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6. Human interactions with australian mangrove ecosystems
Eric C. F. Bird
Australian mangrove ecosystems (fig. 1) occupy about 20 per cent of the coastline, and have a total area of about 11,500 km² (Galloway 1982). They extend as far south as Corner Inlet in Victoria (38°55'S), but reach their greatest extent and diversity on the tropical coasts of northern Australia (fig. 2), especially the humid tropical sector of northeast Queensland between Ingham and the
Daintree River. There are at least 30 species of mangroves in the swamps bordering the Daintree estuary. Mangrove ecosystems in southern Australia are generally scrub rather than woodland, the white mangrove (Avicennia marina var. resinifera) being the commonest, and south of latitude 35° the only, species present. In northern Australia the mangroves include areas of forest, usually with scrub communities along their landward and seaward margins, and fringing rivers and tidal creeks (Bird 1972a; Gill 1975).
The greater extent, diversity, and luxuriance of mangrove ecosystems in northern Australia is related to the higher air and water temperatures and to the abundance of muddy sediment in coastal areas, derived from the tropically weathered rock formations of the hinterland and delivered to the coast by rivers. Other factors include the generally large tide ranges, the many sectors protected from strong wave action (e.g. inlets, intricate embayments, and the lee of spits, islands, or coral reefs), and the generally low relief of coastal regions in northern Australia. High salinity is a limiting factor for mangrove growth in the relatively arid parts of northwestern Australia, where there are only narrow fringes of mangroves alongside estuarine gulfs such as King Sound. In Shark Bay, on the west coast, sea salinity is too high for mangrove growth. The bulk of the mangrove area is thus on the coasts of the Northern Territory (Wells 1982) and north Queensland (Dowling and McDonald 1982).
Mangroves colonize the upper part of the intertidal zone and spread seaward, usually to about mid-tide level; at low tide they are fronted by exposed tidal flats, muddy or sandy, often with marine plants such as sea-grasses (e.g. Zostera spp.) (fig. 3). While mangroves can grow on rocky, gravelly, or sandy substrates, their best development is on muddy habitats. Sectors of coast receiving muddy sediment prograde as the mangroves spread forward, and there is often a zonation of mangrove species parallel to the coastline (Macnae 1967). It has been shown that mangroves can influence patterns of muddy sedimentation in such a way as to build up a depositional terrace, eventually to high spring tide level (Bird 1972b; Bird, in press). Once this level has been attained, the mangrove vegetation gives place to other ecosystems: in the wetter areas swamp forest, and in the drier areas savanna, salt marsh, or bare hypersaline flats. A feature of many Australian mangrove areas is the presence of cheniers, which are narrow sandy or shelly ridges washed up by storm surges, particularly during tropical cyclones, and left stranded within or behind the mangroves. Hopley (1974) described the emplacement of a chenier in a mangrove area during the storm surge that accompanied Cyclone Althea in the Townsville region in December 1971.
Aborigines and Mangrove Ecosystems
Archaeological research has shown that Aborigines arrived in Australia at least 40,000 years ago, during the Last Glacial phase of the Pleistocene, when the sea level was 100 to 150 metres lower than it is now. Mainland Australia was then enlarged, and linked by "land bridges" northward to Papua New Guinea and eastern Indonesia, and southward to Tasmania (Jennings 1971; White and O'Connell 1982). Nothing is known of the distribution of mangroves at this lowsea-level stage: the present pattern has developed only within the past 6,000 years as a sequel to the world-wide Holocene marine transgression, which brought the sea up to its present level, submerging Torres Strait and Bass Strait and establishing essentially the modern outlines of Australia (Bird 1 984).
Aboriginal tribes who occupied what is now the sea floor around Australia must have retreated as this marine transgression took place. Between 18,000 and 6,000 years ago the sea rose at an average of a metre per century. The rate of land submergence depended on the transverse gradient: off north-western Australia, where the continental shelf is up to 160 kilometres wide, the coastline must have retreated at an average of 13.3 metres per year, whereas off New South Wales, where the continental shelf is only 20 kilometres wide, the average retreat of the coastline was 1.6 metres per year (Bird, in preparation). When the marine transgression came to an end, some Aboriginal tribes remained in coastal areas (Bowdler 1977), and, as the mangrove ecosystems developed and spread, they made use of them as a place to hunt and collect food items.
