distribution and biogeography of oribatid mites (acari: oribatida) in antarctica, the sub-antarctic...

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Distribution and biogeography oforibatid mites (Acari: Oribatida) inAntarctica, the sub-Antarctic islandsand nearby land areasJosef Starý a & William Block ba Institute of Soil Biology , Academy of Sciences of the CzechRepublic , Na sádkách 7, 37005, České Budějovice, CzechRepublicb British Antarctic Survey , Natural Environment ResearchCouncil , High Cross, Madingley Road, Cambridge, CB3 OET, UKPublished online: 17 Feb 2007.

To cite this article: Josef Starý & William Block (1998) Distribution and biogeography of oribatidmites (Acari: Oribatida) in Antarctica, the sub-Antarctic islands and nearby land areas, Journal ofNatural History, 32:6, 861-894, DOI: 10.1080/00222939800770451

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JOURNAL OF NATURAL HISTORY, 1998, 32, 861-894

Distribution and biogeography of oribatid mites (Acari: Oribatida) in Antarctica, the sub-Antarctic islands and nearby land areas

J O S E F S T A R Y t and W I L L I A M BLOCK:~

"~[nstitute of Soil Biology, Academy of Sciences of the Czech Republic, Na sddkdch 7, 37005 Ceskd Bud~jovice, Czech Republic {British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK

(Accepted 20 November 1997)

Oribatid mites are an ancient group of cosmopolitan terrestrial arthropods with limited trans-oceanic dispersal abilities. They provide an opportunity to answer questions concerning the role played by Gondwanaland, either as a migration route for terrestrial organisms or as a centre for their origin and subsequent glacial destruction, in the development of the biota of Antarctica, the sub- Antarctic islands and nearby land areas. Biogeographical studies of present-day oribatid mite faunas of the Antarctic region, New Zealand and South America (particularly the Andes Mountains) also allow insight into the historical development of such biota after the break-up of Gondwanaland. No records of fossil oribatid mites are known for the Antarctic and their main dispersal mechanisms within the biome are likely to be via sea-birds and possibly ocean currents. A total of 105 species from 20 families of oribatid mites are recorded from the Antarctic which, together with species records from South America, including Patagonia and Tierra del Fuego, and New Zealand, allowed faunal similarities to be examined using the similarity coefficient of Jaccard and principal co-ordinate analysis. Species endemism is high in both the continental (60%) and the sub- Antarctic zones (63%) compared with the maritime Antarctic zone (18%) and the Falkland Islands (19%), but lower than in the New Zealand fauna (83%) and in the Neotropical areas of South America (89%). Species diversity of oribatid mites in the Antarctic is low (five species in the continental Antarctic zone, 22 species recorded for the maritime Antarctic zone, and 78 species found in the sub-Antarctic zone) compared with New Zealand (330 species) and the Neotropical South American region (1193 species). The numerically-dominant species are from the families Oppiidae and Ameronothridae in the Antarctic region, but only a single endemic family (Maudheimiidae) occurs there. Several conclusions are drawn regarding the relationships of the oribatid mite faunas within Antarctica and between them and those of the surrounding land areas. The high similarity of the present faunas of the Andes Mountains and New Zealand at both generic and family levels suggests a genetic continuity of these areas in the past, but reduced species similarity indicates that the majority of the present oribatid mite species arose after the break-up of Gondwanaland (17 oribatid mite species found in both areas at present, have not been recorded elsewhere). Two possibilities regarding the possible land connection between these two geographical areas are (i) via what are now the sub-Antarctic islands, and (ii) via what is now the continental Antarctic. The latter is more probable in view of the disharmonic nature of the present sub-Antarctic island oribatid mite faunas. The present distribution of the most common family of oribatid mites, the Ameronothridae, in the Antarctic suggests that it is not a faunal relict but results from post-glacial recolonisation possibly from Australasia, where there are many species and high generic diversity. The sub-Antarctic islands have a

0022-2933/98 $12"00© 1998 Tay lo r& Francis Ltd.

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862 J. Star~ and W. Block

distinctly richer oribatid mite fauna than the continent, probably influenced by a longer period of habitat development uninterrupted by volcanic activity. Their re-colonisation by physiologically pre-adapted, cold-hardy taxa is likely to have occurred from South America and Australia, a long-term and often accidental process. Studies are required to determine the possible mechanisms which underlie this process.

KEYWORDS: Mites, Oribatida, Antarctica, biogeography, distribution, Gondwana.

Introduction The Antarctic continent as well as the islands contiguous to it are one of the

most interesting parts of the world from a biogeographical point of view. The theory of continental drift, the break-up of Pangaea and ultimately of the southern super- continent of Gondwanaland as well as the consequent long-term climatic isolation of the Antarctic continent resulting in approximately 98% glaciation of it at present, propound many questions regarding the historical development of the Antarctic terrestrial fauna as well as the faunas of the nearby land areas.

Was Antarctica, as a central part of Gondwanaland, an important migration route for dispersion of plant and animal taxa or was it the centre for their origin and development, which was destroyed by later massive glaciation? The biogeographical study of phylogenetically ancient groups of terrestrial cosmopolitan animals with limited dispersal abilities across oceans is important for the solution of such problems.

Soil oribatid mites are such a group. The Oribatida (Cryptostigmata) or 'beetle mites' (moss mites) are a cosmopolitan group of Acari, which comprise, in part, relicts of an ancient fauna which has existed for 150 200 m.y. as well as taxa which evolved later, but whose distributions are compatible with the movements of the continental land masses since the Permian period (Hammer and Wallwork, 1979). The majority of species are strongly sclerotised and slow-moving as adults. They are primarily fungivorous, algivorous or saprophytic. A total of 27 species has been reported in the Antarctic and 78 species in the sub-Antarctic (Block, 1984; Pugh, 1993; Block and Star~, 1996). The microclimatic characteristics of soils, especially their hydrological and thermal regimes, are critical limiting factors for the geographical distribution of oribatid mites. The High Arctic regions appear to be the most microclimatically similar part of the earth to Antarctica, especially the maritime Antarctic zone (Longton, 1988), but they have never been isolated climatically and geographically from other regions such as Europe, Asia and North America. Climatic conditions as well as ecologically similar ecosystems and biotopes may determine the structure and species composition of communities of soil oribatid mites. Polar oribatid mites have similar ecophysiological strategies, through their long pre- adaptation to low temperature conditions. However, the Arctic and Antarctic are very different in terms of species composition of their oribatid mite communities, with distinct taxa being found in each region. These taxonomic differences between the Arctic and Antarctic oribatid mite faunas suggest that they have had long and independent development without any possibility to cross the geographical and climatic barriers of the tropical and subtropical regions between the two cold polar regions. Antarctica and Australasia separated c.40m.y, ago, whilst the Arctic

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Biogeography of Antarctic oribatid mites 863

developed later at ca.3m.y, ago. On the other hand, the similarity between the oribatid mite faunas of South America, especially the Andes Mountains (Hammer, 1958, 1961, 1962a,b), and New Zealand (Hammer, 1966, 1967, 1968) poses the question of the role of the Antarctic continent and the sub-Antarctic islands in the historical development of these faunas before and after the break up of Gondwanaland.

The Antarctic region is divided into three main biological zones, based primarily on climatic differences (Holdgate, 1964, 1977; Smith, 1984): (a) the continental zone; (b) the maritime zone; (c) the sub-Antarctic zone, the last having the least severe environmental conditions.

The present study was undertaken to provide new information and analyse the present-day distributions of species of oribatid mites in the Antarctic region, to assess the similarities of the faunas of sub-Antarctic, maritime Antarctic and continental Antarctic sites, and to review the faunal relationships within and without the Antarctic region. The results contribute to an understanding of the biogeography of this ecologically important and widespread group of arthropods and to questions on the development of the terrestrial faunas of Antarctica and its nearby land areas.

