Beautiful Benefits of Biodiversity Part II: Coral Reefs and Marine Systems

Biodiversity is the fabric and engine of life. The biodiversity of marine and coral reef ecosystems is crucial for fully functioning and healthy ecosystems, resilience against stochastic events, and a healthy and stable climate. It is fundamental to the existence, security and well-being of all life on Earth, including for each one of us and our economic, social, governing, food, industrial, and other systems in society (photo above © Greg Lecoeur)

(Excerpts from Augeri, 2025)

© Dave Augeri, Ph.D.

 “Look closely at nature. Every species is a masterpiece, exquisitely adapted to the particular environment in which it has survived. Who are we to destroy or even diminish biodiversity?” (Wilson, 2017)

Biodiversity is both the fabric and engine of life. The millions of species and their roles are what make life possible on Earth. They provide the essential services, processes, products and other resources that we entirely depend on, not only for our livelihoods and economies, but for our very existence. They are vital for the proper functioning of both natural and human systems, including crucial life sustaining services that support all people, economies, societies, cultures, governments, and nations around the world. Their products and services range from creating the oxygen we breathe and clean water we drink to a broad variety of foods, fibers, protein, medicines, raw materials, genetic and natural resources, pollination, nutrient cycling, CO2 sequestration, and the production of biomass, soils, and other essential needs, to mention a mere fraction of what is incalculable (Millennium Ecosystem Assessment, 2005). Diversity in this biological array is what makes all of this is possible. Consequently, the relative condition of biodiversity is a barometer of the health of the biosphere and, therefore, our own wellbeing and that of our society.  

Parts I and II of this blog series on biodiversity briefly summarize the benefits from what most scientists consider two of the most biodiverse and endangered ecosystems on Earth: native tropical forests and coral reefs. Part III is focused on the critical life support services we receive from ecosystems. All three are crucial for the survival of each one of us, our society, the biosphere, and all life on Earth.

Coral Reefs

Coral reefs are one of Earth’s oldest ecosystems, emerging approximately 500 ± million years ago. However, many are more recent due to changing environmental conditions throughout the Earth’s history. Reefs are occasionally referred to as the “rainforests” of the sea, harboring some of Earth's most diverse biogeographic ecosystems. They exist in 100 countries and territories and are considered by some scientists to be en par with rainforests and soils as one of the top three most biodiverse ecosystems on Earth, possessing highly significant ecological, intrinsic, and practical values for the world.

Corals are the foundation of, and are at the interface between, exceptionally complex marine and terrestrial ecological communities. While they occupy less than 0.1% of the world's ocean area, they support the life-cycles of at least 25% of all marine life on Earth, including 4,000 - 6,000 fish species that depend on coral reefs for shelter, food, reproduction, and rearing their young, among other needs (EPA, 2022; NOAA, 2022; Pozas-Schacre et al. 2021). In addition to the exceptionally high diversity of coral species themselves (at least 6,000 coral species worldwide), some individual coral reefs also support more than 7,000 other known species of fishes, invertebrates, plants, marine mammals, birds, reptiles, and many other organisms, all of which contribute to essential ecosystem services (NOAA, 2022). On a global scale, it is estimated that approximately 830,000 (95% confidence limits of 550,000–1,330,000 species) to perhaps 9 million multi-cellular species exist on coral reefs (Fisher et al. 2015; GCRMN, 2021; Plaisance et al., 2011; Reaka-Kudla, 1997; Small et al, 1998). However, given the exceptional difficulty of underwater studies, which are still in their infancy, and particularly relative to smaller invertebrates and micro-organisms that are difficult to observe, the true figure is likely higher. Coral reefs also support high diversities of terrestrial and semi-aquatic species and protect other systems as well, like intertidal zones, coastal mangrove forests, and seagrass beds, which are also nurseries and feeding grounds for thousands of marine and semi-aquatic species.

It is well known that ecological interactions are a crucial foundation of an ecosystem’s structure and functioning. Given the high species and taxonomic diversity and richness in coral reefs across a vast array of trophic levels, the sheer volume of relationships is extraordinarily high, resulting in an innumerable diversity of trophic pathways and interactions that are critical for reef ecosystem functions (Pozas-Schacre et al., 2021). However, those relationships are both maintained and dominated by species with very narrow and specialized niches, which makes these trophic pathways vulnerable to biodiversity loss and ultimately threatens the healthy functioning and services coral reef ecosystems provide for the world (Pozas-Schacre et al, 2021).