The Aboriginal population of Australia is now about 125,000, but only a few thousand Aborigines still live in the traditional manner on the coasts of northern Australia, and even these have modified their life-style in response to European influences. Aborigines have Australian citizenship rights and obligations, and their socioeconomic conditions have been transformed by the availability of welfare payments and services and by the purchase and use of retail goods. Particularly in the vicinity of urban centres (such as Darwin, Broome, Cooktown, and Cairns) and mining settlements (such as Gove in north-east Arnhem Land and Weipa on the Cape York Peninsula) Aboriginal culture has been greatly modified; and on the east coast of Queensland the traditional life-style and economy of Aborigines whose predecessors used mangrove areas has been almost forgotten. Even in reservations such as the Yarrabah Mission near Cairns and the Palm Island Aboriginal Settlement off Ingham there is now only incidental use of mangrove areas. South of the Tropic of Capricorn, where mangrove species are fewer and mangrove ecosystems less extensive, there appears to have been little use of mangroves by Aborigines.
It is only in the more remote parts of the north coast of Australia that the traditional use of mangrove areas by Aborigines can still locally be observed. In the Northern Territory, Davis (1984) studied coastal Aboriginal tribes within the region of Arnhem Land, which was legislatively reserved in 1931 for the use of the Aboriginal tribes that had traditionally occupied it. Davis dealt with coastal tribes of the Yolngu Group, which consists of about 30 tribes with a total population of 4,000 to 5,000, living up to 60 kilometres inland.
Some of these tribes inhabit the Crocodile Islands and the shores of Castlereagh Bay, about 450 kilometres east of Darwin (fig. 4). In this remote part of Australia the coastal tribes retain much of their traditional relationship with, and use of, the mangrovefringed coastline, the main interactions with European Australians having been through the Methodist Milingimbi Mission Station, established in 1921 on Castlereagh Bay, and recently replaced by an independent community with an elected, governmentfunded Aboriginal Council.
Spring-tide range on this part of the Arnhem Land coastline is about 8 metres, and wide inter-tidal areas are exposed and available for foraging as the tide falls. Mangroves fringe sectors of the coastline, especially bordering Hutchinson Strait and alongside the estuaries of the Glyde and Woolen Rivers, and behind these are broad coastal plains, extensively flooded in the wet season, and a hinterland of hilly sandstone country. The Yolngu tribes utilize each of these landscapes, frequenting and using the mangrove areas in the traditional way, drawing upon them for food and other products, but also ranging landward over the coastal plains to the sandstone bush country, and seaward as the tide ebbs. There is no evidence of inland trading of products obtained from the mangroves, and little contact with tourists, although some income is now obtained from the sale of mangrove wood carvings through the former Mission Station.
Within the lands of the Yolngu, coastal tribal estates are defined to include areas of mangrove, bounded by rivers and creeks and occasionally by vegetation features. The Australian Aborigines have never established settlements in mangrove areas in the way that is common in parts of South-East Asia, but the Yolngu tribes certainly use the mangrove ecosystem. They go into the mangroves daily in search of shellfish, including the oysters that grow attached to mangrove roots, as well as mud crabs, worms such as latju (Teredo spp.), which bores into mangrove roots and sodden logs, and fish such as mullet and barramundi, caught in the tidal creeks. Some tribes catch and eat crocodiles; others venerate them as sacred. From the low-growing mänyarr (Avicennia marina) on the landward fringe of the mangroves they collect honey from native bee-hives, but this is only partly a product of the mangrove ecosystem, as the bees also range to the flowering eucalypts in the hinterland.
The mangrove ecosystem is thus a primary source of food supply for these Aborigines, especially in the wet and stormy summer season, when it is more difficult to catch fish or hunt mammals in the rough sea. Davis (personal communication) estimates that in the wet season the mangroves provide more than 80 per cent of the total food intake of the Yolngu: It is not possible to assess this in conventional economic terms, because it is a hunting and gathering activity, without monetary implications.
In the absence of permanent settlements in mangrove areas, the Aborigines occupy temporary hunting camps for periods of two or three days, either at the landward margins of the mangroves or on the cheniers, mentioned previously, which provide zones of relatively high and dry terrain. During these forays into the mangroves the older women of the tribe camp on the cheniers and look after the young children, while the young women and older children search for oysters and other shellfish gleaned from the mangrove roots. The shellfish are usually cooked and eaten near the landward margins of the mangroves or on nearby higher ground, and shelly middens of waste material, including charcoal from the fires, have accumulated at these sites. On the Cape York Peninsula some of these middens are mounds up to 20 metres high, accumulated over many centuries and with a stratigraphy showing variations in the nature and abundance of the shellfish utilized (Mulvaney 1975). Although large quantities of such waste material have been generated, the shelly material has not been used to make track-ways or build up camp sites in mangrove areas.
