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State of the Marine Environment Report for Australia: The Marine Environment - Technical Annex: 1

Compiled by Leon P. Zann
Great Barrier Reef Marine Park Authority, Townsville Queensland

Ocean Rescue 2000 Program
Department of the Environment, Sport and Territories, Canberra, 1995

ISBN 0 642 17399 0

Biogeography and diversity of Australia's marine biota

Gary C. B. Poore

Department of Crustacea
Museum of Victoria
71 Victoria Cresent
Abbotsford, Vic, 3067

The Australian continent's shallow water marine biota, though less well known than our bizarre terrestrial flora and fauna, is notable for its high level of endemicity and for groups of animals and plants shared only with other southern hemisphere continents. The reasons for this are now emerging after nearly two centuries of taxonomic and biogeographic research.

Geological history

Although the present biota of Australia's coast and continental shelf can be explained in part with reference to modern conditions it is also the result of a long history in a changing environment. Since the mid1960s theories of continental drift and plate tectonics have been applied to our interpretation of the Australian marine fauna - in particular, its position on the globe; its size, especially of its shelf; its connections to other land masses; the currents surrounding it; and the temperature and chemical nature of the sea surrounding it. The main events took place over the consecutive periods: Mesozoic, Tertiary and Quaternary.

Mesozoic

The history of Australia's modern marine biota began in the Mesozoic. Earlier and for most of this era Australia, South America, New Zealand, Antarctica, India and southern Africa were joined as a large southern continent, Gondwana. The fossil record suggests that most phyla had originated by the close of the earlier Cambrian times, perhaps 600 million years ago (600 mya) but new classes continued to appear until the Lower Carboniferous (300 mya). During the late Palaeozoic era the affinities of Gondwanan benthic marine faunas fluctuated between Palaeoaustral (cool temperate) and Tethyan (warm temperate). After the PermoTriassic extinctions, rediversification of the marine fauna was mainly through evolution of new families, genera and species and the Gondwanan fauna became much like the fauna of today.

During the Triassic era (c. 200 mya) Gondwana to the south and Laurasia to the north were connected in the west but separated in the east by a major incursion from the surrounding Tethys Sea. The wide epicontinental shelf of Tethys included, in Australia, areas of present day northern and southern Western Australia and southeastern Queensland, then between 40oS and 65oS. Climates were warm and two faunal realms probably existed: a restricted cold or coolwater Maorian or Austral Realm in the New Zealand-New Caledonia area; and a warm water Tethyan Realm in what is now northern and western Australia.

Australia was positioned between 35oS and 65oS during the Jurassic period (200-129 mya) almost at right angles to its present orientation, and still connected to Antarctica (Figures 1A, 1B). A continental shelf surrounded only its present northwestern and northeastern coasts. Early Jurassic marine faunas were quasicosmopolitan but provinciality increased until the Australian (Tethyan) fauna became differentiated from the Boreal fauna of the northern hemisphere. The northern and southern coasts of Gondwana were at quite different latitudes but climatic differentiation was slight. The northern coast (incorporating northwestern Australia) lay on the Tethyan Sea at about 30oS, while the southern coast (New Zealand, eastern Australia, West Antarctica, southern Africa and southern South America) lay on a southern arm of the 'Pacific Ocean' at 60oS.

The breakup of Gondwana commenced with the separation of Africa (c. 125 mya) and India (c. 118 mya) from Australia-Antarctica during the Cretaceous period by an arm of the Tethys becoming the protoIndian Ocean by 100 mya. Several incursions of the sea from the west occurred along the line of the south coast of Australia. Much of the Australian land mass was submerged during Cretaceous incursions of the Tethyan Sea, especially from the north and northwest (Figure 1C). Temperatures warmed to about 10oC warmer than at present but cooled later. The Austral Province, which included Australia, New Caledonia, New Zealand, New Guinea, southern South America and eastern India, could be contrasted with the East African Province and accords with the tectonic isolation of Australia and western India-Africa at that time.

