Modern Ecological and Palaeoecological Contributions Towards
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Postglacial Colonization Conference: Mind the Gap 2006 An ever-closing gap? Modern ecological and palaeoecological contributions towards understanding the Irish post-glacial insect fauna EILEEN REILLY* Botany Department, School of Natural Sciences, University of Dublin, Trinity College, Dublin 2 Over the last 25 years, since the 1983 conference, a range of new evidence has emerged on the changing nature of the post-glacial insect fauna of Ireland. Modern entomological research, especially new surveys and re-evaluation of existing collections, is continually improving our understanding of the Irish fauna (e.g. Alexander 2002, Speight 2004). However, findings from palaeoentomological analyses, particularly sub-fossil beetle remains, from various locations throughout Ireland have been particularly illuminating in filling gaps in our knowledge (e.g. Reilly 2005, Whitehouse 2006). This paper will attempt to evaluate these findings in order to address the following questions: " Do we know more now than we did in 1983 about the Irish native insect fauna? " Do we know more about colonization, dispersal mechanisms and extinctions of this fauna? " What gaps still exist in our knowledge and how do we fill them? Background At the 1983 conference, two contributors, Martin Speight and T.K. McCarthy, discussed the general state of knowledge at that time regarding the Irish postglacial insect fauna (Speight 1986, McCarthy 1986). The following key points were identified: The present day Irish fauna of approximately 14 000 species represented gradual accretion over 10 000 years and was highly mobile for the most part. It was generally depauperate in nature compared to Great Britain or north-western Europe and many groups were poorly represented, particularly among terrestrial insects. There are 'Lusitanian' elements in the Irish fauna i.e. insects that are absent from Britain today and have predominantly south-western European distributions. This might hint at south-westerly origins for the Irish fauna. However, there is also a distinct Arctic/Boreo-Alpine element to the Irish fauna. Both writers cautioned against an attempt to generalize as to the origin of the whole Irish fauna on the basis of a small number of insects - whether they were glacial relicts that survived in refugia or very early colonizers was still open to debate. Lack of known endemics, they believed, cautioned against the glacial refugia idea. Mobility, wind, accidental transport appeared to discount the need for a land-bridge. Speight highlighted the lack of particular groups of insects, most notably woodland species associated with oak, pine and 'old woodland' in general. He highlighted the need at that time for increased palaeoentomological research to understand if this lack of species represented an accurate picture of the Irish native woodland fauna and, if not, the timing and causal factors of the loss of these species (Speight 1986). Both writers emphasized the need to examine the ecological requirements of insects, climate change and habitat diversity through time. Did insects arrive and become extinct quickly because their habitat requirements were not met, or disappear gradually or hang on in isolated pockets of certain relict habitats? What if anything could they tell us about their colonization/dispersal mechanisms? All this needed to be addressed by increased research. * Email address: eireilly@tcdie 63 This content downloaded from 143.239.131.220 on Tue, 11 Sep 2018 10:35:16 UTC All use subject to https://about.jstor.org/terms Ir. Nat. J. Spec. Suppl. 2008 Modern entomological research Since that time, modern entomological work has continued apace and has made a substantial contribution to our understanding of the origins of the Irish insect fauna. This has mostly been in the form of new surveys of different habitat types and revisions/reviews of particular families/genera of insects. Surveys Surveys have added numerous 'new' species to the Irish List and this paper can merely touch on a few of them. The beautiful damselfly Coenagrion lunulatum Char. was discovered during surveys of Co Fermanagh/Tyrone habitats (Nelson et al. 1989). At the time it was unknown in Britain and rare in western Europe. In Finland it is most common north of the Arctic Circle (Hamalainen 1984 cited in Nelson et al. 1989). It occurs in the sparse vegetation around high altitude lakes (85-310m) and is probably an example of a very early, mobile colonizer, overlooked until this time. Four species of Diptera were added to the Irish List during a survey of Blackditch Wood, The Murrough, Co Wicklow and Glendine, Co Waterford (Speight et al. 1990). Antichaeta brevipennis (Zet.) was discovered in thickly vegetated parts of wet woodland in the Murrough. It is a rare insect across Europe and on the British Red List for insects (Shirt 1987). Leucophenga maculata (Du.), discovered in Glendine, is an ancient deciduous forest insect, its larvae feeding in saproxylic fungi (Alexander 2002). It is known from various parts of