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The Invertebrate Life of Peatlands

The freshwater fauna of Irish rivers and lakes is well known and, indeed, routinely used to evaluate water quality. The fauna of peatlands in all their variety, with hummocks, pools and man-made cuttings, is much less well known, although bogs are the most extensive type of wetland in Ireland. We now understand a great deal about peatland vegetation. However, the invertebrates of Irish peatlands were relatively neglected until the 1970s, when an increasing awareness started to develop of the rapid disappearance of these habitats as a result of direct exploitation, afforestation and agriculture (Reynolds, 1984a, 1990).

This fact sheet looks at the different faunal communities living in the 'terrestrial' and 'aquatic' habitat zones of peatlands, and how their community functions maintain the coherence of the system.

TERRESTRIAL INVERTEBRATES OF BOGS
The obvious 'terrestrial' fauna of peatlands includes the large black slug Arion ater which is conspicuous on blanket bogs, the myriad ants tunnelling in the drier hummocks, and a surprising diversity of moths and spiders. The smaller fauna may be associated with a particular plant or rootstock rather than with a habitat (Ryan, 1981; Speight and Blackith, 1983; Webb et al., 1984).

. A number of studies have identified a range of bogland insects and arachnids. Blackith (1974) described springtails from Glenamoy, Co. Mayo and Nixon et al. (1975) recorded the spittlebugs occurring on blanket bogland. In midland raised bogs, Higgins (1984) lists around 40 spider species, noting also the common occurrence of centipedes (Chilopoda) in the drier hummocks. The arachnids do not appear to be specialist "tyrphobiont" species, but are opportunist, very exploitative of flying and vagrant insects in these windswept spaces. However, Good (1985) recorded some insects particular to bogland, Ashe (1987) noted some tyrphobiont chironomid midges, Foster et al. (1992) identified some rare peatland beetles, including the flightless beetle Agabus labiatus, and Bond (1984) listed around 60 microlepidopterans.

AQUATIC INVERTEBRATES OF BOGS
There are a variety of aquatic and semi-aquatic habitats in acid peatlands (bogs), including lakelets, pools, lawns, soaks and flushes. Not all organisms found here are bog specialists. In fact, as Reynolds (1984b, 1985) shows, some aquatic invertebrates of these habitats are widespread and common forms of small ponds, broadly tolerant of a wide range of conditions.

The few invertebrates of bog pools are chiefly insects and some small chydorid crustaceans, while under the moss surface, ciliate protozoans, nematodes, springtails, and enchytraeid worms are often abundant. The tiny carnivorous waterflea Polyphemus pediculus may be common. This cladoceran is most usually associated with nutrient-deficient situations such as bog pools although it may occur elsewhere.

On the surface of each bog pool is often a Water Strider (Gerris sp.) or Water Cricket (Velia sp.) which patrols the fringes waiting for an unwary ant or moth to fall or be blown onto the water surface. The strider rushes out and feeds on the struggling insect; its frass or droppings passing through the meniscus to enrich the pool below. Around the margins, the carnivorous great Raft Spider (Dolomedes fimbriatus) will occasionally appear, by its predation transferring nutrients from one small bogland habitat to another.

Where some groundwater is involved the fauna may be richer, and include lime-requiring larger crustaceans (e.g. the Hog Louse Asellus) and molluscs such as the Pea-clams (Pisidium spp.), which are important herbivores and detritivores. These are important food for aquatic vertebrates such as Smooth Newts and 3- and 9-spined Sticklebacks (Gasterosteus aculeatus and Pungitius pungitius) which may be present in the marginal laggs and drains of raised bogs, and also in fen peatlands.

The fauna of new pools on cutaway bog shows much similarity to that of bog marginal drains and of fens. Among the most abundant colonists of cutaway ponds are Lesser Water Boatmen (Corixidae), which fly easily in summer and are attracted to any shiny water surface. Eight corixid species were found by O'Connor et al. (2000) in Turraun Lake, Co. Offaly, in the first ten years of its existence.

INVERTEBRATE FAUNA OF FENS
Fens are often rich in nutrients, and their 'terrestrial' fauna is, like their vegetation, correspondingly varied, too much so to be discussed further here. Insects are particularly diverse, many of them characteristic of other water-influenced habitats such as marshes, turloughs and damp grassland.

There is a wide range of aquatic habitats within fens, such as tufa springs, rainwater- and spring-fed pools, drains, openwater areas and reedswamps. Here, because nutrients are not limiting, and with a pH of 7 or greater and often high levels of calcium, a diversity of crustaceans and molluscs join the large variety and abundance of insect forms ­ caddis-flies, water-boatmen and beetles.

Pollardstown Fen was described by Doyle (1990), and is among the best studied of Eastern Irish fens. It has been man-modified for centuries and contains a rich variety of small aquatic habitats, from canals and marginal drains drains with much Common Reed Phragmites, to acidic or deep spring-fed pools, watercress-filled or tufa-depositing springs and seepage areas (Reynolds, 1980). A shallow central 'lake' resulting from the recent flooding of disturbed ground shows marked similarities to the shallow lakes recently created on midland cutaway bog described by Reynolds et al. (1998) and O'Connor et al. (2000).