Later (82 mya), the Tasman Sea opened to separate Australia and New Zealand and later rifting between Australia and Antarctica began by intrusion from what is now the west. The colonising biotas of the Australian southern coast were therefore of tropical Tethyan origin (Figure 1D). The biota of what is now the eastern coast of Australia was part of a Weddellian Province which extended across Australia and northwards up the coast of Chile (Zinsmeister 1982).

Figure 1: The relative position of /Australian coasts and shelves among the southern continents (Gondwana) during the Mezozoic.

A. Early Jurassic (180 mya)-the Tethys Sea with its broad shelves separates Laurasia to the north from the fused continents of Gondwana. B. Late Jurassic (145 mya)-Africa and Antarctica begin to drift apart. C. Early Cretaceaous (125 mya)-an ocean basin separates Africa and India from Antarctica; continental Australia is largely submerged by waters of the Tethys. D. Late Cretaceous (90 mya)-New Zealand has separated and Australia and Antarctica begin to rift from the north-west. The bold line indicates coastlines and shading shelf waters.

Figure 1

Tertiary

By midPalaeocene (60 mya) the Coral Sea had formed and Australia had begun to rotate anti-clockwise towards its present orientation and to move northwards. Water temperatures were warm with a northerly current on the west coast and a south-directed offshoot of the Pacific Equatorial Current on the east coast. Although Australia moved northward during the Tertiary, the continent experienced successively cooler, rather than warmer, temperatures as a result of worldwide cooling. Sediments at this time were predominantly biogenic carbonates, much as they are now.

By the late Eocene-early Oligocene (40-35 mya) the sea was able to pass from the west between Antarctica and the southeastern corner of Tasmania over the South Tasman Rise at a latitude of about 65oS (Figure 2A). In the late Oligocene (30 mya) oceanic water moved freely between the two continents (Knox 1979) and water temperatures cooled quickly. For the first time biotas from the eastern coast of Australia and the western and southern coasts (mainly Tethyan) were able to mix along the south coast.

Although Australia was by then a separate continent with an isolated marine fauna two other tectonic events are relevant. The first event was the separation of West Antarctica and South America by the opening of Drake Passage during the Oligocene or Miocene (30-22 mya), thereby allowing the CircumAntarctic Current to form. Global cooling resulted in polar ice caps and a permanent steep water temperature gradient away from the poles. The Antarctic Convergence (a steep temperature gradient) formed during the early Miocene (22 mya) (Figure 2A) thus producing an important biogeographical boundary. This convergence and the Subtropical Convergence further north have persisted from those times but their latitudinal movements have had a profound effect on biogeographic regions of southern continents (Knox 1979).

At about the same time there was a notable invasion into southern Australian shelf faunas of warm water elements. In a detailed analysis of the Tertiary Mollusca of the southeastern coast, Darragh (1985) concluded that during the Tertiary the Tethyan IndoPacific element increased with time at the expense of the Australian and New Zealand elements. Cosmopolitan elements decreased to be replaced by greater endemism. It has been suggested that the development and decline of this warmer interval, which contrasts with the overall Cenozoic glacial cooling pattern, resulted from Australia's northward drift towards the equator overtaking the contraction of marine temperature zonation (Talent 1984). At the time (3-4 mya) the Subtropical Convergence crossed in the latitude of Tasmania and a cold water mass separated the eastern and southern coasts of Australia (Knox 1980).

The second tectonic event of significance to Australia's biogeography was its collision with Southeast Asia in the north. This event began in the early Miocene (c. 20 mya) when the New Guinea margin of the Australian block came into contact with the Sunda Arc (eastern Indonesia). The two blocks continued to overlap and with the evolution of island arcs the Tethys Sea was no longer a barrier to marine shelf and coastal biota. This event contributed to a reduction in the easttowest faunal differences across the tropical Pacific.

