The Diversity of Land Molluscs Questions Unanswered and Questions Unasked

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1 The Diversity of Land Molluscs Questions Unanswered and Questions Unasked Author(s): Robert A. D. Cameron Source: American Malacological Bulletin, 31(1): Published By: American Malacological Society DOI: URL: BioOne ( is a nonprofit, online aggregation of core research in the biological, ecological, and environmental sciences. BioOne provides a sustainable online platform for over 170 journals and books published by nonprofit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne s Terms of Use, available at Usage of BioOne content is strictly limited to personal, educational, and non-commercial use. Commercial inquiries or rights and permissions requests should be directed to the individual publisher as copyright holder. BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research.

2 Amer. Malac. Bull. 31(1): (2013) The diversity of land molluscs Questions unanswered and questions unasked* Robert A. D. Cameron Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, United Kingdom, and Natural History Museum, London SW7 5BD, United Kingdom Correspondence, Robert A.D. Cameron: Abstract. Studies on the diversity of land molluscs raise a number of crucial questions that remain unanswered, or in some cases have not been asked. At the most fundamental level, we need to question the comparability of species concepts used in different studies, which influence our assessment of overall diversity and of regional differentiation. Current estimates of global diversity are likely to be too low. Beyond that, the assumption that all species can be represented by equivalent digits needs to be challenged: the range of size, habits and trophic levels needs to be taken into account. While higher taxonomic categories can be used as rough proxies for similar ecology, we have very little detailed information other than for shell size and shape, where the huge range suggests some radical differences within and among faunas. The range of niches occupied may differ among faunas: are we dealing with comparable taxonomic entities? Our knowledge of microhabitat requirements is severely deficient. Regional species richness varies much more than maximum local (site) richness across faunas. The poor regional faunas of areas subject to glacial or periglacial Pleistocene conditions are not always impoverished at local level. While this might suggest the presence of competitive interactions, the evidence for competition is scarce. There are many cases where closely-related species live in sympatry at the smallest scales. Nevertheless, there are cases of parapatric or microallopatric distributions of congeners, and metapopulation dynamics can facilitate the co-existence of potential competitors. There may be no general rule; cases of both character displacement and convergence in sympatry are reported. While most studies accept the traditional model of allopatric speciation to account for the development of diversity, the balance between vicariance and dispersal as driving forces remains undetermined, and clearly varies from case to case. Poor powers of active dispersal may permit genetic differentiation over very short distances, but there are a number of spectacular cases of long-distance passive dispersal that suggest that our direct estimates of dispersal are questionable. Better knowledge of actual dispersal and of the ecological requirements of land mollusc species should be priorities for the future. Keywords: speciation, species equivalence, microhabitats, species richness, ecological requirements In 1983, Alan Solem and A. C. van Bruggen organised a symposium on snail diversity within the UNITAS congress in Budapest. Its proceedings were published a year later (Solem and van Bruggen 1984). Although not solely concerned with land molluscs, many of the papers within it were so dedicated, and in the opening chapter Solem (1984) attempted to draw A world model of land snail diversity and abundance. As Schilthuizen (2011) said, his hypotheses, some of which have been refuted since, were designed to provoke more studies, and certainly had that effect (Cameron et al. 2005). He asked How many species are extant? Live in one area? Occur sympatrically? Why are the observed diversity levels so different? Can we identify factors that might have causal relationships to these observed differences? The data necessary to give fully satisfactory answers are lacking. We do, of course, have more data today, and new analytical techniques. The 1983 symposium antedates the explosion in model-based biogeography and macroecology (Brown and Maurer 1989), though not the seminal source (MacArthur and Wilson 1967). Solem himself adopted what I have called elsewhere the nitpicker naturalist approach to general propositions (Cameron et al. 2005), a bottom-up approach relying on detailed knowledge of particular cases. We also have an increasing body of molecular data, enabling us to start answering questions not addressed directly by Solem relating to the origins of diversity and the mechanics of speciation. Nevertheless, there are many unanswered questions within terrestrial malacology, and there are also questions that we ought to ask that might challenge some of the approaches taken in the broader field of diversity studies. The * From the Mollusks: The Great Unanswered Questions. The James H. Lee Memorial Symposium presented at 77th Annual Meeting of the American Malacological Society on 24 July 2011 in Pittsburgh, Pennsylvania. All symposium manuscripts were reviewed and accepted by the Symposium Organizer and Guest Editor, Dr. Timothy A. Pearce. 169

