01 — The basics
Off parts of Australia's southern and eastern coast, divers describe the same eerie sight: smooth rock stretching into the blue, dotted with spiny sea urchins, and almost nothing else. These are urchin barrens — the spot where a kelp forest used to be.
What is an urchin barren?
An urchin barren is an area of rocky reef that sea urchins have grazed so heavily that kelp and other seaweeds can no longer survive there. With the canopy gone, the reef loses the species that depended on it. What remains is bare rock — often tinged pink by the encrusting algae urchins leave behind — and a population of urchins dense enough to eat any new kelp the moment it tries to grow.
Ecologists describe this as a stable state: once a barren forms, it tends to stay a barren. The urchins are no longer just a symptom of decline — they actively hold the reef in its degraded state, which is what makes barrens so hard to reverse without intervention.
What causes urchin barrens in Australia?
The driver is a warming ocean. A strengthening East Australian Current is carrying warm water further south than it used to, and with it the long-spined sea urchin (Centrostephanus rodgersii). Once largely confined to New South Wales, this urchin has extended its range down into Tasmania — arriving faster than the local ecosystem, and its natural predators like rock lobster, can keep it in check.
The result is a population boom. Where urchins were once one species among many, they now occur in numbers high enough to clear-cut a reef. Overfishing of urchin predators in some areas has compounded the problem, removing one of the few natural brakes on their spread.
- Bare rock
- Kelp and seaweed grazed away, leaving little for other species to live on.
- Self-locking
- High urchin density eats new kelp before it can establish, holding the reef barren.
- Reversible
- Reduce urchin numbers and re-establish kelp, and a forest can return.
Why urchin barrens matter
It's tempting to see this as a problem only for marine biologists. It isn't. Kelp forests — the forests the barrens replace — do an enormous amount of quiet work:
- Biodiversity: kelp forests shelter hundreds of species, from abalone and rock lobster to fish and weedy seadragons. A barren supports a tiny fraction of that life.
- Fisheries: Australia's temperate reefs underpin valuable abalone and rock-lobster fisheries. When the forest goes, so does much of the catch.
- Carbon and coastlines: fast-growing kelp draws down carbon as it grows and helps buffer coastlines from wave energy. Bare rock does neither.
Because barrens spread quietly and underwater, they attract a fraction of the attention given to events like coral bleaching — even though the scale of loss along Australia's Great Southern Reef is significant.
Can urchin barrens be reversed?
Yes — and this is the hopeful part. Because the barren is held in place by urchin density, reducing that density changes the equation. Clear enough urchins from a stretch of reef, give native kelp a chance to re-establish, and a forest can return. The challenge is doing it at meaningful scale, and in a way that doesn't simply run out of funding.
That's the approach Ocean Greens takes: combining urchin removal with actively replanting native kelp on engineered seaweed farms, then harvesting a sustainable share of that seaweed to help fund the next stretch of reef. You can read the full method on our kelp restoration page.
Common questions
What is an urchin barren?
An area of rocky reef overgrazed by sea urchins until almost no kelp or seaweed remains — bare rock where a forest once grew.
Which sea urchin causes barrens in Australia?
The long-spined sea urchin, Centrostephanus rodgersii, is the main driver in south-eastern Australia. Warming water has let it extend its range south into Tasmania.
Are urchins themselves the problem?
Not exactly — urchins are a natural part of healthy reefs. The problem is unnaturally high densities, driven by warming water and the loss of predators, which tip the reef into a barren state.
Can a barren become a forest again?
Yes. Lowering urchin density and re-establishing kelp can restore a forest. The hard part is doing it at scale and sustainably funding the work over time.