Figure 2: The changing shoreline and shelf of Australia during the Cenozoic.
A. Late Eocene (38.5 mya)-Australia and New Guinea still connected to Antarctica by the South Tasman Rise. B. Middle Miocene (11 mya)-extensive marine transgressions in the south and north. C. Pleistocene (18 000 ya, solid line, and 10 000 ya, dotted line)-a period of minimum shelf area with the 10C minimum Subtropical Convergence appearing across south eastern Australia. D. Modern-both New Guinea and Tasmania separate.

Figure 2

Quaternary

At the beginning of the Quaternary period (2 mya) the arrangement of land and sea in the southwest Pacific was essentially as it is today except that neither Torres Strait nor Bass Strait existed. However, the size and shape of the Australian coastline and shelf continued to change during the Pleistocene due to sea levels fluctuating over 200 m (Galloway & Kemp 1981; Figures 2C, 2D). Both Torres Strait and Bass Strait opened and closed repeatedly. The changing shelf size, coastline and latitudes of the passages between the east coast of the Australian mainland and the remainder of the continent would have affected the continuity of their biotas.

The coastal marine fauna of Australia did not respond to the breakup of Gondwana in the same way as did the terrestrial fauna because the separation of land masses created shallow shelves for marine invasion (barriers to terrestrial faunas) before these shelves were split by sea-floor rifting.

In summary, the most important tectonic events affecting the biota of Australia's coasts and shelf are:

Distribution patterns

An understanding of the marine biogeography of Australia has been achieved through investigation of the geology and palaeontology of the continent and by examination of modern distribution of the biota. This distribution reflects past events and present-day environmental conditions.

Wilson and Allen (1987) compared the number of species and species composition of fishes, molluscs, echinoderms and corals throughout the continent. These groups were chosen because their taxonomy is relatively well-known, and they probably demonstrate general principles which can be applied to other groups.

Fishes

Some of the 3400 species of Australian marine fishes are pelagic or oceanic and wide-ranging in tropical or temperate seas. About three-quarters occur on the shelf and nearshore. The greatest number of species are in the tropics where approximately 120 families, 600 genera and more than half of the species (1900) are found; and most of these are common to the Indo-West Pacific region. Although most species have pelagic eggs and larvae a moderate level of endemicity (13%) is maintained with the help of southerly flowing currents on both the east and west Australian coasts.

In contrast, the fish fauna of southern temperate Australia comprises about 600 species of which 85% is endemic and 11% is shared with New Zealand. One contribution to this high level of endemicity is radiation in a few families with low dispersibility: viviparous clinids, brooding syngnathids and nesting gobiescocids and gobiids. The pipefishes and seahorses (Syngnathidae), leatherjackets (Monacanthidae) and fishing frogfishes (Antennariidae) are especially rich in species in southern Australia.

Shallow-water reef fishes provide one of the best studies in zonation along the southern coast. Wilson and Allen (1987) recognised four ecological barriers which appear to inhibit dispersal: a sharp temperature gradient around Albany near the cessation of the Leeuwin Current; and the absence of nearshore rocky reefs in the centre of the Great Australian Bight, at the mouth of the Murray River, and in eastern Victoria. These barriers may act today to maintain allopatric eastern and western species pairs. Eighteen pairs of closely related fish species in 12 families were reported by Wilson and Allen (1987: Table 3.9).

Molluscs

The biogeography of molluscs is very much like that of fishes although there are many more species. Most families are represented in the Australian fauna and have more species in the north of the country than in the south. The level of endemicity in the north is low (about 10%) with most species distributed widely in the Indo-West Pacific region. In contrast, endemicity of the southern Australian fauna is about 95% and several endemic genera occur. Some endemic genera are relicts of the once widespread Tethyan fauna (as are the endemic families, Trigoniidae, Campanilidae and Diastomatidae, each with a sole living representative). Other endemic genera are relicts of the ancient Palaeoaustral fauna.

