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
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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 of data. This recent research emphasises 1768)) (Rowe et al. 2006), Leislers Bat {Nyctalus that differences in animal and plant communities leisleri (Kuhl, 1817)) (Boston 2008), Common across Europe owe more to climatic variables, Frog (Rana temporaria Linnaeus, 1758) (Teacher rather than post-glacial dispersal-mediated
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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 Never mind the gap: limits to Ireland's species richness colonisation patterns, although these may still be temperature gradients, this climatic zone can important for habitat specialists, endemic species, also be divided into the Atlantic Central and or those with poor dispersal ability, including Atlantic North Climate zone, with Scotland, ancient woodland plants (Normand et al. 2011), Wales, Northern Ireland, NW Germany and beetles (Schuldt and Assmann 2009, Baselga et Denmark belonging to the Atlantic North zone al. 2012), amphibians and reptiles (Araujo et al. (Metzger et al. 2005). The climate is characterised 2007) and endemic mammals (Flojgaard et al. by relatively small annual temperature ranges, 2011). mild winters, cool summers and high rainfall Tree species richness within Europe, in throughout the year. At a smaller regional scale, particular, has been the subject on ongoing the Irish climate is similar to that of Britain, debate between dispersal- and climate-mediated but differs in several (biologically) important richness. Currie and Paquin (1987) showed ways (Fig. 1). Mean winter temperatures across that tree richness in Britain and Ireland could Ireland are relatively high, with most of England be accurately predicted from realized annual and Scotland experiencing colder winters. Irish évapotranspiration. Adams and Woodward summer temperatures, in contrast, are similar to (1989) found that differences in species richness those of Northern England, Wales and Scotland, between three northern temperate regions, with most of southern England considerably Europe, eastern North America and eastern Asia, warmer (Fig. 1). could be largely explained in terms of present-day In terms of mean annual temperature, the climatic factors, notably those associated with southern third of Ireland is similar to Wales and thermal energy and water availability. Further, the south of England. In addition to warmer Bennett (1995) found, from palaeoecological summer temperatures, South East England evidence of Holocene distributions, that the receives considerably more daily bright sunshine majority of trees and shrubs in Britain and Ireland hours than Ireland, which is likely to be critical were capable of spreading across channels 10- for ectothermic species that need high threshold 100 km in extent, such that insularity would be temperatures for activities such as movement, unlikely to limit tree distributions in the present feeding and digestion for short periods during or previous interglacials. the day (including plant pollination). Ireland Svenning and Skov (2007), on the other shares a similar annual precipitation with the hand, have argued that tree dispersal rates from West of Britain, with Eastern England being likely glacial réfugia are too slow for climate to markedly drier. The low annual temperature account solely for the current tree distribution range gives Ireland a very low continentality in Europe, but they base this assumption on the index, in common with North Scotland, West maximum northern limit of temperate tree full- Wales and South West England (Fig. 1). In glacial réfugia being not greater than 46° N (mid- summary therefore, relative to the rest of Europe, France). The putative northern cryptic réfugia Ireland typically has warm wet winters and cool discussed above, however, would have allowed wet and cloudy summers, a particular climate deciduous trees to persist in multiple isolated it shares only with localised parts of Wales and locations during the LGM, allowing even slow- South West Scotland. dispersing species to have reached the climatic Whilst Fig. 1 illustrates broad climate limits of their distribution within Europe (Provan differences within and between Britain and and Bennett 2008, Feurdean 2013). Similarly, Ireland, a species' biological climate envelope Mitchell (2006) has argued that many European is much more complex, encompassing climate tree species are absent from Ireland because of extremes both within, and between, years, their failure to migrate over land connections probability of favourable and unfavourable from mainland Europe, (and rather less