While minor fen drains are often peat-stained, canal habitats are exceptionally clear and those richest in invertebrates are well provided with submerged macrophytes ­ Mare's-tail (Hippurus), Water Milfoil (Myriophyllum) and the Reed Sweet-grass Glyceria maxima. The dominant organisms are the Hog Louse, Asellus aquaticus, and many species of caddis larvae (Trichoptera) including the rare Leptocerus tineiformis noted by O'Connor et al. (1990), while water boatmen (Corixidae) and beetles (Coleoptera) are well represented.

Despite the high lime content of the water in Pollardstown Fen, aquatic snails are restricted in abundance (apart from the Olive Snail Physa fontinalis) and diversity compared with the adjacent Grand Canal; the greatest variety of snails being found in old cutover pools in fen peat rather than in the canals or springs. Dragonfly nymphs (Odonata) are also scarce outside marginal drains or cuttings. Flatworms (Polycelis tenuis, Dendrocoelum lacteum) and leeches both occur in parts of the feeder canals, neither in high abundance. Two native species of freshwater shrimp (Gammarus) are present, G. duebeni occurring alone in and near springs but mingling with G. lacustris in canals (Reynolds, 1980). Of particular conservation interest are tiny snails of the genus Vertigo, here associated with tufa springs.

COMMUNITY INTERACTIONS
Despite extensive studies of peatland vegetation, we still know very little of how the peatland fauna and flora interact, and whether such patterns are common across the range of peatland communities (Reynolds, 1990; Reynolds et al., 1998). It is important to understand how the breakdown of vegetation and its transformation by processes involving fungi, bacteria and animals sustains the wider community and maintains the slow growth and development of these peatlands. Plant matter breaks down faster in water than in air, chiefly because wetting allows easy colonisation of surfaces by bacteria. Thin-walled, submerged aquatic plants break down faster than the stout emergent reeds and sedges. Alternate wetting and drying of shallow peatland depressions may help speed breakdown; the anoxic subsurface slows the same processes. Some nutrients remain locked up in peat, and are not bioavailable until it has passed through detritivores (Schell, 1983) or is liberated by the action of ultra-violet light.

In water, dissolved oxygen is in short supply, particularly at high temperatures, and it must be replenished by underwater photosynthesis or by water turbulence, both of which are restricted in small peatland pools. The peaty sediments may be anoxic beneath a thin surface zone. Thus, most wetland dwellers are exposed to lowered levels of oxygen and limited water circulation, which Blackith (1974) showed may lead to considerable mortality among the smaller, less mobile forms.

Aquatic invertebrates chiefly attack senescent plants, usually in the second half of the year. The strong jaws of these 'shredders' enable them to chew off fragments of dead plants. Other detritivores will further break down this material, passing their frass and fine residues to the 'collectors', who also take in peat particles. Wetland shredders are chiefly insects - species of caddis and beetles - and crustaceans, while collectors in these habitats include some tiny entomostracan crustaceans, midge larvae and pea-clams. Algae may be food for filter-feeding crustaceans and rotifers, which also eat bacteria and protozoans. The periphyton (a greenish scum or film, mostly of algae) is a further food-source for wetland herbivores.

Since much terrestrial peatland vegetation has a strong protective cuticle, resistant to animal attack, and many contain tannins which inactivate digestive enzymes, herbivores have to specialise; ants feeding on seeds; spittle bugs sucking sap, or leaf miners burrowing into plant stems and leaves. Apart from the Emperor Moth Saturnia pavonia most bog flies (Diptera) and moths (Lepidoptera) are small and inconspicuous, many emerging from leaf mines.

The higher predators in peatlands are less predictable; they depend in part on the productivity of the habitat and the range and numbers of detritivores it sustains. The large mobile predators ­ snipe, frogs, newts and the Raft Spider Dolomedes - provide links and move nutrients around between peatland habitats. Invertebrate and vertebrate carnivores are less fundamental than detritivores in the context of wetland functioning, though some (e.g. dragonflies, beetles) may attract more conservation interest.

THE VALUE OF CONSERVATION
Bogs are of European importance among Ireland's specialised wetland habitats. Some of their biota is equally specialised, and a proportion of species may be uncommon or threatened. But should conservation interest be directed at such small, genetically isolated stocks? Peatland species which are rare or declining in Ireland, but relatively stable elsewhere, may be affected by large-scale changes such as global warming, as suggested by Grainger (1996). If many rarer invertebrate species are to survive, it is vital to know not only their biogeography, but also something of their biology - longevity, fecundity, population structure and conditions for life within their plant-animal community.

Often the best conservation approach is to conserve habitat. However, a habitat defined solely by its vegetation may be protected from perceived 'disturbance factors', which may be vital for the survival of some species or life stages of both plants and animals. Habitat protection is thus more appropriate for some rare species than for others. In all cases, to maintain the functioning of these ecosystems it is centrally important to know how the species interact in peatland communities. The new, often fen-like habitats developing on reflooded cutaway bog provide valuable additional refugia for a number of species and will be of increasing conservation value as they mature.