Echinoderms

Echinoderms, with only a few hundred Australian species, repeat the biogeographic patterns of the fishes and molluscs with 13% of species endemic in the tropical region and 90% in the temperate region. Many echinoderms have a long larval life which may explain why 22% of Tasmanian species occur also in New Zealand where they are transported by the West Wind Drift.

Corals

Reef-building scleractinian corals are essentially tropical and their distribution is substantially different from that of the three groups discussed above. The Australian fauna is a subset of Indo-West Pacific corals and is largely confined to the Great Barrier Reef and smaller reefs of the west coast. There is not a gradual reduction in the number of species with increasing latitude but rather an abrupt depletion of species at the termination of the Great Barrier Reef and at the Houtman Abrolhos. Coral reefs do occur further south -eg at Lord Howe Island and Solitary Islands - but the number of species is few. Few species of coral occur on the southern coast of Australia and reefs are not formed there.

Other taxa

Analyses of other groups in the manner of Wilson and Allen (1987) are more difficult because species composition is less well known. Similar patterns are expected but not all taxa are more diverse in tropical than in temperate environments. While there are more species of decapod crustaceans (crabs, hermit crabs, lobsters, shrimps, prawns) in warmer waters the same is not true for other groups such as peracarids (amphipods, isopods, and others). These biogeographic distributions have repercussions in functional ecology - for example, the taxonomic composition of crustacean scavenger guilds. Cypridinid ostracodes, cirolanid isopods and lysianassoid amphipods play this role but the last of these become relatively more important as one moves from tropical to temperate waters (J.K. Lowry pers. comm.).

East-west species pairs such as occur in the fishes occur in other groups. Dartnall (1974) figured pairs of brachyuran crabs, molluscs and asteroids whose distributions overlap or are contiguous in Bass Strait. Their distributions imply that this region is or has been a barrier stimulating speciation. Such an interpretation depends on establishing the sisterhood of the species pairs but in few cases have phylogenetic treatments been done.

Present Australian biogeographic provinces

In his 1953 classification of the world's marine environments, the biogeographer, S. Ekman, placed tropical and subtropical Australia within his IndoWest Pacific region. He recognised that southern Australia was separate and placed it within the warm temperate fauna of the Southern Hemisphere. The IndoWest Pacific was recognised by Ekman as containing 'the greatest wealth of animal life' (1953:11). His view was not disputed by the most recent discussion of Australian marine biogeographic components (Wilson & Allen 1987).

Several marine provinces within Australia have been proposed and reviewed many times (Knox 1963). Suggestions of as many as three tropical provinces and three or four temperate provinces no longer have currency partly because their boundaries are doubtful and not defined quantitatively (for example, Figure 3A) and their definitions are intuitive. Edgar's (1984) presentation of a matrix of indices of similarity between sampling sites in Tasmania and the nearby mainland is one exception. Poore et al. (1994) analysis of the distribution of nearly 300 species along the Victorian coast found only weakly defined boundaries in this short section of the Australian coastline.

Nevertheless, division into tropical and temperate regions has never been seriously disputed. Wilson and Gillett (1971) and Wilson and Allen (1987) simplified the picture by recognising northern and southern Australian regions (Figure 3B) with transition zones between them - one on the east coast and one on the west coast. Wilson and Allen's (1987) conclusions on Australian biogeography derived more from inferred origins of the fauna than from its present classification based on species composition. The distribution patterns seen today are the result of contributions from two different early Tertiary biotas:

  1. the panPacific Tethyan biota and its successor, the IndoWest Pacific biota, have dominated the northern coasts of Australia since the beginning of the Tertiary and also contribute to temperate biotas, especially in the southwest. To the north, barriers to interchange of shelf and coastal biotas with South-east Asia are only slight. There are therefore many widespread tropical elements in the northern Australian biota and a low percentage of endemicity. At its southern limit the Tethyan element is limited by the latitudinal temperature gradient.
  1. the temperate Palaeoaustral fauna has dominated southeastern Australian coasts also from the early Tertiary and is now the major element of the biota of the entire southern coast. Its high level of endemicity results from its isolation by ocean basins from other southern continents and from a latitudinal temperature gradient to the north.

