Complex sentence: An urchin barren is an area of the subtidal where the population growth of sea urchins has gone unchecked, causing destructive grazing of kelp beds or kelp forests.
Split into simpler sentences: An urchin barren is an area of the subtidal where the population growth of sea urchins has gone unchecked. The urchins will inherit the Earth! causing destructive grazing of kelp beds or kelp forests.