likely climate within a given year and, more importantly, from Britain), including the Small-leaved Lime daily weather. Hosseil et al. (2003) constructed a and Common Beech. Sykes et al. (1996) and bioclimatic classification for Britain and Ireland, Huntley (2012) have however shown clearly that based on a multivariate pattern of 89 monthly both these tree species reach their northern and climate variables and bioclimate indices, chosen western limits of distribution within Britain due to reflect limits to species distribution. The to the interplay of several climate factors. principal bioclimatic variables that discriminated areas of different climate across Britain and The Irish climate envelope Ireland were related chiefly to rainfall, mean temperature and growing season, wind speed and Ireland lies within the European Atlantic climate the drying power of the air (Fig. 2). In Hossell et zone, which is restricted to north-central France, alls holistic analysis, Ireland is strikingly different Belgium, the Netherlands, Britain, North West to Britain in terms of drying power and sunshine Germany and Denmark. Based largely on (Fig. 2), largely due to the high humidity and
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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. authors also point out that many plant species climate and weather, both in the aquatic larval reach their northern or western limits of European stage and in the adult stage. Active flight in distribution within Britain, such that Ireland is adults is critically important for mating, territory beyond their natural range. This is also true for defence and prey capture and feeding and has many faunal groups. In fact, Ireland as a region been shown to be highly dependent on ambient would appear to be beyond the northern and temperature and weather conditions (Corbet western range limits of the majority of European 1980, Vogt and Heinrich 1983). The number of flora (Webb 1983, Preston and Hill 1997) and sunny days within a season has also been shown fauna, including reptiles and amphibians (Gase to be a critical factor in determining reproductive et al. 1997), breeding birds (Hagemeijer 1997), success in the Azure Damselfly (Coenagrion terrestrial mammals (Heikinheimo et al. 2007), puella Linnaeus, 1758) (Thompson 1990). Adult freshwater fishes (Reyjol et al. 2007), butterflies feeding is unusually important in odonates, (Settele et al. 2008) and dragonflies (Kalkman et unlike many other aquatic insect groups, as it al. 2010). provides crucial protein for egg development Turner et al. (1987) showed that there was a (Harrison and Dobson 2008). Latitudinal (and strong correlation between butterfly diversity thus climatic) effects on Odonata within Britain within Britain and sunshine and temperature include changes in voltinism (the number of during the summer months May to September. breeding cycles per year) (Corbet et al. 2006) and They also showed a negative correlation with morphology (Hassall et al. 2008). average temperature for the three winter As for plants (Grace 1987) and ants (Baroni months December to February. They attributed Urbani and Collingwood 1977), odonates can these relationships to the extreme ectothermic be placed into distinct groups according to their behaviour of adult butterflies, which benefit spatial distribution within Britain and Ireland, greatly from high temperatures and direct The distribution of odonate groups within Britain sunlight in summer to fly effectively. Menendez and Ireland, up to 1990, is shown in Fig. 3. Eight et al. (2007) also showed that butterfly species species show a widespread even distribution richness within Britain was closely associated across Britain and Ireland, although Coenagrion with growing degree-days > 5 °C (which puella does not occur in northern Scotland and relates to developmental thresholds for larvae), Black Darter fiSywrperrww r&«
112 Irish Naturalists' Journal Mind the Gap II
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FigureFigure 3a. 3a. (above) (above) Maps Mapsof odoriates of odonates in Britain in (a) Britain andand Ireland Ireland with with widespread widespread distribution distribution withinwithin Britain Britain and Ireland and Ireland(records pre-1950to(records pre-1950to 1990):1990): (a) (a)Pyrrhosoma Pyrrhosoma nymphula, nymphula, (b) Enallagma (b) Enallagma cyathigerum,cyathigerum, (c) Lestes (c) Lestessponsa, (d)sponsa, Coenagrion (d) Coenagrion puella,puella, (e) (e)Ischnura Ischnura elegans, elegans, (f) Libellula (f) Libellula quadrimaculata,quadrimaculata, (g) Sympetrum (g) Sympetrum danae, (h) danae, (h) AeschnaAeschna juncea. juncea. Adapted Adapted from: Merrittfrom: etMerritt al. : et al. (1996).(1996).