The boundary between the tropical and warm temperate provinces coincides approximately with 18-20oC winter minimum surface temperature but this is variable and dependant on the influence of the East Australian Current and the Leeuwin Current.

Figure 3: A. Biogeographic provinces of the Australian intertidal according to Bennet and Pope (Knox 1963).
B. Major faunal regions of the Australian coast (after Wilson & Gillett 1971).
Figure 3

This gross picture of Australia divided into a northern region with low endemicity and a southern region of high endemicity is superimposed on other patterns as yet poorly understood. Most obvious of these is the separation of the Great Barrier Reef community from that of the adjacent coast but it could be argued that this distinction is an ecological rather than a biogeographic division.

Similarly, George (1969) divided tropical coasts on the basis of water turbidity: northern Western Australia, Northern Territory and Queensland where high monsoonal summer rain and dry winters result in grey mud sediments inshore and well developed mangrove creeks; and northwestern Western Australia where rainfall is low and irregular, with occasional cyclonic disturbances and flash flooding resulting in brown sediments. The two tropical regions contrast with the southern half of the continent where rainfall is more reliable - uniform throughout the year in the east and falling in the winter in the west. Again, this division may reflect modern ecological regimes rather than more ancient biogeographic events and is covered in the chapter on ecosystems.

Diversity

In an earlier section, numbers of fish, mollusc and echinoderm species in Australia were estimated. Also of interest is the number of species of all taxa in a circumscribed habitat: so-called 'diversity' or, in modern parlance, 'biodiversity'. Such data are known to correlate with environmental conditions and are a useful tool in understanding biogeographic trends.

The few quantitative attempts in Australia to obtain data on numbers of species are rarely comparable. The result depends on the habitat chosen, its size, methods, and on the effort and skills of the taxonomists involved. The most serious impediment to completion is the poor state of knowledge of the fauna: rarely are more than 40% of the true species complement known to science.

Less is known of the marine environments. Birtles and Arnold (1988) recorded 103 species of echinoderm and 196 species of mollusc from four sites on the Great Barrier Reef lagoon, and Ward and Rainer (1988) reported 308 species of decapod crustaceans from the North West Shelf. Both studies are taxonomically limited.

Attempts to identify all species (above a minimum size) in a defined small area have been made in estuaries. Poore (1982) reported that the number of species in the Gippsland Lakes (90) was 30% lower than in estuarine systems studied by others. Explanation might be sought in the relative sizes of the estuaries, degree of marine influence, and biogeographic history.

Less is known of marine environments. In Port Phillip Bay 713 macrobenthic species were taken from 430 samples of sandy and muddy habitats (total area = 43 m_) (Poore et al. 1975). In Western Port fewer samples took 572 species (Coleman et al. 1988) but a total of 2000 has been estimated from all habitats.

A survey of 1.2 m_ of sea-floor in eastern Bass Strait turned up 353 species (Parry, Campbell & Hobday 1990) but more detailed work at the same place has discovered about 800 species in 10 m_ (unpubl.). The comparisons ofParry, Campbell and Hobday (1990) showed that even the lower figure was much greater than in other parts of the world.

Poore and Wilson (1993) and Poore, Just and Cohen (1994) summarised data on the number of species of isopod crustaceans on the south-eastern Australian slope where 359 species were identified. This is many more than found in similar studies in the northern hemisphere.

Less information has been published from macrobenthic communities in other parts of Australia and where data exists they are unlikely to be comparable. This points to the need for some basic protocols for quantification of 'biodiversity' before regions can be compared and latitudinal gradients measured.