Most mangrove swamps are threaded by branching tidal creeks, which diminish in size landward but often remain relatively deep and navigable by canoe or raft. These provide harbours where boats may be sheltered between expeditions out into the estuary or into coastal waters. The Yolngu cut wuduku (Camptostemon schultzii, a light and buoyant wood, to make impromptu rafts when they want to cross an estuary. They catch fish by damming small tidal creeks and diverting them into narrow side channels, from which the fish are taken in hand-held baskets. Alternatively, they make fences of woven mangrove saplings that can be set in creeks to trap the fish.
Camp sites are often located where tidal creeks intersect cheniers. On the Arnhem Land coast, men returning from turtle hunts offshore will often take their boats upstream to such sheltered sites, where they can cook and eat the turtles.
In the wet summer season along the northern coast of Australia the maximum tides occur, and these, augmented by river floods, submerge wide areas of the coastal plain. Mangroves adjacent to high ground are then of particular value, being accessible for shellfish harvesting at low tide. The sea is then too rough for fishing and hunting, but some fish can still be caught in the mangroves and on the flooded coastal plain. Mosquitoes are a great nuisance at this season, and when infestations are severe the Aboriginal people coat themselves with a thick layer of mangrove mud, and light smoky fires to drive away the insects.
In the dry season, especially when strong southeasterly trade winds are blowing, the mangroves offer a cool, shady environment in contrast with the wide open coastal plains. Mosquitoes are then less abundant, and Aboriginal people move into the mangroves on hot days to escape heat and dust.
In the dry season, especially when strong southeasterly trade winds are blowing, the mangroves offer a cool, shady environment in contrast with the wide open coastal plains. Mosquitoes are then less abundant, and Aboriginal people move into the mangroves on hot days to escape heat and dust.
Davis (1984) has demonstrated that Aboriginal languages include names for many mangrove species. The most revered is the giyapara (Rhizophora stylosa), a stilted mangrove which is seen as embodying the ancestral being who created the coastal area, and which plays a part in ritual and story. Since estuarine shoals are colonized by this species, it is easy to envisage it as a maker of land. Possibly the Aborigines 6,000 years ago, witnessing the cessation of the marine transgression, saw Rhizophora stylosa as the mangrove pioneer and derived this legend of stilted mangroves walking in from the sea.
The activities of the Yolngu in the mangroves of Castlereagh Bay, documented by Davis, are similar to those of other coastal tribes. In Western Australia, for example, Kenneally (1982) described the building of fishing platforms by Aborigines in the mangroves. The soft wood of Camptostemon is widely used for carvings, some of which are now sold to tourists, and for the making of floats for use with harpoons in hunting marine animals. Bruguiera parviflora is used for making spears. Foods from mangroves include the fruits of Avicennia and the hypocotyls of Bruguiera, while the lemon fragrance of crushed leaves of Osbornia octodonta is used for flavouring dugong and turtle meat. Medicinal properties are claimed for a number of mangroves: Camptostemon ash is used to treat ringworm and scabies, while Excoecaria sap cures skin ailments (Hegerl 1982).
Such traditional use of mangrove ecosystems by Aborigines is thought to have had very little ecological impact, partly because the Aboriginal population has remained small and partly because the techniques of fishing and collecting are still primitive. Conflicts have arisen between Aboriginal and non-Aboriginal commercial fishermen where the latter have been catching barramundi in coastal waters and thereby depleting the numbers of this fish catchable within mangrove areas by Aborigines. Hostilities, marked by mutual threats and the brandishing of weapons, have given way to discussion and conciliation through legal proceedings. The Aboriginal Land Rights Act (Northern Territory) of 1976 introduced legislation to determine fishing rights and the tenure of intertidal and near-shore areas by Aborigines on the Arnhem Land coast (Davis 1984).