FigureFigure 3b. 3b. (right) (right) Maps ofMaps odonates of inodonates Britain in Britain andand Ireland Ireland with with a predominantly a predominantly southern southern distributiondistribution within within Britain Britain and Ireland and Ireland . (records pre-1950 pre-1950 to 1990): to 1990): (a) Cordulia (a) Cordulia aenea, aenea, (b) Libelluladepressus,Libelluladepressus, (c) Platycnemispennipes, (c) Platycnemispennipes, (d) AnaxAnax imperator, imperator, (e) Gomphus (e) Gomphus vulgatissimus, vulgatissimus, (f)(f) Aeshna Aeshna cyanea, cyanea, (g) Calopteryx (g) splendens,Calopteryx splendens, 3 (h) CoenagrionCoenagrion pulchellum, pulchellum, (i) (i)Brachytron Brachytron if pratense.proteose. Adapted Adapted from: from: Merritt Merritt et al.et (1996).al. (1996).
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(a) 4 (b) •f (c) vM. i 4®|j. i '-If V '.?A j.: ' a ■ '<-• ■. tte tl v* h - .p. v «• . «■ r .jfcj (d) (e) 4 (f) -r"2 *Y—} M x? ■f'M <®r ! v /it >t«T- t5 \ 'Sk t-4\ x'v, •. ■ j. • • , > > .7 2: % ii . '^dh 7 ^pffTi&frf!? .' 4 "V- *1P-tfc*■*>*- T * jMOWi „vv; Figure 3d. Maps of odonates in Britain and Ireland with a predominantly western/south-western distribution within Britain and Ireland (records pre-1950 to 1990): (a) Cordulagaster boltonii, (b) Coenagrion mercuriaie,mercuriale, (c) Ceriagrion tenellum, (d) Ortethrum coerutescens,coerulescens, (e)(e) CalopteryxCaiopteryx virgo,virgo, (f)(f) IschnuraIschnura pumillio.pumillio. AdaptedAdapted from:from: MerrittMerritt etet al.al. (1996).(1996). 114 Irish Naturalists'Journal Mind theGap II 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 Never mind the gap: limits to Ireland's species richness (a) (b) (c) & $ 4? ?J ( &£? wi fiMj -1*V ' 11*•.? - LT)T( i-H J/ ' J.J 0 V; ffr Pi? tl 4 -4^ V i if 9" "-1—^ j y'~ j—A 9-y;VT ~\ || V--l~4* ,9-'V" , v , r. 5T i" ~P blT t-»:-■' ' Uifcl ^ 7t f B 4vU : rfc^| j4--" (d) (e) s? jyy/7 £»V II-if, . Jr&£? f'M T"? jtfjf O ✓&jr I / ! ✓cgf 1 svxH i i Ai^-% < * ^ Tr1 -i ;. * • • ; x? i Figure 3e. Maps of odonates in Britain and Ireland $T with a predominantly Scottish or Irish distribution c_ within Britain and Ireland (records pre-1950 to CrJ 1990): (a) Aeshna coerulea, (b) Somatochlora arctica, Jr (c) Leucorhinia dubia, (d) Coenagrion lunulatum, ' (e) Coenagrion hastulatum. Adapted from: Merritt et al. (1996). Southern Hawker (Aeshna cyanea (Müller, that are clearly more related to climatic factors 1764)). The fourth group consists of species with than dispersal limitation. They also demonstrate a predominantly south-western distribution. the complex suite of climatic factors that Three of these are either very rare or absent from may limit distributions. Odonates with a Ireland: Golden-ringed Dragonfly (Cordulegaster predominantly south-eastern distribution may boltonii (Donovan, 1807), Southern Damselfly plausibly be limited by a lack of sunshine in (Coenagrion mercuriale (Charpentier, 1840) other parts of Britain and Ireland, and species and Small Red Damselfly (Ceriagrion tenellum with a predominantly southern distribution (De Villers, 1789)). Of the three common to limited by temperature elsewhere, but these Britain and Ireland: Keeled Skimmer (Orthetrum simple parameters are unlikely to explain why coerulescens (Fabricius, 1798)), Beautiful species confined to south-east England within Demoiselle (Calopteryx virgo (Linnaeus, 1758) Britain, such as Aeshna grandis and Sympetrum and Scarce Blue-tailed Damselfly (Ischnura sanguineum, are also reasonably common in pumilio Charpentier, 1825), C. virgo is more Ireland, or why species common in England and common in the south of Ireland. The fifth Wales, such as Libellula depressa and Aeshna cyanea group consists of five species, three of which are are absent from Ireland. Perhaps the most curious confined to Scotland: Azure Hawker (Aeshna distribution is that of Cordulagaster boltonii, caerulea (Ström, 1783)), Northern Emerald which is common in Scotland, Wales and the (.Somatochlora arctica (Zetterstedt, 1840)) and southwest of England, yet is virtually absent from Northern Damselfly (Coenagrion hastulatum central and eastern England and Ireland. Charpentier, 1825), one which is most common The complexity of climate responses within in Scotland, but also found in a few locations in the odonates is perhaps unsurprising considering England: White-faced Darter (Leucorrhinia dubia their life histories. Aquatic larvae are long zander Linden, 1825); and