Diversity has classically been viewed as being greatest at low latitudes and decreasing towards the poles. Recent evidence from terrestrial animals (Platnick 1991) and marine amphipod crustaceans (Barnard 1991) supports the view that this is not always the case, especially in the southern hemisphere. Australia has a very long coastline (both latitudinally and longitudinally) but trends in species diversity are poorly documented. Gradients in both direction are to be expected but not all taxa will behave in the same way. While fishes, corals, molluscs, echinoderms and decapod crustaceans decrease in diversity from north to south in Australia the same trends are not apparent in other taxa. In amphipods (Barnard 1991) the reverse is true. In isopod crustaceans a diversity gradient overall is not yet apparent but relative dominance of families certainly changes with latitude as it does in many other taxa.

Conclusions: implications of biogeography for management

Biogeographic regions

One of the many criteria used in the selection of areas for management is 'biogeographic zone', the implication being that areas within zones have biotas more similar to each other than with areas in different zones. Thus it is argued that each area is representative of the zone from which it was selected. The division of Australia into a northern tropical region and a southern temperate region with broad transition zones is clearly not adequate for making such decisions.

However, I would argue that finer division of the coast (and continental shelf and slope) has not yet been achieved satisfactorily. The divisions of the early biogeographers were entirely intuitive and therefore hotly debated. Most were based on a single taxonomic group or habitat: molluscs, echinoderms, algae, fishes, ascidiaceans or intertidal habitats (see review by Knox 1963). But in reality, steep environmental gradients that might explain some of the provinciality do not exist and do not affect all taxa and habitats equally.

In 1986, the Australian Committee of the International Union for the Conservation of Nature and Natural Resources adopted in its policy for protection of marine and estuarine areas a classification of the Australian habitats and coastline prepared by the Australian Bureau of Flora and Fauna (Figure 4). Fourteen coastal (shallower than 200 m) geographic zones plus 18 oceanic zones and external territories are mapped. Boundaries between the 14 coastal zones do not coincide with suspected biogeographic boundaries and the geographic categories are inappropriate for environmental management. The absence of comparative quantitative data on the relative abundance of species in many taxa for most of the coast is a serious impediment to a classification which reflects distribution patterns of plants and animals.

A considerable body of data exists (mostly in museum collections) on the distribution of species around the coast. However, coverage of the coast and of taxa is certain to be uneven. These data could be collated and analysed objectively to determine where, if anywhere, biogeographic boundaries can be recognised. But success will be slight since the data were not collected for this purpose. Much better would be to gather new strategic quantitative data. Modern multivariate techniques such as hierarchical classification and ordination are appropriate analytical techniques worth investigating.

Ideally, the discovery of biogeographic boundaries - past and present - depends on the revelation of phylogenetic relationships within numerous taxa at the family and generic level. The congruence of their distribution patterns will indicate barriers as are hinted at in the study of species pairs on the southern coast and in Bass Strait. The costs of obtaining data and performing the required analyses will be high but until it is done biogeographic considerations can play little part in environmental management.

The concept of endemism as an important criterion in the selection of areas for management must be investigated. Such a criterion will shift emphasis from the north of Australia (low endemicity) to the south (high endemicity). It is in the south that Australian 'native' marine biota resides. Further, it is the southern temperate ecosystems which are the most threatened by the largest population centres.

Diversity

Management of marine environments must be geared to the management of communities of species rather than towards individual species. There are of course rare exceptions where a large and obvious species may warrant special attention.

Coral reefs in particular are said to be of special interest because of their high 'biodiversity' but this has not been quantified and the relative importance of different reefs from the point of view of diversity is unknown. By and large the species inhabiting tropical reefs are widespread through the Indo-West Pacific. Concentration of research effort and management on Australian coral reefs at the expense of more southern ecosystems with a much more endemic biota can only be justified on the grounds of international responsibility.

Figure 4: Coastal zones within the 200m bathymetric contour and the 200 nautical mile Exclusive Economic Zone adopted by the Australian Committee of the I.U.C.N. in 1986.

Figure 4

In fact, many temperate marine environments are inhabited by communities rich in species, with no species especially more abundant than others. Investigations in Bass Strait and the south-eastern slope have revealed soft-bottom benthic communities more diverse than elsewhere in the world

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