The mangrove habitat consists of several different species of plants that have the ability to tolerate and reside in the intertidal zone, a hostile environment defined by high variable factors such as temperature and sun exposure, anoxic soil, water-logging, tidal currents, and salinity. Despite these harsh conditions, mangroves have become an incredibly diverse group of plants, encompassing over 39 genera in 26 plant families worldwide. Each species has adapted and endured the harsh conditions that the intertidal zone brings forth, evolving ingenious adaptations such as prop root systems, viviparous seeds, and the ability to secrete salt.
Endurance
The intertidal zone is one of the harshest and most erratic environments defined by its constant contact with excess salt, anoxic soils, and tidal fluctuations. High tides and rain bring forth unpredictable flooding of saltwater and/or freshwater inundation. Low tides, on the other hand, bring forth extreme sun intensity and thus rapid increases in temperatures. Consequently, convergent evolution led mangroves to adapt a variety of unique characteristics found in a variety of terrestrial plants, such as the smaller, thicker leaves shared with desert plants used to prevent water loss from evaporation and the buttresses found in many rainforest trees, which provides stability and support against the high energy wave action.
The most unique adaptation is the mangroves’ tolerance to salt, an attribute that sets mangroves apart from other types of plants. The ecological success of mangroves is due to a variety of morphological and physiological adaptations, with each mangrove species independently evolving to endure the excessive salt-toxic to plants-and to ensure it receives enough water despite the high negative water potential caused by salinity.
There are three main salt elimination methods in mangroves: secretion, exclusion, and accumulation. Several species of mangroves are secreters, using salt glands to secrete excess salt; expelling it onto the plants’ leaves, bark, and stems; and then shedding them to get rid of the accumulating salt. Thus, the plants stay relatively salt free. Salt excluders, on the other hand, have the ability to overcome the negative osmotic pressure by changing its own hydrostatic pressure through transpiration, and therefore allow themselves to draw water in and keep salts out. Similarly, salt accumulators also overcome the negative osmotic pressure by accumulating high concentrations of salt in its cells.
Interactions
The diverse array of salt tolerant adaptations led to the formation of vast mangrove forests along the coast of Australia and subsequently formed ideal habitats for numerous marine and terrestrial fauna. As a result, there are several symbiotic interactions between organisms, some of which are so unique that they are only found in the mangroves.
The mutualistic relationship between the sesarmid crab and multiple mangrove tree species is one such example. Like other trees, mangrove trees produce leaf litter, forming a source of potential nutrients. During low tide, the crabs (Sesanna spp.) scurry along the mangrove forest floor in search of these fallen leaves, carrying them into their burrows beneath the trees, shredding them into smaller pieces, and feeding on them. The crabs are essentially recycling the leaves’ nutrients by breaking the leaves down and thus allowing them to decompose right back into the soil at a much quicker rate. According to Robertson and Micheli, the removal of leaf litter from the surface and back into the soil is essential for secondary plant production (1986; 1993), and without the crabs, leaves would have been flushed away with the next high tide, depriving the habitat with the additional sources of nutrients.
Therefore, these crabs serve as a keystone species in maintaining forest growth and development. The crab-mangrove relationship is just one of several unique interactions that are only found in the mangroves. As a keystone organism, the crab is responsible for expanding the entire mangrove habitat, one that supports several microhabitats that are unique to the mangroves and found nowhere else.
Furthermore, mangroves are not only an irreplaceable habitat filled with extraordinary interactions between marine and terrestrial organisms, but they also provide people with several resources and ecological services that our society is highly dependent upon.
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