3 170 AMERICAN MALACOLOGICAL BULLETIN questions we ask, or should ask, are connected; they often relate to the same data, and there is no evidently correct order in which to present them. Some apparently factual questions conceal conceptual confusion, and are considered first. A major theme here is the need to take account of the immense range of size, shape, trophic levels, habits, life histories and phylogenetic relationships within terrestrial molluscs. We set up false equivalencies when species spanning such a range are reduced to the level of one species, one digit for analysis. In the different context of defining rarity, Gaston (1994) quotes from Reveal (1981):...rarity is merely the current status of an extant organism that is restricted either in numbers or area that is demonstrably less than the majority of other organisms of comparable taxonomic entities. (my emphasis). Are our molluscs comparable entities? If not, how do we analyse molluscan diversity on land? Another theme, related but distinct, is the need to account for diversity at local and regional levels in both ecological and evolutionary terms. What, if any, are the assembly rules for local faunas? What role, if any, is played by competition? Is the land mollusc fauna an entity to be studied without reference to other potentially interacting animals? For both themes, though, the starting point is adequate and consistent knowledge of faunas. HOW MANY SPECIES ARE THERE, AND WHAT ARE THEIR DISTRIBUTIONS AND HABITATS? These might seem like factual, housekeeping questions. We know that there is a vast amount of alpha- and revisingtaxonomy still to do. New monographs add many new species (e.g., Koehler 2011). Solem s (1984) total of 30 35,000 species worldwide now seems likely to be an underestimate. He did not explain how his estimate was arrived at, though he clearly extrapolated from the numbers already described. Since 1984 there have been many more studies of diversity, especially in tropical regions. These report many identified but formally undescribed species-level taxa; the backlog of alpha taxonomy is great. Bank (2010) lists at least 1800 nominallyvalid species within the relatively narrow confines of Europe. While some of these might not survive detailed revision, there are also many described subspecies (see below). Given the relatively unexplored faunas of large parts of Asia and South America where local endemism will be high, it is possible that the true figure is in excess of 50,000. At some scales, and in some regions, we can give reasonably complete answers; elsewhere coverage is at best patchy and new surveys extend known geographical ranges or increase known incidence within them (e.g., Nekola 2009). Analyses based on incomplete knowledge have already led to erroneous conclusions (Triantis et al. 2008). These are the basic data on which any analysis depends, and it might seem that all we need are more resources and more malacologists. There are, however, conceptual issues embedded in these questions. Many species are defined on shell characters alone; anatomical and molecular studies frequently challenge established taxonomy (e.g., Giokas 2000, van Moorsel et al. 2000, Nekola et al. 2009, Duda et al. 2011) even at levels higher than that of species (Rowson et al. 2011). In large scale comparisons, can we equate the 22 species of shell-defined species of Cochlicopa A. Férussac, 1821 within the former U.S.S.R. (listed, with doubts, by Sysoev and Schileyko 2009) with the possibly three largely selfing clades of the same genus delimited by Armbruster (1997) on molecular evidence in Europe? Within the European fauna (Bank 2010), especially bedevilled by ancient names and practices, 22 Alopia H. and A. Adams, 1855 species have 62 subspecies among them; 57 Vitrea Fitzinger, 1833 species have just four. Clausilia dubia Draparnaud, 1805 has 22 recognised subspecies. Are the same criteria applied to each? Nor is the problem confined to ancient names. 