one found only in lived, spending between 1 and 4 years in water the north of Ireland: Irish Damselfly (Coenagrion before emerging as adults. Annual cumulative lunulatum (Charpentier, 1840)). temperatures would then be a major determinant Together, the odonates demonstrate of larval growth rates, and hence relative distributional limits within Britain and Ireland competitive abilities of species within similar Irish Naturalists'Journal Mind the Gap II 115 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. aquatic habitats. Variables such as mean daily Ectothermic vertebrate distribution sunlight hours, wind, rainfall parameters {e.g. intensity and duration), humidity, maximum The very low number of native Irish reptiles, temperatures and number of hours above amphibians and freshwater fish, compared threshold temperature values, are more likely to to those of Britain and particularly mainland govern adult behaviours and competitiveness. Europe, is one of the more striking features of The ability of a given species to be able to fly Ireland. Post-glacial dispersal limitation onto earlier in the morning, or longer during the day, Ireland is usually held to be the cause of this low than a sympatric competitor (and larval predator) species richness. It is often stated, for example, may well determine whether a species is present that the only freshwater fish native to Ireland or absent from a climatic region. are euryhaline species that were able to migrate Grace (1987) argued that the distributional to Ireland's rivers and lakes across the Irish sea, limits of flowering plants within Britain and such as salmonids, stickleback, shad and Pollan Ireland were also largely determined by a complex {Coregonus autumnalis (Pallas, 1776)) {e.g. suite of climatic variables, including heat, cold Fitzmaurice 1984, Griffiths 1997). Historical and drought and particularly sporadic climatic sources, including the accounts of writers such as extremes such as those that occur once or twice a Giraldus Cambrensis also make no mention of century. WallisDeVries et al. (2011) also showed many freshwater fish species now found in Ireland more recently that weather, and in particular (such as Pike {Esox lucius (Linnaeus, 1758)) which extreme weather, was a more realistic driver of are therefore held to have been introduced into regional butterfly population trends than simple Ireland in recent centuries (Fitzmaurice 1984). climate envelope models. More recently, Huntley There are several flaws in this view of Irish (2012) has argued strongly that climate variables post-glacial insularity being solely responsible taken independently are unlikely to explain for the low number of ectothermic vertebrate current or past plant and animal distributions species. Firstly, many species {e.g. Natterjack and in fact often interact together in complex Toad {Epidalaea calamita (Laurenti, 1768)), ways. Common Frog {Rana temporaria Linnaeus, Grace (1987) also highlighted the interplay 1758) and Pike) have now heen shown to be between climatic limitations to a species growth native to Ireland (see above). They either survived and its competitive pressure, such that a species in cryptic northern glacial réfugia in or near to may be uncompetitive within a certain climate the current south west of Ireland, or were able to envelope even though it is able to tolerate the colonise new regions from southern réfugia, very climatic conditions within the envelope. A soon after the retreat of glaciers. The colonisation distinction needs to be made therefore between of Ireland by these species, and presumably tolerating a certain climatic environment and other ectothermic vertebrates, were thus clearly being competitive within that environment. not limited by significant post-glacial dispersal Recent research has shown that the competitive barriers. Secondly, there are many anecdotal balance between two dragonflies - one a southern accounts of freshwater fish, amphibians and species expanding its range: Scarlet Dragonfly reptiles (including the Grass Snake {Natrix natrix {Crocotbemis erythraea (Brullé, 1832)); and (Linnaeus, 1758)) and the Common Toad {Bufo the other a native northern European species: bufo (Linnaeus, 1758))) being introduced by man Leucorrhinia dubia, shifts towards the southern into Ireland in the past, together with the more species at higher temperatures (Suhling and documented introductions of species such as the Suhling 