This extraordinarily high marine biodiversity provides significant benefits for society. In brief, coral reefs are a direct source of some of our most commonplace medicines, including treatments for cancer, heart disease, Alzheimer’s disease, bacterial infections, arthritis, and asthma, among others (NOAA, 2023a). In addition, billions of people around the world rely on coral reefs for their direct and consequent production of fish; more than one billion people worldwide depend directly on coral reefs for personal food security, economic well-being, and cultural identity; and millions more rely on reefs for recreation and physical, mental, and emotional health and well-being (Beck et al., 2018; Costanza et al., 2014; IOC, 2017; IPCC et al., 2019; NOAA, 2022). Consequently, reefs provide the U.S. economy as much as $3.4 billion per year via the food and service sectors alone (NOAA, 2022), and the annual global value of coral reefs is estimated to be $9.9 trillion USD (Costanza et al., 2014).

The oceans in general provide us with countless other essential benefits and services. These include climate regulation, food for billions of people, a major source of fresh water (i.e., evaporated ocean water eventually precipitates as freshwater on land), renewable energy, and protection from storm surges, among many other services. In addition to the more than 1 billion people who benefit directly from coral reefs, at least 350 million people have jobs directly related to the oceans. Fishing and fish farming alone employ 56 million people around the world and support the livelihoods and families of 660-880 million people (FAO & UNEP, 2020).

Coral reefs also avert and mitigate substantial flood and storm damages and thus provide significant protection benefits for most sectors of society (Beck et al., 2018; Costanza et al., 2014; IOC, 2017; IPCC et al., 2019; NOAA, 2022). Reefs reduce wave energy by as much as 97% (NOAA, 2022) and, as a result, reefs reduce annual damages from storms by more than $4 billion USD (Beck et al., 2018). In fact, without reefs, annual damages would be more than 118% higher, flooding of land would increase by 69% and affect 81% more people annually, and costs from frequent storms would triple (Beck et al., 2018). During serious storm events that occur about once every 25 years, coral reefs reduce flooding for more than 8,700 km2 of land and 1.7 million people worldwide, and provide $36 billion USD in avoided damages to infrastructure (Beck et al. 2018). Without reefs for shoreline flood and storm protection, damages would increase by 141% for these 25-year events (Beck et al., 2018). For 100-year storm events, just the top layer of reefs alone (1± m) reduces flooding so much that approximately $130 billion USD in damages are avoided (Beck et al. 2018). Without reefs, damages would increase by 91% to $272 billion USD for 100-year events (Beck et al. 2018). Given the rising costs since these studies were conducted in 2018, data show such damages are significantly higher today, in some instances by orders of magnitude.

Fragility of Coral and Marine Ecosystems

Coral Polyps

Most corals, particularly shallow-water corals, are only able to live within a very narrow range of specific environmental conditions, such as water temperature, salinity, pH, and water chemistry, etc. Even minor changes in these can lead to their degradation or death. In reality, coral reefs are perhaps the most fragile and vulnerable ecosystem on Earth and are particularly susceptible to climate change and associated greenhouse gas emissions that cause global warming (GCRMN, 2021).

As described in various texts (e.g., Fagerstrom, 1987; Sheppard et al. 2018), corals and reefs are formed of colonies of individual coral polyps, primarily held together by calcium carbonate (i.e., limestone). The coral polyps are tiny soft-bodied animals related to anemones and jellyfish. Polyps absorb calcium carbonate (CaCO3) dissolved in the seawater and eventually secrete it as a hard protective limestone “skeleton”, initially beginning as a “calicle” at the base of polyps. Reefs begin when a polyp attaches itself to a rock or other hard structure on the sea floor via its calicle, which provides the initial structure of the reefs, and then it buds or divides (sexually or asexually) into thousands of clones or larvae.

Coral polyp anatomy.

Coral polyps live in a symbiotic relationship with zooxanthellae (dynoflagellates) algae, which are photosynthetic organisms that live inside the polyps’ tissues. In this arrangement, the coral provides the zooxanthellae with shelter while the zooxanthellae provide compounds that give energy to the coral through photosynthesis. This relationship has allowed corals to survive for millions years, including in low nutrient environments. However, shallow-water corals can only survive within extremely narrow environmental conditions in the quality, temperature, level, salinity, and pH of the ocean water.

