Introduction

• Introduction
• Coral reef ecosystem services
• Corals: The foundation of coral reefs
• Trophic levels
• Threats to coral ecosystems
• How you can help coral reefs
• Dig deeper
• Ready to begin?

Introduction

When people think of corals, they might picture colorful coral reefs bursting with coral and fish. While this is true of a healthy reef, there is more than meets the eye. Dive below the ocean surface to explore the significance of coral reefs. We’ll learn what coral really is and how we can do our part to keep reefs healthy and beautiful.

Coral reefs are found in warm, shallow waters in tropical and subtropical oceans around the world. Though they cover less than 1% of the ocean floor, coral reefs support roughly 25% of all marine species. Such diversity has earned them the nickname "rainforests of the sea."

Coral reef ecosystem services

Coral reefs don’t just support marine life. They also provide vital services to people…these benefits are called ecosystem services. Coral reefs provide us with some of the most important ecosystem services on Earth.

Storm protection
Healthy coral reefs are able to absorb wave energy from storms or hurricanes. This function can prevent coastline damage and flooding.

Eco-tourism
Colorful, diverse reefs attract visitors and can bring in money to help support local communities. These visitors might take part in activities like snorkeling, scuba diving, and kayaking.

Fisheries
Coral reefs provide habitat for many fish and shellfish species that are essential to local food webs. The reef is both a nursery and a hunting ground for commercially important species.

Corals: The foundation of coral reefs

Corals may look like colorful rocks or plants, but they are actually colonies of tiny animals called polyps. Coral polyps build hard outer skeletons of calcium carbonate, the same material that shells are made of. Over time, these skeletons form the massive reef structures we see today.

Most reef-building corals have a special partnership with tiny algae that live inside their tissues. These algae, zooxanthellae, use sunlight to make food through photosynthesis and they share the energy with their coral host. This partnership gives corals their vibrant colors and provides up to 90% of the energy they need to survive and grow.

Coral polyps

Another important reef algae is crustose coralline algae (CCA). This hard algae helps cement the reef together and provides a surface for new corals to settle on.

Corals come in many shapes. Some are complex and grow in branches that create nooks and crannies for other organisms. Others are structurally simpler but more resistant to storm damage. The health of the reef depends on a balance between coral growth and algae that compete with corals for space. When conditions are healthy, grazers keep algae in check so corals can thrive. When this balance shifts, algae can take over and smother corals.

Crustose coralline algae

Trophic levels

The sequence of organisms that eat one another is known as a food chain. A trophic level refers to an organism's position in that chain. At the base are primary producers like the algae inside coral tissue, turf algae, and phytoplankton. These organisms use the sun's energy to grow and make their own food.

Grazers make up the next level. Parrotfish, sea urchins, and surgeonfish feed on algae. They play a crucial role in keeping algae from overgrowing corals. Without grazers, algae would quickly smother the reef.

At the higher levels are predators and scavengers. Butterfly fish feed on coral polyps, while moray eels and snappers hunt fish and crustaceans. Larger predators like groupers and reef sharks help control populations of smaller fish. Scavengers eat dead animals or parasites. Predators and scavengers help keep the whole system in balance.

In a real reef ecosystem, many species don't fit neatly into a single trophic level. But understanding trophic levels helps us see why losing even one group, like grazers, can send ripple effects through the entire reef.

Threats to coral reefs

Coral reefs are facing an unprecedented combination of threats, many of them driven by human activity. Some threats strike locally, while others affect reefs across the globe.

Warming temperatures and coral bleaching
Rising ocean temperatures cause corals to expel their zooxanthellae. Expelling their algae turns them white in a process called coral bleaching. Bleached corals aren't dead yet, but they're starving. If the heat persists, they will die.

CO₂ emissions and ocean acidification
Carbon pollution also changes ocean chemistry. The ocean absorbs roughly 30% of the CO₂ humans produce, making the water more acidic. This ocean acidification makes it harder for corals to build and maintain their skeletons.

Hurricanes and storms
Powerful storms can physically break apart coral colonies and destroy a reef. Many reefs no longer have time to fully recover between disturbances.

Overfishing
When too many fish, especially grazers, are removed from the reef, algae can grow unchecked and smother corals. Overfishing of predators can also trigger effects that affect the whole food web.

Runoff and pollution
Runoff from agriculture, sewage, and development can have major effects on the reef. That wastewater carries sediments, nutrients, and chemicals into reef waters. Many algae grow out of control in such conditions, and algal blooms can block sunlight and suffocate corals.

Disease and predator outbreaks
Coral diseases have become more common as ocean temperatures rise. Outbreaks of crown-of-thorns starfish, a coral predator, can also devastate large areas of reef.

Many of these threats don’t act alone. It's the combination of multiple stressors that makes the current crisis so serious.

How you can help coral reefs

There are many different ways to help that don’t require you to be near an ocean!

Use your vote and your wallet.
Buy from businesses that care about the environment, and use your vote to support people and policies that help protect the Earth.

Be conscious of your energy and water consumption.
Turn off lights, faucets, fans, and heaters when you’re not using them and uses renewable energy sources when you can.

Eat sustainably sourced seafood.
Overfishing and bottom trawlers are just two negative ways humans can harm reef ecosystems.

Be a responsible ocean visitor.
If you get the chance to visit any coral reefs, take only pictures and leave only bubbles! Use reef-safe sunscreen and remember not to touch corals.

Choose greener transportation.
Walking, biking, carpooling, and public transit help cut down on carbon pollution.

Reduce, reuse, recycle.
Reducing waste also keeps plastics and pollutants out of the ocean.

Share what you know.
Learn more about coral reef communities and climate change with your friends and family. Being knowledgeable about the subject is the first step in understanding why corals are important and how you can protect them.

Donate to conservation.
Donate to ocean conservation campaigns to help protect coral reefs and combat climate change.

Support marine protected areas.
Support marine protected areas that limit overfishing and destructive development, giving reefs space to recover.

Even small changes can make a big impact when many people do them. It is up to companies and governments to make many of the changes needed to protect coral reef ecosystems, but we can help make this change happen.

Dig deeper

Want to learn more about some of the subjects covered in the game? Visit Cruising Around Coral Reefs to learn about corals on Ask A Biologist.

Ready to begin?

See for yourself how coral reef species depend on one another. Build food chains in your ecosystem and protect your reef from real-world threats. Try to maintain a thriving, biodiverse reef that can support food webs and provide essential ecosystem services.