2013). The distribution of birds within Roach {Rutilus rutilus (Linnaeus, 1758)) and Britain has also been argued to be only weakly the Dace {Leuciscus leuciscus (Linnaeus, 1758)) related to climatic factors, and that biotic factors (Fitzmaurice 1984) the Slow Worm {Anguis may overwhelm the direct impacts of climate on fragilis Linnaeus, 1758) (McCarthy 1976) and a a species (Beale et al. 2008). In fact, the role of whole suite of European reptiles and amphibians environment versus competition in determining onto the island of Lambay, off the coast of Dublin a species range is also well known from classical (Baring 1915). ecological literature. Connell (1961) showed An account in the Irish Literary Gazette in that the realized niche of Poli's Stellate Barnacle 1857 states that "Reptiles are few in Ireland: the {Chthamalus stellatus (Poli, 1791)) was the result common toad has been more than once introduced of both its fundamental (environmentally- with a view to employ it in gardens as a devourer determined) niche and competition from another of the smaller slugs, but it has not prospered barnacle Semibalanus balanoides (Linnaeus, here". Similarly, from the Dublin Penny Journal 1767). in 1850: "About 1797, a gentleman is said to have imported from England into Wexford a number of vipers; but mark the result of his patriotic 116 Irish Naturalists'JoumalMndfbeGap// 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 Never mind the gap: limits to Ireland's species richness labours: - no sooner were they loose among our chiefly members of the family Cyprinidae, are shamrocks and shining daisies than they were at the extreme western limit of their European as dead as a herring before you could have said distributions, and have optimum feeding and Jack Robinson". In the same article, the author growth water temperatures above 20-25 °C - {anon) states that: "In the summer of 1831, a conditions which are rarely achieved in Ireland, gentleman by way of experiment, to ascertain Cool summer temperatures in Ireland, coupled whether snakes would survive in Ireland, brought with the low daily sunlight hours are then likely from Scotland a few pairs of what are usually to act as a major climate filter, called the Common Snake {Coluber natrix). Average summer temperatures may not be as These he put into a plantation at Milecross, important to ectotherms as the amount of daily near Newtownards, where they soon from their sunshine a region, or microhabitat, receives, number, gave evidence of becoming as fruitful as Many reptiles seek out hotspots to bask for a few if they had been placed in South Carolina". hours a day, allowing relatively short periods of Although anecdotal, these reports suggest high temperature in which to feed and digest that not only were several reptile and amphibian (Skoczylas 1970, Forsman 1993, Isaac and species introduced into Ireland from Britain Gregory 2004). A climate with short periods of in the past, they failed to establish viable direct sunlight on a regular basis over the summer populations, in the short- or long-term. Their (even in latitudes with low mean summer absence from Ireland may therefore be due to temperatures) may then still support many reptile population establishment failure, rather than species. Conversely, the cloudiness of the Irish a lack of colonisation. The Common Carp summer and lack of direct sunlight, despite the {Cyprinus carpio Linnaeus, 1758), for example, relative warmth of summer in the south of the is known to have been introduced to Ireland island, may therefore act as a climate barrier to on multiple occasions since medieval times, but viable reptile populations, has only established breeding populations in The body temperatures of ectotherms a very small number of shallow lakes in recent track environmental temperatures, which in years (Ross Macklin, pers. comm.). Thirdly, turn control feeding, digestion, growth and as for flowering plants and odonates discussed reproduction. All ectotherms have minimum above, the British distributions of many species and maximum lethal temperatures, optimum of ectothermic vertebrate absent from Ireland temperatures and critical lower temperature shows distinct range limitations to the south and thresholds below which feeding, digestion and east of England, which cannot be due to dispersal growth slows greatly (Fitzpatrick 1976, Naulleau barriers. Common Barbel {Barbus barbus 1983, Hardewig and van Dijk 2003). Above