Europeans and Mangrove Ecosystems
The various explorers who discovered and charted the Australian coastline - the Dutch in the seventeenth century and the British and French in the eighteenth and nineteenth -described mangroves and included them on their maps, but few of them actually went into mangrove areas. Navigators like Matthew Flinders steered clear of mangrovefringed coasts, knowing their inshore waters to be shoaly, and it was land-based exploration that led people such as Edmund Kennedy on the Cape York Pensinsula in 1848 and John McKinlay on the coastal plains east of Darwin in 1866 to traverse mangrove swamps. They found them difficult areas, and the settlers who followed them regarded mangroves as hostile environments, habitats for snakes and crocodiles, breeding grounds for biting midges and mosquitoes, waste areas of little or no value until reclaimed as dry land. It is only within the past 20 years that Australian scientists have publicized the ecological importance of mangrove systems and argued for their conservation.
Sundry uses have nevertheless been made of mangrove ecosystems by European settlers and their Australian descendants during the past two centuries. An early use was the harvesting and burning of mangrove wood to produce soda ash, known in Australia as barilla, for soap manufacture. This occurred in the Sydney district in 1810 and also around Moreton Bay near Brisbane, Cairns Bay in northeast Queensland, Port Adelaide in South Australia, and Westernport Bay in Victoria during the early decades of the nineteenth century (Bird 1981). The practice declined only when the industrial production of alkalis by the Le Blanc process, invented in France in 1976 but only slowly disseminated, became established in Australia in the mid1840s.
In Westernport Bay, Victoria, extensive areas of white mangrove (Avicennia marina var. resinifera, the only species present here) were cleared and burned to produce barilla in the early 1840s. Records of shipments to nearby Melbourne in 1843/44 have been used to calculate the area of mangroves cleared in that year: 8.4 hectares, or about 7 per cent of the mangrove area in Westernport Bay at that time {Bird 1975). Several of the areas then cleared have failed to regenerate, largely because the clearance of mangroves exposed sandy shores to the rear to erosion by wave action, mobilizing sandy deposits that drift to and fro, preventing the establishment of Avicennia seedlings (Bird and Barson 1975).
Mangrove timber has been extracted on a small scale in many areas for jetty construction, boat building, fish traps, mooring poles, and road foundations, and mangrove wood has occasionally been used in carpentry (Hegerl 1982). However, mangrove forest management for sustained yield, based on the systematic cutting of trees and controlled regeneration, has not been practiced in Australia in the way that is common in South-East Asia. A minor local industry has been the extraction of tannin from mangrove bark: it is particularly used to make fishing nets more durable. In some areas cattle have browsed mangrove areas or have been supplied with mangrove leaf fodder, usually in times when drought has depleted their usual pasturelands.
Some mangrove swamps have been cleared to make way for salt pans. Areas near Lake Macleod in Western Australia have been converted for salt production, and at Port Adelaide a seaward fringe of mangroves was retained to prevent waves damaging the salt pans.
Estuaries and tidal creeks in mangrove areas have been much used by commercial fishermen and anglers, and mud crabs and oysters are harvested from the mangroves, but fishponds of the kind widely developed in South-East Asia have not yet been constructed in mangrove areas. Salt-water crocodiles, once numerous in the mangrove-fringed estuaries of northern Australia, have been intensively hunted commercially for their skins and are now rarely encountered. For some years they have been declared a protected species, but crocodile farming is now developing as a means of reviving the skin industry while at the same time ensuring conservation of the species (Bustard 1972).
Mangroves have been destroyed in many areas by dumping garbage on them or by deliberate land reclamation. Embanking and filling of mangrove areas has been a prelude to coastal land development, especially in northern New South Wales and southern Queensland, adjacent to resort areas. The cutting of access channels through mangroves to allow boats to reach the land has occurred at many places around Australia -for example at Broome, in the north-west, where such channels were cut to enable pearl luggers and fishing boats to come in and land their catches (fig. 5). Such excavations have had little impact where the dredged material was taken away, but where it has been dumped alongside the channel to form artificial levees, mangroves are killed and replaced by other vegetation (fig. 6). In parts of Westernport Bay the cutting of channels through the mangrove fringe to allow boats to land and collect cattle in the midnineteenth century has been followed by a widening of these cut areas, due to the killing of bordering mangroves by drifting sand (Bird and Barson 1975).
In recent years the mechanical excavation of mangrove areas to form marinas for recreational boating has become extensive. In such areas the mangroves have also been modified by the construction of causeways to carry access roads and by the reclamation of land adjacent to the marina for clubhouses and dryland facilities such as boat storage. Similar replacement of mangroves by canal networks, with housing on intervening reclaimed land, has been carried out in several places, for example on the Nerang River estuary in southern Queensland. Here it was found that the canals greatly extended the breeding area for biting midges, which previously bred only in a narrow zone between high neap and high spring tides. The resulting increase in the population of these midges was accompanied by an increasing abundance of mosquitoes, which have a similar breeding habitat, and it has been necessary to introduce widespread spraying of pesticides to combat this nuisance (Hegerl 1982).