15 species of Romanian Deroceras, Rafinesque, 1820 all described since 1960, have recently been subsumed into one (Bank 2010). To the previously known 22 Monacha Fitzinger, 1833 species from Turkey, Hausdorf (2000) has added a further 27, most separable only by dissection (Schütt 2005). Schütt (2005) also shows that 25% of the known clausiliid fauna of Turkey are known only from their type localities. They are defined mostly on shell characters, yet recent work (Szybiak and Leśniewska 2008) has shown how plastic such characters can be. While there are cases where molecular taxonomy confirms conchologically determined species, but not their relationships, as in one group of North American Vertigo Müller, 1773 (Nekola et al. 2009), there are others in which it does not. Emberton (1988, 1991) raised key questions about the convergence and phylogenetic significance of shell characters within and among subfamilies and genera of North American Polygyridae, a group in which detailed molecular taxonomy would yield rich returns. I am not here concerned with the correctness or otherwise of any particular determinations. It is clear, however, that even at the level of regional species richness, or a simple analysis of faunal similarity and difference, data are by no means always comparable. We have only just begun to appreciate the consequences of selfing (Jordaens et al. 2001, Reise et al. 2001, Geenan et al. 2006), and I suspect that ongoing studies of speciation may reveal more species arising from hybridization among previously separated stocks than we have imagined (Martins 2005). From Bank (2010), I might draw the conclusion that the countries of Greece and the former Yugoslavia have more species than the whole of America north of Mexico, the more so if the mass of subspecies in the former were raised in status. It may well be true; equally, differences in taxonomic traditions and criteria may have biased

4 DIVERSITY OF LAND MOLLUSCS 171 this comparison. These differences do not prohibit large scale comparisons, but they should oblige us to ask whether the data sets we use have comparable definitions of the entities (usually species) involved. ONE SPECIES, ONE DIGIT? The impetus for the modern macroecological approach stemmed from a justified impatience with the accumulation of a mass of particular cases, and a desire to look for general, overarching, testable hypotheses. Large amounts of disparate data generate patterns conforming to very general mathematical distributions. These must reflect a mix of biological processes, but since they also occur in data sets beyond the realm of biology, they do not add much to our understanding (Nekola and Brown 2007, McGill and Nekola 2010). Nevertheless, any rigorous analy sis needs quantitative data suitable for testing. How, then, can we categorise the fauna in such a way that we have batches of species that can reasonably be regarded as comparable taxonomic entities? Taxonomic categories Relatedness is often used as a proxy for similarity in species properties, and might also illuminate history in a useful way. Unfortunately any attempt to use genera for large scale comparisons in terrestrial malacology is subject, with added force, to all the doubts raised above about the commensurate nature of species (Cameron et al. 2006). Some generic designations are extraordinarily unstable. They often reflect radically different approaches to taxonomic practice, and recent molecular studies have overturned as convergent some characters used in their definition (van Moorsel et al. 2000, Rowson et al. 2011). Subgeneric divisions are even worse. Genus or higher group cladograms based on molecular data are far better (e.g., Rundell 2008; Jordaens et al. 2009, Rowson et al. 2011), especially for any study of differentiation over time. We have rather few of them. In general, though there are many exceptions, familylevel taxa are a more reliable proxy for properties held in common. At the very least, following Borges and Hortal (2009), analyses of patterns of distribution by family have already demonstrated radical differences in the patterns shown by different family groups. In terms of island biogeography, for example (Figs. 1 and 2), disaggregation shows that particular families may show opposite trends in richness with island area or age (Cowie 1995, 1996). The same differentiation is present in continental faunas (Fig. 3), and it can apply at much smaller scales (Table 1). Schilthuizen et al. (2005) have similarly reported a different response to disturbance between pulmonate and operculate Figure 1. Numbers of native species in selected families in the Hawaiian archipelago. Above: black bars, Amastridae; stippled bars, Achatinellidae. Below: black bars, Succineidae; heavy stipple, Pupilloidea; light stipple, Helicinidae. Data mainly from Cowie et al. (1995). Central includes the islands of Lanai, Molokai and Maui. The overall pattern is dominated by the most species-rich families, but others do not show the same pattern. snails on limestone outcrops in Malaysia. In all these cases, the use of all data, undifferentiated, will either give a meaningless average, or will reflect only the trend in the dominant group in terms of number of species. Figure 2. Numbers of native species in selected families on some islands in the Canary archipelago. Black bars, Enidae; heavy stipple, Helicoidea; light stipple, Vitrinidae; vertical lines, Ferussaciidae. Mahan combines Fuerteventura and Lanzarote, which are arid. Data from M. Ibáñez and M. Alonso (personal communication).