Fossil mites Knowledge of fossil mites in the Antarctic is very poor. The oldest fossil oribatid

mites in the world have been found in terrestrial deposits from the Devonian period 376 379m.y. ago in north-east USA. Norton et al. (1988) described two primitive oribatid mites (Enarthronota) from these deposits. Sivhed and Wallwork (1978) recorded the brachypyline genus Hydrozetes from Lower Jurassic deposits (150 m.y. old) in Scandinavia. In addition, Krivolutsky (1973) has recorded fossil oribatid mites from terrestrial deposits of the Upper Jurassic period ( 140 m.y. old) in southern Siberia, which belong to recent families of both lower and higher oribatid mites (Trhypochthoniidae, Cymberemaeidae, Astegistidae and Achipteriidae). From the same material, Krivolutsky and Druk (1986) described species of brachypyline mites: Cultoribula jurassica Krivolutsky, 1977 and Achipteria obscura Krivolutsky, 1977. Krivolutsky (1979) mentioned other fossil records from the Mesozoic in the middle Cretaceous of northern Siberia in the families Camisiidae and Plateremaeidae. Records of mite fossils from the Cenozoic era are more abundant, especially from the Baltic, North American and Sakhalin amber, which contain recent forms or closely-related forms (Sellnick, 1918).

There are no records of fossil mites from the Antarctic region or from the other continents which comprised Gondwanaland. However, from the small amount of other fossil evidence it may be concluded that oribatid mites are a very ancient group of terrestrial arthropods with very slow rates of evolution, exhibiting radiation at the generic and family levels before the separation of Gondwanaland and Pangaea in the Cretaceous period (figure 1). These conclusions are supported by the semicosmopolitan distribution patterns of some recent genera and species of lower oribatid mites, especially the hygrophilous and tyrphophilous species Mucronothrus nasalis (Norton et al., 1996) and Trimalaconothrus horus (Sellnick, 1921) (Hammer and Wallwork, 1979) and, for example, the euedaphic Gehypochthonius rhadamanthus Jacot, 1936, all with very limited dispersal abilities (J. Star2~, unpublished).

Dispersal of oribatid mites The dispersal abilities of oribatid mites are limited by their microscopic size and

their habitats in different types of soils. Three main types of passive dispersion of

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I3

C D FIG. 1. Palaeocontinental maps of the Southern Hemisphere. (A) late Devonian (360m.y.);

(B) late Jurassic ( 140 m.y .); (C) mid-Cretaceous ( 100 m. y .); (D) present time. Modified after Smith, Hurley and Briden (1981).

oribatid mites over long distances including oceans are theoretically possible: anemohoria (wind-borne), hydrohoria (water-borne) and zoohoria (animal-borne).

Anemohoria is a very common transport mechanism for phytophagous mites (Tetranychidae), their predators (Phytoseiidae) and some plant mite parasites (Eriophyoidea). These mites are arboricolous and they have different adaptations for dispersal by air currents (Evans, 1992). There is no evidence for oribatid mites being a component of the aeroplankton around Antarctica although many insects have been trapped (Gressitt et al., 1961; Yoshimoto, et al., 1962; Yoshimoto and Gressitt, 1963). A single part specimen (prodorsum) of an unidentified oribatid mite was collected in aerial nets flown at sea near Enderby Island (Auckland Island) at 50 ° 40' S, 166 ° 25' (Yoshimoto and Gressitt, 1963). This may be due to the inability of oribatid mites to survive desiccation in dry air without contact with a moist

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substratum for long periods. However, a common frontal system experienced over the Southern Ocean, which deflects the strong prevailing westerly winds southwards towards the Antarctic Peninsula, may carry air-borne organisms and exotic propag- ules from South America as suggested by Smith (1991) and Marshall (1996). However, trapping of airborne microbiota over two years at Signy Island, South Orkney Islands in the maritime Antarctic yielded very few arthropod fragements and no complete specimens of oribatid mites (Chalmers, et al., 1996).

Transport by sea currents offers greater possibilities for oribatid mites, particularly for salt-tolerant species which are adapted for living on the seashore. Stranded tree trunks of Nothofagus spp. transported from Patagonia by the West Wind Current have been recorded on most of the sub-Antarctic islands and the South Shetland Islands (Smith, 1984). Bamboo stems carried southwards from tropical regions have been washed up on Marion and Prince Edward Islands (van Zinderen Bakker, 1971). Luxton (1964) observed tolerance to high salinities in salt-marsh Acari, whilst Weigmann (1973) and Schulte and Weigmann (1977) noted the ability of littoral mites for hypertonic regulation of their body fluids. Significant levels of tolerance of both salt and fresh water by different types of oribatid mites have been reported (Schuster, 1979); some thalassobiont species of the family Ameronothridae have the capability to survive six months submersion in sea water at 18 °C. Halozetes marinus (Lohmann, 1907) had an LTso of >20 days with continuous immersion in 25-100% seawater (Pugh, 1995).

Phoresy on some groups of insects is a very common method of mite dispersion especially for some Gamasida, Uropodina and the hypopi of Acaridida. Additionally, some gamasid mites and Actinedida are phoretic or ectoparasitic on many vertebrates. Norton (1980) surveyed phoresy in oribatid mites and reported that some adults of the genera Mesoplophora, Paraleius, Metaleius, Oppia, Euscheloribates and Tectocepheus are phoretic on Coleoptera, especially passalids, as well as Metaleius on Diptera and Mesoplophora on Dictyoptera. No records of mite phoresy on Coleoptera or any other group of higher insects have been reported for the Antarctic region and Pugh (1997) found no evidence of insect phoresy for any of the acarine colonizers.

Transport on vertebrates, especially sea-birds, would be much more advantageous for trans-oceanic crossings to aid dispersion of oribatid mites throughout the sub- Antarctic region. It is well-known that some Antarctic oribatid species are very common and abundant near or in the nests of sea birds. Wallwork (1972, 1976) reported large differences between the frequency of different oribatid species in nests of sea birds at South Georgia, with 12 out of a total of 18 oribatid species being found in such habitats and five species showing a preference for such nests. Covarrubias (1968) and Convey and Quintana (1997) recorded three species of oribatids from sea-bird nests in the South Shetland Islands and the Antarctic Peninsula. Oribatid mites are the superdominant (dominance=98.5%) group of arthropods in this type of habitat, but this may reflect trophic relationships rather than indicate a dispersal mechanism (Marshall and Pugh, 1996). Strong (1967), however, noted similar oribatid mite affinities with sea bird nests especially Alaskozetes antarcticus (Michael, 1903) and he found this species in the feathers of a fleshly-killed Antarctic skua. Pugh (1997) concluded that flying sea birds are the most effective vectors of zoohoric mites to the sub-Antarctic islands.

Biogeography of Antarctic oribatid mites The biogeography of oribatid mites in the Antarctic region and sub-Antarctic

islands has been aided by a series of papers by Wallwork (1967, 1973, 1976). The

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866 J. Star) and W. Block

fauna of this area can be divided into three zones (after Smith, 1984), which are further separated by the number of species and genera as well as the level of endemism. The fauna of the continental Antarctic zone is characterized by a high level of generic and specific endemism and the four species (Coetzee, 1997), of the genus Maudheimia are relicts which probably survived the Pleistocene glaciation, that eliminated other oribatid taxa (Marshall and Pugh, 1996). A low level of species endemism occurs in the maritime Antarctic zone which appears to have a transition fauna with possible penetration of elements from the sub-Antarctic zone, which itself has a distinctly higher number of endemic species. Conspicuous differences are seen between the oribatid mite faunas of older continental and younger oceanic islands. The main penetration of elements from the surrounding temperate areas is most likely from South America to South Georgia and from New Zealand to Macquarie Island.

