Why the story of urchin barrens and kelp forests is more complex than we thought

It’s not hard to find a long-spined sea urchin (Centrostephanus rodgersii) on rocky walls and in underwater forests comprised of golden kelp (Ecklonia radiata) and crayweed (Phyllospora comosa) in southeast Australia.

They also live in “urchin barrens”, areas of bare rock and encrusting algae. In media reports, these areas are often described as “lifeless moonscapes”. In this telling, large numbers of urchins eat their way through kelp forests and create new barrens.

This isn’t quite fair on the barrens – or the urchins. Much concern has focused on how fast barrens have expanded their range into Tasmanian waters in recent decades as oceans have warmed up. But in New South Wales, barrens do not appear to be increasing. Barrens are vital habitat. By some measures, they can be richer in species and host different invertebrates than kelp forests.

Urchins, too, may be being judged unfairly. Across their native NSW range, long-spined urchin populations have been stable over long timeframes.

Long-spined urchins have long been suggested as the key cause of kelp forest declines, because disappearing kelp coincides with more urchins. In response, authorities have launched urchin control programs, where divers pull urchins from the sea.

But recent research suggests it’s not that simple. Kelp species like cold water, and the oceans are warming fast. Urchins may well be a symptom, not the key cause.

Of urchins and underwater barrens

Like many urchin species, long-spined urchins are omnivores. They eat kelp as well as small worms and other invertebrates, tiny organic particles and drifting algae.

The question is, how heavily do they depend on kelp? Long-spined urchins graze differently to most other urchins. They don’t actively move towards kelp and cannot climb into the kelp canopy to feed. Unlike other native urchins, they don’t form dense “feeding fronts” typically associated with creating barrens.

Across their native range in southeastern Australia, long-spined urchins prefer shallower waters (down to ~15–20 metres) and their mobility to access kelp is limited compared to other species. That means they eat kelp within well-defined boundaries.

In recent research, we collected 100 long-spined urchins from barrens and kelp forests off the coast of New South Wales. We examined their gut contents and found they had been eating invertebrates, kelp and particulate matter.

We then used a different approach, stable isotope analysis, to estimate the long-term dietary proportion for each food group. We found urchins were using kelp as food, but that invertebrates and particulate organic matter were often equally or more important.

In southern NSW, the recent decline of kelp forests has been attributed to grazing urchins. But our results show urchins in these waters relied far more strongly on particulate matter. Even within dense kelp forests, urchins can rely more on food other than kelp.

Are urchin predators in short supply?

So why are long-spined urchins so common?

One possibility is a lack of large predators. It is often suggested overfishing of eastern rock lobsters (Sagmariasus verreauxi), eastern blue groper (Achoerodus viridis) and pink snapper (Chrysophrys auratus) has removed natural controls on urchin numbers.

But do these predators often eat long-spined urchins? To test this theory, we analysed the gut contents of gropers and snapper and eastern rock lobsters and ran wild and caged feeding trials with lobsters. We found urchins were being eaten by these species, but only rarely. Predator size didn’t appear to matter.

When we used isotope analysis on these predators, we found little direct evidence the two fish species relied on long-spined urchins. Snapper appeared more likely to be long-spined urchin predators than blue groper, while rock lobsters showed far less potential reliance on them.

Are our interventions helping?

It’s natural to be concerned about disappearing kelp forests. Culling urchins and protecting their predators have been proposed as ways to help kelp.

The problem is, culling costs money, takes continual effort, is very hard to scale and has limited long-term effectiveness. Recent research found when long-spined urchins were removed across large areas, those remaining reproduced faster.

What about protecting urchin predators? Even when large predators are present, long-spined urchins appear to be at the bottom of the menu.

In long-protected areas such as Jervis Bay Marine Park, large urchin predators are abundant, but kelp-free urchin barrens persist. This suggests the role of predators has been overstated.

Urchins may not be the major cause of kelp forest loss. But once kelp is gone, their grazing of young kelp does stop it regrowing, maintaining the barrens.

So what else might be killing kelp forests?

Kelp likes cold, salty water. But Australia’s coastal waters are warming fast, while large volumes of fresh water from intense floods kill swathes of kelp. Climate change is worsening each of these threats.

The same climate-boosted threats also affect urchins. Pulses of fresh water and storms rapidly reduce urchin numbers across a large area.

Long-spined urchins do eat kelp and predators do eat urchins. But our body of research shows these relationships are more complex than often made out.

We can’t just say urchins are the problem and that the solution is to keep pulling them from the sea.

Protecting gropers, snapper and lobsters may have little effect on urchin control. But it’s worth doing because they’re ecologically valuable in their own right.

Spotting more long-spined urchins after kelp disappears may be a symptom of broader collapse, rather than the main cause. Long-term solutions will require confronting the elephant in the room – the rapid warming of our oceans due to climate change.