Britain, from Scandinavia to Iberia and throughout Eurasia. At All Saints Bog, Speight surveyed the birch woodland and found a very important saproxylic fauna consisting of click and longhorn beetles (including Leptura quadrifasciata Linn. - only five records this century) and various tipulid flies (Speight 1990). The ground fauna was similarly indicative of ancient woodland, including the arachnid Araneus umbraticus Cl. and earthworm Eisenia eiseni Lev. The fauna as a whole appeared to indicate prolonged stable ground conditions. Speight noted that other beetles and hoverflies present as well as the rare grasshopper, Stethophyma grossum (L.), indicate that All Saints Bog is supporting a more complete, rounded fauna than equivalent sites in Ireland and can be considered a 'relict Irish biotope', in this case, birch woodland (Speight 1990). A survey of turloughs in north Clare and south-east Galway by Good and Bulter (2001) added five new Staphilinid beetles to the Irish record and these beetles are now considered 'indicator species' of turloughs. Eight new beetle species were added to the record from a wide-ranging survey of east Clare, Galway and Offaly sites by coleopterists in 2003 (Regan and Anderson 2004) most notably many rare species associated with wetland habitats but also the longhorn beetle, Grammoptera ustulata (Sch.). It is classified as rare in Britain, and is an indicator of ancient woodland, mainly recorded from the midlands and southern England (Alexander 2002) Its find in old estate woodland in Charleville, Co Offaly is therefore of national importance. Revisions/Reviews Of the many insect groups reviewed or revised in the last 25 years the following is a mere sample to highlight the changes in our knowledge since 1983. O'Connor, Liston and Speight (1997) reviewed the sawflies (Hymenoptera: Symphyta) resulting in nineteen 'new' additions to the Irish list. The fauna is rich in 'secondary woodland' species (larvae host trees like birch, hawthorn and willow) but poor in oak and pine associates (excluding new introductions associated with spruce and larch). Two species with a Euro siberian distribution and not known from Britain include Cimbex conatus Sch., associated with alder, and Arge metallica (Klug), associated with birch. Carabid beetles have been subject to detailed reassessment and revision (e.g. Speight et al. 1983, Anderson et al. 1997, Anderson et al. 2000). Two species, Bembidion argenteolum Ahr. and Agonum Iugens (Duff.), are not currently recorded in Britain but are both found in Scandinavia (www.habitas.org.uk/groundbeetles, Anderson 1985). They are 64 This content downloaded from 143.239.131.220 on Tue, 11 Sep 2018 10:35:16 UTC All use subject to https://about.jstor.org/terms Postglacial Colonization Conference: Mind the Gap 2006 characteristic of bare or sparsely vegetated ground and may have been more widespread in the early postglacial. Anderson et al. (2000) note that other boreal species are more widespread here than in Britain. They also note that the weakest categories of ground beetle species in Ireland are the Southern-temperate and Mediterranean-Atlantic groups - 62 such species occur in Britain but there are no Irish records (www.habitat.org.uk/groundbeetles). Morris, in his 1993 review of Irish weevils (Coleoptera: Curculionidae), confirmed 246 Irish species, an increase of 21 per cent from Johnston and Halbert's 1902 list of 203 (Morris 1993). Some possible 'Lusitanian' species include Otiorhynchus auropunctatus Gyll., which has only been recorded once from Scotland, otherwise is only known from southern France and Spain. Also, Barypeithes curvimanus du Val, a very rare Irish weevil is not recorded in Britain and only known from France. However, Morris noted that the taxonomy of some genera remained unworked at this time so it may be that unrecognized examples of these species exist elsewhere in collections and in nature (Morris 1993). Summary This all too brief summary of modern entomological work highlights some important issues. Lots of species added to the Irish record in the last 25 years are described as 'new to Ireland'. In fact, this could be broken down into those that were 'overlooked' and those that are 'recent arrivals' the former being the majority because they weren't identified before or were not captured before. This is an important point because it means there is huge potential for many more species to be added to the record as a result of such work. The new total for the insect fauna of Ireland may be upwards of 16 000 species (O'Connor and Ashe 2000). Many of these findings have important biogeographical implications. The examples above give us some more evidence of the boreal elements within the Irish fauna but also others with a more southerly distribution today. Alexander describes the Irish saproxylic fauna as a 'relict hybrid of Atlantic temperate and Boreal fauna' (Alexander 2002). This appears to be true of other elements of the Irish fauna indicating multiple potential routes for the arrival of insects and, possibly, multiple phases of arrival, making the 'land-bridge' theory less and less likely.