Species richness in Madeiran land snails, and its causes

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L. M. Cook A second approach concerns the size and shape of shelled molluscs. The relation of shell height to diameter for modern gastropods falls into two groups: high-spired, or globular to disc shaped (Cain, 1977). Divergence into one of two distinct modes suggests an adaptive cause, possibly related to the way species use their habitat. High-spired species tend to be active on vertical surfaces, while low-spired species use horizontal substrates, both in the European (Cain & Cowie, 1978; Cameron, 1978) and Madeiran faunas (Cameron & Cook, 1989). Differences in shape, as well as in size, are associated with different microhabitats, and animals similar in size and shape are more likely to compete. If competitive exclusion limits overlap, there will be a minimum distance apart in the height/diameter space. The mean distance should be greater for species associating together than for arbitrarily pooled groups of such associations. A test of this expectation showed no reduction for pooled subsets of the Madeiran fauna (Cook, 1984) or for regional components of the Turkish molluscan fauna (Cook, 1997). We could look harder for competition in experimental situations, but the results may not mirror field conditions, and there are many theoretical reasons to doubt that competition directly determines presence or absence. SPECIES FORMATION All speciation processes may involve isolation, divergent selection and reinforcement. Sympatric and parapatric speciation both require divergent selection and reinforcement, but are aided by anything that helps to isolate diverging subgroups. If allopatric, isolation could lead to speciation on its own, but there may be divergent selection while groups are apart and reinforcement when they meet. Intergradations are sometimes encountered in the molluscan fauna. On the Madeiran eastern peninsula, Discula polymorpha (Lowe) changes from one shell form to another over a short distance (Cook et al., 1990), while on Porto Santo, similar taxa do not intergrade and are recognized as good species (Fig. 2a). There is sufficient character variation in Heterostoma to have led to three species names (Waldén, 1983), although the morphological characters do not associate consistently (Lace, 1992; Cameron et al., 1996a), and there are slight ecological differences between forms on Porto Santo (Craze & Lace, 2000). Both these examples could indicate parapatric evolution. The majority of Madeiran taxa are well defined, however, and the general relationship of species richness to geographical location points to isolation as an essential feature, with divergent selection in a subordinate role. Taking all lines of evidence together, species proliferation might typically proceed as follows. Porto Santo is the oldest island, available to colonizers for perhaps 10 Myr without major volcanic activity. Colonizers arrived by island-hopping from Europe through the chain now remaining as sea mounts. There may be 300 endemic taxa in total on all the islands, including extinct forms, derived from at least 20 colonizing founders (Cameron & Cook, 1992). If the colonizers arrived 10–5 Ma, then the doubling time for number of species (Turner, 1999) is 1.25–2.5 Myr. If there were more colonizing species, or more time was available, these times are lengthened, and they are shortened if the islands are younger. This is not rapid species formation. Hawaiian Drosophila and Baikal amphipods evolved more slowly; Hawaiian crickets and Malawian cichlids much faster (Turner, 1999). After a molluscan fauna was established, other islands became available. If migration to these was high compared with the speciation rate, little or no divergence would occur; if very low, new territory would remain empty. Between these limits speciation would be likely, especially where the environment differed from that of the donor population. Some of the new species would recolonize the original island, possibly to diverge from their parental populations, while extinctions would simplify the faunas at rates likely to be inversely proportional to area. The topography and habitability of all the islands altered periodically. Sea-level change joined islets and exposed new territory or reduced continuous land to isolates. On Madeira there were volcanic eruptions. Climate change may have resulted in merged or fragmented habitat types. The islands rise steeply from the sea bed and there is evidence of slumping in several places. One such event may have modified the fauna in both eastern Madeira and western Porto Santo (Cameron et al., 2006). Given these structural changes, many isolates must come and go. Migration allowed Porto Santan endemics to reach Madeira, but was rare enough for unique species to develop on small islets. Examples are Actinella laciniosa (Lowe) on Ilhéu Chão, Discula turricula (Lowe) on Ilhéu de Cima, and the sporadic distribution of Discus guerinianus/calathoides (Lowe) (Cameron & Cook, 1999b). Different animal and plant groups in the Madeiras show levels of endemism inversely related to their migration expectations (Cook, 1996). It is the threefold interaction of production of isolates, migration rate and speciation rate that has been the critical determinant of species richness. Zonitid molluscs of the Azores (Frias Martins, 2005) and clausiliids in Crete (Gittenberger, 1991) show similar diversification patterns and, at a general level, the European molluscan fauna tells the same story. The British Isles have no endemics and show almost no geographical variation, while there is high diversity in the much smaller area of the Alps. The British fauna reflects the ability to migrate and to occupy suitable territory, while the Alps are highly dissected with isolated habitable pockets. There, and in Mediterranean refugia, divergence has been promoted by isolation (Bouchet, 2006). DISCUSSION The patterns on the Madeiras fit clearly into one of Solem’s (1984) categories of land snail diversity. Local (sympatric, alpha) diversity is low and species richness in the island group relates to microgeographical (allopatric, beta) diversity in conditions described by Solem as a mosaic. Species are undoubtedly adapted to the environments in which they live. That is most obvious when we compare forest species, 650 Journal of Biogeography 35, 647–653 ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd
