ZOOGEOGRAPHY OF SOME TERRESTRIAL MICRO‐ARTHROPODA IN ANTARCTICA

Abstract

Summary1. A review is presented of the information relating to the distribution of free‐living terrestrial Cryptostigmata, Prostigmata and Collembola in the south polar region.2. The Maritime zone, comprising the Antarctic Peninsula and its neighbouring islands, forms a less‐clearly definable faunal province than does the Continental zone where generic and specific endemism is high. The Maritime distribution of the micro‐arthropods forms no consistent pattern, although in the case of the two groups of mites a distinction can be made between a southern and a northern Maritime fauna. The boundary between these two elements appears to lie between the South Orkneys and the South Sandwich Islands.3. As far as distribution in the Sub‐Antarctic is concerned, all three groups of micro‐arthropods show certain general similarities, although in each case particular features can also be distinguished. All three show relatively high specific endemism in the Sub‐Antartcic, which sugge:ts that this zone is a faunal province distinct from that of the cold temperate zone to the north and other parts of the Antarctic to the south. It seems equally reasonable to recognize that this province can be subdivided into western and eastern parts, each with its own distinct group of species, although the extent to which this separation is expressed varies from group to group. In all three groups there is an element which is circum‐Sub‐Antarctic in distribution, but this element is more in evidence in the Collembola than in the mites. From the study of the distribution of endemic species in the Sub‐Antarctic it is concluded that the fauna of the eastern part has been isolated from the south temperate zone fauna for longer than it has in the west.4. In all three groups of micro‐arthropods a relic element can be identified, the present distribution of which is consistent with the idea of a former continuous distribution extending across a southern land mass incorporating both the eastern and western parts of the continent, the islands on the Scotia Ridge, and the older Sub‐Antarctic islands of South Georgia, Macquarie and, possibly, Kerguelen. In the Cryptostigmata in particular, this relic element, which is represented by members of the Podacaridae, is joined in the Sub‐Antarctic and Maritime zones by a penetrant element which probably has invaded the south polar region from the north during the post‐Pleistocene period.5. An alternative hypothesis is also discussed, namely that present distribution patterns of micro‐arthropods can be explained solely in terms of post‐Pleistocene colonization. It is accepted that limited overseas dispersal could have occurred in recent times, indeed may still be occurring between, for example, the various volcanic islands in the eastern Sub‐Antarctic, between the south temperate zone and the Sub‐Antarctic, and between the latter and the Maritime zone. However, there is no evidence to suggest that any appreciable amount of long‐range dispersal is occurring, on the scale required to support this hypothesis. In addition, there is no evidence that rates of speciation in the south polar region are rapid enough to produce the kind of evolutionary divergence which is implicit in the theory of post‐Pleistocene colonization.