Mark Langdon

Herbivore grazing is a well-documented cause of habitat decline in terrestrial systems, but marine examples from seagrass meadows are rare. Here we present evidence that isolated urchin grazing events have caused further localized losses to seagrass meadows already degraded by eutrophication or other anthropogenic disturbances. By 1992 a substantial scar in Posidonia meadows at Luscombe Bay in Cockburn Sound, Western Australia, had been caused by grazing urchins. When seagrass transplants were placed at the site more than a decade later most were grazed and did not survive. GIS analyses on imagery from 1985 to 2004 indicated that rapid seagrass meadow decline coincided with the presence of an unusually large aggregation of the grazing urchin Heliocidaris erythrogramma. Evidence of some seagrass recovery after 1993 was also apparent after the manual removal of the urchins in late 1992. Restoration efforts in seagrass meadows should consider the potential for grazing damage, as is commonplace in terrestrial systems.