5 172 AMERICAN MALACOLOGICAL BULLETIN Figure 3. The proportions of all species in three families of European snails occurring in increasing numbers of countries. Black bars, Clausiliidae (N = 407); heavy stipple, Chondrinidae (N = 57); light stipple, Vertiginidae (N = 33). Data from Bank (2010). Countries act as a very approximate proxy for geographical range. While it is clear that nominal families are not always commensurate, it appears that there are regional differences in the relatedness of co-existing species (Table 2). Species in faunas resulting from Holocene colonization appear, on average, to be less closely-related than those in faunas with a longer history and in situ diversification. Can we explore this more rigorously? It relates to the topics of dispersal, speciation, assembly rules, competition, and metapopulations examined below. Common characteristics There are other ways of categorizing our species: size and shape, slug or snail, life histories, reproductive strategies, microhabitat requirements, trophic level, powers of dispersal. All except the first two are difficult. We lack adequate data for a complete fauna, or for even a more limited taxonomic group within it, and obtaining them is hard (and unlikely to lead to publication in high-rank journals until the complete set is available for analysis). This slow work is being done by some, for example Maltz and Sulikowska-Drozd (2008) on clausiliid life cycles. There is much more to do. Because size and shape are easy to measure, they have been extensively studied. The range of size alone is colossal. For example, in the snail fauna of eastern Australia, the largest species, Hedleyella maconelli (Reeve, 1853), is around 5 x 10 4 times the weight of the smallest, Letomola contortus (Hedley, 1924). For comparison, an average Homo sapiens is only around 5 x 10 2 times the weight of Hedleyella maconelli. Similar extremes occur in other faunas. In Europe, the difference between the smallest species Punctum pygmaeum (Draparnaud, 1801) and the largest, Helix pomatia Linnaeus, 1758, is equivalent to that between a house mouse and a rhinoceros. There is also a great range of shell shape, and indeed of other features that might relate to species habitats and ways of life. Cain (1977, 1981) drew attention to the paucity of species in many faunas between the two arrays of those with flattened or globular shells and those with shells distinctly taller than wide. He suggested that this indicated an adaptive trough that was hard to cross, and that similarities among members of the same family were maintained by competition with species occupying other parts of the size and shape spectrum. By contrast, Emberton (1994, 1995a) attributed some of these differences to phylogenetic constraint, deeply embedded patterns of development within families. While some families are indeed very conservative in size and shape (e.g., Clausiliidae), others (e.g., Streptaxidae, Rowson et al. 2011) span a huge range, and such a range has been found even within a single genus (Gould 1989). In this last case, Stone (1996) pointed out changes in a very few variables that can produce the observed range; none of them is evidently more difficult than others. Hence the concept of phylogenetic constraint may act as a cover for our ignorance. However, efforts to investigate the adaptive significance of size, shape, and other shell characters have produced few clear and consistent generalities (Goodfriend 1986, Schilthuizen 2011). Flattened species with sharp keels are often associated with rocks, and are found in many diverse families. There are other cases of remarkable convergence (e.g., the helicid Isognomostoma isognomostomos (Schröter, 1784) in Europe, and the polygyrid Stenotrema barbatum (Clapp, 1904) and others in North America) that strongly suggest adaptive significance. Comparisons of the Table 1. Site occupancy by species of land snail in the Kimberley district of Western Australia, data for Camaenidae separated from the remainder, which are all much smaller. The proportion of the fauna occurring in any one site is far greater in the latter. Data and original sources in Cameron et al. (2005). A more extreme case is given in Solem (1988). Region Camaenidae All others Sites No. of species Mean species per site No. of species Mean species per site Napiers/Oscars Rainforests

