The biogeography of British Isles invertebrates as a subset of the European or Palaearctic faunas has had relatively little attention, mainly because of lack of knowledge of wider distribution patterns within many of the less well-known groups. The British flora has been better served in this respect, and distribution patterns can be set in a global context using a simple but effective binary range classification pioneered by Preston & Hill (1997). There are potentially useful applications for this work to the invertebrate fauna, and this is explored for the Carabidae in the following section.
The Preston & Hill (1997) system as applied to the Irish ground beetle fauna provides a more quantifiable expression of origins and affinities. For a given species, the numeral for major biome is combined with the numeral reflecting easterly limits to give a combined numeric (and verbal) definition. For example, Leistus montanus merits the numeric descriptors 43, derived from the major biome which is Boreal-montane (4), and the easterly limit which is European (3). The categories used for individual species definitions were arrived at from distributional information given by Lindroth (1985, 1986,1992), Marrgi (1992), Herrera & Arricibita (1990), and Turin (1981).
Clearly the categories assigned depend to a large extent upon the accuracy of distributional information, so some latitude in interpretation must be accepted. Some of the Preston & Hill categories are scarcely used in the following analysis. In ground beetle terms, Mediterranean-montane (93) seems to be an irrelevant category since distributional information is insufficiently precise at present to determine whether southern elements of the Britannic fauna are largely montane in southern Europe or not. Submediterranean-Subatlantic (92) is also difficult to visualise for many species since it requires an ability to distinguish between species with a strong western bias in the Temperate Biome (Mediterranean-Atlantic, 91), and those with only moderate continentality in the Temperate Biome (Submediterranean-Subatlantic, 92). There have also been difficulties in separating Temperate from Southern-temperate distributions in Europe, since many species which are properly Temperate may exist in montane or submontane habitats in the Southern Biome, although such information is rarely if ever available to distinguish from true Southern-temperate species. I have distinguished Wide-temperate species from the two latter categories on the northern limits of their range which are better known i.e. Temperate and Southern-temperate species only get into the extreme south of Fennoscandia, if at all, whereas Wide-temperate species may be widespread in southern to central Fennoscandia.
Ireland north of a line from Sligo Bay in the west to Carlingford Lough in the east has been chosen as a rational area to represent northern parts of Ireland, and the area south of it, to represent southern Ireland. This principle has been adopted in Appendix I in which the species are listed with their codes. In practice, there is only one species, Asaphidion pallipes, included within this wider definition of northern Ireland, which is not included within the narrower political entity of Northern Ireland.
Species which have a predominantly southern range in Europe are classified as Southern-temperate (8) or Mediterranean-Atlantic (9). These comprise nearly 18% of the southern Irish fauna, and 16% of the northern Irish fauna. Of the 40 species found in the south but not the north of Ireland, 10 belong to this southern group, whereas of the 10 species found in the north and not the south, most have a northern range in Europe. Despite its small size (less than 450km north to south), Ireland therefore appears to show some evidence of climatic zonation. However, great caution is needed in interpreting these results as other differences between north, and south such as the relative distribution of soil types and drainage, outcropping of basic strata, and occurrence of high mountains, need to be taken into account.
In the north of Ireland, there are 5 species of the Mediterranean-Atlantic (91) category. In the south there are 8 species of this category present. The three species not found in the north are Trechus subnotatus, Pogonus littoralis and Bembidion normannum. These can all be found on the southern half of the east coast around Dublin. Of these three species, only Pogonus littoralis, ranges much further north. The east coast around Dublin has, with the extreme east of Down, the lowest rainfall (600-800mm per annum) in the island (Jordan, 1997), but insolation is higher in Dublin than in Down. The highest insolation in Ireland (>1600hr per annum bright sunshine) is found in the extreme south-east, in Wexford (Collins & Cummins, 1996), but this area is wetter than Dublin (800-1000mm). The area with the highest continentality in Ireland, as measured by low precipitation and sunshine hours, is therefore Dublin, which may explain the presence of essentially southern species.
