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.
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