At Trinity Inlet, near Cairns, a 726-hectare area of mangroves was embanked in 1972 to be reclaimed and used for sugar-cane cultivation (fig. 7). The damage to mangroves was extensive even outside the embanked area, because the embankment was constructed without regard to the pattern of tidal creek systems, and cut-off loops became stagnant backwaters where mangroves died back. The project has been something of a failure because of an unexpectedly high incidence of flooding from neighbouring steep catchments after heavy rains. It is hoped that any future mangrove reclamation will proceed only after studies of the hydrology and ecology of the area have indicated where such reclamation can be successful and will cause a minimum of disruption to the mangrove ecosystem.
Dredging in estuaries to improve navigability or aid flood abatement produces large quantities of mud and sand, and in many areas this material has been dumped on adjacent mangroves, killing them. In Cudgen Creek, New South Wales, dredging of rutile deposits from the estuary produced large amounts of overburden that have been dumped in and around bordering mangroves. Here, as elsewhere, the destruction of mangroves has been followed by a decline in the local fishery.
It is now widely acknowledged that the mangrove ecosystem is a key part of fisheries ecology, because mangroves are sheltered, nutrient-rich areas where many fish species breed and feed. However, it is difficult to assess this accurately, because other marine ecosystems, such as salt marshes and seagrass beds, also contribute to the productivity of fisheries, while fish populations fluctuate independently of habitat factors: a reduced catch may be due to overfishing, increased predation, or the onset of a disease. But mangrove ecosystems, with their high biological production, rich associated flora and fauna, and accumulated nutrient-rich substrates, are now considered worth conserving for their scientific and educational interest as well as their role in maintaining the productivity of fishery resources.
The role of mangroves in trapping and stabilizing sediment has become obvious in areas where mangroves have been cleared or have died back and erosion has ensued (Bird and Barson 1975). A healthy, spreading mangrove fringe is correlated with the building up of bordering depositional terraces and the maintenance of relatively deep water channels. When mangroves are cleared or die away, erosion of their substrate leads to dispersal of sediment and shallowing of channels, thereby diminishing navigability.
Mangroves have been damaged locally by oil spills and herbicides, as on parts of the shoreline of Westernport Bay, but the effects of pollution by urban sewage have been generally to impoverish or destroy the associated marine organisms rather than to kill the mangroves. It should be noted that some cases of mangrove die-back are the outcome of natural changes, such as the in-washing of sandy deposits by storm surges (fig. 8).
In recent years there has been some planting of mangroves to restore a vegetation cover that has been depleted or destroyed in the course of mining activities (e.g. near Gove in Arnhem Land), seaport development (e.g. at Gladstone, Queensland), or airport development (e.g. Brisbane, Queensland). In the latter case over 50,000 seedings of Avicennia marina and Aegiceras corniculatum have been planted adjacent to a runway that extends into the mangroves (Bunt 1985).
Conclusions
Whereas the impact of Aboriginal hunting and gathering appears to have had little effect on mangrove ecosystems in northern Australia, the more widespread and intensive impact of European settlers and their Australian descendants has modified and reduced the mangrove ecosystems during the past two centuries. Dumping of waste materials and deliberate land reclamation have reduced the mangrove area in Australia by about 10 per cent over this period. In recent decades, attempts to conserve mangrove ecosystems have included the declaration of national parks, coastal parks, marine parks, and nature reserves in each of the states of mainland Australia and in the Northern Territory, which include areas of mangroves. However, many mangrove areas remain unprotected, and it is necessary to establish many more reserves that will include mangrove ecosystems with a large number of species, or with features such as zonations that are of scientific interest, as well as those that may contribute to the continued productivity of local fisheries. One difficulty is that mangroves occupy areas submerged at high tide, and it is often simpler to treat them as parts of the marine rather than the terrestrial environment. Yet it is from the adjacent land area that most of the human impacts and influences of mangroves derive.
Acknowledgement
I am grateful to Stephen Davis (Landsearch, Darwin) for information on coastal Aboriginal tribes in Arnhem Land, and for discussion of this paper.
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