Why the story of urchin barrens and kelp forests is more complex than we thought
Sea urchins eat kelp – but new research shows their diets and relationships with kelp forests are more complex than commonly thought.
The story of urchin barrens and kelp forests is more complex than initially believed. Often, media reports describe barren areas as "lifeless moonscapes," implying that large numbers of urchins consume all the kelp, creating these barren habitats. However, this oversimplification overlooks the roles of both urchins and barrens themselves.
In New South Wales, barrens have not been expanding in recent decades, despite ocean warming. In fact, barrens may be richer in species and host different invertebrates than kelp forests. Moreover, long-spined urchin populations in their native range have remained stable for long periods of time.
Long-spined urchins are omnivores that consume kelp, small worms, other invertebrates, organic particles, and drifting algae. However, they rely differently on kelp than other urchin species. They do not actively seek out kelp and cannot climb into the kelp canopy to feed, limiting their mobility. Instead, they graze within well-defined boundaries, primarily feeding on invertebrates and particulate organic matter.
Predation on long-spined urchins has been suggested as a potential cause for their abundance. However, recent research indicates that these predators rarely consume urchins. For example, when analyzing the gut contents of eastern rock lobsters, eastern blue groper, and pink snapper, it was found that urchins were being eaten by these species, but only rarely.
Additionally, isotope analysis revealed minimal direct evidence of snapper relying on long-spined urchins as a primary food source. Similarly, eastern blue groper was rarely found to eat urchins in wild settings.
The effectiveness of interventions, such as culling urchins and protecting their predators, has also come under scrutiny. Removing urchins from large areas can lead to the remaining urchins reproducing faster. Furthermore, even in areas where large predators are present, such as Jervis Bay Marine Park, kelp-free urchin barrens persist. This suggests that the role of predators in controlling urchin populations may be limited.
Written by urgent.news from The Conversation AU's reporting — not their text. Machine-written — may contain errors; check the original before relying on it.