The Southern-temperate group (8) numbers 29 species in southern Ireland of which 7 have not been recorded in the north. The seven are, Nebria complanata, Perileptus areolatus, Bembidion lunulatum, Bembidion varium, Platyderus ruficollis, Harpalus rufipalpis and Syntomus foveatus. Within this sub-group there is one riparian south-western species, Platyderus ruficollis, with the remaining six species being coastal and south-eastern. The main distinguishing characteristic of the south-eastern element is that members exhibit a close association with dry, warm coastal sands i.e. have greater thermoxerophilicity. The south-western element represented by Platyderus is characterised by hygrophilicity plus a degree of thermophilicity. Climate in south-western Ireland is defined by high relative humidity (75-80%) over the whole year, combined with much lower risk of air frost in winter than other regions of Ireland, and with relatively high summer temperatures (Collins & Cummins, 1996). Hygrophilous species, which in Britain are predominantly southern, seem able to survive under these circumstances in what is a much more oceanic regime than applies in most of the rest of their European range. Apart from Platyderus, five of the seven Badister species are confined to the south-west, as are Elaphrus uliginosus, Bembidion fumigatum, Agonum livens, A. lugens, Acupalpus consputus and Stenolophus mixtus. There is clearly a mixture of elements in this group, but all species are hygrophilous. Within this restriction, these species have a relatively southern distribution in Europe. Perhaps indicating some need for greater warmth, either in the way the local climate operates, or in the physical characteristics of the substratum. The Burren turlough fauna comprising Bembidion fumigatum, Agonum livens, A. lugens, Acupalpus consputus and Stenolophus mixtus, with several of the Badister spp. including B. meridionalis, appears to be very dependent upon a local micro-climate generated by large areas of karstic limestone on the sheltered landward side of the Burren massif. The effect of relatively high levels of evapotranspiration, and of the high heat storage capacity of the limestone, upon flora, has been well demonstrated (D'Arcy & Hayward, 1992), and this no doubt contributes to the maintenance of a distinctive fauna also.
From the arithmetic mean of the binary range of each species, an 'average' member of the British and Irish carabid faunas would have a range corresponding to European or Eurosiberian Wide-temperate (63 or 64). However, this only emphasises gross biogeographical similarities. Histograms of the distribution of ranges, both in terms of major biome (Fig. 2.1), and easterly range (Fig. 2.2) illustrate more clearly the contrasting profiles of the two faunas. In terms of total species, the Irish fauna represents 61% (211 species compared to 347; Appendix II) of the British fauna. Figure 2.1 gives the percentage of Irish species compared to British in each biome category so that this can be compared to expected proportion of 61%. Clearly there are major differences in emphasis apart from the expected differences in size due to the smaller land area of Ireland compared to Britain.
Major deficiencies in the Irish fauna when compared with the British are concentrated in the Arctic-montane and Southern-temperate (or Mediterranean-Atlantic) species groups. The Britannic Arctic-montane fauna is a relic of the early stages of Postglacial history. It has survived better in the mountains of Scotland than in Ireland which has no peaks over 1000m, and in which conditions are strongly oceanic. This has probably disadvantaged species such as Amara alpina, which, as a representative of the Scandinavian fjeld fauna, is adapted to more continental (drier) conditions. Nevertheless, a surprising number of Boreal and Arctic-montane species have survived in Ireland. Compared with Britain, Ireland lacks a relatively high proportion (60% or three species) of the Arctic-montane group (Nebria nivalis, Elaphrus lapponicus, Amara alpina). However, only one each of the British Boreo-arctic montane (Bembidion virens) and Wide-boreal (Bembidion nigricorne) species are lacking, and four (or 33%) of the Boreal-montane group (Bembidion prasinum, B. schueppelii, B. stomoides, Amara quenseli). It is not unlikely that several of these missing species will ultimately be found in Ireland, as Irish montane habitats have not been well researched. Donisthorpe (1903), for example, recorded Bembidion nigricorne from Kerry but this species had to be excluded from the Irish List as corroboration of the record was lacking (Speight et al., 1983). Being a rare species generally, it could still occur undetected in the mountains of western Ireland. The potential for new discoveries is further indicated by several significant additions to the montane fauna during recording for this atlas, including Bembidion geniculatum and Miscodera arctica found in several places in the relatively well known mountains of Down and Antrim.
