Ecosystems are often hard to define, and they depend on the scale of the observer. Tropical seashores are here considered an ecosystem defined by their tropical geography, intertidal location and characteristic seashore vegetation.
One of the best ways to define an ecosystem is simply by describing them. This tropical seashore itself can then easily be divided into different recognizable zones. These are examined and described below. These zones occur in either of the two overlapping worlds that make up the tropical seashore: the marine intertidal and the terrestrial vegetation communities.
The marine intertidal zone is that region which is covered by water at some time during the regular tidal cycle. The dividing up of this intertidal area into various zones can be based on different factors such as the substrate (sand, mud, rock, etc) or their proximity to the ocean (submerged, intertidal, splash zone, etc).
Just above the intertidal area, there is a similar zonation, but this based on vegetation type, and divided into ‘vegetation communities’. There are various plants that have adapted to the life on the tropical seashore. These plants are characteristic of the shore, and most species do not grow too far into the forest or woodlands behind it (although there are often closely related species). They are usually technically referred to as littoral plants. But they are often commonly called ‘beach’ or ‘island’ plants. These plants are widespread throughout the tropical oceans, and include species such as Beach Callophyllum, Pandanus, and today, Coconut. (Some of these species found here overlap with the Australasian Mangroves and Asian Mangroves). Each species’ different requirements and adaptations to certain environmental conditions determines where they grow on the seashore, and there tends to be clumps of certain species that grow together in particular zones. Like the intertidal zone, these conditions are usually set by factors such as substrate and the proximity of the ocean. This often results in zones that develop roughly parallel to the ocean. There are various examples of different authors and their classification of zones based on seashore vegetation in the Tropical Pacific (eg: Whistler 1992).
Rocky shores are a much loved and explored part of temperate coasts around the world. However, they are not as obviously common along the tropical coast. For here, the more abundant shallow continental shelves or fringing reefs reduce the wave action, and this tends to instead promote the development of deposition landforms, such as sandy beaches and muddy bays.
However, within the limited rocky shore habitat, there is a huge range of different types along the tropic coasts of the world. For example, between the coral offshore and the sand of the beach, there often lies a transitional rubble zone of what appears to be dead material, made up of broken coal, interspersed with sandy areas. And just under the tide and up onto the beach there is also often a zone of ‘beach rock’, a cemented mix of coral, shells and sand, that usually form platforms. And on continental shelf islands and along certain coasts, rocky habitats range from resistant granite boulders to cliffs and caves of sedimentary sandstone.
Life on tropical seashores is not always as obvious as it is along temperate shores. First time or casual visitors to tropical rocky shores who have grown up along cooler coasts are often disappointed at the apparent scarcity of life here. This is largely due to the harsher conditions. Warmer waters hold less oxygen than cooler waters, so there is generally less life in the tropical seas off the coast. Less oxygen means less nutrients in the water, and therefore there is an obvious lack of the filter-feeding animals such as barnacles, oysters and mussels. And in the exposed areas and rock pools, the heat of the tropical sun results in higher temperatures, which leads to higher rates of evaporation, and thus higher salinity levels.
However, many of the ecological principals of tropical rocky habitats are similar to their temperate counterparts. There are distinct zones, distinguished by colour and shape of the rocks, and the species living there. Above the intertidal zones is the normal bedrock. Below that is a salt-whitened ‘splash zone’, a severe place where few intertidal animals live, but lizards may scurry. Below this is the intertidal area. This is frequently blackened by cyanobacteria and has a variety of different animals grouped at different levels. There is sometimes an eroded shelf or even platform at this intertidal level and, as in temperate shorelines, there are invariably rock pools in these flatter areas.
Within the intertidal area, different animals tend to live at slightly different heights. Small shells called winkles graze the upper intertidal areas. Within the upper middle areas, barnacles feed when covered by water at high tide, but can survive exposure by closing up at lower tides. In the middle areas there are several animals that roam around. Some are extremely slow, such as the grazing limpets and chitons. Much faster are the cockroach-like marine slaters. In the lower middle zones are more sedentary animals, such as the sharp-shelled tropical oysters, and in the lower tidal areas are the mussels.
