Bioerosion is the regular process in which calcium carbonate substrate from coral reefs is broken down by bioeroders into rubble and fine sand, which is typical of tropical beaches. Bioerosion is a naturally occurring process that is essential in making room for new corals to grow. However, if bioerosion is excessive, expedited by human factors, then coral destruction will be faster than coral growth, weakening the coral structure.
Bioeroders are the biological agents that break down coral reefs and include three main groups:
- Microborers are organisms that bore microscopic holes into the surface of the coral substrate. These include tiny endolithic algae, cyanobacteria and fungi. In Australia, microborers erode about 0.35 kg of calcium carbonate per meter square each year. Assuming this rate is accurate for the entire Great Barrier Reef, then microborers alone could produce enough sand to fill 1.1 million school buses in a year .
- Macroborers are small marine animals that erode the internal structure of coral, which can leave it weak and fragile. Macroborers include small marine worms, sponges, bivalves and barnacles .
- Grazers are animals that scrape the surface of the coral skeletal framework while trying to eat the algae that live on the coral reefs. These scrapings change the surface area of the reef framework, which can have implications for physical processes, such as water exchange, on the reef. Grazers include urchins and fish, particularly parrot fish .
- 800 million years ago: Limited evidence of Microborers reaches as far back as the Precambrian period.
- 500 million years ago: Macroborers (boring worms) emerged in reefs in the Lower Cambrian period.
- 250 million years ago: More conclusive research has found Microborers in fossilized reefs from the Triassic period. Boring sponges and bivalves become important in reefs during this time. Grazers were not relevant to reef bioerosion until later geologic periods.
- 175 million years ago: Patellids (sea snails) and Echinoids (sea urchins, sand dollars, etc.) began impacting bioerosion starting in the Jurassic period.
- 20 million years ago: Scarid Fishes are first seen in the Miocene period.
Throughout the time periods, the intensity of bioerosion has changed drastically. The general trend has been an increase in bioerosion as more types of bioeroders have evolved .
Effect of excessive bioerosion on reefs:
Coral reefs are able to absorb about 90% of energy from wind-driven waves before they reach the shore. This helps protect the shoreline and coastal property from destruction, especially during storms and tsunamis. When the skeleton becomes weakened, these natural events are much more destructive and can wreak havoc on coastal communities.
About 114 reef fish species, like butterflyfish, parrotfish and damselfish, eat the surface of live coral without damaging the underlying skeleton. Damage to coral reefs takes this food source away from them . The coral reefs also provide shelter for sea animals, so if the reefs are broken down, there is less shelter available for fish, crabs and shrimp. This is a problem for the species in that they do not have a habitat and for the humans that eat these ocean species.
If bioerosion occurs at a faster rate than corals build, the reefs will not be able to keep up with sea-level rise. Humans have contributed to the increased speed of coral reef degeneration.
Bioerosion is a naturally-occurring process and is necessary in order to maintain a healthy coral reef environment. However, recent research has shown that human-induced factors can accelerate bioerosion rates and negatively impact the delicate balance between bioerosion and coral growth . The increase in ocean acidification, caused by global warming, has been shown to significantly increase bioerosion, especially by the sponge Cliona orientalis.  Researchers have also found a correlation between increasing human coastal populations and the growing rate of excessive bioerosion. Increasing ocean temperatures are detrimental to coral reefs, but research into its impact on bioerosion has been inconclusive .
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