuta, found only on Safonidi, and Xerotricha apicina, which is absent from the whole group, even from the nearby island of Pserimos, but is present on Kos. Krevatti, due to its very small size, is almost impossible to have been affected by the usual anthropogenic activities in the area. Consequently, the presence of two strongly anthropophilic species such as C. aspersus and C. acuta cannot be attributed to human interference. Thus, it must be considered as the clearest example of the strong influence that the nearby island of Kos and the Bodrum peninsula have on the malacofauna of this island group, with species dispersing towards the islands of the group. Another example of increased stochasticity characterizing the structure of the Kalymnos malacofauna, compared to the other groups, is the absence of Eobania vermiculata from the island of Kalavros. Eobania vermiculata is one of the most common species in the Mediterranean and is found on almost every island and islet in the area (Mylonas 1982; Vardinoyannis 1994; Botsaris 1996; Triantis et al. 2004, 2005a; Hausdorf and Henning 2005). In the present study it was recorded on 35 out of the 37 islands. The second island not hosting this species was E. Diavatis, a
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484 K.A. Triantis et al. non-calcareous island of the Skyros group. Despite the fact that Kalavros lies very close to Kalymnos and Telendos, not even an empty shell of the species could be found. In total, although the three island groups have the same structure (one main large island surrounded by smaller ones), the Kalymnos group is the only one where 11% of the species (five out of 42) of the group are not present on the largest island. In the two other island groups all the species, except for one in the case of Astypalaia, are found on the main island. In conclusion, although the three island groups have similar characteristics as far as the malacofauna is concerned, with reduced ‘‘island character’’, it seems that the island groups of Astypalaia and Skyros can be considered to be in the initial phase of ‘‘faunal relaxation’’, with highly nested faunas and without any significant or obvious extinction of species caused by the isolation and the reduction of area. On the other hand, for the Kalymnos group, the proximity of the group to large species pools and the very recent isolation results in a higher stochasticity of the structure of the malacofauna, with environmental heterogeneity and ‘‘passive sampling’’ of individuals becoming the most probable determinants of species diversity (Triantis et al. 2005a, 2005b; Triantis 2006). Completeness of data about land snails of the Aegean area Many of the questions addressed in biogeography require data on the distribution of multiple species among many localities. It is often impractical for an individual investigator to do the field research required to collect all the necessary data. It is necessary to compile, either from the literature or from museum collections, electronic databases, government reports or other unpublished sources, data collected by other investigators (Kodric-Brown and Brown 1993). Concerns about declining biological diversity and global environmental change have increased the need for medium to large scale studies from data compiled from multiple sources. However, the validity of these studies depends on the completeness of the data. No large data set is likely to be perfectly accurate, so incomplete data sets might affect the qualitative results of analyses and the conclusions drawn from them (KodricBrown and Brown 1993; Cameron and Pokryszko 2005). The results of the present work indicate that a large number of islands of the Aegean Sea still remain understudied and more extensive studies are needed to improve and complete knowledge. The case of Astypalaia and Skyros, along with some of the smaller islands such as Pserimos and Skyropoula are typical, since the present work almost doubled the number of species reported in the past, indicating a significant incompleteness (see Tables 1–3). The main causes for the inefficiency of data are: sampling during unfavourable periods, and limited sampling effort. The most favourable sampling period for land snails in the Aegean area, is October to April, when snails reproduce, feed and grow (Mylonas 1982). In some cases even surveys during April should be considered as deviating from the favourable sampling period, if the rainfall during the previous months was limited. Unfortunately, a significant number of studies in the Aegean area focusing on land snails have been conducted during the dry period (e.g. Reischu¨tz 1983, 1985, 1986; Bank 1988; Schultes and Wiese 1990, 1992; Welter˝ rstan et al. 2005). Two major problems arise Schultes and Wiese 1997a, 1997b; O from sampling during unfavourable periods: the first one is the fact that a number of
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species will not be collected, since they probably aestivate, hidden in undetected habitats, and the second is the limited number of live specimens in the samples. The first problem is directly correlated with the underestimation of the total species richness, since slug species and species represented by small numbers of individuals will most probably not be found. Especially on very small islands where a species can be represented by a small population, sampling during an unfavourable period will most certainly result in overlooking it. The second problem is strongly correlated with the (mis)identification of the species, since for many taxa features of the internal anatomy are crucial for their taxonomic identification. The second cause of incompleteness is the sampling effort invested in the survey. This is correlated both with the number of sites surveyed on islands, since sampling the entire area is unrealistic, and the time spent at each surveyed site. For the latter the most efficient way is to consider the survey complete when no new species has been added in the samples for the past 30 minutes (see Mylonas 1982). As far as the number of sites surveyed, it is clear that as the number increases, the probability of finding new species increases. Typical is the case of Skyros Island: among the 42 extant species found, 13 species (31%) were found in one or two of the sites surveyed. This limited presence of a number of species in the Aegean area requires a significant number of sites to be surveyed on each of the Aegean islands, and especially on the larger ones. Last but not least, is the problem of taxonomy, and the way that species are considered and counted. Although important progress has been established over the last decades regarding the taxonomy of various genera and families of terrestrial molluscs of the Aegean (see Riedel 1992; Subai 1996, 2005; Wiktor 2001; Hausdorf 2003; Parmakelis 2003; Gittenberger and Hausdorf 2004; Parmakelis et al. 2005), there are still a great deal of problematic taxa, such as Albinaria (Giokas 1996; Nordsieck 1999; Welter-Schultes 2000), where the approach taken can strongly affect counts of species richness and consequently the analyses of biodiversity patterns. Future studies will probably reveal similar patterns on other Aegean islands, where the faunas have been considered to be well-known. In particular, islands where the reported malacofauna is based on surveys during unfavourable sampling periods and/or low number of sites, should almost certainly be considered as undersampled. It is probable that in the eastern Aegean islands (Lesvos, Kos, Limnos, Chios, Rodos) as well as in some of the northern Aegean islands (Thasos, Alonisos, Skiathos, Skopelos) future studies will significantly increase the number of land snail species. Hence, the extent to which conclusions of research based on incomplete faunal datasets will change or not (e.g. Welter-Schultes and Williams 1999; Hausdorf and Hennig 2005) is an important issue and should be investigated in the future. Similar analyses of other invertebrate taxa, also based on sporadic and disproportional surveys, i.e. tenebrionid beetles (see for example Fattorini et al. 1999) should also be regarded with caution. It is stressed here that regardless of the significant advances in the knowledge of the Aegean’s malacofauna over the last decades, single-visit surveys which are the rule, especially for the small, isolated and difficult to reach islands, must be performed during the most favourable period and by investing a lot of sampling effort. This is the only way to obtain a data set as complete as possible in order to avoid serious errors of fact, interpretation and application.
486 K.A. Triantis et al.
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Acknowledgements We thank P. Lyberakis and A. Parmakelis for their valuable help during the field trips. A. Parmakelis provided valuable and constructive comments on an earlier version of the manuscript. RAD Cameron and anonymous referees made valuable suggestions improving greatly the science of the manuscript. The present study was funded by the Natural History Museum of Crete and the Heraclitus Scholarship Program for Basic Research of the European Union and the Hellenic Ministry of Education.
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