this (Linnaeus, 1758)), Bleak {Alburnus alburnus temperature, the nearer their body temperature (Linnaeus, 1758)), Bitterling {Blicca bjoerkna), is to their optimum temperature, the more the Ruffe ( Gymnocephalus cernuus (Linnaeus, 1758)), animal can feed, accumulate energy and grow. The Roach {Rutilus rutilus (Linnaeus, 1758)) and metabolic rate (energy expenditure) of ectotherms Bullhead {Cottus go bio Linnaeus, 1758), for also tracks external temperature, however, largely example, all have restricted ranges historically irrespective of feeding (energy intake). Net energy within Britain, being largely confined to south gain is the difference between energy intake and and east of England and all except bullhead, expenditure. Below the threshold for feeding and absent from Wales and South West England digestion (i.e. no energy gain) and above the lethal (Wheeler, 1977). The distributions of Bream minimum, energy expenditure is directly related {Abramis brama Linnaeus, 1758) and Rudd to temperature (Costanzo 1989, Hölker 2003). {Scardinius erythrophthalmus (Linnaeus, 1758)) Seasonal temperature regimes that minimise within Ireland, which are now widespread, were energy loss in winter {i.e. low temperatures above until recently restricted to the central north of the the lethal minimum) and maximise energy gain country (Kennedy and Fitzmaurice 1968, 1974). in summer (high temperatures below lethal As for plants and many insects, ectothermic maximum) for a given species will lead to high vertebrates are also strongly limited by multiple growth and fecundity. Cold winters (low energy climate variables. Reptiles can be described loss) and hot summers (high energy gain), typical as extreme solar ectotherms and global and of continental climates, are then more suitable for European species richness is strongly related most ectothermic vertebrates than warm winters to temperature and sunshine (Hawkins et al. (high energy loss) and cool summers (low energy 2003a, Rodriguez et al. 2005). Amphibian and gain), typical of the Atlantic fringe climate, fish species richness is also temperature-related Recent research has demonstrated that warm but not to the same extent as reptiles (Hawkins winters are deleterious for many species of et al. 2003a, Rodriguez et al. 2005). Most of northern latitude amphibians, reptiles and fish, the fish species introduced into Ireland recently, In a long-term field study of Common toads in Irish Naturalists'Journal Mind the Cap II 117 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. Britain, Reading et al. (2007) revealed a clear with very restricted Atlantic coastal distributions relationship between poor body condition of within Europe which are clearly well adapted to female toads and the occurrence of warmer than the prevailing climatic conditions (sometimes average years. In those years, toads were unable to referred to as 'Lusitanian' species). There has hibernate fully and so expended more metabolic been intense debate over the years about the Irish energy during the non-feeding period. Warm and European distribution and origin of these winters were also associated with high hibernation Atlantic fringe species, with most authors relating mortality of Natterjack Toads in South East their current distribution to their ice age survival England (Beebee 2011). Similarly, Brodersen et in putative cryptic northern glacial réfugia (e.g. al. (2011) showed that warm winter temperatures Webb 1983). Few authors have attempted to in food-poor lake systems can bring about rapid explain their distribution in terms of climatic, energy depletion of overwintering Roach. Low rather than historical or dispersal factors. As inter-annual survival of Great Crested newts argued above, if climatic factors, rather than (Triturus cristatus (Laurenti, 1768)) was also dispersal limitations, are largely responsible for found to be related to mild, wet winters on a the absence of many species from Ireland, then regional scale within Britain, with rainfall having it would seem reasonable to assume that climatic a greater effect than temperature (Griffiths et cd. factors are equally responsible for the current 2010). High winter rainfall had a greater effect restricted distribution of the Atlantic fringe than mild winter temperatures, attributed to species, rather than post-glacial dispersal factors, a reduced cuticular respiration by hibernating The same argument can be made for species with newts in