Never Mind the Gap: Climate, Rather Than Insularity, May Limit Ireland's Species Richness *Simon Harrison
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Never mind the gap: climate, rather than insularity, may limit Ireland's species richness *Simon Harrison School of Biological, Earth and Environmental Sciences, University College Cork The species paucity of Ireland, in comparison to Britain and Europe, is widely held to be due to many species' inability to disperse post-glacially across the Irish Sea. This view is being increasingly challenged. Many winged animals and wind-dispersed organism will not be limited by such a barrier and a growing number of less mobile species are now thought to have survived in a cryptic northern refugium in or near Ireland. Many British and continental species, furthermore, have been introduced into Ireland yet fail to establish. Global and regional species richness for most plant and animal groups has been shown in a large body of research to be driven by climatic environment, rather than dispersalfactors. The prevailing climate of the island of Ireland, with characteristically cool, cloudy and dull summers and warm, wet winters is likely to present a sub optimal climate for the majority of temperate Europe's flora andfauna. Cool summer temperatures can limit species feeding ability, activity and growth; low sunshine hours will restrict activity for ectotherms that need to reach high temperatures for short periods during the day; and warm winters may hinder the ability of species to conserve energy or hibernate in winter. The insularity of Ireland is nevertheless likely to limit species presence in Ireland, but rather by impeding the periodic re-colonisation of species outer range limits following extreme episodic climate events, than by preventing a single post-glacial colonisation event. Keywords: biogeography, climate, dispersal barrier, Ireland Introduction species than mainlands has long exercised ecologists and three explanations are commonly The island of Ireland has fewer resident species put forward: (1) the smaller area of islands, than the larger island of Britain, which in turn relative to mainlands, support lower populations has fewer species than mainland Europe. The of individual species, which are then more explanation of this relative species paucity has vulnerable to stochastic extinction events (2) usually centred on Ireland's insularity, with islands support fewer habitats than larger the Irish Sea being an effective barrier to the mainlands and so have fewer niches for species dispersal of many species to Ireland from Britain to occupy and (3) islands accumulate fewer and continental Europe (Mitchell and Ryan species over time than mainlands due to dispersal 1997). Larger islands, however, may also differ limitations (MacArthur and Wilson 1967, climatically from mainlands, notably in oceanicity Ricklefs and Lovette 1999). - the degree to which an area is influenced by the Although Ireland is smaller than Britain, sea. Ireland's position at the extreme western populations are unlikely to be limited to the fringe of Europe, and the dominating influence point that they would become vulnerable to of the Atlantic Ocean on its climate, gives Ireland stochastic extinctions, unlike the smaller islands a markedly different climate to most other lying off the British and Irish coasts. Ireland does, countries in Europe. Given that the centres of however, possess fewer habitats than Britain. The the geographical ranges of most European species high altitude montane habitat of the Scottish lie within continental Europe, rather than the Cairngorms, for example, does not occur in Atlantic fringe, an unfavourable climate may Ireland, which lacks mountains of sufficient provide an equally if not more plausible reason height. Chalk downlands and sandy lowland for Ireland's species paucity, rather than its heaths (and their associated freshwater habitats, insularity. such as winterbournes and heathland ponds) do not occur in Ireland due to differences in Irish species richness geology and climate. Certain types of woodlands, including those dominated by Common Beech The observation that islands tend to have fewer (Fagus sylvatica Linnaeus) and Small-leaved Lime ( Tilia cordata Miller, 1768), do not occur 'Corresponding author - [email protected] naturally in Ireland, where these two tree species Irish Naturalists'Journal Mind the Gap II 107 This content downloaded from 143.239.131.220 on Tue, 11 Sep 2018 10:30:48 UTC All use subject to https://about.jstor.org/terms Harrison, S. are not native. Even for habitats common to both et al. 2009), Hairy Wood Ant (Formica lugubris regions, however, Ireland is relatively species- Zetterstedt, 1838) (Mäki-Petäys and Breen poor. For example, Ireland has fewer species of 2007), European Badger (Meies meles (Linnaeus, freshwater insects across all the major orders than 1758)) (O'Meara et al. 2012), Pike (Esox lucius Britain (McCarthy 1986, Ferris et al. 2009), Linnaeus, 1758) (Pedreschi et al. 2013) , Irish despite the great abundance of lake and stream Stoat (Mustela erminea hibernica (Thomas and habitats in Ireland. Barrett-Hamilton, 1895)) and Irish Mountain It is possible that Ireland has lost more habitats Hare (Lepus timidus hibernicus Bell, 1837) through a higher anthropogenic pressure than (Sleeman 2014). Britain. Woodlands, for example, have been very Other than dispersal limitation, then, what extensively converted to grassland in Ireland else can account for the relative species poorness since the arrival of Neolithic Man (Mitchell and of Ireland? Observed patterns in global and Ryan 1997, Crawford 2005, Hall 2011), whereas regional species richness have generated much Britain may have retained a greater percentage debate for decades, and given impetus in recent cover (Rackham 2006). The historic low level of years by the need to understand how ecosystems old-growth woodland in Ireland may thus account may respond to anthropogenic climate change, for the low diversity of woodland specialist Wright (1983) proposed that the energy available plants, insects and birds in Ireland (Cudmore to plants and higher trophic groups (a measure 2012). Overall, however, it seems unlikely that a therefore of productivity) in a given area could greater human footprint can explain the general explain much of the difference in species diversity lower species diversity in Ireland. between islands, rather than area or habitat The relative species paucity of Ireland has diversity. Subsequent research has confirmed often in the past been attributed to the barrier that energy and water availability can account of the Irish Sea to post-glacial colonisation from for much of the global and regional diversity southern réfugia {e.g. the 'steeplechase' model of of a wide range of organisms, including plants, Mitchell and Ryan 1997). Successful (non-flying the four main vertebrate classes ana insects or non-wind dispersed) colonists to Ireland would (Currie 1991, Guegan et al. 1998, Lennon et then have required some form of terrestrial/ al. 2000, Hawkins and Porter 2003, Hawkins et freshwater colonisation route from Britain or al. 2003a,b, Rodriguez et al. 2005, Willis et al. Europe to have been present for some time after 2007, Keil, et al. 2008, Field et al. 2009, Qian the retreat of glaciers, or for there to have been 2010, Oberdorff 2011). Many of these studies a glacial refugium in, or near to, Ireland. The also showed that historical and colonisation nature, location and duration of this putative factors only rarely explained species richness, Europe-Ireland colonisation route, particularly indicating strongly that at large scales, climate, for plants, freshwater fish and mammals, has rather than species' dispersal abilities, determined generated much debate over the years (Yalden the number of species at a given location. 1999, Davenport et al. 2008, Searle 2009). Further, there is evidence from the Many groups, however, including winged palaeoecological record that post-glacial climate insects ana birds, plants with small, easily- change is sufficient to explain most continental transported seeds, and smaller invertebrates scale patterns of plant migrations (Pearson and capable of being carried by wind, are unlikely Dawson 2003). Recent analysis of European to be constrained by the barrier of the Irish Sea. mammal distributions has shown that the main There is also growing evidence that many species controls on their meta-community distributions did not in fact need to re-colonise Ireland via a within European regions, including Ireland, land bridge. It is now thought that, in addition to remain predominantly related to natural factors the established southern European réfugia, there such as topography and climate, rather than were several 'cryptic' northern European réfugia dispersal history or anthropogenic disturbance during the Late Glacial Maximum (Provan and (Heikinheimo et al. 2007). These findings Bennett 2008), including a refugium within were corroborated by Rueda et al. (2010), who Britain (Hänfling et al. 2002) and in, or near found that spatially coherent structuring of the to, southern Ireland (Stewart et al. 2010). An ranges of European trees, butterflies, reptiles, increasing number of DNA studies are showing amphibians, birds and mammals are associated that many temperate Irish species are now thought with climate variables, and for faunal groups, the not only to be native, but to have survived in such spatial pattern of tree communities. However, a cryptic 'Biscayan' refugium. These include the they excluded Ireland from their analysis due Natterjack Toad {Epidalaea calamita (Laurenti, to lack