Oribatid mites in a changing world

Abstract

The main scope of this thesis is to illustrate the validity of oribatid mites as tools for palaeoecological reconstructions. Palaeoecology studies the responses of past organisms to past environmental changes. This can be accomplished through the use of biological proxies, which are indicators of past conditions. The search for additional means of distinguishing climate change has only recently led to the use of other commonly found biological proxies such as tiny oribatid mites known as moss-mites. Oribatid mites are among the most numerous biological remains in anoxic sediments, yet until now oribatids have not been widely used due to the uncertainties about their present distribution and the lack of expertise to identify them to species level. This thesis contains four papers which provide evidence about how oribatid mites, when they are properly identified to species level and their background distribution is adequately known, can give useful additional and supporting information for reconstructing past habitat and environmental conditions. Paper I studied oribatid preferences and ecology in different habitats, mainly forested, in western Norway. One hundred and ninety two species were found of which 64 were new records for Norway. The species Chamobates borealis, Oppiella nova, Moritzoppia neerlandica, and Rhinoppia subpectinata characterised the oribatid communities of Betula, mixed, and Picea forest subsets. Deciduous forest oribatid communities were characterised by Achipteria coleoptrata, Acrotritria ardua, Ceratozetes gracilis, and Oribatella calcarata. Hemileius initialis, Nanhermannia dorsalis, C. borealis, Tectocepheus velatus, and Atropacarus striculus characterised wet habitats. In water-logged habitats, Limnozetes ciliatus, Mucronothrus nasalis, and Trimalaconothrus glaber dominated. Carabodes labyrinthicus, C. marginatus, Melanozetes mollicomus, and T. velatus characterised the oribatid community of the lichen and moss subset. The tree-line ecotone was dominated by the euryceous species H. initialis, T. velatus, and Oribatula tibialis. This study represents a thorough survey of oribatid communities in western Norway, and the insights it gives are an important tool for habitat reconstructions, as they provide the background knowledge about modern oribatid fauna needed to identify the type of past plant community and past environments represented in Quaternary sediments. Paper II studied the oribatid communities at the tree-line in western Norway and compared them with the oribatid fossil assemblages found in Lake Trettetjørn. The modern oribatid assemblage provided a guide to the reliability of the fossil assemblages to reconstruct ecological and environmental changes and, in addition, to find the most favourable coring point within the small lake. Results showed that the core retrieved from the middle of Lake Trettetjørn basin represented the oribatid fauna from the catchment area. Aquatic oribatids were the best group represented in the lake sediments, followed by oribatids from the habitats adjacent to the lake. This constitutes good evidence that oribatids are excellent indicators of local habitats. Comparison of the oribatid fauna found in the lake traps with the oribatid assemblages from paper III illustrated the importance of identifying the mites to species level, as this increased the ecological indicator value and, therefore, the reliability of the palaeoreconstructions. In Paper III, sub-fossil oribatid mites, pollen, plant macrofossils, and diatoms from a lake sediment core from western Norway were studied. This multi-proxy study attempted to reconstruct tree-line fluctuations and their impact on Lake Trettetjørn’s environment. Evidence from pollen, plant macrofossils, and oribatids complemented and corroborated each other in the reconstruction of the vegetational development. A semi-open grassland developed into forest. Mires began to replace forested areas on the landscape as a more oceanic climate began to prevail. All proxies indicated increasingly intensive human land-use as the Upsete settlement grew to accommodate the construction of the Bergen-Oslo railway. In Paper IV, oribatid mites and pollen were used to reconstruct the local habitat at an archaeological excavation. The study aimed to identify the start of cereal cultivation at Kvitevoll farm, on Halsnøy island, western Norway. The high number of oribatid remains identified to species level and the close match to pollen stratigraphy led to a detailed palaeoenvironmental reconstruction. Oribatids and pollen indicated the development of a moist forest followed by vegetation openings and mire expansion over the site. At the top of the sequence, the presence of oribatids such as Tectocepheus velatus and the increase in members of the family Oppiidae indicated a higher degree of disturbance, probably from grazing. Pollen of Cerealia indicated the start of cultivation around the same time.