REFERENCES
Ashe, P. (1987) Lasiodiamesa sphagnicola, Cardiocladius capuchinus and Orthocladius (Eudactylocladius) fuscimanus (Diptera: Chironomidae) new to Ireland. Irish Naturalists' Journal 22: 194-195
Bilton, D.T. (1988) A survey of aquatic Coleoptera in central Ireland and in the Burren. Bulletin of the Irish Biogeographical Society 11: 77-94
Blackith, R.E. (1974) The ecology of Collembola in Irish blanket bog. Proceedings of the Royal Irish Academy 74B: 203-226
Bond, K.G.M. (1984) Invertebrates of Irish midlands raised bogs: Part III. Lepidoptera. Bulletin of the Irish Biogeographical Society 8: 103-110
Doyle, G.J. (1990) Pollardstown Fen. In: Ecology and conservation of Irish peatlands (ed Doyle, G.J.), Royal Irish Academy, Dublin
Foster, G.F., Nelson, B.H., Bilton, D., Lott, D.A., Merritt, R., Weyl, R.S. & Eyre, M.D. (1992) A classification and evaluation of Irish water-beetle communities. Aquatic Conservation 2: 185
Good, J.A. 1985 Invertebrates of Irish midlands raised bogs: Part IV. Notes on terrestrial insects. . Bulletin of the Irish Biogeographical Society 9: 2-9
Grainger, J.N.R. (1996) Climate change and the Irish aquatic fauna. In: The conservation of aquatic Systems (ed Reynolds, J.D.) pp 181-183. Royal Irish Academy, Dublin
Heery, S. (1993) The Shannon floodlands ­ a natural history. Tir Eolas, Ireland
Higgins, D.G. (1984) Invertebrates of Irish midlands raised bogs: Part I. Araneae, Opiliones, Chilopoda. Bulletin of the Irish Biogeographical Society 8: 91-97
Nixon, D., Okely, E.F. and Blackith, R.M. (1975) The distribution and morphometrics of spittle bugs on Irish blanket bog. Proceedings of the Royal Irish Academy 75B, 305-315
O'Connor, A., Kavanagh, B. & Reynolds, J.D. (2000) Corixidae (Hemiptera Heteroptera) in two artificial lakes on Irish cutaway raised bog. Verhandlungen Internaionale Vereinigung fur Limnologie 27 (in press)
O'Connor, J.P., Good, J., Bond, K.G.M. (1990) Leptocerus tineiformis Curtis: a caddisfly new to Ireland from Pollardstown Fen, Co. Kildare. Entomologists' Record 102: 115-116
Reynolds, J.D. (1980) The controversy over Pollardstown Fen. Canaliana 1980: 53-56
Reynolds, J.D. (1984a) Invertebrate survey of Irish midlands raised bogs. British Ecological Society Bulletin 9: 41-45
Reynolds, J.D. (1984b) Invertebrate fauna of Irish midlands raised bogs: Part II Odonata, aquatic Hemiptera and Trichoptera. Bulletin of the Irish Biogeographical Society 8: 98-102.
Reynolds, J.D. (1995) Invertebrate fauna of Lough Roe, a rare and endangered bogland habitat. . Bulletin of the Irish Biogeographical Society 9, 41-45
Reynolds, J.D. (1990) Ecological relationships of peatland invertebrates. In: Ecology and conservation of Irish peatlands (ed Doyle, G.J.) pp 135-143. Royal Irish Academy, Dublin
Reynolds, J.D. (1998) Ireland's Freshwaters. Marine Institute, Dublin
Reynolds, J.D., Duigan, C., Marnell, F., O Connor, A. (1998) Extreme and ephemeral waterbodies in Ireland. In: Studies in Irish Limnology (ed Giller, P.) pp 67-99. Marine Institute, Dublin
Ryan, J.K. (1981) Invertebrate faunas at tundra sites. In: Tundra ecosystems: a comparative analysis. (eds Bliss, L.C., Heal, O.W. & Moore, J.J.) pp 516-539. Cambridge University Press
Schell, D.M. (1983) Carbon-13 and carbon-14 abundances in Alaskan aquatic organisms: delayed production from peat in arctic food webs. Science 219: 1068-1071
Speight, M.C.D. & Blackith, R.E. (1983) The animals. In: Mires: swamp, bog, fen and moor, A. General Studies (ed Gore, A.J.P.) pp 349-365. Elsevier, Amsterdam
Webb, F.E., Clarke, R.T. & Nicholas, J.T. (1984) Invertebrate diversity on fragmented Calluna-heathland: effects of surrounding vegetation. Journal of Biogeography 11: 41-46

Copyright Julian D. Reynolds
Department of Zoology, University of Dublin, Trinity College, Dublin 2, IRELAND
Tel. (01) 608 1063, fax 677 8094, email: jrynolds@tcd.ie