waterlogged soils. In a striking parallel, genetically different populations in the Atlantic it has been suggested that the warm moist fringe compared to more southerly and easterly conditions of the oceanic winters of the North populations (e.g. Searle 2008, Searle et al. 2013). Atlantic may also be detrimental for woody The explanation usually put forward to explain plants, due to poor root health in waterlogged these differences centres around allopatric genetic anoxic soils and reduced frost hardiness and divergence of populations separated in different depletion of over-wintering carbohydrate reserves réfugia during the Late Glacial Maximum (e.g. (Crawford 2005). The sub-optimum climatic Martinkova et al. 2007, O'Meara et al. 2012, environment of the west of Ireland for trees and Herman and Searle 2013). Equally plausible, shrubs may have facilitated the conversion from however, is that genetic differences between woodland to grassland during the Neolithic, and Irish, British and continental populations arose then to blanket bog, once grasslands had been through evolved adaptations to different climate abandoned (Crawford 2005). regimes. Such adaptations may have occurred It can be seen that the Irish climate is clearly post-glacially, or over longer time periods if sub-optimal for most ectothermic vertebrates, Atlantic climate-adapted populations survived providing a plausible reason why there are so few the LGM in climatically suitable réfugia (perhaps species and why many that are introduced fail. An the Atlantic coastline of southern Europe?), exception can be seen for several freshwater fish and subsequently re-colonised the emerging species with a largely continental distribution, post-glacial Atlantic fringe climate zone. Such a such as the non-native cyprinids Roach, Dace model of current and past climatic adaptation has and Tench (Tinea tinea (Linnaeus, 1758)). been proposed to explain contemporary genetic These are known to have been introduced into differences in continental European weasels and Ireland in the 19th century and have in several the importance of climatically appropriate LGM cases greatly expanded their ranges. Widespread réfugia (McDevitt et al. 2012). eutrophication in recent decades (McGarrigle An example of the emphasis placed on et al. 2010) has led to greatly increased food dispersal and colonisation, rather than climate, supply for these fish in lakes and rivers, giving is seen for the stoat in Britain and Ireland. The critically higher energy gain in summer for these Irish Stoat very rarely develops a white winter warm water fishes that do not feed in autumn pelage, even in severe winters (Moffat 1937). and winter, and may have facilitated their Stoats in Britain show a distinct geographic establishment in Ireland. pattern of winter pelage, with most stoats in the North East of Scotland turning white in winter, Species adaptations to the Irish climate pied in South West Scotland and rarely if ever turning white in England (Hewson and Watson Although the European Atlantic fringe climate, 1979, King 1979). This pattern was associated characterised by cloudy, cool and humid with regional differences in the number of days of summers and warm humid winters, is likely snowfall and snow-lie, and in monthly minimum to be sub-optimal for many European species, temperature, but not with mean temperature or there is a small number of plants and animals exposure (Hewson and Watson 1979), indicating 118 Irish Naturalists'JournalM/ndfheGap// 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 Never mind the gap: limits to Ireland's species richness that winter pelage is an evolved adaptation to temperature and moisture) over timescales of winter climate. Observed genetic differences hundreds to thousands of years since the end of found between Irish, British and continental the Ice Age, many of which are associated with stoats have, however, been ascribed to spatial the historical collapse of local human populations replacement of one type by another, with the or cultures (McDermott et al. 2001, Turney et Irish stoats proposed to be a cold-adapted form al. 2006). These periodic climate perturbations that colonised Britain and Ireland immediately can result in the local extirpation of many post-glacially, before being replaced by warm- species at the northern limits of their ranges. For adapted forms in Britain, but not Ireland, owing example, Turney et al. (2006) found that