6 DIVERSITY OF LAND MOLLUSCS 173 Table 2. Numbers of families and species recorded in forest sites close to one another in each region in regions subject to colonization in the Holocene, and in regions where molluscan populations are of much greater age. Data from Pokryszko and Cameron (2005), de Winter and Gittenberger (1998), Schilthuizen and Rutjes (2001) and Stanisic et al. (2007). Site Families Species Species per Family Latvia S. England SE Poland Queensland Cameroon Borneo ecology of such convergent pairs might help us understand the selective forces responsible for such precise phenotypic similarities. Similarly, Schilthuizen et al. (2006) show a connection between details of shell morphology among very closely-related species and the distribution of invertebrate predators. In general, however, we lack conclusive evidence on the evolution and functions of features such as apertural barriers (Pokryszko 1997). Certainly, the range of size and shape differs greatly between faunas at both regional and local scales. Why are there no tall shelled species larger than Cochlicopa Férussac, 1821 (ca. 6 mm) in most parts of North America, when, in terms of numbers of species, such forms are one of the largest components of European faunas at comparable latitudes (Cameron 1987)? Why is the New Zealand fauna dominated by tiny species, while those of many other regions contain a far larger proportion of much larger species (Barker 2005)? Do these differences reflect phylogenetic constraints among the families involved, or radical differences in the range of niches occupied, perhaps constrained by the presence of competitors or predators in other phyla? Are there adaptive troughs that constrain the pattern of evolutionary change? We have at least some means of associating shell size and shape with patterns of distribution. Small species may survive extreme cold better than larger ones (Ansart and Vernon 2003), and faunas in very cold climates tend to be dominated by such species (Nekola 2005, McClain and Nekola 2008). It is a repeated finding that small species tend to have larger geographical ranges than large ones (Cameron et al. 2010), although there are many exceptions. Long-distance passive dispersal is clearly easier for very small species, and there is a greater incidence of uniparental reproduction. For those small species with restricted ranges we can sometimes see evident causes, as for example in the many tightly restricted species of Vitrea Fitzinger, 1833 recorded from caves, contrasted with those, for example V. contracta (Westerlund, 1871), living in the open, and with very large ranges (Riedel 1992). We should surely be examining the differences, if any, between closely-related and similar sized species with different range sizes. There are many cases associated with Holocene recolonization of previously glaciated or periglacial regions where only one or a few species within speciose genera have spread, while others of similar size, morphology and apparent ecology have remained restricted (see, for example, the maps for Abida Turton, 1831 and Trichia Hartmann, 1840(now Trochulus Chemnitz, 1786) in Kerney et al. 1983). Do the colonising species have characteristics in common, distinguishing them from less successful congeners? Work on bats (Tello and Stevens 2010) suggests that environmental correlates with range may be weaker in species with very restricted distributions that are failing to occupy all theoretically available locations. Is this an intrinsic failure of dispersal, or a consequence of niche pre-emption by others? Of course, the accidental and deliberate transport of molluscs by humans, going back perhaps five millennia (Preece 1998, Lubell 2004, Lee et al. 2007, Giokas et al. 2010, Ó Foighil et al. 2011), complicates our search for patterns. Beyond shell size and shape, our knowledge of other properties is very limited. Most faunas appear to be dominated by presumed detritivores/herbivores, but many African faunas have large numbers of the morphologically diverse and carnivorous Streptaxidae (de Winter and Gittenberger 1998, Rowson et al. 2011). This again raises the usually unasked question: are faunas in different biogeographical regions commensurate in the range of niches occupied? LOCAL RICHNESS, COMPETITION, SPECIATION, AND DISPERSAL Local richness In reviewing land mollusc species richness at the local scale, Solem (1984) concluded that the richest local faunas were to be found in wet subtropical or temperate forests with a long history of habitat stability. By comparison, he asserted that tropical rainforests, known to be a habitat sustaining very high levels of local species richness in other taxa (Hubbell 2001) were impoverished, a phenomenon he attributed among other causes to high soil acidity and the absence of long-lasting food and shelter provided by litter accumulation. This conclusion was comprehensively overturned by later work (de Winter and Gittenberger 1998, Schilthuizen and Rutjes 2001), and appears to have been a product of inadequate sampling where densities are low (Cameron and Pokryszko 2005, Schilthuizen 2011). Solem s dismissal of recorded high levels of local diversity in colder temperate forests as mosaic diversity has likewise been overturned by later work; as Schmid (1966) and Nekola and Smith (1999) have demonstrated: 20 to 30 species may be found in quadrats of 1 m 2 or less.