which often have reduced or delicate shells, with robust dryfacies species. Original colonizers survive in a given habitat, and after establishment may become better adapted. As a result, the mosaic of habitat types is occupied. Although not ruled out, evidence of faunal assembly with competition is lacking. The idea that species richness could be understood in terms of introductions and losses is at least as old as MacArthur and Wilson’s equilibrium model of insular biogeography. More recently, Hubbell (2001) discussed a model including intraspecific but not interspecific competition and the possibility of species formation. Species arise, enter new territory and become locally or globally extinct, giving rise to equilibrium communities. Empirical data from ecology, biogeography and phylogenetics can be explained using formulations that include a quantity h =2Jm, described by Hubbell (2001) as the fundamental biodiversity number, where m is the speciation rate and J the size of the metacommunity. Extinction rate, obviously an essential feature, is not an independent parameter but varies inversely with population. The metacommunity is the total number of individuals of all species that are connected by migration to the community studied, including, for an island group, continental populations that provide colonists. If colonization consists of only a few non-recurrent events, the metacommunity is the total population of the archipelago. Species richness depends on, and increases more or less linearly with, h. Much variation in species number can be put down to variation in J, while m could remain constant. However, species richness is often enhanced on archipelagos, pointing to an effect of physical conditions. These generate a dynamic balance involving archipelago J, rate of migration and rate of species formation. Hubbell treats the latter in a manner analogous to mutation, with one or a few individuals giving rise to new species. An equilibrium number of species arises because abundant species are more likely than rare ones to give rise to new species, while those that are rare are more likely to become extinct. Abundant species survive for longer than rare ones, so there is a wide range of residence time. If speciation results from partitioning of an extensive range, then species richness and evenness are increased, essentially because extinction rate is reduced. In genetic terms, speciation usually depends on divergence at several to numerous loci, where allelic substitutions accumulate (Nei, 1976). The number of substitutions required and the probability that they result in incompatibility are therefore important variables. Speciation rate differs between groups due to intrinsic genetic differences, but for a homogeneous group it varies inversely with mutation rate if selection is absent (Nei, 1976). Evolution is related to selection, when it is present, and local adaptation and selection for reinforcement can dramatically increase speciation rate (Gavrilets, 2003). The genetic conditions can be accommodated in Hubbell’s system by addressing the environmental factors that influence them. In a defined archipelago, speciation requires a level of migration high enough for new territory to be colonized but not so high as to limit divergence. Speciation is speeded by local adaptation so that changing environmental conditions can be advantageous; it is modified by breeding population size so that there is an optimum area of occupancy for new isolates. To describe general conditions for evolution, we therefore need a further factor that takes into account the geological, geographical and climatic dynamics of the territory occupied. For want of a more comprehensive term, this could be referred to as geodetic, relating to the layout of the Earth. Species richness is proportional to Jmg, g being the rate of geodetic change. Geodetic change has to be concordant with m. If too slow, no opportunities for divergence occur; if too fast, there is ecological disturbance, leading at worst to catastrophe and decrease in J. Similarly, a moderate amount of habitat change promotes divergence, but there are limits to rates of adaptation. For these reasons, g is likely to be about zero for short-term disturbance, rising to above 1 at some intermediate level, followed by a decline towards or below 1 for increasingly long periods of structural and ecological stability. Maximum species richness requires large J and the right mix of m and g. Hubbell’s theory is close to the neutral theory of genetic diversity (Kimura, 1983), with the same theoretical attractiveness and practical problems. Provided selection is left out, genetic diversity is predicted from the quantity 4Nu, analogous to Hubbell’s 2Jm, where N is effective population size (like J, a carefully defined but not intuitively obvious quantity) and u the mutation rate. It also provides time scaling, as the rate of divergence is determined directly by the mutation rate. However, mutation rate is difficult enough to determine, we can almost never get good estimates of effective population size, and selection is not necessarily absent. Studies at any level show the primacy of selection as an explanatory factor in individual genetic studies. Similarly, interspecific competition does occur in communities, speciation is probably rarely a mutation-like process, and the neutral theory of biodiversity becomes less tidy if a quantity like g is included. With both theories, however, the scale at which the problem is viewed may make these types of explanation appropriate. Molecular clock assumptions can provide a valid measure of the rate of genetic divergence despite selection on parts of the system; this is the assumption on which standard time-scale estimation is based. In the same way, a neutral explanation of the ensemble distributions of species abundance can accommodate the manifest evidence of adaptedness and competition. All the phenomena of evolution and ecology are then explained in terms of numbers and the rate of genetic change, which is bound up with geodetic history. ACKNOWLEDGEMENTS Madeiran land snails I thank Robert Cameron for collaboration, expertise and discussion of these problems over many years, and Alison Bates for the illustrations. The paper was developed at the Payamino Field Station, Orellana Province, Ecuador. Journal of Biogeography 35, 647–653 651 ª 2007 The Authors. Journal compilation ª 2007 Blackwell Publishing Ltd
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Species richness in Madeiran land snails, and its causes

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