Material and methods

Sample material The present analyses are based on the determination of extensive collections of

oribatid mites in the British Antarctic Survey's Data and Resource Centre, Cambridge, U.K., with some comparative material from the Division of Entomology, CSIRO, Canberra, Australia, and on published records of oribatid mites, which were compiled from many different literature sources for the whole Antarctic region, the Andes Mountains and New Zealand. Figure 2 shows the areas and geographical regions covered in this study. A total of 19,205 specimens of oribatid mites was determined to species level in samples collected from 30 localities in the Falkland Islands, the Scotia Arc area, Bouvetoya, Peter I Oy, Heard Island and numerous other locations in the Antarctic region. A total of 49 species was found. The oribatid fauna of some localities (Beauch6ne, Fredriksen, Atriceps, Vega, Cockburn, Seymour, Brabant, Adelaide, Lagoon, L6onie, Horseshoe, Alexander and Peter I islands) was studied for the first time. Many are new faunal records and five new species were described (Star~, 1995; Star~ and Block 1995, 1996; Block and Star~, 1996; Star~, et al., 1997).

For the analyses of faunal similarity a list of oribatid mite species, genera and families from a wide geographical area ranging from the Ecuadorian Andes Mountains, through the Antarctic region, including the sub-Antarctic islands, to the islands of New Zealand was compiled. The list also included data from Amsterdam and St Paul Islands in the South Indian Ocean. For the Neotropical region only the faunas of Tierra del Fuego, Patagonia and the Andes Mountains were included, because only from these areas are there records of oribatid mites in common with the Antarctic region. Also, the information available for the oribatid fauna of other parts of South America is comparatively poor and fragmentary, thus precluding the possibility of significant comparison. The region of the Andes Mountains was divided into six sections using semi-artificial boundaries except for the Colombian sector, where the oribatid fauna is much less well known than the other sections. The oribatid fauna of Australia, and especially of Tasmania, was not included for the same reason. A list of 95 families, 292 genera and 1001 species of oribatid mites was used for the analysis of faunal similarity. These data were extracted from our studies of the oribatid mites of the Antarctic region and Falkland Islands (loc. cit.), the

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publications of Hammer (1958, 1961, 1962a,b, 1966, 1967, 1968), Luxton (1967), Spain and Luxton (1971), Balogh and Balogh (1988, 1990) and Pugh (1993).

Faunal similarities The aim of this analysis was to examine the similarity between sites using

presence-absence data of oribatid mites at the family, genus and species levels. The data originated from a variety of sources and are therefore likely to vary in quality. In particular, the absence of a species may in some cases be a reflection of low sampling intensity. The main aspects of the analysis were:

a) calculation of similarity coefficients between sites; b) principal co-ordinate analysis to provide a graphical display of the similarity

between sites; c) hierarchical classification of the sites. Similarity coefficients. Various similarity coefficients have been proposed for

presence-absence data (Digby and Kempton, 1987). For two sites A and B with presence-absence species data the coefficients utilise the following statistics:

a the number of species occurring at both A and B; b the number of species occurring at A but not at B; c the number of species occurring at B but not at A; and d the number of species absent from both A and B.

In practice, it is usually helpful to present similarity coefficients as a triangular matrix with elements showing the similarity between pairs of sites. Sometimes the main features of the analysis can be seen from the matrix of similarity coefficients, which were calculated for family, genus and species data. The Jaccard coefficient (Sj) was utilised to examine faunal similarities in the oribatid mite data. It is equal to the ratio of the number of co-occurrences (a) to the total number of occurrences for the two sites (a+b+c), i.e.

a

sj= (a+b+c)

Co-absences (d) do not contribute to similarity between two sites and this may be ignored.

Principal co-ordinate analysis'. Principal co-ordinate analysis (PCA) is a method for representing sites as a configuration of points such that the distances between the points reflect the similarities between the sites--those which are similar have points that are close together in the configuration. More precisely in PCA, the square of the distance (D) between points is related to the similarity coefficient (S) by D2=2(1 --S). Generally, with more than four points a three-dimensionai plot is required. As the number of sites increases so does the number of required dimensions and the configuration becomes impossible to visualize. In practice, the main features can be seen in two- or three-dimensional plots. Thus, in some cases PCA may provide a useful diagrammatic representation of the similarity between the sites. This may be convenient for presenting conclusions in relation to other information such as the spatial locations of the sites, indicating possible pathways for mite colonization, etc. PCA analyses were undertaken for family, genera and species data using the Jaccard coefficient. Analyses were implemented using the statistical package Genstat 5 (Payne, 1987).

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868 J. S t a r ) and W. Block

SG

FIG. 2.

(a)

Maps of the areas included in the present study. (2a) South America showing the seven sectors (together with the Falkland Islands

(FI ) and South Georgia (SG)) used in this study. EM: Ecuadorian Andes; PM: Peruvian Andes; BM: Bolivian Andes; NM: north Argentinian and Chilean Andes; MM: central Chilean and Argentinian Andes; PT: Patagonia; TF: Tierra del Fuego.

(2b) the Antarctic Region including the sub-Antarctic islands. Bouvet I. = Bouvet~ya; Peter I I. = Peter I Oy; Antarctic Pen. = Antarctic Peninsula which covers Graham Land in its northern half and Palmer Land in the south.

Results

Distribution patterns o f oribatid mites in the Antarctic and sub-Antarctic The mos t pr imi t ive Fami ly A r c h e o n o t h r i d a e has two genera Andacarus and

Stomacarus with four species in the An ta rc t i c region. Thei r d i s t r ibu t ions are shown

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Biogeography of Antarctic oribatid mites

, G o u g h I 45

869

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60

: Prince Edward I I \

K e r g u e l e n Is

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Chatham Is w

Macquane I

* A n t i p o d e s iS

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1~0

(b)

N E W Z E A L A N D \ ~ / AUSTRALIA

in figure 3 a. They have been found in the eastern sub-Antarctic islands and on islands to the south of New Zealand. Stomacarus macfarlanei Grandjean (1957) has been recorded in the Neotropical Region at Tucuman in northern Argentina. Andacarus ligamentifer Hammer, 1967 was described from New Zealand (Hammer, 1967) as was Stomacarus ciliosus Luxton, 1982 by Luxton (1982). Mites in the Family Archeonothridae, which is one of the most primitive in the oribatid mites, have semicosmopolitan distributions. The family is not recorded from the Oriental region. Archeonothrus natalensis Tr~igfirdh, 1906 is found in South Africa and Loftacarus siefi Lee, 1981 in southern Australia•

The Family Phthiracaridae has a cosmopolitan distribution, but is represented by only a single species Phthiracarus (Neophthiracarus) neotrichus Wallwork, 1966

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870 J. StarT~ and W. Block

from Campbell Island in the Antarctic region (figure3a). The subgenus Neophthiracarus also has a semicosmopolitan distribution and is represented by Phthiraearus (Neophthiracarus) insignis Balogh and Csiszar, 1963 in the Neotropical region (Balogh and Csiszar, 1963).

The cosmopolitan Family Brachychthoniidae (figure 3b) is represented in the Antarctic region only by two genera with four species, but there are two records determined only to the genus. From the distribution patterns of this taxon a strong affinity to the Neotropical region is evident. Interesting distribution patterns are shown by the semicosmopolitan species Eobrachychthonius oudemansi Hammen, 1952 known from the Holarctic region and Liochthonius fimbriatissimus (Hammer, 1958) recorded from the Andes Mountains as well as New Zealand. Liochthonius australis Covarrubias, 1968 is probably endemic to the sub-Antarctic and the South Shetland Islands. It is interesting that no species of the Family Brachychthoniidae have been recorded from Macquarie and Campbell Islands, because brachychthoniid mites are common in the mite fauna of New Zealand, which also has four species in common with the South American Andes.

The Family Crotoniidae (figure 3 c), which is very important from a biogeographical point of view (Hammer and Wallwork, 1979) is represented in the Antarctic region by two genera and four species. Species of this family are found in Central America, Mexico, New Zealand, eastern Australia (parts of former Gondwanaland) and islands such as Campbell, Macquarie and Falkland Islands in the Southern Ocean.

The cosmopolitan Family Camisiidae (figure 3c) is represented in the Antarctic by two cosmopolitan genera Platynothrus and Camisia each with a single species and one subspecies Platynothrus skottsbergi expansus Wallwork, 1966 from South Georgia, whose validity is unclear. The distribution of Platynothrus skottsbergi Tragfirdh, 1931 shows this to be a very common species in the southern Andes and

FIG. 3. Distribution patterns of oribatid mites in the Antarctic and sub-Antarctic regions, and the Falkland Islands.

