Jökull


Jökull - 01.12.1986, Síða 52

Jökull - 01.12.1986, Síða 52
occasional variations in the fossil biota imply varying degrees of local wetness on the surface, there is insuffi- cient evidence to regard this as anything other than the result of the natural processes of bog growth (cf. Moore & Bellamy, 1973), rather than as indications of climatic change. The limited evidence from the fossil seeds and megaspores is shown in table 1. Only 5 taxa, distri- buted among 4 samples are recorded, the matrix of each being dominated by poorly preserved Sphagnum. Most of the plants can be found on the bog at the pre- sent day but clubmoss, Selaginella selaginoides, re- presented by large numbers of megaspores in several samples, is now confined to the upper slopes of Geita- fjall, presumably having been shaded out by the denser growth of grasses and sedges on the now drier bog surface. The bog bean, Menyanthes trifoliata, appears in KEl/4, a sample which is considerably earlier than Landnám, the drier nature of the surface is probably the reason for its absence from the bog at the present day. In virtually all samples, the beetle fauna (table 2) is, not surprisingly, indicative of a bog environment. Small pools of open water are indicated by the Dytiscid water beetles Hydroporus nigrita and Agabus bipustulatus. Hydraena britteni, on evidence from elsewhere in Europe (Lohse, 1971; Balfour-Browne, 1958), probably lives in amongst the wet Sphagnum cushions at the sides of the pools. Several of the Staphylinids would also appear in the wet habitats on the bog surface, including species of Stenus and Lath- robium brunnipes. Somewhat drier areas are implied by some of the Carabids but all might be found in an intermittently rather less damp tussock and pool en- vironment, where beetles such as Pterostichus dili- gens, P. nigrita and Quedius umbrinus could occur among the grasses and sedges and in the accumulation of decaying plant debris. The paucity of phytophages in the Icelandic fauna means that little can be added to the list of plant species by way of the insects which feed upon them. Both of the weevils Otiorhynchus nodosus and Tropiphorus obtusus are polyphagous on a wide variety of plants, the larvae feeding on the roots (Lindroth et al., 1973). Post-Landnám Above the Landnám tephra, the impact of man rapidly becomes apparent in the succession. The most diverse flora occurs in sample BRl/2, from immedi- ately below the —1357 tephra fall, a reflection of the increased habitat variety created by human activities. The flora includes the chickweed Stellaria media, a plant if not introduced by man (Steindórsson, 1962) much encouraged by his disturbance of the ground, and the buttercup Ranunculus cf. acris may also belong to the group of anthropochorous plants. The limited nature of the flora, however, allows few con- clusions and the beetle fauna can add little in terms of information on the species of plant rather than the general nature of the vegetation. Byrrhus fasciatus is a moss feeder usually found in drier localities among stones and mosses rather than in bogs but it is a capable flier (Lindroth et al., 1973). The larvae of the click beetle Hypnoidus riparius are generalised feeders upon the roots of grassland vegetation and it is sugges- tive of a somewhat drier situation. Since the imago is flightless (op. cit.), it is probable that the area became at least intermittently less boggy after Settlement. In part, this may reflect the increased sediment input to the basin (fig. 6) but peat cutting for fuel (fig. 4) must also have altered the drainage pattern. The weevils are more informative. Whilst Stropho- somus melanogrammus has been taken in Iceland in localities away from scrub (Larsson & Gígja, 1959; Lindroth et al., 1973), it is more characteristic of birch woodland, where the imago eats the leaves and young shoots; it may be regarded as an Urwaldrelikt in the Icelandic fauna, now surviving in such conserved areas of woodland as those in Thórsmörk. Crucifers are absent from the fossil record at Ketilsstadir, a con- trast with the modern flora, where the shepherd’s purse, Capsella bursapastoris, is frequent on the dis- turbed ground around the farm and along the sides of the road to Vík, and rock cress, Cardaminopsis petraea, grows abundantly on the bare slopes of Geitafjall (Greig, pers.comm.). The small weeVil, Ceutorhynchus contractus breeds in many different species of Crucifer (Dieckmann, 1972) but the few modern Icelandic records, including one from Vík (Larsson & Gígja, 1959), tend to be coastal, where the species perhaps breeds in scurvy grass, Cochlearia officinalis; Lindroth (in Lindroth et al., 1973) also records the beetle from a whitlow grass, Draba nemoralis, in southern Iceland. Both Ketilsstadir records are post-Landnám but this weevil appears beneath the Thjórsárhraun at Thjórsárbrú in deposits of about 8,000 B.P. (Buckland et al. 1986). Although the faunas become considerably more diverse after Landnám (fig. 7), only two species of beetle are directly associated with man. The dung beetle, Aphodius lapponum, could not have lived in Iceland before the introduction of large herbivores; it first appears in the Ketilsstadir succession in sample BRl/10, probably belonging to the eleventh century. 50
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