7 174 AMERICAN MALACOLOGICAL BULLETIN There is a similarity in the maximum numbers of species reported from sites of between 100 m 2 and 1 ha from many regions and climates in the range species (Schilthuizen 2011). Given appropriate habitat, sites occupied by Holocene colonisers are not evidently poorer than those continuously occupied for millions of years (Pokryszko and Cameron 2005, Stanisic et al. 2007, Pokryszko et al. 2011). Even on oceanic islands, some local faunas achieve levels of species richness comparable to those found on continents (Cameron et al. 2007, 2012). Local variations in site richness and composition appear to relate to the range of microhabitats available (Waldén 1981, Horsák 2006, Juřičková et al. 2008), or to very recent anthropogenic disturbance (Götmark et al. 2008). Given the much greater variation in the richness of the regional faunas from which these local faunas are drawn (Cameron 2004), there are clearly great differences in the regional to local ratio of richness (Srivastava 1999). Although great care is required in interpreting such ratios, they raise questions about the roles of competition, speciation, dispersal and environmental change in the development of diversity: assembly rules in time and space. We have plenty of material on which to speculate, but little hard evidence for any particular mechanisms. What role does competition play in structuring local faunas? The consequences, indeed the existence, of interspecific competition is perhaps the most contentious issue in the study of land mollusc faunas. Following the early declaration of Boycott (1934), I do not think that a snail is ever excluded from a locus suitable to it because other snails are there already or that one species can expel another, most later studies have similarly failed to find convincing evidence for competitive interactions, other than in a few specific cases involving pairs of very similar species, often in rather extreme, simplified habitats such as sand dunes. Solem (1984) documented cases in which many very closely-related species co-exist in microsympatry and later work reports the same phenomenon (Stanisic 1997, Tattersfield 1998, Seddon et al. 2005). In Georgian (Asia) forests, five out of eight species of Leiostyla Lowe, 1852 found regionally were found in a single 400 m 2 site (Pokryszko et al. 2011); on Santa Maria (Azores) seven out of eight endemic Oxychilus Fitzinger, 1833 species were found in a single small site (Martins 2005, Cameron et al. 2012). There are many cases in which closely-related species with very similar morphology share not only the same site, but may be found together on the same rock ledge, tree trunk, or rotting log. There is little support for limiting similarity among co-existing species (Huntley et al. 2008, Schamp et al. 2010), a potential indicator of competitive effects. Where introduced species have been added to a native fauna (even on an oceanic island), their effects appear to be minimal (Cameron and Cook 1996), unless the introduced species prey on the native species (Lydeard et al. 2004). Waldén (1981) and Hylander et al. (2005) showed that variations in local richness related to the number of microhabitats available with little or no complementarity. Local richness is nested. Indeed, studies at very small scales show that some individual quadrats may contain nearly all the species to be found in a larger site, while others may be devoid of snails (Nekola and Smith 1999, Sharland 2001, Horsák 2006). Microhabitats of many different species appear to coincide, but little is known of the specific requirements of each species. At face value, the absence of competition would lead to the conclusion that local, syntopic richness is a product solely of the range of microhabitats available, the properties of regionally available species, and their capacity to disperse. Certainly, some local faunas in areas colonized in the Holocene may contain nearly all the regionally available species appropriate to that habitat (Pokryszko and Cameron 2005). I believe we should be very cautious before accepting this widely held view as the general rule. Two closely-related and partially sympatric species of Partula Férussac, 1821 on Moorea show character displacement (Johnson et al. 1993), and Chiba (2004) and Chiba and Davison (2007) demonstrated character displacement and both morphological and microhabitat preference differences among co-existing species of Mandarina Pilsbry, 1894 (Bradybaenidae) on the Ogasawara (Bonin) Islands, Japan. By contrast, Emberton (1995b) reports a case of sympatric convergence between two polygyrid species. More detailed studies of this kind are needed; clearly there is no consistent generality. The rather surprising uniformity of maximum site richness referred to above may be fortuitous, given the range of size, shape and habits encompassed by the fauna, but it might also reflect an upper limit on the number of ways the various available niches can be divided while maintaining minimum population sizes (Soulé 1986) for all species. There are two further reasons for doubt, and both have implications for our understanding of speciation and dispersal. One is the major breakthrough in our understanding of ecosystems by adopting spatially-explicit models, the field of metapopulation dynamics (Hanski 1999). If suitable microhabitats are transient and partly isolated from each other, the extent to which they are occupied will be a function of their turnover and isolation relative to the powers of reproduction and dispersal of the species that can use them (Pinto and MacDougall 2010). Species with identical niches can then co-exist in an area large enough to contain many patches. At the extreme, this becomes the famous neutral model of Hubbell (2001), which explicitly involves competition among individuals. It is a well-known but poorly-documented fact that most land mollusc species have extremely patchy distributions

