Monday, July 1, 2013

On the diversity of land snails Down Under

By Frank Köhler
Australian Museum, 6 College Street, Sydney NSW 2010, Australia
     Ever since Europeans have set foot here, Australia has been a pretty exciting place for biologists. While the first reports on hopping creatures and duck billed water moles were initially met with suspicion back in Old Europe, it soon became clear that the continent indeed harbours a unique fauna and flora, which resembled little the biotas found elsewhere in the world. And because the place is big, there has been a lot to discover – and trust me, there still is. In fact, it is estimated that Australia is home to about 600,000 native species of plants and animals, or in other terms, perhaps 10 per cent of Earth’s biodiversity. However, you may be surprised to learn that a whopping three quarters of these species are awaiting discovery. On the other hand, Australia has also suffered the largest documented decline in biodiversity of any continent over the last 200 years due to the grave impact of humans on the natural world. As a result, Australia ranks high amongst the global hotspots of endangered and extinct species (Department of Environment, 2009).
Amplirhagada boongareensis

     If we seriously want to stop or at least slow down the current rate of species loss, we need to better understand how Australian biodiversity is made up, how it is distributed throughout the continent, and which are the main factors that cause the decline and ultimately extinction of species. Only then can we develop appropriate conservation strategies that meet the urgency of the case. Frankly, this seems to be an awful lot of work. While it is utterly unrealistic to hope we could gain adequate knowledge of all groups of organisms at once, focussing on a number of model groups might be the one way to go. Such model groups would then serve as surrogates for ‘the other 99% of biodiversity’ as so dubbed by Ponder & Lunney (1999). And that is where land snails come into play. A number of factors render land snails promising model organisms for conservation related research. Firstly, Australian land snails display exceptionally high levels of endemism, which in fact exceed levels found in most other faunal groups. About 98.6% of all native species are endemic to Australia (that is, they occur nowhere else) (Slatyer et al., 2007). Moreover, marked endemism is found on much finer spatial scales, with some species having extremely small ranges of a few square kilometres. This endemism is tightly correlated with low mobility and acute moisture sensitivity of the organisms – a combination that renders snails susceptible to changes in their habitat, be it due to climate change or more direct and localized disturbances from urban development, mining, oil and gas exploration or other forms of land use.
     Exactly for these reasons, land snails are increasingly targeted in biological surveys and used to monitor the effects of landscape degradation, environmental change or to evaluate the conservation significance of certain areas. For example, land snails were the only invertebrate group surveyed in the Kimberley Island Survey, during which between 2008 and the little known biota of islands off the remote north-western Australian coast was surveyed. This project gathered new records of endangered vertebrates, but most significantly, it also led to the discovery of nearly 100 new species and five new genera of land snails (Kimberley Island Survey). This brings us to another favourable aspect of land snails: Their taxonomic and ecological diversity. Briefly, snails are diverse enough to allow meaningful analyses of diversity patterns but are not too diverse, causing ongoing taxonomic nightmares.
     Molluscan studies in Australia look back to nearly 250 years of history. The first species were collected during explorations and voyages along the coastal fringes. Most significantly, the early French voyages to Australia yielded over 50 newly described species (Dance, 1986). These species were often described with dubious or imprecise locality information, which continues to cause taxonomic difficulties and ambiguity until today (e.g.,  Köhler, 2012). From 1850 on, when the continent became better known and more densely settled, a small but proliferous scene of Australian taxonomists begun to flourish. Within just a hundred years, workers such as James Charles Cox, John William Brazier, Ralph Tate, Charles Hedley and Tom Iredale, together named more than 1,300 species. While these early works were entirely shell-based, the American malacologist Alan Solem initiated the modern era of Australian malacology. In his hallmark work, which incorporated comparative anatomy, particularly genital anatomy, Solem revised the taxonomy of most Western and Central Australian taxa and described hundreds of new species (Cameron et al., 2005).
     Despite the on-going descriptions of new taxa, by end of the millennium the number of recognized species had dropped dramatically to 504 because many previously introduced names were relegated to synonymies (Smith, 1992). However, Slatyer et al. (2007) argued that 2000 was a more realistic estimate for the total number of species throughout Australia. This notion has largely been confirmed recently. In the last five years alone, the number of recognized species increased to about 1,200. This increase results primarily from new descriptions of species but also the removal of names from synonymies in a monograph of the eastern Australian fauna (Stanisic et al., 2010). Accordingly, about 750 species are now documented from the mesic eastern fringes of the continent. In addition, a recent series of taxonomic treatments of north-western Australian camaenids has probably doubled the number of known species from this part of the country approximately 350. 
     Eventually, the current decade has also seen the first more comprehensive molecular phylogenetic studies of Australian land snails. Molecular phylogenetic studies, even though still in their infancies, contributed significantly to disentangle systematic relationships within several main groups, aided the recognition of morphologically cryptic species, and provided intriguing insights into patterns of land snail evolution across Australia (e.g., Hugall & Stanisic, 2011; Criscione et al., 2012; Köhler & Johnson, 2012). We learned that the diversity of land snails throughout Australia is governed mainly by availability of water and complexity of habitat. In addition, factors, such as distribution of surface layers of limestone and historical habitat fragmentation due to increased acidification since the Tertiary have played an important role in lineage diversification by creating faunal refuges both on spatial and temporal scales. These factors shaped the distributions of many taxa, which display recurrent patterns of western vs. eastern, peripheral vs. interior and northern vs. southern distributions. The highest diversity of species is found in the eastern Australian mesic zone, which harbours several major land snail radiations, such as the Camaenidae, Charopidae, Helicarionidae and Rhytididae. Additional hotspots of diversity are found in the Western Australian Kimberley as well as the rocky ranges of Central Australia. These regions, however, boast radiations of only one land snail group, the Camaenidae.
     On-going works continue the taxonomic description of the Australian fauna demonstrating that the process of discovery is far from over. Large efforts are being made to close survey gaps (i.e., by conducting helicopter-based surveys in remote regions) but the vastness and remoteness of the continent and the comparatively small workforce of taxonomists set limits to the pace of progress. Our on-going studies are aiming to tie together taxonomic description, phylogenetic inference and collection-based biodiversity analyses in order to gain deeper insights into the evolution of Australian land snails and a more comprehensive understanding of patterns in order to improve current conservation management.

Literature Cited
Australian Government (2013). Conservation of biodiversity. Online resource at (looked up 14 February 2013).

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