• Leila Sulentic

Coral Degradation: Impacts on Reef Biodiversity & Reef-Dependent Human Communities

Updated: May 26, 2020

How is coral bleaching and extinction around the globe affected reef biodiversity and health, and what are the implications, economically and culturally, on reef and fish dependent communities?

Before we dive into how coral reef degradation is effecting reef biodiversity and reef-dependent human communities around the globe, I first want to take a few minutes to explain what coral reefs are and why they are bleaching and dying.

Coral reefs have formed over millions, sometimes hundreds of millions of years like the Great Barrier Reef which is believed to be over 500 million years old. They are made up of hard coral calcium carbonate remains, living hard corals, and soft corals. Even though coral reefs only cover about .2% of the sea floor, an estimated ¼ of all known marine life inhabit coral reefs. Additionally, coral reefs help sustain various other important marine ecosystems by protecting them from strong currents and surge including seagrass beds which remove large amounts of CO2 from the ocean and clean the waters they live in, mangroves which act as breeding grounds for countless species including ¾ of known tropical fish species, and shorelines and beaches.

There are 4 main types of shallow reefs. Fringing reefs, the most common type, grow near coastlines around islands and continents and are separated from the shore by shallow lagoons. Barrier reefs also parallel coastlines but are separated by wider and deeper lagoons and at their shallowest points can reach the water’s surface, forming a barrier to navigation. Patch reefs are small isolated reefs that grow up from the open platform of a continental shelf or island platform usually between fringing and barrier reefs and rarely reach the surface. Lastly are atoll reefs which are rings of coral that create protected lagoons and are usually located in the middle of the sea. They form when islands surrounded by fringing reefs sink into the sea or sea levels rise around them. There are also deep sea reefs but little is known about them, their importance, and the effects of climate change on them so I will be focusing on these four shallow reefs.

Living corals are actually made up of many small organisms called polyps that secrete layers of calcium carbonate beneath their bodies to create limestone skeletons and live in symbiotic relationships with single cellular algae called zooxanthellae. Zooxanthellae provide coral with a majority of their nutrients via photosynthesis and are also what give corals their brilliant colors. In exchange for nutrients, the polyps provide the zooxanthellae a home and protection. (notice those menacing stinging tentacles in the photo below.)

For corals, the water they live in is like their body temperature. When this temperature becomes higher than normal, the coral gets sick and expels its life-giving zooxanthellae. Without the zooxanthellae, the polyps have no color and are translucent, and all we see is the white calcium carbonate structure of the coral beneath hence the name bleaching. If water temperatures do not decrease, the coral will eventually die from disease or starvation. With global warming, sea temperatures have increased worldwide, and we have begun to see mass coral bleaching and extinction around the globe.

Now that we have a basic understanding of what coral reefs are and why they are bleaching and dying, let’s move to the focus of this article beginning with how coral degradation is impacting fish and other coral-dependent organism biodiversity.

The first case I looked at was a study conducted from 1996-2003 on reefs in four marine preserves in the Tuman Puli Conservation Area of Papua New Guinea to observe the effects of coral degradation on fish populations. The results of this study tell a warning tale for the effects of coral degradation on fish species. It was found that the diversity of fish for the four focal families studied declined by 22% between 1996 and 2002 as coral reef cover fell from 66% to a record low of 7%. This number increased slightly to 15% in 2003, a recovery attributed to a small increase in coral cover. Perhaps even more troubling is that 75% of the fish species studied declined in abundance between 1996-2003, 50% by more than half.

The second case I looked at was a 2015 study of the Hawaii Islands to investigate patterns of diversity and what they could be attributed to such as thermal changes, habitat loss, or ocean currents. This study yielded multiple important results. First it was found that coral species richness, which ranged from 10 to 25 species per island, positively correlated with fish species richness. Second, this study revealed that herbivores such as sea urchins and mollusks tended to respond positively to coral species richness because corals provided them algal substrate, aka food, and habitat. Lastly, an overall positive correlation between habitat area and genetic diversity was found.

