Coral Reef Ecology Lecture 2 - Importance of Coral Reefs

What are coral reefs?
Global distribution of coral reefs
Environmental constraints to reef formation
Importance (or value) of coral reefs, and what is at threat
- Ecological value
- Socioeconomic value
Further Reading (Prescribed readings)
- Connell, J.H. (1978). Diversity in tropical rain forests and coral reefs. Science, 199: 1302-13010.
- Kleypas, J.A. et al. (1999). Environmental limits to coral reef development: Where do we draw the line? Amer. Zool., 39:146-159.
- Eddy, T.D. et al. (2021). Global decline in capacity of coral reefs to provide ecosystem services. One Earth, 4(9):1278-1285.

Coral Reefs are geomorphologic structures formed by the accretion of calcium carbonate, specifically aragonite produced by hermatypic (reef-building) corals.

The exact amount of carbonate accretion necessary to classify an area as a coral reef is still debated.
Example: At Lord Howe Island, which is the world’s southernmost “coral reef,” a thin veneer of carbonate exists, indicating limited accretion.

Aragonite: One of the two naturally occurring forms of calcium carbonate (the other being calcite). Currently, it is the dominant form of calcium carbonate on coral reefs.
Aragonite crystals are less stable than calcite, implying that some early corals may have produced calcite skeletons under extreme environmental conditions (e.g., very high CO2 levels).
Conversion of aragonite to calcite is possible but can take up to 10 million years under ambient temperatures and pressure.

Latest estimates show that coral reef area is approximately 348,361 ext{ km}^2 (Lyons et al. 2024).
Coral reefs are primarily restricted to tropical zones (between 30°N and 30°S) and are mostly found along the margins of continental shelves where waters are shallow.

Scleractinian corals can thrive in a variety of environments; however, coral reef development is contingent upon adequate calcification rates.
Factors affecting calcification rates:
- Temperature
- Light
- Salinity
- Aragonite saturation
Example Study: Pocillopora aliciae in Sydney Harbour (Lough and Barnes, 2000).

Coral communities at high latitudes often do not form reefs, referred to as “marginal reef environments.”
Environmental conditions influencing reef formation:
- Temperature: Required range is 18 - 32 ext{ °C}.
- CaCO3 saturation: Must be greater than 3.4 ext{ Ω}_{ ext{arag}}.
- Light Availability: >50 ext{ μE m}^{-2} ext{ s}^{-1}.
- Salinity: Ranges from 25-42 ppt.

Low temperatures below 18 ext{ °C} inhibit coral growth and calcification.
High temperatures exceeding 35 ext{ °C} (e.g., in Persian Gulf) can lead to coral bleaching. Previous resilience observed, but increasing temperatures are a growing issue.

Reference: Hoegh-Guldberg et al. (2007).
The energy required by calcifying organisms for biogenic calcification increases as aragonite availability decreases.
The threshold for successful reef formation is acknowledged to be 3.4 ext{ Ω}_{ ext{arag}} (Kleypas et al., 2009).

High latitude environments may provide a refuge for hermatypic corals from increased ocean temperatures.
However, coral establishment in these regions is limited by aragonite saturation, as well as light availability, nutrient levels, and suitable habitat.
The global distribution of coral reefs is more likely to contract than expand. (Abrego et al., 2021).

Reef Tourism: Estimated at 35.8 ext{ billion USD annually} (Spalding et al., 2017).

Coral reef fisheries make up less than 2 ext{ ext{%}} of total global fisheries production but are vital for subsistence and artisanal fisheries, critical for food security in many tropical nations.
- Underreporting of fisheries yields is common, leading to potential underestimation of their significance.

Economic evaluations suggest fisheries are undervalued compared to tourism, representing under 10 ext{ ext{%}} of total economic impact. Yet, fisheries provide crucial animal protein in many tropical island states:
- Example statistics: Consumption percentages of animal protein from various fish types across places like Solomon Islands, Vanuatu, Kiribati, etc.

Despite increased fishing efforts, coral reef fisheries yields are declining; there has been a 60 ext{ ext{%}} decrease in catch per unit effort since the 1950s, attributed largely to habitat degradation (Eddy et al., 2021).
Growing demand for coral reef fisheries exceeds supply due to environmental changes, leading to food production shortfalls (Pratchett et al., 2011).

Much remains unknown concerning the full diversity of species residing in coral reefs due to the existence of undescribed species, particularly small-bodied cryptic invertebrates.
- Biodiversity is more than just species richness; it includes the functional roles species play in ecosystem resilience (Fisher et al., 2015).

Explains that a highly dynamic nature of coral reefs contributes to great biodiversity; moderate disturbances prevent dominant species from monopolizing space.
Evidence shows significant coral diversity declines at very high coral cover levels, indicating diversity peaks at intermediate coral cover.

Coral reefs are highly productive ecosystems renowned for their rich biodiversity.
The distribution and health of reef-building corals are largely influenced by environmental conditions, particularly temperature.
Although some corals endure high temperatures, rising ocean temperatures remain a predominant threat to coral reefs globally.
The environmental changes threatening coral reefs pose significant risks to the livelihoods and food security of populations in tropical island nations as well as developing countries.