Consequently, corals can die via many direct and indirect means. The most commonly known process is bleaching, which occurs when changes in temperature, light, pH, or other inhospitable environmental conditions, including the influx of pollutants and toxins, cause stress to the polyps. Bleaching occurs when coral polyps expel the zooxanthellae, which causes the coral to turn white and eventually die in many cases. It has also been documented recently that species of soft corals have been “disintegrating” and their tissues have been falling off due to hotter sea temperatures (Readfearn, 2023).

Healthy polyp with abundant zooxanthellae.

Dying polyp with few zooxanthellae.

Because the zooxanthellae are photosynthetic, as water levels rise by even small amounts less light is able to reach the corals and their symbiotic zooxanthellae. Consequently, the capacity of the zooxanthellae to photosynthesize declines, which impacts the survival of the corals. In addition, as water temperature rises, the zooxanthellae can die or produce reactive species that are toxic to the polyps. As a result, the polyps expel the zooxanthellae from their tissues. In some cases, the dead or damaged zooxanthellae remain and can gradually accumulate in the polyps’ tissues, which can result in the coral’s death. A spike of just 1o – 2o C (1.80 – 3.60 F) sustained over a few weeks can cause this reaction and kill the corals (GCRMN, 2021; IOC, 2017; IPCC et al., 2019; WMO, 2022; WRI, 2022). While some corals can sometimes temporarily live without zooxanthellae, the zooxanthellae provide as much as 90% of the corals’ energy through the photosynthetic process. Thus, their environment needs to become more hospitable relatively quickly, remain stable over an extended period, and the corals need to replace the zooxanthellae. If these conditions don’t occur, they will be highly susceptible to their environment, including changing water temperatures, salinity, and chemistry as well as increased water levels, acidification, diseases, herbicides, sedimentation, pollution and toxics. Consequently, the corals will die over time.

Healthy reef in Komodo, Indonesia in 2015 (left) and same reef in 2019 (right) dead due to coral bleaching.

By far, climate change is the greatest global threat to coral reef ecosystems. This happens through the interactive threats of increased temperature, acidification, and changing water chemistry (GCRMN, 2021; IOC, 2017; IPCC et al., 2019; WMO, 2022; WRI, 2022). Rising sea temperatures not only cause heat stress on corals and other marine life as described above, but it also reduces dissolved oxygen in the water and decreases the oceans’ capacity to remove carbon from the atmosphere. As a result, more CO2 accumulates in the oceans:

In its simplest form, CO2 and seawater produce carbonic acid that releases hydrogen and bicarbonate ions. When excess CO2 enters seawater, chemical reactions occur through a series of processes resulting in an increased concentration of hydrogen ions and decreased relative abundance of carbonate ions. Consequently, the more hydrogen ions there are, the more acidic the seawater becomes. This interactive threat from climate change of rising ocean temperature, changes in water chemistry, and acidification has been devastating to shallow-water coral ecosystems and the majority of marine life.

So, Why Does Any of This Matter?

Great Sea Reef, Fiji.

The answer is simple…The marine realm covers about 72% of Earth and harbors at least 80% of the planet’s biodiversity – both of which provide critical life support services for all of us and our economies to exist, survive, and flourish. This includes tiny phytoplankton that are at the base of and support the marine food web, absorb 25% of atmospheric CO2 on the entire planet, and perform photosynthesis to produce as much as 70-80% of the world's oxygen (NOAA, 2023b) (versus forests, which produce up to about 28% of the world’s oxygen). However, these microscopic plants and much of marine life that are indispensable for our lives are at risk from numerous human-caused disturbances, including ocean mining, pollution, and the deleterious climate change impacts on ocean temperature, pH, chemical composition, and salinity levels, etc.

The changes and disturbances mentioned above among many others cause impacts beyond their immediate areas and ripple through marine ecosystems with deleterious consequences for the broader ocean environment, the terrestrial realm, and society. In essence:

The oceans are Earth’s life-support system. With every drop of water we drink and every breath we take we are connected to the oceans. The entire planet, including humans and all life, require the Earth’s marine ecosystems to be healthy for our existence. When the oceans die, so will we.

It is unequivocal that the world’s reefs and oceans are indispensable, but are under tremendous pressures and reaching some tipping points. Nevertheless, there’s hope. Although the available window is narrowing, studies (e.g., Duarte et al., 2020) indicate that rewilding and recovery of the structure, function, and abundance of marine life is possible in about 3+- decades, but only IF major pressures and impacts from climate change, pollution, over-fishing, and other disturbances are mitigated. Indeed, “rebuilding marine life represents a doable Grand Challenge for humanity, an ethical obligation and a smart economic objective to achieve a sustainable future” (Duarte et al., page 39, 2020).

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