What ecological impact have purple sea urchins had on kelp forests?

• A. They have increased kelp forest density.
• B. They have caused many kelp forests to be replaced by urchin barrens.
• C. They have not affected kelp forests.
• D. They have enhanced kelp productivity.

Answer: (A)

Kelp forests rely on a balance between kelp growth and herbivory. Purple sea urchins graze on kelp, especially the holdfasts and lower fronds; when their numbers are high, they eat kelp faster than it can regrow. This intense grazing can collapse large areas of kelp, turning dense forests into barren patches dominated by urchins—urchin barrens. The shift reduces habitat structure and biodiversity, showing a clear ecological impact of urchin overgrazing. So the most accurate description is that purple sea urchins have caused many kelp forests to be replaced by urchin barrens.