(3a) Families Archeonothridae and Phthiracaridae: Andacarus campbellensis Wallwork, 1966 (1); Andacarus watsoni Trav~, 1964 (2); Stomacarus macjarlani (Grandjean, 1957) (3); Phthiracarus (Neophthiracarus) neotrichosus Wallwork, 1966 (4).

(3b) Family Brachychthoniidae: Eobrachychthonius oudemansi Hammen, 1952 (1); Eobrachychthonius sp. (2); Liochthonius australis Covarrubias, 1968 (3); Liochthonius fimbriatissimus (Hammer, 1962) (4); Liochthonius mollis (Hammer, 1958) (5); Liochthonius sp. (6).

(3c) Families Crotoniidae, Camisiidae and Malaconothridae: Crotonia brevicornutus (Wallwork, 1966) (1); Crotonia sp. (2); Holonothrus concavus Wallwork, 1966 (3); Holonothrusfoliatus Wallwork, 1963 (4); Camisia segnis (Hermana,1804) (5); Camisia sp. (6); Platynothrus skottsbergi Trfiggtrdh, 1931 (7); PIatynothrus skottsbergi expansus Wallwork, 1966 (8) ; Malaconothrus translamellatus Hammer, 1958 (9); Trimalaconothrusflagelliformis Wallwork, 1970 ( 10); Trimalaconothrus novus (Sellnick, 1921 ) (11); Fossonothrus wallworki Star~ and Block, 1995 (12).

(3d) Famil ies Nanhermanniidae, Hermanniidae, Hermanniellidae, Gymnodamaeidae, Plateremaeidae, Metrioppiidae, Ceratoppiidae, Tectocepheidae and Nodocepheidae: Nanhermannia elegantissima Hammer, 1958 (1), Phyllhermannia falklandica Balogh, 1988 (2); Hermanniella sp.(3); Allodamaeus sp.(4); Pheroliodes austral& (Hammer, 1962) (5); Pheroliodes sp.(6); Macquarioppia striata (Wallwork, 1963) (7); Ceratoppia sp. (8); Teetocepheus velatus (Michael, 1880) (9); Nodocepheus dentatus Hammer, 1958 (10).

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C~

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in Patagonia and that it penetrates to western sub-Antarctic islands. It has also been recorded in New Zealand (Luxton, 1985). Camisia segnis (Hermann, 1804) which has been recorded from many localities in the Andes Mountains was recently synonymised with Camisia nova Hammer, 1966 which occurs in the North Island of New Zealand (Colloff, 1993). It is very probable that specimens of Camisia segnis identified from South Georgia and the Falkland Islands (Star~ and Block, 1995, 1996) are C. nova. The distribution of Camisia nova suggests an affinity of some components of the Neotropical oribatid mite fauna, especially Patagonia, with that of New Zealand. Other lower oribatid taxa show similar patterns (e.g. Liochthonius fimbriatissimus).

The Family Malaconothridae (figure3c) has two cosmopolitan genera Malaconothrus and Trimalaconothrus, which have 12 and nine species, respectively, in the Neotropical region, and three and eight species, respectively, in New Zealand. There are four species of Trimalaconothrus common to both regions. The records show a strong component of Neotropical elements in the oribatid fauna of the Falkland Islands as well as in that of South Georgia. An interesting record is that of a very common South American species, Malaconothrus translamellatus Hammer, 1958 on the relatively young volcanic Amsterdam Island in the south Indian Ocean. The genus Fossonothrus is represented by a single species in southern Chile and another in New Zealand with a third species on South Georgia (Star?~ and Block, 1995). The distribution patterns of these lower oribatid mite genera suggest, once again, close affinities of the faunas of the sub-Antarctic islands with South America and New Zealand.

Records of species in the genera Nanhermannia and Phyllhermannia suggest strong links between the Falkland Islands' oribatid mite fauna and that of South America. Species of the genus Hermanniella (figure 3 d) recorded from the young volcanic Marion Island are interesting because this genus is not found in the Neotropical Region and it may be an immigrant from Africa or a species introduced by Man. The distribution of Pheroliodes australis (Hammer, 1962), as well as other species in the genus Pheroliodes, may indicate an historical connection between South America and New Zealand and the possible migration of this oribatid mite between these areas through the sub-Antarctic islands. On the other hand, the species may date from Gondwanaland. Nodocepheus dentatus Hammer, 1958 may have migrated by a similar route also. The monotypic genus Macquarioppia is endemic to the eastern sub-Antarctic area.

One of the dominant oribatid families in the maritime and especially the sub- Antarctic zone is the cosmopolitan Family Oppiidae (figures 4 a, b). Four elements can be deduced from its distribution. A Neotropical element which has penetrated from South America to the Falkland Islands and into the western and possibly also the eastern sub-Antarctic (Globoppia intermedia Hammer, 1962, Globoppia maior Hammer, 1962 etc.); a sub-Antarctic element (Austroppia crozetensis (Richters, 1908), Globoppia intermedia longiseta Wallwork, 1970, etc.); a group of endemic species (Globoppia scotiae (Wallwork, 1970), Globoppia gressitti Wallwork, 1964, Globoppia campbellensis Wallwork, 1964, etc.); and a maritime Antarctic element (eg. Globoppia loxolineata (Wallwork, 1965)), which has also been recorded recently from Heard Island (Star~, Block and Greenslade, 1997). Records of the genera Solenoppia on the Falkland Islands and Lanceoppia on South Georgia are also interesting in this respect, but unfortunately the higher classification of this family is unclear, so it is not possible to draw firm conclusions at present.

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The second dominant family of oribatid mites in the Antarctic region is the Family Ameronothridae (figures 4 c, d; 5 a). The majority of its genera and species have affinities to marine coastal habitats. It is represented in the Antarctic region by four genera with 23 species and subspecies, and many of them are the commonest oribatid mites in this region. The genus Alaskozetes has a very rare and peculiar distribution pattern, because Alaskozetes antarcticus (Michael, 1903) has a wide circumpolar distribution in the Antarctic region and it has been recorded on the South Island of New Zealand (Luxton, 1990), whilst the type species Alaskozetes coriaceus Hammer, 1955 has been found only on the northern coast of Arctic Alaska. It is the only example of a bipolar distribution of an oribatid mite genus. Alaskozetes antarctieus has two subspecies and the nominate, with limited distributions, in the Antarctic. A. a. intermedius Wallwork, 1967 is found in the Falkland Islands, Bouvetoya in the maritime Antarctic and on sub-Antarctic islands from South Georgia to the Kerguelen Archipelago, whilst A. a. grandjeani Dalenius, 1958 has been recorded only from Heard and Macquarie Islands. Morphological differences between all the species and subspecies of the genus Alaskozetes are very small and a detailed, comparative study of the morphology and variability of this genus is required.

The genus Podacarus is endemic to the sub-Antarctic islands and the genera Antarcticola and Halozetes are endemic to both the Antarctic and sub-Antarctic region. The genus Halozetes (figures 4 d, 5 a) is especially rich in species which have different ranges and distribution patterns, from the circumpolar distribution of Halozetes belgicae (Michael, 1903) to the endemic pattern of H. impeditus Niedbala, 1986, H. plumosus Wallwork, 1966 and H. necrophagus Wallwork, 1967. Comparison of the species richness of this genus distinctly shows most species to be present in the eastern sub-Antarctic. Similar trends are seen in the distributions of other genera

FIG. 4. Distribution patterns of oribatid mites in the Antarctic and sub-Antarctic regions and the Falkland Islands.

(4a) Family Oppiidae: Globoppia campbellensis Wallwork, 1964 (1); Globoppia gressitti Wallwork, 1964 (2); Globoppia intermedia Hammer, 1962 (3); Globoppia intermedia longiseta Wallwork, 1970 (4); Globoppia maior Hammer, 1962 (5); Globoppia scotiae (Wallwork, 1970) (6); Globoppia loxolineata (Wallwork, 1965) (7); Solenoppia pernettyae Balogh, 1988 (8); Solenoppia usheri Balogh, 1988 (9); Lanceoppia elegantula Star~ and Block, 1995 (10).