oak tree to the lack of a land bridge (Martinkova et al. populations were reduced during periods of low 2007). It would surely be more logical to assume temperatures and high precipitation since the end that the Irish stoats are in fact adapted to warmer of the Ice Age. Locally or regionally extirpated winters with low snow cover and that genetic species can then re-establish populations in differences reflect current and past climatic their range extremes from immigrating colonists differences between Britain and Ireland, rather from the range centres. Geographic barriers to than stochastic historical dispersal patterns. these periodic re-colonisation events, including Beatty and Provan (2012) have shown that water bodies such as the Irish Sea, will therefore there was likely a Biscayan refugium during the tend to restrict the occupation of regions at the LGM, from where Atlantic fringe species such as species' range limits. We should think of the Irish St Dabeoc's Heath (Daboecia cantabrica (Huds.) Sea therefore as a filter slowing the periodic re K. Koch) colonised Ireland and northern Spain colonisation of Ireland by species at the far north post-glacially. The question can therefore be and west of their potential distributions, rather asked - why has this species (and other Atlantic than a continuing barrier to a species post-glacial fringe species) not attained a wider distribution reoccupation of pre-Ice Age distribution, in Europe since the retreat of the glaciers and amelioration of climate? Many species currently Conclusions widespread in Ireland are thought to have colonised from southern réfugia at much greater Global and regional species richness for most distances from Ireland and Britain than the plant and animal groups has been shown to be putative Biscayan refuge, so there is little logic in driven largely by climatic environment, rather relating current limited distributions of Atlantic than dispersal factors. The range limits of many fringe species to the geographic location of species of plant and animal which do not occur their ice age réfugia. It would be more logical to in Ireland occur within Britain, with many assume that the European Atlantic fringe species species confined to the south or southeast of are adapted in some way to the extreme western Britain, indicating that climatic factors, rather Atlantic climate and are unable to tolerate climatic than dispersal limitation exclude them from conditions, or are uncompetitive, outside of this Ireland. The prevailing climate of the island of region, thus restricting their European range. Ireland, with characteristically cool, cloudy and Interestingly, there are several moth species humid summers and warm, wet winters, is likely in western Ireland with colour forms and to limit the range of many European plant and morphologies which differ from European animal species. Organisms may be limited by continental populations, including the Silver- low summer temperatures (poor growth, flight, ground Carpet (Xantborhoe montanata Denis feeding, activity), low sunshine hours (inability and Schiffermüller, 1775) and the Yellow Shell to reach optimum temperatures for activity) or (Camptogramma bilineata Linnaeus, 1758) (Ken warm winters (high energy expenditure during Bona, pers. comm.). Both these species also non-feeding periods or inability to hibernate), have morphologically distinct sub-species in Only very few species are confined to the the Shetland Islands, suggesting that they have European Atlantic fringe, indicating that this adapted to the North Atlantic fringe climatic climatic zone has had a limited evolutionary zones since the LGM. What these adaptations are history within Europe. The insularity of Ireland (and for other Atlantic fringe species, sub-species is nevertheless likely to limit species' presence and ecotypes) and to what climate variable(s) in Ireland, but rather by impeding the periodic they are an adaptation, is a fruitful potential area re-colonisation of species' outer range limits of future research. following extreme episodic climate events, than by preventing a single post-glacial colonisation Ireland as an island event. Northern latitudes in Europe have experienced oscillating climatic conditions (both of Irish Naturalists'Journal Mind f/re Gap // 119 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. Acknowledgments ONE 6(10): e24022. Connell , J. 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