The long white sandy beach is the image many visitors often have of the tropical seashore. The reality is somewhat different. There are many different types of sands and beaches. On the more typical large grained beaches of the tropics, the soft sand is often interspersed with sharp coral. On some beaches it may in fact be made up entirely of coral (for more details about the composition of the sand, refer below in ‘Geology’ section).
Tropical beaches are not all that conducive for obvious life. They are exposed and therefore hot. Their shifting substrate of sand in the intertidal area means they are bare of large plants (if plants do colonize the beach, it may develop into a mangrove). Many of the larger animals that are seen here are just occasional visitors, such as some shore birds or breeding sea- birds. The obvious resident animals are often scavengers that wait for debris to wash up from the adjacent ocean, such as ghost crabs. But there is also a range of smaller animals at the surface of the sand, and an even larger range of tiny microscopic animals just underneath.
In areas of higher tide or in protected bays, finer sand and mud will accumulate. These seashores are much richer in life within their fine-grained substrate. They subsequently attract many more of the feeding shore birds. These softer seashores in more protected estuaries commonly end up with mangrove forests growing on their edge.
Many tropical seashores are simply the edge of the ancient and stable continental shelves where they meet the ocean. However, other more remote seashores have been created by extreme volcanic activities. Most of the oceanic islands are the result of land being thrust up relatively suddenly from depth due to land movement, and thus many tropical seashores have recent or ongoing volcanic activity. One of the best (and dangerous) places to climb the rim of an active volcano is the island of Tanna, in Vanuatu.
Among the more dramatic and interesting features along exposed coastlines are sea caves.
These are caused by the constant pounding of the ocean against the land. The hitting of constant waves all day and every single day has a huge cumulative effect. When the water hits the land it not only takes material off (the weathering), it also takes it away (the erosion). It is the particles that are carried in the water, (more so than the water itself) that acts as an abrasive and grinds away the material.
Weathering by water is not just due to simple physical battering. There is also weakening of the rock due to relatively fast and extreme changes in water and temperature where the sea water hits the land. When there is a rapid change in the volume of water seeping into the rock, and the water expands and contracts with re-hydration and dehydration, this hydration pressure weakens the rock. While the land above is consistently exposed to air and the land below constantly covered, the intertidal area is changing in temperature; it is exposed, then covered by cool water, then exposed and dried again in the tropical heat. Along the tropical coast there is also the addition of rain seeping into the rock through the heavy wet season rains, and then drying out again in the high heat of the summer sun. When temperatures change, they expand and contract the rock, and weaken in the process of thermal expansion.
Blowholes occur where holes have been weathered down into caves underneath a rocky coast. When waves hit the shore, the water in the underwater cave is forced into the small eroded gap and squeezed out and high into the air. One of the best places to see blowholes in the tropical Indo-Pacific is along the coast of Tonga at Mapu a Vaea.
Many visitors to the tropical seashore are disappointed when they first walk on a beach, due to the presence of large bits of hard coral and jagged remains of seashells, and they can be a nuisance for bare feet. While the main contributors of tropical beaches are the corals and ‘seashells’, there are also the remains of other obscure, unrelated groups, such as sponges, echinoderms, diatoms, foraminifers, and algae.
The sand of some tropical Indo-pacific beaches is stained pinkish from the broken down 'Organ-Pipe Coral', such as on Pink Beach on Komodo Island in Indonesia.
Some algae have calcified tissues, so when they break down these harder parts contribute to the beach sand.
When the green seaweed Halimeda breaks downs, the calcified harder parts of the alga can contribute to the beach sand. When there are larger areas or entire beaches made up of this, it is like soft piles of styrofoam snow flakes!