8 DIVERSITY OF LAND MOLLUSCS 175 even within a relatively small sampling site (Nekola and Smith 1999, Schilthuizen 2011, see also above), and that some of these patches, for example coarse woody debris, are temporary (Kappes 2005). The second is the occurrence of parapatric or microallopatric distributions of some sets of closely-related species. Gittenberger (1991) described this pattern in Albinaria Vest, 1864 (Clausiliidae) species as non-adaptive radiation, indicating that the species concerned did not differ in their ecology. The case is similar to that for camaenid snails in NW Australia referred to above (Table 1), and Gittenberger (1973) has earlier described a mosaic of restricted ranges within Pyrenean chondrinids. Raheem et al. (2009) reported similar cases in Sri Lanka, and there are many others. In the context of competition we can ask if the failure to find mixed populations is simply a function of poor dispersal (even short distances of unsuitable habitat may present a formidable barrier), or to the disadvantage suffered by individuals moving into the territory of another population, which might relate to local adaptation or, more probably, to some degree of genetic incompatibility. Giokas et al. (2000) demonstrated maladaptive features of hybrids in a contact zone between two species of Albinaria and other narrow hybrid zones are known in this genus (Gittenberger et al. 2001). This brings us to the mechanisms of speciation. How do new snail species arise? The issues of competition and speciation are linked, and often use the same body of circumstantial evidence. With a few exceptions, most accounts of speciation in snails accept the conventional model of allopatric speciation outlined originally by Mayr (1963). Patterns of distribution like those of Albinaria or the camaenids of NW Australia referred to above fit comfortably into this framework, and can be related to known environmental changes or discontinuous habitats. The proverbially poor powers of dispersal in snails have tended to buttress explanations based on vicariance mediated by environmental change, but in some cases a pattern of differentiation following dispersal over barriers (Rundell 2008) is indicated by molecular studies. Recent molecular work on Albinaria suggests that such cases are complex (Schilthuizen et al. 2004, Douris et al. 2007), with successive episodes of range expansion and contraction. Molecular work has also revealed cryptic differentiation, complete with secondary contact and the development of hybrid zones (Dépraz et al. 2009); other similar studies (Duda et al. 2011) separate sister lineages with distinct habitat preferences but overlapping gross distributions. Ecological or parapatric speciation across an ecological threshold or gradient is suggested for other organisms (Schneider et al. 1999); it has not been explicitly documented in land snails, but seems likely. The molecular studies above, with others (Holland and Cowie 2007) in which a complex mixture of vicariance and dispersal are involved, suggest that we need more detailed analyses of particular cases before drawing general conclusions. Martins (2005) documented the development of anatomical differences among populations of the Azorean Oxychilus atlanticus (Morelet and Drouët, 1857) subject to periodical isolation; there were no broad geographical trends, and the possibility of isolates acquiring their characters by hybridisation was emphasized. Martins (2011) also noted the explosive differentiation of populations in the early stages of range expansion, with later stabilization, a form of punctuated equilibrium. The role of dispersal needs reassessment (Ketmaier et al. 2006). Where closely-related species co-exist or have widely overlapping ranges, the patterns shown do not in themselves shed light on the speciation processes involved. We have very little evidence as to how such sympatric diversity originates. It seems likely that it usually represents older patterns of differentiation than in the allopatric replacements mentioned above (Cameron et al. 2007). Nekola et al. (2009) report close multiple sympatry in a group of North American Vertigo, among which radiation started in the Miocene. In the case of Partula on Moorea, up to four species, still sufficiently similar for occasional hybridisation to occur, may occur in local sympatry on an island c. 1.5 Ma (Johnson et al. 1993). Other sympatric monophyletic lineages on small islands reinforce the evidence that distances