[Coral-List] sea urchin removal to prevent bioerosion
psammarco at lumcon.edu
Thu Feb 11 12:01:56 EST 2010
Dear Clement and Colleagues,
Thank you for the alert regarding this decision. Very interesting. I think
this is a classic case of decisions being made and actions being taken based
upon a limited amount of information, or at least an incomplete
understanding of it. I tend not to "blog" very much on this or other sites,
or hardly at all actually. I try to restrict myself to putting my
information into scientific papers and leaving it out there for people to
read, if they have the need or interest. But allow me to comment on this
In a nutshell, there is a delicate balance between grazing and bioerosion.
And bioerosion is not a simple process in itself. There is external
bioerosion, to which the hard-bottom, regular echinoids and grazing fish
contribute. Then there is internal bioerosion, which is effected by
organisms like boring sponges, boring sipunculids, boring polychaetes, and
boring barnacles. These two processes are controlled to an extent by
grazing - a third, (in this case) top-down process.
Basically, if fish densities or echinoid densities are high, external
bioerosion rates are high. The sediment which is produced is more coarse,
and the substrate break-down rates are relatively high. But there is a
higher-order interaction as well. The settling larvae of internal
bioeroders do not survive very well under these circumstances because of
high rates of predation and biological disturbance, and internal bioerosion
is relatively low. The bioerosion process is fast, and one can observe
dramatic changes on the hard-bottom substratum certainly within 18 mos.
If the fish grazing or echinoid grazing rates (densities) are low, the
external bioerosion rates are basically non-existent. However,
sedimentation rates on the substratum are high. More importantly, however,
the success of settlement by internal bioeroders increases due to an
increase in refugia, decreased predation, and decreased biological
disturbance. Thus, internal bioerosion rates increase as a result of
increasing internal benthic bioeroder populations. The sediment being
produced by this bioeroder community in this case is fine carbonate silt -
not coarse sand. This process is much slower and requires years to observe,
depending upon the hardness of the substratum.
In a way, because of this balance between internal and external bioerosion
and its at least partial control by grazing, it's a "darned if you do,
darned if you don't" situation - but on different time-scales.
So, was an approach like this the "right" thing in this case? Based on the
information available at the time, probably. But, if one is interested in
long-term effects, one would probably should re-examine the substrate in
several years. Like termites, internal bioeroders take their sweet time to
do their job.
Paul W. Sammarco, Ph.D.
Association of Marine Laboratories of the Caribbean (AMLC)
Louisiana Universities Marine Consortium (LUMCON)
8124 Hwy. 56
Chauvin, LA 70344
Email: psammarco at lumcon.edu
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