Coral reefs and the importance of ecosystem engineers

Friday, May 20, 2022 — Sunday is Biodiversity Day. But what actually is biodiversity and why is it important? Is it bad if there is one beetle less in Flanders or one or more fish in a coral reef somewhere in the tropics? Prof. Marc Kochzius, marine biologist at the Vrije Universiteit Brussel, is very clear about this:

"Yes, the fish in the coral reef and the beetle in the Flemish forest are more important than ever. If they disappear, they may be the missing link in a larger whole and the whole ecosystem may collapse.

When extinction becomes a problem

Biodiversity is the collection of all kinds of organisms living on Earth. Each species has a particular function in a larger whole, known as the ecosystem. Usually, several species perform the same function, which is why an ecosystem can continue to function when a species disappears. The extinction of species is in fact a natural process of evolution: species decay and new species emerge. The resilience of an ecosystem is greater if there are more species and thus more biodiversity.

Some organisms form an ecosystem by themselves, like trees that form a forest or corals that build a reef. These are called ecosystem engineers. If an ecosystem engineer disappears, you immediately lose an entire ecosystem.

Kochzius: "Normally, for an ecosystem with high biodiversity, it is no problem if one or two little beetles or fish disappear. Of course, it is regrettable if a species disappears because of human activity, and you have to ask whether this is ethically justified. Biodiversity certainly has a value in itself and must be protected for that reason."

Around 1.75 million species have been described, but it is estimated that anywhere between 7 million and 30 million species exist. The problem today is that the rate of species extinction is 50 to 100 times faster than the natural rate. Species are disappearing so quickly that there are no species left for certain functions, so the ecosystem no longer works. The cause is due to human activities such as land use, overexploitation and pollution and their consequences such as climate change, which are having such a great impact on the entire Earth ecosystem that this era is called the Anthropocene, the human age.

Biodiversity in the Coral Triangle

Kochzius and his team are investigating coral reefs in the Coral Triangle in South-East Asia, the world’s largest centre of marine biodiversity with more than 580 coral species and around 2,700 fish species. To find out why this is so, the team studies the genetic biodiversity in anemone fish, corals, starfish and giant clams. Their larvae drift with the sea currents as part of the plankton, sometimes for only a few days, sometimes for several weeks. They can travel up to thousands of kilometres. The adults remain attached to their coral reef afterwards.

The results of this research in the Coral Triangle show that each species has three genetically distinct groups in clearly delimited geographical areas: the west, the centre and the east of South-East Asia. These differences came about through evolution, which is the change of species due to mutations in their DNA. At a certain point, groups within a species are genetically so different that they form a new species.

During the last ice age, around 15,000 years ago, the sea level was around 120 m lower than it is today. This created land bridges between the islands of Southeast Asia, forming a barrier between the Indian and Pacific Oceans. As a result, exchange between the different populations was no longer possible and, over time, they built up a genetic difference through mutations. After the last ice age, the sea level rose again, but because of the pattern of sea currents, the exchange was limited and the genetic difference remained. Ultimately, this is how the coral triangle has become the centre of marine biodiversity.

Ecosystem engineer coral doomed?

The biggest problem for the biodiversity of coral reefs is the rising temperature of the ocean due to climate change. Coral reefs live in symbiosis with single-celled algae, which photosynthesise and use the energy of sunlight to make sugar from water and carbon dioxide. When it gets too hot, this is very stressful for the corals, and they expel these symbiotic algae. This causes them to bleach because their colour depends on the algae. In losing the algae, they immediately lose a very important source of sugars and after a few weeks they starve to death.

Kochzius: "In this way, very large areas of coral reefs can bleach and die. At the moment, for example, 90 % of the Great Barrier Reef in Australia is damaged by coral bleaching because there was a heat wave. This is the fourth heat wave in the Great Barrier Reef since 2016.

Another problem of climate change is the increase in tropical storms, which can cause great damage to coral reefs. Coral reefs are also in danger from an increase in the crown-of-thorns starfish that feeds on corals and has become more of a pest in recent years. Moreover, in many countries there is overfishing and pollution of the coral reefs.

If this continues, we will have completely lost the coral reefs by the end of this century, especially if coral bleaching causes the ecosystem engineers to disappear. A high biodiversity with many species that can replace each other is the only chance we have of not losing the coral reefs altogether.

So, it is definitely important to protect biodiversity in the Coral Triangle and elsewhere, because a lot of people along the coast depend on the fish they catch in the coral reefs."

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