Mangrove Distributions in Australia
Species and Sea Temperature
Histogram bars of species numbers by subregion affected by gradients in average sea surface temperature. Shaded zones show temperatures in °C.
Species and Rainfall
Histogram bars of species numbers by subregion affected by gradients in coastal mean annual rainfall. Shaded zones show levels of annual rainfall in mm.
Average Annual rainfall, based on a 30-year climatology 1961 -1990 © Commonwealth of Australia 2003
Australia’s mangrove species today are primarily distributed according to temperature and rainfall regimes that define and characterise the Australian climate. However, these factors are always changing. As noted in the previous section on Mangrove Origins, mangroves apparently once inhabited Tasmania around 50 million years ago in an apparently much warmer climatic setting. Fossilised remains of Nypa, the Mangrove Palm, were unearthed from Eocene rock facies on the south-west coast near Strahan. Curiously, this was also a time when the Australian landmass was much closer to the Antarctic continent, a feature that further contrasts with the present tropical distribution of Nypa fruticans in Australia. We live in a dynamic world. It is essential for us to understand both the dynamic nature of such factors, and how they affect local distributions of natural habitats like mangroves.
Latitude and Temperature
Australia’s mangroves are influenced and limited by temperature. This is shown by declines in species numbers that match Sea Surface Temperatures (SST) taken in winter. There are comparable but notably different declines in species numbers along western and eastern shorelines. In the Table (opposite), note the different southern-most occurrences of each species. In the upper map, notice how the histogram bars along west and east coasts decline to the south. The gradients closely match SST isotherm contours shown, for instance, by the 20-25° C isotherm that is roughly central on each coast.
Rainfall and Moisture
Australia’s mangroves are influenced and limited by moisture conditions and rainfall. Overall, there is maximal diversity along the uniformly warm, northern tropical coast. However, there are notable variations in species number with successive coastal sections. In the map to the lower right, see how the histogram bars, representing species numbers, along the north coast reflect seemingly erratic differences. These differences match mean annual rainfall isohytes on the map. Greatest species numbers are found along certain sections of the coast. These coastal areas are those with higher mean annual rainfalls. In the Table (opposite), rainfall conditions vary with each species, and some species are limited by drier conditions, but not always. Generally, the maximal number of species are found on the north east coast where temperatures and rainfall are highest.
Tidal and Estuarine Effects – Local Distribution
Australia’s mangroves are distributed within local areas according to the influences of salinity and tides. These factors define and characterise mangrove communities, especially within sheltered embayments and estuaries, shown by their location within an estuary and their position along the tidal profile. Hence a species might, for example, be known as a low intertidal, upstream specialist. However, this is not always easy to interpret where compounding factors like competition between species and selective predation by fauna also influences local distribution patterns.
Estuarine Location and Salinity
Individual mangrove species rarely occupy entire estuaries from sea mouth to the tidal limit upstream. Each species has a preferred upriver estuarine location based on its salinity tolerance range. Estuaries are influenced by a salinity gradient that can be conveniently considered in three parts, including: ‘downstream’ as the lower third of the estuary and off-shore islands where salinities vary infrequently from sea water; ‘upstream’ represents the upper third where salinities are influenced either by freshwater or hypersaline runoff; or, ‘intermediate’ represents the middle third. Particular mangrove species will occupy one or all of these estuarine locations, depending on its salinity tolerance in combination with the local climate and catchment conditions. For instance, species like Avicennia marina, Rhizophora stylosa, Osbornia octodonta, Sonneratia alba commonly occur in downstream locations. By comparison, Rhizophora mucronata, Sonneratia caseolaris, Sonneratia lanceolata, Bruguiera sexangula and Barringtonia racemosa are found upstream in freshwater dominated estuaries. Other species, like Avicennia integra, are notably located in intermediate estuarine locations.
Right - Daintree River estuary, Queensland, showing large downstream mangrove area and a narrowing fringe upstream.
Tidal Position and Inundation Frequency
Individual mangrove species rarely occupy the entire tidal profile from mean sea level to the highest tide levels. Each species occupies a distinct part, defining its characteristic tidal position. Mangrove species have a special relationship with tidal inundation plus the frequency of wetting, and soil type. These influences commonly result in distinct bands of species ecotones that follow tidal contours, referred to as zonation. Tidal position of mangroves can be conveniently considered in three parts, influenced by different degrees of tidal inundation, including: ‘low’ representing areas inundated by medium high tides and flooded >45 times a month; ‘mid’ representing areas inundated by normal high tides and flooded from 20 to 45 times a month; and ‘high’ representing areas inundated less than 20 times a month. For example, species like Avicennia integra, and Sonneratia alba commonly occupy low intertidal positions. By comparison, Heritiera littoralis, Xylocarpus granatum and Lumnitzera racemosa are found in high intertidal positions. Other species, like Avicennia marina, Acanthus ilicifolius and Aegiceras corniculatum are observed at high and low intertidal positions. These occasional bimodal distributions across the tidal profile are apparently the result of competition with other mangroves and predation by fauna such as small crabs.