MANGROVES - KIDNEYS OF THE COAST
Mangroves and tidal saltmarshes are amongst the most endangered marine wetland habitats worldwide. This is despite a wealth of benefits, ranging from fish habitat supporting commercial and recreational fisheries, to shoreline defence protecting valuable coastal real estate. These wetlands also act as filters of coastal waters, but they are disappearing around the world at up to 2 per cent a year on average. As a consequence, there has been a dramatic loss of ecosystem services from mangroves with vast losses in area and function as remnant patches progressively deteriorate.
Let’s have a look at what is going on in Australia - by hovering your mouse over the map.
Mangrove plants are found throughout tropical regions of the world. Distributions into higher latitudes are generally constrained by the 20° C winter isotherm in each hemisphere. Deviations from this pattern mostly correspond with the path of oceanic circulation currents where mangrove distributions are broader on eastern continental margins and more constrained in the west. For instance, in the North and South Atlantic Oceans, the cooler currents (each moving toward the equator from the north and south respectively) reduce the lineal extent of warmer coastline for west Africa. Conversely, on the east coasts of North and South America, the warmer poleward moving currents extend the warmer coastline. And, because mangroves match these warmer conditions so closely, it implies they are profoundly limited by low temperature. However, there are at least three important deviations from this pattern, and they all occur in the southern hemisphere. These are along the coastlines of eastern South America, across the North Island of New Zealand, and around Australia. This could be the result of specific, small-scale extensions of warmer currents, but it appears more likely these populations are relict, representing refuges of more poleward distributions in the past.
Global dispersal and continental barriers
Present day global dispersal patterns are dependent on the specialised water-buoyant propagules of most mangroves. Their dispersal is constrained by wide bodies of water and land masses that block equatorial circulation. There are four major barriers around the globe today that generally restrict dispersal of warm coastal marine organisms. These include the continents of Africa and Euro-Asia, North and South American continents, and the oceans of the North and South Atlantic and the eastern Pacific. The relative effectiveness of each of these barriers differ, depending on geological history, dispersal/establishment ability, and the timing of evolutionary development of respective species. There are two barriers that appear to have been effective during recent geological time, namely the African Euro-Asian continents and the Pacific Ocean. Accordingly, mangrove species, along with other tropical shallow-marine coastal inhabitants, are divided into two global hemispheres, namely, the Atlantic East Pacific (AEP) and the Indo West Pacific (IWP). These regions span more-or-less equal portions of the earth, and each has equivalent areas of mangrove. They further each represent centres of secondary radiation and they share major mangrove genera, like the Rhizophora and Avicennia. The AEP has fewer species, and fewer additional genera, although these span two existing barriers. The most diverse mangrove flora occurs in the IWP, and this is constrained between two existing barriers. Based on species presence, each of these regions is divided into three subregions making a total of six in the world, including: Western America, Eastern America, Western Africa, Eastern Africa, Indo-Malesia and Australasia