required to speciate may be very small (Pearce 1993). In the context of speciation, populations far apart relative to the dispersal capacity of a species may have virtually no genetic exchange, and can diverge by genetic drift or locally-differing selection regimes (Kisel and Barraclough 2010). Differences in powers of dispersal certainly influence the pattern and process of speciation (Holland and Cowie 2009). Short distance genetic differentiation is well known even in large and relatively mobile species such as Cepaea nemoralis (Linnaeus, 1758) (Thomaz et al. 1996, Cook 1998) or Mandarina Pilsbry, 1894 (Davison and Chiba 2006). We do not know if such sympatric diversity arises from allopatric differentiation, perhaps over very short distances, followed by back migration, or by differentiation in situ. In predominantly selfing species, in situ speciation is certainly possible (e.g., Craze and Lace 2000), and our knowledge of breeding systems is very sparse. How far, how fast, and how often do snails disperse? The capacity for dispersal is a crucial element in many of the questions raised earlier. While snails are proverbially limited in active dispersal, they may be carried passively over great distances. Many very isolated oceanic archipelagos have rich and diverse faunas resulting from multiple colonization events (Vagvolgyi 1975, Cowie and Holland 2006), and within

9 176 AMERICAN MALACOLOGICAL BULLETIN them dispersal from older to younger islands, the progression rule (Whittaker and Fernández-Palacios 2007), is found in some snail faunas (Parent and Crespi 2006). Recent work (Wada et al. 2012) has re-established the case for survival even in the passage through the guts of birds, and Gittenberger (2012) gave references to the evidence on aerial dispersal accumulated since Rees (1965). The islands created by the Krakatau eruption of 1883 had accumulated 19 species by 1985, with an idiosyncratic pattern of occurrence among islands; human transport was thought unlikely for any species (Smith and Djajasasmita 1988). Dispersal tends to be leptokurtic, with rare long-distance movements even in large species contributing to patterns of genetic variation within species (Cook 1998), and to relationships among sister species. Perhaps the most remarkable case is that documented for the clausiliid genus Balea Gray, 1824 (Gittenberger et al. 2006). Such rare events are hard to quantify in terms of frequency, and will often be missed in studies of active dispersal. In practice, we also face the complication of recent passive dispersal by humans (Uit de Weerd et al. 2005), not always documented or clearly identifiable. There seems to be a paradox: we have evidence for a capacity for long-distance dispersal on the one hand, yet evidence of very local differentiation implying genetic isolation and some tiny geographical ranges not evidently constrained by present environment on the other. Studies on the ecology of invasions (Williamson 1996) show that most dispersal events fail to result in establishment. The true extent of dispersal in space and time, and the circumstances in which dispersal is successful, either in altering the genetic composition of single species populations, or in adding species to a local fauna, remain open questions. CONCLUSIONS The questions raised here range from the relatively factual (how many, and which species are present in areas of varying size?) to the very conceptual (is it useful to study specifically molluscan diversity?). The former is deceptive; we need to be sure that comparable entities are being considered when we attempt to explain the patterns detected. The latter, not generally asked in public by malacologists, is best answered by using the huge range of size, shape, and ways of life among land molluscs to approach the basic questions relating to biodiversity from the bottom up while retaining data sets large enough for quantitative and statistically meaningful analyses. To do this, we need far more understanding of the properties of individual species. In particular, we lack evidence on the microhabitat requirements of most species, yet the evidence available suggests that many share the same requirements. We should not make the simplifying assumption that within such a large and diverse group all species are equivalent digits. Given the impossibility of gathering all relevant information for all species, we need reliable proxies for the species properties that affect outcomes. Already, a case has been made for using land mollusc distributions as a proxy for detecting hotspots of diversity (Moritz et al. 2001). Finally, questions relating to distribution, competition, and speciation all depend on our knowledge of dispersal, at present very limited, and presenting severe technical challenges to improvement. ACKNOWLEDGMENTS I thank the American Malacological Society both for the invitation to present this paper and for help with the costs involved. Timothy Pearce gave support and encouragement throughout. Jeff Nekola made very pertinent comments on an earlier version, and provided me with additional information. LITERATURE CITED Ansart, A. and P. Vernon Cold hardiness in molluscs. 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