A most unusual barrens: Complex interactions between lobsters, sea urchins and algae facilitates spread of an exotic kelp in eastern Tasmania

Sea urchin (Heliocidaris erythrogramma) 'barrens' in central eastern Tasmania differ from typical barrens elsewhere in that they seasonally support dense cover of the introduced kelp Undaria pinnatifida. Sporophytes of this kelp establish in dense stands only when the native algal canopy is reduced, and experimental removal of native algae results in greatly increased cover of U. pinnatifida and sediment. Urchin barrens are integral to widespread establishment of U. pinnatifida because urchin grazing and sediment on barrens prevents native canopy-forming algae from colonizing, but is insufficient to prevent proliferation of U. pinnatifida. Estimates of population growth of the urchin and in situ rates of lobster predation on urchins reveal that fishing of lobsters can account for increases in urchin densities, ostensibly to levels enabling barrens formation. Lobsters are far more important predators of H. erythrogramma than are fishes. Thus, fishing of rock lobsters may ultimately account for the proliferation of U. pinnatifida in this area.