The weakest categories in the Irish fauna are predictably the Southern-temperate and Mediterranean-Atlantic groups. No less than 56 British species in the former category and 6 in the latter have no Irish records. The genera Ophonus and Harpalus are very poorly represented in Ireland, with only 2 out of 10 British Ophonus species and 8 out of the 20 British Harpalus species present. This is predictable as both genera are characteristic of dry grassland in Europe, being particularly attached to chalk downland in Britain (Lindroth, 1974) which provides a warm substratum in a geographical area marginal to their continental range. Irish grasslands are almost entirely, if not entirely, anthropochorous in origin, and there is no equivalent to the English chalk downs. Even the extensive outcropping limestone of the Burren in western Ireland was originally covered with hazel scrub, and in any event lies too far west for a continental fauna. Some Harpalus/Ophonus survive in Ireland in sandy pockets along the coast, but the downland species are absent as are many of the British coastal species e.g. Harpalus neglectus, H. servus, H. serripes.
Clearly, while strong oceanicity and cloudy coolness of the Irish climate is a disincentive to thermoxerophilous species, it favours desiccation-prone animals adapted to survive in cool, wet forest litter environments. Irish slug species total 31 which is all but 2, or 94%, of the total British Isles fauna (including one undescribed arionid) and the same total (31) as the fauna of the island of Britain. Of these, at least 11 are introductions, several of very recent date. An initially impoverished fauna has therefore been augmented by immigration. According to Barber & Jones (1996), the Irish Diplopoda comprise 38 species, or 76%, of the British fauna of 50, although the Irish total has since been augmented by the discovery of a southern European species, Anamastigona pulchellum Silvestri, near Belfast (Anderson, 1996), the discovery of the native supralittoral Thalassisobates littoralis (Silvestri) in Cork (Cawley, 1997) and of a further introduction, Nopoiulus kochii (Gervais), from the Belfast area (R. Anderson, unpublished). A total of eleven Irish species (out of 41 established out-of-doors) are probable introductions, and the acquisition of additional species seems likely to continue. With the Carabidae, wetness and lack of insolation in Irish habitats must be considered critical factors in determining faunal composition. Thus, while apparently similar habitats may exist in Ireland, differences in climate from that pertaining in, particularly the southern and eastern, parts of Britain, mean that uptake of 'vacant' niches is likely to be limited until or unless global warming significantly increases the small element of continentality in the Irish climate.
The Irish Carabidae has certainly been augmented to some degree during the present century, but this is generally considered to be small. There is persuasive evidence that Bembidion guttula was either not present in Ireland or was very rare at the beginning of the 20th century (Johnson & Halbert, 1902) (see Chapter 3). This species is currently widespread in riparian habitats across the island, and appears to have achieved this status in a comparatively short time, beginning, probably, in the nineteen-twenties or thirties. The only pre-1940 specimens we have seen were collected by W. M. Crawford at Belfast in 1938.
Apart from Bembidion guttula, a total of 29 carabid species have been added to the Irish List since the first checklist of Johnson & Halbert (Anderson et al., 1997). However, of these only Amara montivaga, which Speight (1972) found among the collections of E. F. Bullock from the Killarney area, is likely to be a recent immigrant. Amara montivaga is a late arrival in Britain, and presently in the process of expanding its range northwards (Lindroth, 1974). Of the 27 other additions, 8 have been added by taxonomic research into new segregates of known species, and 19 by the discovery of hitherto overlooked native species. The pace of natural change in the Irish fauna is therefore very slow, and not at the level evidenced by Lindroth (1985 & 1986) for Scandinavia. There, climate change and re-investment of northern territories by insects exterminated in the Quarternary glaciations, have produced significant additions to the fauna in the last two centuries. The slow pace of change in Ireland may be partly due to the paucity of observers in the historical period. However, the island status of Ireland, and its position to the west of Europe in a cool, wet airstream opposing the natural direction of migration, must be the most decisive factors. The Irish climate, as discussed above, remains a major impediment to colonisation by a continental European fauna.