(4b) Family Oppiidae: Belloppia beemanensis (Wallwork, 1964) (1); Austroppia crozetensis (Richters, 1908) (2); Austroppia crozetensis anareensis (Wilson, 1958) (3); Subiasella diaphora (Wallwork, 1964) (4); Membranoppia disjuncta (Wallwork, 1964) (5); Gressittoppia pepitensis brevipectinata (Covarrubias, 1968) (6); Rectoppia dispari- seta (Hammer, 1958) (7), Oppia nitens brachytrichinus Wilson, 1958 (8); Oppia sp.1 (9); Oppia sp.2 (10).

(4c) Family Ameronothridae: Alaskozetes antarcticus (Michael, 1903) (1); Alaskozetes antareticus intermedius Wallwork, 1967 (2); Alaskozetes antarcticus grandjeani Dalenius 1958 (3); Alaskozetes sp. (4); Antarcticola georgiae Wallwork, 1970 (5); Antarcticola meyeri Wallwork, 1967 (6); Antarcticola sp. (7); Podacarus auberti Grandjean, 1955 (8); Podacarus auberti occidentalis Wallwork, 1966 (9).

(4d) Family Ameronothridae: Halozetes belgicae (Michael, 1903) (1); Halozetes belgicae brevipilis Wallwork, 1963 (2); Halozetes belgicae longiseta Wallwork, 1967 (3); Halozetes crozetensis (Richters, 1908) (4); Halozetes edwardensis Pletzen and Kok, 1971 (5); Halozetesfulvus Engelbrecht, 1975 (6); Halozetes impeditus Niedbala, 1986 (7); Halozetes intermedius Wallwork, 1963 (8); Halozetes littoralis Wallwork, 1970 (9).

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3~4 ~ 10 •

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in the family Ameronothridae, which are very diverse in the Australian, and New Zealand regions to the east (Capillibates in New Zealand, Chudalupia--Australia, Pseudoantarcticola--New Guinea), whilst there are no records of this family from the Neotropical region.

The distribution patterns for oribatid mites of the families Oribatulidae (figure 5b), Haplozetidae (figure 5c), Oribatellidae (figure 6b) and Parakalummidae (figure 6b) show differing trends. For example, there is the very local distribution of two species in the genus Maculobates on Tierra del Fuego, the Falkland Islands and the South Shetland Islands compared with the wide distribution of the genus Totobates from South America to Macquarie and Campbell Islands. In addition, the genera Maculobates and Totobates are found in South America as well as in New Zealand, but neither is common in both areas. The genus Sandenia is endemic to the sub-Antarctic islands and the older records of Galumna alata (Hermann, 1804) from the Antarctic are very probably mis-identifications of species of this genus (J. Star), unpublished).

The Antarctic region, especially the continental zone, has only a single endemic family (Maudheimiidae) (figure 6a) with the endemic genus Maudheimia containing four species found on widely-separated mountain ranges inland on continental Antarctica (see Marshall and Pugh, 1996; Coetzee, 1996, 1997 for discussion).

The cosmopolitan family Ceratozetidae (figures. 5d; 6a is represented in the Antarctic region by seven genera containing ten species. These can be divided into four main groups. An abundant Neotropical group from South America which reaches as far as the South Sandwich and South Shetland Islands, e.g. Edwardzetes dentifer Hammer, 1962. Those found in the sub-Antarctic and maritime Antarctic form a second group comprised of Magellozetes antarcticus (Michael, 1895) and Magellozetes processus Hammer, 1962, which are probably conspecific, and these species also occur on Tierra del Fuego and near Punta Arenas in southern Chile. Some species and genera are endemic, such as Scotiazetes bidens Wallwork, 1966 and Ceratozetes gaussi (Richters, 1908) and form a third group, whilst some occur or have affinity with the New Zealand fauna, e.g. Macrogena monodactyla Wallwork, 1966, and constitute a fourth group.

Biogeography of oribatid mites in the three Antarctic zones Records of the species of oribatid mites from the three zones of the Antarctic

region, namely the continental Antarctic, the maritime Antarctic and the sub- Antarctic are presented here. Data from other areas, especially the southern cold temperate, which may serve as a source of possible immigration of oribatid mites into the Antarctic region, are also included. These areas include the Falkland Islands, the Andes Mountains and New Zealand. The basic characteristics of all the areas examined with their geographical positions are listed in table 1 and shown in figure 2. The data for oribatid mite diversity and endemism at the family, genus and species level are shown in table 2.

Continental Antarctic zone. The characteristic features of this zone are the extensive glaciation (>98% of the surface being covered by ice and snow) and extreme climatic conditions resulting in ecologically as well as geographically limited living space for soil animals. Suitable terrestrial habitats are found only in a very narrow band along the coastal margins and in isolated groups of nunataks inland, often at considerable distances from the sea. Such extensive ice and permanent snow cover results in the elimination of ecological niches suitable for colonisation by

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Table 1. Geographical position, surface area, glacial status and age of oldest rocks on maritime and sub-Antarctic islands, and the Falkland Islands (Headland, 1984; LeMasurier and Thomson, 1990; Selkirk, et al., 1990).

Surface Latitude area Glaciated Ice-free Age

Island/island group (south) (km 2) (%) (km 2) (m.y.)

South Sandwich Is. 56°30 59°47 310 80 62 3.1-0"3 South Orkney Is. 60 ° 50 60 ° 83 622 85 93 5-0 2-3 South Shetland Is. 61 ° 00 63 ° 37 7215 80 1203 29-5-0"001 Bouvetoya 54 ° 42 54 93 4 5-0 4.5 Peter I COy 68 ° 85 157 95 8 12-8 Falkland Is. 51 ° 00-52 ° 30 14567 0 14567 200 South Georgia 53°50 55°00 3755 57 1615 120 Prince Edward Is. 46°60-46°97 317 0 317 0-45-0"11 Crozet Is. 45 ° 95-46 ° 50 325 0 325 9-0 l'3 Kerguelen Is. 48 ° 58 49 ° 73 4662 10 4196 40-25 Heard Is. 52 ° 97-53 ° 20 385 80 77 20 Amsterdam Is. 37 ° 83 85 0 85 0-7 Saint Paul Is. 38 ° 72 7 0 7 0-5 Macquarie Is. 54 ° 62 128 0 128 10-0.5 Campbell Is. 52 ° 55 113 0 113 1-1 1"5

oribatid mites. These unfavourable ecological conditions restrict the oribatid mite fauna, with a high propor t ion o f endemism occurr ing at all taxonomic levels. The endemic Family Maudheimiidae has low species diversity and increased endemism at the family level compared with the New Zealand and the Neotropical oribatid mite faunas. However, the validity o f this family is not clear at the present time. The endemism of genera in the continental Antarct ic zone is similar to that in these two areas, but specific endemism is much lower than in the continental Antarctic. It is thought that the inland acarofauna of continental Antarct ica (e.g. Dronn ing M a u d Land) had a different origin f rom that o f the marit ime Antarctic, being

Fro. 5. Distribution patterns of oribatid mites in the Antarctic and sub-Antarctic regions and the Falkland Islands.

(5a) Family Ameronothridae: Halozetes macquariensis (Dalenius and Wilson, 1958) ( 1 ); Halozetes marinus (Lohmann, 1907) (2); Halozetes marinus devilliersi Engelbrecht, 1974 (3); Halozetes marinus minor Wallwork, 1966 (4); Halozetes marionensis Engelbrecht, 1974 (5); Halozetes necrophagus Wallwork, 1967 (6); Halozetesplumosus Wallwork, 1966 (7); Halozetes sp.1 (8); Halozetes sp.2 (9).

(5b) Family Oribatulidae: Campbellobates acanthus" Wallwork, 1964 ( 1 ); Dometorina marionensis Pletzen and Kok, 1971 (2); Paraphauloppia sp. (3); Oribatula australis (Hammer, 1962) (4); Zygoribatula subantarctica Pletzen and Kok, 1971 (5); Scheloribates flageIlatus Wallwork, 1966 (6).

