Coral reefs are among the most biologically diverse and productive ecosystems on Earth. Found in tropical and subtropical oceans, they are often called the “rainforests of the sea.” For IB ESS students, understanding how coral reefs form and function is essential for exploring ecosystem dynamics, symbiosis, and human impact — all key topics within Environmental Systems & Societies.
Quick Start Checklist for IB ESS Students
- Learn how coral polyps build reefs over time
- Understand the symbiotic relationship between coral and algae
- Recognize biotic and abiotic factors influencing reef development
- Study human threats to coral reef systems
- Use RevisionDojo ESS resources for data-based and essay questions
How Coral Reefs Form
Coral reefs are built by tiny animals called coral polyps, which belong to the phylum Cnidaria. These polyps live in colonies and secrete calcium carbonate (CaCO₃) to form hard, protective exoskeletons. Over time, as polyps die and new ones grow on top, their skeletons accumulate, gradually forming large reef structures.
Step-by-step formation process:
- Colonization: Free-swimming coral larvae attach to hard surfaces such as submerged rocks or dead corals.
- Growth and calcification: Polyps secrete calcium carbonate, building a limestone base.
- Reef expansion: Continuous growth and reproduction create large colonies and complex structures.
- Ecosystem development: Other marine organisms — fish, mollusks, algae — inhabit the reef, forming a balanced ecosystem.
The Role of Zooxanthellae
A key feature of coral reef formation is the mutualistic symbiosis between coral polyps and microscopic algae known as zooxanthellae.
- The algae live inside coral tissue and perform photosynthesis, providing oxygen and glucose to the coral.
- In return, the coral provides carbon dioxide and a protected environment for the algae.
This relationship drives reef productivity and enables the deposition of calcium carbonate, especially in warm, shallow, and sunlit waters.
Types of Coral Reefs
- Fringing Reefs: Directly attached to shorelines (e.g., the Red Sea coast).
- Barrier Reefs: Separated from land by a lagoon (e.g., the Great Barrier Reef).
- Atolls: Circular reefs surrounding a lagoon, often formed from submerged volcanoes.
Ecological Importance of Coral Reefs
- Biodiversity: Coral reefs support around 25% of all marine species.
- Coastal Protection: Reefs act as natural barriers, reducing erosion and storm damage.
- Carbon Cycling: Reefs play a role in carbon sequestration and nutrient cycling.
- Economic Value: Reefs provide food, tourism income, and medicinal resources.
Human Impacts on Coral Reefs
Coral reefs are highly sensitive to environmental changes. Climate change, ocean acidification, and pollution have caused widespread coral bleaching and degradation. Rising sea temperatures disrupt the coral-algae relationship, leading to loss of color and vitality.
IB ESS Focus
In IB Environmental Systems & Societies, students explore how coral reefs illustrate key ideas about interdependence, sustainability, and systems balance. Exam questions often require explaining reef formation, energy flow, and management strategies for coral conservation.
Students using RevisionDojo’s IB ESS course gain structured notes, diagrams, and case studies on coral reef ecosystems — ideal for Paper 2 data analysis and essay-style answers.
FAQs
What are coral reefs made of?
They are built from calcium carbonate secreted by coral polyps and hardened over time into limestone structures.
Why are coral reefs important?
They protect coastlines, support biodiversity, and contribute to carbon cycling and human economies.
What causes coral bleaching?
When water temperatures rise, corals expel their symbiotic algae, losing color and vital nutrients — a major threat to reef survival.
