Hands-On Activities: The Reef City Experiment
Big Idea:
Coral reefs are not just pretty, they are among the most productive ecosystems on Earth because their physical structure creates thousands of microhabitats. This activity demonstrates why structural complexity drives biodiversity.

Materials:

• A flat tray or baking sheet
• 10 to 15 small objects to represent sea creatures (marbles, beads, small toys, or buttons)
• A reef structure: a large kitchen sponge, a cluster of Lego bricks, or crumpled aluminum foil
• Optional: a second flat tray for side-by-side comparison

What to Do:

Step 1: The Empty Seafloor Test
Spread the small objects across the flat tray. Gently tilt the tray or blow across it. How many objects stay in place? Where do they go? "This is a flat, sandy seafloor: plenty of space, but almost no places to hide or shelter."

Step 2: Build the Reef
Place the sponge, Lego cluster, or crumpled foil in the center of the tray. Now tuck the same 10 to 15 objects into the holes, crevices, and underside of the reef structure.

Step 3: The Reef Test
Tilt the tray or blow across it again at the same force. How many objects stay in place this time? Record your counts on the lab sheet.

Step 4: Count and Compare
How many survived on the flat seafloor versus the reef? What percentage difference does that represent?

Step 5: Discuss

• What does the physical structure of the reef provide that a flat seafloor cannot? (Hiding places, anchoring points, shelter from currents and predators.)
• Different species live at different "levels" of a real reef. Some near the top, some in crevices, some underneath. How does this reduce competition between species?
• What happens to all the species that depend on a reef if the reef is destroyed?

What's Really Happening (Caregiver Explanation):
In the Ordovician and Silurian seas, corals and sponges began building the first large reef structures, and the effect on marine biodiversity was dramatic. A flat sandy seafloor offers very few places to hide, shelter, or anchor, limiting the number of species that can coexist. A reef creates hundreds of distinct microhabitats in the same area of ocean floor, allowing many more species to live alongside each other without competing directly for the same resources. This concept, that structural complexity drives species diversity, applies to every ecosystem, from coral reefs to rainforest canopies to prairie grass.

Digging Deeper:
Research modern coral reef bleaching and find out what causes it. How does the loss of coral reefs today compare to what happened at the end of the Ordovician mass extinction? Look up the Great Barrier Reef and find the most recent data on its condition. What would be lost if coral reefs disappeared from Earth entirely?