(5c) Family Haplozetidae: Maculobates breviporosus Mahunka, 1980 (1); Maculobates nordenskjoeldi (Trfigfirdh, 1908) (2); Totobates anareensis (Dalenius, 1958) (3); Totobates elegans (Hammer, 1958) (4)," Totobates elegans antarcticus Wallwork, 1966 (5); Totobatesmarionensis Pletzen and Kok, 1971 (6); Tuxenia manan- tialis Hammer, 1962 (7).

(5d) Family Ceratozetidae: Ceratozetes gaussi (Richters, 1907) (1); Edwardzetes dentifer Hammer, 1962 (2); Edwardzetes elongatus Wallwork, 1966 (3); Edwardzetes australis Star) and Block, 1995 (4); Granizetes curvatus Hammer, 1961 (5); Macrogena monodactyla Wallwork, 1966 (6); Furcobates lewissmithi Star) 1995 (7); Magellozetes antarcticus (Michael, 1895) (8); Magellozetes processus Hammer, 1962 (9).

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composed of pre-Pleistocene endemic oribatid mite species (Marshall and Pugh, 1996).

Maritime Antarctic zone. This zone may be divided into two parts, the Antarctic Peninsula and the northern maritime Antarctic islands. An account of the biota of the western Antarctic Peninsula is given by Smith (1996). The zone has a higher number of oribatid mite taxa, but endemism at all levels is much lower, than the continental Antarctic zone. The maritime Antarctic zone has only four endemic species and endemism is the lowest of all three Antarctic zones. The Antarctic Peninsula, which is poorer in the number of taxa than the maritime Antarctic islands, shows a trend of increasing number of taxa from south to north (from Palmer Land to Graham Land). The relatively younger volcanic and isolated Bouvetoya and Peter I Oy are extensively glacierized with extremely small land areas devoid of ice. Only a single species of oribatid mite has been found on these two islands. It seems that the more northerly location of Bouvetoya and the older geological age of Peter I Oy have not influenced their faunal richness. The South Shetland Islands have only one endemic species Halozetes impeditus, which may be a result of the larger ice-free area of this island group. Generally endemism is very low for the oribatid mite fauna in the maritime Antarctic zone. The greater geological age of some of its islands may not have been important, because older and closer continental land masses such as Graham and Palmer Land, although possessing similar habitats, do not have a richer fauna and are without endemic species.

Sub-Antarctic zone. The sub-Antarctic zone has a higher oribatid mite diversity than the maritime Antarctic zone, but a lower diversity than the Neotropical Region and New Zealand. The levels of specific and generic endemism are comparable with the continental Antarctic zone, but no endemic family has been found in the sub- Antarctic zone. The numbers of taxa recorded from the western, central and eastern sectors of the sub-Antarctic are similar, but no endemic oribatid mites have been found in the central sector, unlike the other two sectors. Similar levels of generic endemism are observed for the western and eastern sectors, although the eastern sector has more endemic species. The most important influences on the level of endemism in this zone are the younger geological age as well as the oceanic character of the volcanic islands comprising the central group, including the Kerguelen Islands. The western sector is formed by South Georgia, which was geologically connected with the southernmost part of South America until c. 30m.y.a, and Macquarie Island to the east, which was formed by the uplift of the ocean floor 30 25 m.y.a, without conspicuous volcanism. When the faunas of the islands in the central sector

FIG. 6. Distribution patterns of oribatid mites in the Antarctic and sub-Antarctic regions and the Falkland Islands.

(6a) Families Ceratozetidae, Maudheimiidae and Haplozetidae: Maudheimia petronia Wallwork, 1962 (1); Maudheimia wilsoni Dalenius, 1958 (2); Maudheimia marshalli Coetzee 1997 (3); Maudheimia tanngardenensis Coetzee 1997 (4)," Porozetes polygonalis Hammer, 1962 (5); Porozetes polygonalis quadrilobatus Wallwork, 1966 (6); Scotiazetes bidens Wallwork, 1966 (7); Sphaerozetes sp. (8); Antarctozetes crozetensis (Richters, 1907) (9); Antarctozetes sp. (10); Cryptobothria monodactyla Wallwork, 1963 (11 ); Neomycobates tridentatus Wallwork, 1963 (12).

(6b) Families Oribatellidae, Galumnatidae and Parakalummidae: Oribatella palustris Hammer, 1962 (1); Oribatella blocki Star~, 1995. (2),' Galumna alata (Hermann, 1804) (3); Galumna sp. (4); Sandenia georgiae (Oudemans, 1914) (5); Porokalumma rotunda Wallwork, 1963 (6).

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,9

67 ,. 9,,lo

8,z9

11 12,

(a) k,

354 ",) 6 ~

6",,

i .

(b)

6~3

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884 J. Starj~ and W. Block

0.70

0.23 P~ = ~ ' i is~ L i . c , i ~ ' NN og

/ ]~ .... i :fz''~ ..................... /~: i I Hi'i~ / ....

. . . . %. 7-o . . . . . . . . . . / ' , / - . . . . . . . . . . . . . . / . . . . .

..... ~.~o ....

FIG. 7.

(a)

Three-dimensional ordination graphs of the oribatid mite similarities for the Antarctic and sub-Antarctic regions, the Andes Mountains and New Zealand. (a) Families. (b) Genera. (c) Species. AC: Antarctic continent; AI: Amsterdam and Saint Paul Islands; BI: Bouvetoya; BM: Bolivian Andes; CA: Campbell Island; CI: Crozet Island; EM: Ecuadorian Andes; FI: Falkland Islands; GL: Graham Land; HI: Heard island; KI: Kerguelen Island; MI: Marion and Prince Edward Islands; MM: middle/central Argentinian Andes; MQ: Macquarie Island; NM: northern Argentinian Andes; NN: North Island of New Zealand; NS: South Island of New Zealand; PI: Peter I Oy; PL: Palmer Land; PM: Peruvian Andes; PT: Patagonia; SG: South Georgia; SH: South Shetland Islands; SO: South Orkney Islands; SS: South Sandwich Islands; TF: Tierra del Fuego.

are compared, the distinctly higher land area of the Kerguelen Islands does not seem to have influenced either the number of taxa or the number of endemic species. Heard Island occupies an exceptional position. It is a very young volcanically-active island, not too remote from the Kerguelen Islands, but much colder and heavily glacierized. Its oribatid mite fauna is similar to that of islands in the marit ime Antarctic zone in lacking endemism and having few taxa.

Influence of geographical position, geological age and glacierization on the oribatid mite fauna

Latitude appears to influence the number of oribatid mite taxa only along the Antarctic Peninsula in the maritime Antarctic zone, where southern Palmer Land

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-0.70 ~-~ ,"

0.23 ~-~-~_~

AXIS1

' , /

4" . . . . :i

& O (F?, , ' " ~!

• : L .

/-<"

- 0 . 7 0

Qi

KI HII

• 2 •

/ /

/

FIG. 7. (continued).

(b)

has much lower numbers of species, genera and families than the more northerly situated Graham Land. Latitude does not seem to have determined the fauna of Antarctic islands, for example Heard Island (sub-Antarctic zone), has an oribatid mite fauna similar to islands in the maritime Antarctic zone; at all taxonomic levels it is like the more southerly South Sandwich Islands and South Orkney Islands. The South Shetland Islands have a comparable fauna also, but unlike other areas in this zone, few endemic species have been found, which may be related to the larger ice- free land area and greater age of some of these islands.