The findings from these two cases are important for several reasons. For one, the correlation between genetic diversity and habitat area indicates that as sea temperatures rise and coral populations are reduced, coral genetic diversity will also be reduced. Less genetic diversity is not beneficial to a species because the more genetic diversity a species has, the more mutations will occur, and mutations are what allow species to adapt, evolve, and survive in changing environments. The loss of genetic diversity for coral species will make it even harder for them to adapt to warmer seas and survive climate change. Second, the clear correlation between coral cover and richness and fish/marine herbivore diversity and numbers tells us that as coral reefs are lost, marine herbivore and fish biodiversity and numbers will be drastically reduced with some species that rely more heavily on living corals for food and protection going extinct. These losses in biodiversity and population sizes will likely cause disruptions to the entire marine food chain and could be incredibly catastrophic.

(butterflyfish like the one pictured above would likely not be able to survive coral extinction as much of their nutrition comes from feeding off of live corals.)

The other part of my research centered around human’s dependence on coral reefs and the economic and cultural implications of coral degradation. Coral reefs bring in a TON of money worldwide from reef tourism and fishing. In Australia, for example, it is estimated that visitor and resort industries on the Great Barrier gross over 1 billion each year. In some small island countries, this money is the main contributor to the nations' economies. Coral reefs also provide excellent natural protection from tropical storms and hurricanes. For example, in Guam and the Northern Mariana Islands, it is estimated that without reefs whole communities would be devastated by storms and tens of billions spent in federal disaster assistance. Coupled with more powerful and frequent storms due to warming sea temperatures, the disappearance of reefs could be incredibly disastrous for these small island communities. Reefs even aid in sea passage, creating calm channels and passes for boats to navigate and protecting harbors from strong currents and surge. Lastly, and perhaps most importantly, reefs are incredibly important to many island communities’ cultures and traditions from diet to traditional occupations. In Fiji, for example, much of the traditional cuisine such as kokodo, raw fish salad, fish suruwa, a type of fish curry, and lovo, meats, usually chicken and fish, that are wrapped in banana leaves and cooked in a special underground oven, rely on fish harvested from the nearby reefs. Food is just one element of culture but it is often underestimated in its importance. What is the South without barbecue or Italy without pasta, risotto, and pizza? Likewise, what is Fiji without these traditional dishes that require fish harvested from the local reefs?

“What is the South without barbecue or Italy without pasta, pizza, and risotto? Likewise, what is Fiji without these traditional dishes that require fish harvested from the local reefs?”

Roughly 665 million people, nearly 10% of the world’s population, live within 100km of coral reefs, and over 91% of these people are in developing nations. Excluding reasonably wealthy and densely populated developing nations including some Middle Eastern and Caribbean Countries, China, and Brazil, 75% of people living within 100 km of coral reefs are in the poorest developing countries, and 65-70% of these people live outside of high-density urban areas. These poorer rural communities are often highly dependent on reefs for income from fishing and reef tourism and food. In many island nations where agricultural land is limited, fisheries are the main source of income and protein. In the Philippines, for example, over 1 million Filipinos are engaged in marine fishing, an estimated 800,000 of them small-scale fishermen using traditional, low cost techniques.

Ironically, many of the people most reliant on coral reefs contribute the least to GHG emissions. In fact, the average per capita GHG emissions rate for the 424 million people living within 100km of coral reefs in developing nations is 1.78 metric tons/person. By contrast, in the US it is 20.1 metric tons/person about 11 times more. This is just one example of environmental inequity where the smallest contributors to the issue reap the worst consequences of environmental degradation and vise-versa.

It is obvious from my research that coral reef degradation and the loss of fish diversity and numbers will have drastic effects on human populations, especially poor communities in developing countries that rely on reefs for income from fishing and tourism and for their main sources of protein. Economies will be hit hard around the globe as coral reefs disappear, especially those of small island countries that rely heavily on reefs for tourism and storm protection. In places like Belize, the Philippines, Fiji, and American Samoa, coral reef degradation will likely impact identity, culture, and tradition which have been heavily influenced by the reefs from cuisine to traditional occupations.

It remains to be seen what will happen to coral reefs, whether they will be able to adapt and survive, with or without human aid, to warming seas, or if they will slowly bleach and die out. But one thing is for sure, coral extinction would be disastrous, not just more millions of ocean fish species and other sea dwellers but also for human communities around the globe.


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