Diadema antillarum, also
known as the spiny urchin, was a key herbivorous species in the
Florida Keys. It exerted massive downward pressure on algal species
in coral reefs, rocky bottoms, and sandy banks. Numbers were so
great, that at one point it was even suspected that D.
antillarum feeding on hard
bottoms removed more calcium carbonate than any other natural erosion
process (Lessios, 2015,
https://www.stri.si.edu/sites/publications/PDFs/2015_Lessios_annurev-marine-122414-033857.pdf).
The massive abundance of the spiny urchin was extinguished seemingly
overnight in a massive die off event that lasted from 1983 to 1984.
The die off obliterated essentially every spiny urchin population in
the Caribbean, with reef populations being reduced 93-100% only a
week after the first signs of urchin death appearing. The dying is
suspected to have been due to some water born disease, but we do not
have enough information to determine for certain what exactly it was.
What we do know for certain is that the removal of these top grazers
resulted and a phase shift in coral reefs from coral dominated, to
algal dominated systems. The effects are related to the drastic
reduction of coral cover in the keys, as more macrology crowd out
space, outcompeting the slower growing corals for light and space. D.
antillarum on the
other hand is very
slowly recovering, but is still not even close to reigning in the
algae dominance which is becoming more and more dominant all across
the keys. This is mostly thought to be due to poor larval supply and
recruitment, resulting in a very slow recovery of key populations
(Miller et al, 2009,
https://link.springer.com/article/10.1007/s00338-008-0458-4).
A pair of Spiny Urchins were found during our dive on Sweat bank.
This is encouraging because there are almost none found on the gulf
side of the keys, and may mean the species is finally expanding out
in this direction. However it is far too soon, and there is far too
little data, to make any sort of assumption. The fate of D.
antillarum, and in
truth the fate of the entire structure of Florida key reefs, is far
from certain. Continuing changes in climate and ocean acidification
may continue to drastically alter the ecology of these reef systems.
Only through careful observation, adequate protection and management
strategy can we hopefully prevent any further disruption of these
cherished environments.
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