The sub-Antarctic zone shows several trends which set it apart from the other zones. Again, latitude does not seem to influence the composition of the oribatid mite fauna because Campbell Island, situated farther to the south than Crozet and Prince Edward Islands, has a comparable faunal diversity, but a distinctly higher species endemism. Geological age is more important. The older 'continental' island of South Georgia as well as 'semicontinental' Macquarie Island have endemic genera unlike other oceanic volcanic islands and species endemism on continental islands is higher than on oceanic ones (excluding Campbell Island). The oribatid mite fauna of Campbell Island is intermediate between those of the sub-Antarctic islands and New Zealand. Oceanic islands of the central sector of the sub-Antarctic zone,

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AXIS3

O170

q~.__ SGI

, ill

I

PT

/" ~7- 0.70 / /

/

--'/0.23

AX~S2

--0.23

--0.70

/

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/ . . . . I / . - . . . .

o. 2 ~ - ~ - ~ ." / / ~---~'-_. // ~ - + -0.70

-0.70

FIG. 7. (continued).

(c)

including Crozet and Kerguelen Islands, do not show differences in species richness or endemism, probably because of their older geological age and larger ice-free areas. Conversely, the geologically younger Prince Edward Islands have greater species endemism than the older and larger Kerguelen Islands.

Oribatid mite faunal relations within the Antarctic region Relationships and faunal similarities at the family level are shown in figure 7 a.

The faunas of the continental Antarctic and Palmer Land are well separated. The faunas of the young, isolated volcanic Peter I Oy and Bouvetoya are identical at the family level and closely related to that of the continental Antarctic. The maritime Antarctic, including Graham Land as well as the sub-Antarctic islands (Campbell and the Falkland Islands are without the sub-Antarctic zone) form separate groups. The maritime Antarctic group is very well consolidated. The widely-separated sub- Antarctic island group does not cluster and South Georgia is most similar to the Falkland Islands and to the maritime Antarctic group. The faunas of Crozet and Kerguelen Islands appear to be least similar to continental Antarctica. Heard Island occupies an intermediate position between the sub-Antarctic and continental Antarctic groups.

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Relationships and faunal similarities at the generic level are shown in figure 7b. In general, similar groups are formed as at the family level, but these are not so consistent, especially in the sub-Antarctic group. South Georgia is more similar to the Falkland Islands and to the islands of the maritime Antarctic (except Bouvetoya and Peter I Oy) than to other sub-Antarctic islands. The oribatid fauna of Heard Island appears again in an intermediate position between the sub-Antarctic and the continental Antarctic groups and the maritime Antarctic group respectively, which have a high similarity. Crozet Island appears to be very different from both the maritime and continental Antarctic faunas.

Relationships and faunal similarities for species are shown in figure 7 c. They are different to those of the families and genera, as continental Antarctica and Palmer Land are not strictly separated from the maritime Antarctic, and the fauna of the South Sandwich Islands has a closer affinity with the sub-Antarctic group especially South Georgia. Within the framework of the sub-Antarctic group a west-east gradient can be seen and the fauna of South Georgia is similar to that of the Falkland Islands and the South Sandwich Islands. Campbell Island is located in a comparatively isolated position whilst Peter I Oy and Bouvetoya are quite separate from the continental and maritime Antarctic at the species level.

Relations of the Antarctic oribatid mite fauna with nearby land areas Figure 7 a demonstrates the relationships and faunal similarities at the family

level. The ordination diagram divides the localities into four distinct groups. New Zealand is closely allied with the Andes Mountains localities, but excludes Tierra del Fuego and the Falkland Islands, which occupy an intermediate position with another group and are more similar to the sub-Antarctic islands. The latter group is not particularly consistent, especially South Georgia, which lies intermediate to the maritime Antarctic islands group (including the South Sandwich Islands, South Orkney Islands, South Shetland Islands and Graham Land) and which is more similar at the family level to this group than to other sub-Antarctic islands. Heard Island is also intermediate lying between the sub-Antarctic islands, the maritime Antarctic islands and continental Antarctica, Palmer Land, Bouvetoya and Peter I Oy, but it is most similar to the continental Antarctic group.

Generic level relationships and faunal similarities are shown in figure 7 b. The ordination produces three main faunal groups. The first group contains New Zealand together with the Andes Mountains. Unlike at the family level, the character of this group is more distinct, with New Zealand on one side opposite Patagonia and Tierra del Fuego. The Falkland Islands lie between the second group of sub-Antarctic islands, which again show a clear relationship to Tierra del Fuego through South Georgia and on to New Zealand through Campbell Island. Heard Island lies intermediate between islands of the sub-Antarctic and a third group, which is formed by the maritime Antarctic islands and the Antarctic continent. The South Orkney Islands fauna appears to b e m o s t similar to that of Heard Island as well as to the sub-Antarctic group.

Species relationships and faunal similarities are given in figure 7 c. The data are aggregated again into three main groups, but there are some interesting features apparent which arc different from those of the previous taxonomic levels. The first group of South American localities has a distinct south--nor th gradient and the position of the Falkland Islands indicates a strong relation to the second distinctly separated and more consistent group of sub-Antarctic islands. However, South

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888 J. Star~, and W. Block

Georgia does not occupy a distinct intermediate position with respect to South America. The separation of New Zealand is very conspicuous as well as the position of Campbell Island, which is linked to the first, but not to the second, group of sub- Antarctic islands. The sub-Antarctic islands group appears to be quite consistent. Unlike the family and genus analyses, Heard Island does not appear to be related to the third group of maritime Antarctic islands and the Antarctic continent and is suprisingly more similar to Macquarie and Marion Islands in its fauna. The inclusion of the South Sandwich Islands in the second group, of mainly sub-Antarctic islands but not the maritime Antarctic islands is intriguing, as well as its high faunal similarity with South Georgia.

Discussion This investigation of the oribatid mite fauna of the Antarctic region and its

nearby land areas is incomplete and future studies of the faunas of Patagonia, Tierra del Fuego and the Falkland Islands will probably yield species new to science as well as new records for particular areas. On the other hand, knowledge of the depauperate oribatid mite faunas in the relatively simple terrestrial communities of the Antarctic is comparatively good. We believe that such differences between the levels of knowledge of these areas do not alter our analysis of the main trends in the biogeography and distribution patterns of these soil mites, which are the focus of this study. The high similarity of the present fauna of the Andes Mountains with that of New Zealand at both the generic and family level confirms the genetic continuity of these two parts of Gondwanaland in the past. The majority of oribatid mite species arose later, probably after the division of Gondwanaland, which is supported by the reduced similarity, at the species level, of New Zealand and the Andes Mountains. This comparatively strong link is reflected also in the occurrence of 17 species of oribatid mites, distributed contemporaneously in New Zealand and in South America, which are found nowhere else (Hammer, 1968). Hammer and Wallwork (1979) supported the land connection between New Zealand and South America and based it on the existence of two morphological types in the genera Trimalaconothrus and Crotonia found together in New Zealand as well as in South America. These are evidence for a harmonic pattern of distribution. Thus evidence supports a geographical and faunal continuity between these two regions at some earlier time. Indeed Carlquist (1974) suggested that Antarctica was a possible migration route during the Cretaceous. What was the form and character of such land connections? We suggest two alternatives. Firstly, a connection through what are now the sub-Antarctic islands and, secondly, via the mainland of what is now Antarctica. In relation to the first alternative, it is conspicuous that most of the sub- Antarctic islands have disharmonic faunas comprising few groups of oribatids, many of which are common in the present day harmonic compositions of the continental land masses of South America, Antarctica and New Zealand. In addition, all the sub-Antarctic islands, except South Georgia, appeared long after the breakup of Gondwanaland. We conclude therefore that the land connection between New Zealand and South America was very probably through the Antarctic continent. Unfortunately, as yet there is no direct evidence in the form of fossil oribatid mites recorded from Antarctica, but information from fossil floras, e.g. Nothofagus, supports this conclusion.

The West Wind Sea Current originated after the separation of South America from Antarctica about 28 m.y.a. This cold ocean current surrounds and completely

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isolates climatically the entire Antarctic continent. Although there is evidence (Miller, et at., 1987) of a polar ice cape at 40m.y.a., the succeeding isolation brought about the almost total glacierization of the Antarctic continent. This had catastrophic effects on the Antarctic flora and fauna and eliminated most of the terrestrial invertebrates (Wallwork, 1976). Only four species of the endemic Family Maudheimiidae have survived on very widely separated groups of isolated nunataks as relicts of the earlier oribatid fauna.

