Australia is home to some of the world’s most beautiful reefs.

This includes the lush Great Southern Reef, which wraps around Australia’s southern coastline, and the world-renowned Great Barrier Reef.

But the corals of the Great Barrier Reef and the kelp forests of the south are both plagued by prickly problems – voracious starfish and sea urchins.

The coral-eating crown-of-thorns starfish is one of the greatest threats to the Great Barrier Reef. And on the temperate Great Southern Reef, long-spined sea urchins have eaten their way through thousands of hectares of kelp forests. Both species are native but can boom in numbers, and the urchins have spread to new areas due to climate change.

Scientists tend to study these two species as isolated threats. But our new study shows that while they pose high but differing risks, investing in control programs and innovative research could help curb these two prickly problems.

Read more: Coral reefs are secretly connected across vast oceans – and that’s crucial for their survival

Crown-of-thorns starfish and long-spined sea urchins threaten reefs by building up large populations that overgraze coral or kelp.

Crown-of-thorns starfish are native to the Great Barrier Reef. Their population growth follows a boom-and-bust cycle. Since data were first available in the 1950s, the number of crown-of-thorns starfish has surged by more than 1,500 per square kilometre roughly every 15 years. But once they eat all the available coral – the foundation of........

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