There is some indirect evidence that Antarctica may have played an important role as a land connection over a long period of time and that it may have been a centre for the spread of oribatid mite taxa. Fossil records of trees confirm the earlier presence of forest in Eastern Antarctica during the early Tertiary (Denton, et al., 1971). It is thought that Australia separated from Antarctica comparatively late at about 40m.y.ago, but the initial breakup began around 85m.y.a. (Miller, et al., 1987). The fossil plant remains from this period in southern Australia indicate a much warmer climate than at the present time. Wellman, et al., 1969 suggested that Victoria Land and George V Land were warmer than southern Australia. The South American tropical flora expanded to the south during the Palaeocene and the Lower and Middle Eocene and was gradually confined to the north at the end of the Eocene (Menendez, 1971). Fossil Nothofagus forests have been recorded from the Lower and Middle Miocene of Western Antarctica (Hill, 1989), whilst Nothofagus remains were reported from the Beardmore Glacier area within 500km of the south pole (Burckle and Pokras, 1991).

The present composition of the oribatid fauna in the maritime Antarctic zone and possibly the sub-Antarctic zone has resulted from the gradual and slow re-colonization by cold-tolerant taxa from northern temperate zones as evidenced by the disharmonic nature and patterns of their faunas, the extensive past glaciation and the relatively young age, geologically, of most of the islands.

Wallwork (1973) and Hammer and Wallwork (1979), on the other hand, introduced the concept of the distribution of the Family Ameronothridae in the south polar region as an example of a harmonic pattern, and proposed that its present circumpolar distribution is a relict of a widespread distribution in the past. If there was a more continuous distribution of the ameronothroid mites before the glacial periods and the separation of Gondwanaland, it would be expected that related species and genera of this family would survive to the present time on all parts of former Gondwanaland, which were unaffected by glacierization. It is evident from the distribution patterns of this, the most common family of oribatid mites in the south polar region, that no species of the Ameronothridae has been recorded from South America and that the present distribution may not be a relict one. It is more likely that it is a result of post-glacial re-colonization from Australasia, where this family has many species and high generic diversity. The low incidence of species endemism, especially in the maritime Antarctic zone, and the almost complete absence of endemism at the genus and family levels as in the continental Antarctic zone, provide evidence for the comparatively recent re-colonization of this region.

A distinctly richer oribatid mite fauna inhabits the sub-Antarctic zone than the continental zone. This is reflected in the comparatively disharmonic faunas of those islands which have been studied. A comparison between the oribatid mites of older continental islands and younger volcanic oceanic islands is illuminating. The differences in species endemicity and especially generic endemism are very striking with significantly higher levels of both on the continental islands. The longer time for the

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development of biotopes, uninterrupted by periods of volcanic activity, together with the greater geological age of continental islands must have played important roles. The re-colonization of sub-Antarctic islands by cold-hardy species probably occurred mostly from South America through South Georgia and from Australasia via Macquarie Island. This process was both long-term and accidental (Wallwork, 1976). The present data, as well as those of Wallwork (1967), show a strong influence of South American elements on the oribatid mite fauna of the western part of the sub-Antarctic zone and on the faunas of some islands in the maritime Antarctic zone, e.g. South Sandwich Islands, South Shetland Islands. It is possible that these elements, for example Edwardzetes dentifer Hammer, 1962, could be transported on drifting Nothofagus spp. trunks by the West Wind Sea Current, because most of these species inhabit South American biotopes where such tree species also occur. Most of these South American oribatids have not been recorded from the nests of sea birds, so reducing the possibility of this mode of transport into the Antarctic region. Hammer (1982) and Schuster (1979) have documented the possible important role of ocean currents in dispersing oribatid mites across considerable distances. The possible introduction of a wood-boring weevil on a stranded Nothofagus log to Beauch6ne Island close to the Falkland Islands (Smith and Prince, 1985) is a case in point.

The oribatid mites from southern Australasia, especially the Ameronothridae, have rather different possibilities for migration because most of the species live close to the sea shore. Such forms are not connected with forest habitats and the chances of using trees and other such plant material to cross the vast South Pacific may not be realistic, because the tolerance of some oribatids to sea water is limited according to Schuster (1979). However, adults of the podacarid Alaskozetes antarcticus (Michael, 1903) have been reported as surviving immersion in sea water for over 40 days (Goddard, 1979). The second possibility for crossing oceans against the prevailing direction of the West Wind Sea Current is by aerial transport on sea birds. Unfortunately, information on this possibility is almost non-existent and there is no direct evidence of such transport occurring, although living specimens of A. antarcticus have been found on a freshly-killed skua near Palmer Station, Anvers Island (Strong, 1967) and more recently on specimens of Wandering Albatross at Bird Island, South Georgia (W. Block and J. Star), unpublished). There is some indirect evidence which suggests that this type of oribatid mite transport could occur and constitute an effective migration process. Covarrubias (1968) and Wallwork (1972) found comparatively high frequencies of some oribatid species which exhibit wide distribution ranges in the nest materials of sea birds. For example, specimens of Austroppia crozetensis (Richters, 1908) have been found in the nests of sea birds on South Georgia (Wallwork, 1972), and A. antarcticus has a very high relative abund- ance (98%) in nest material in the South Shetland Islands (Covarrubias, 1968). On the other hand, Sandenia georgiae (Oudemans, 1914), a species endemic to South Georgia has never been found in birds nests or their constituent materials (Wallwork, 1972).

Few comparable data exist for other groups of Antarctic terrestrial invertebrates apart from a study of Tardigrada (McInnes and Pugh, in press), that of Collembola (Greenslade, 1995) and Diptera (Convey and Block, 1996). The Antarctic tardigrade fauna exhibits a high level of endemism compared with other regions, and there is evidence suggesting long-term isolation of the Antarctic fauna, especially in East Gondwanaland (Antarctica, Australia and New Zealand) since the Jurassic/ Cretaceous period.

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Confirmation or contradiction of these theories regarding the possible mechanisms of oribatid mite recolonization of the sub-Antarctic and maritime Antarctic zones from the South American continent as well as from Australasia requires further studies, both in the field and experimentally. Surveys are required not only of oribatid mites in the nests of potential bird vectors, but especially of the occurrence of oribatid mites amongst the feathers of living sea birds, to allow statements to be made concerning the possible mechanisms of distribution and provenance of particular species. In particular, experiments on the physiological resistance to sea water of Antarctic and circum-Antarctic species of oribatid mites are needed, both in controlled laboratory simulations and also under field conditions. Information is also required on the life history traits and genetic systems of species of oribatid mites that occur in Antarctica. In addition, the oribatid mite fauna of decaying wood as well as that of drifted trunks of Nothofagus spp. and other trees should be studied in Antarctica. Smith (1985) reported on 56 specimens of driftwood found on South Georgia, the South Sandwich and the South Shetland Islands and highlighted their potential for transport of terrestrial organisms into the Antarctic region. The investigation of fossil oribatid mites, if found, in Mesozoic and Cenozoic terrestrial deposits in the Antarctic would furnish crucial evidence for some of the problems highlighted here and may provide the key for the explanation of the development of the Gondwanaland oribatid mite fauna.

Acknowledgements We are grateful for the support of the British Antarctic Survey (BAS) and the

British Council in Prague to enable Josef Star~ to visit Cambridge to undertake this collaborative study. The use of the specimen material in the Data and Resource Centre of the BAS Terrestrial and Freshwater Life Sciences Division is much appreciated. We also thank Dr P Greenslade (CSIRO, Canberra, Australia) for kindly providing oribatid mite samples from Heard Island for study, Peter Rothery (BAS) for assistance in calculating the similarity indexes and the Mapping and Geographic Information Centre (BAS) for provision of a base map for figures 3-6. The construct- ive comments of Drs May Block, David Cantrill, Peter Convey, Malcolm Luxton, Philip Pugh, Ronald Lewis Smith and an anonymous Referee on the manuscript were much appreciated.

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