[Coral-List] parrotfishes and coral reef health

[Charles Delbeek]

Les could you comment on the role that hurricanes and severe storms play in
this scenario? I would think that such events would scour reef surfaces as
well as any herbivore but the lack of grazers may not prevent their rapid
recolonization?


*J. Charles Delbeek, M.Sc. *Assistant Curator, Steinhart Aquarium
California Academy of Sciences

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On Tue, Feb 14, 2017 at 10:25 AM, Kaufman, Leslie S <lesk at bu.edu> wrote:

> Hi everybody.
>
> So I’ve been following this herbivore engagement, and I am puzzled about
> one thing.  As experienced coral reef scientists, why are we insisting on
> taking such an over simplistic view of things?  I think Gene was on the
> right track here.  Or maybe he just wasn’t on any track, and this was
> healthy.
>
> Where the field is at, is that we’ve acknowledged that the relationship
> between herbivory and reef benthic communities is not best approximated by
> linear (as in straight line) models, and may not even be best fitted to
> curvilinear relationships (as in still assuming global consistency, or a
> single equation for the entire graph volume).  Like everything else on a
> coral reef (or in your mouth) herbivore-benthic dynamics on oral reefs are
> complex.  Within certain limited domains in space and time, a simple,
> straightforward model is fine.  Speaking broadly, circumstances vary, a
> lot, and so do the rules of community organization.
>
> What we’ve been trying to figure out is how the rules work in any
> particular segment of the parameter space within which coral reefs can
> exist, and where you fall out of that space (or off of the manifold
> encompassing the range of possible conditions in which coral reefs of some
> sort can exist).
>
> So, for example, take Diadema antillarum.  Several decades ago, when
> seemingly healthy coral reefs were still a thing in the Caribbean, Diadema
> played a critical role in maintaining hard coral dominance, as a grazer,
> most particularly on the reef crest and shallow fore and back reefs, to a
> depth of about 8 meters, and then rapidly falling off in abundance and
> influence as you headed out into deeper water.
>
> What many had not noticed is that the herbivorous fish communities over
> much of the tropical west Atlantic were already severely overfished.  Well,
> in Jamaica it was hard not to notice this because the low fish biomass was
> so extremely low, but elsewhere I suppose it could slip by.  Grazing fishes
> also peaked in biomass in shallow water, where the reef crest was attended
> by Sparisoma rubripinne, S. viride, and Scarus vetula particularly, plus in
> some places sizable schools of Scarus coelestinus and to a lesser extent S.
> guacamaia.  The parrotfishes were coeval with large schools of Acanthurus
> coeruleus with varying amounts of the other two acanthurids mixed in.
> Except in places where they’d been fished out.
>
> The urchins must have been extremely important because when they croaked
> in the early 1980’s epizootic, there were big blooms of macro algae
> followed in many places by the establishment of a blanket of recalcitrant,
> long-lived macrophytes, usually including Dictyota spp. and Lobophora
> variegate.  Poorly grazed shallows became infested in many places with
> Sargassum and Turbinaria.  In Florida, where the taking of herbivores was
> banned as early as 1980, grazing pressure was (and now is) fairly high in
> many places.  This has not guaranteed a return to coral dominance, though..
> Sometimes, for some reason, corallines fail to resurge.  Sometimes there
> are corallines but still very little settlement of framework-significant
> corals- possible recruitment limitation now that coral reef condition is so
> diminished as to compromise both self-recruitment locally, and rescue
> through larval connectivity.  We are living in the Age of Lags.  Under such
> circumstances, it is dangerous to jump to conclusions about how things are
> working…how they are working may be working very slowly.
>
> Within this broad setting there are an amazing number of bizarre
> possibilities.  For example, even decades ago, the eastern Antilles were
> dominated by fleshy algal pavements (and still are).  Herbivore biomass
> there varied, but was mostly concentrated in browsers (both urchins and
> fishes), not grazers, which may have operated so as to perpetuate the reign
> of the macrophytes.  At least we suggested as much back in 1977.  This is
> still the basic nature of fish and urchin assemblages associated with
> fleshy algal pavement.  They are similar to seagrass communities in this
> regard.
>
> In other places the densities of Diadema were so incredibly high (and they
> were BIG) that what you’d see is a carbonate moonscape with big gouges
> taken out of the coral rock, which itself was shot through with clionid
> sponges and other bioeroders.  One place this was very obvious was in
> Guadaloupe, but recently I saw something of this sort right outside
> Half-moon Bay on the west end of Jamaica.  It can still happen, even now.
>
> I think before we make broad generalizations about herbivores and coral
> reefs, we need to define the state space and clearly indicate where in it
> we are talking about.
> Much more interesting than assuming the whole world works exactly the same
> way, and ultimately, a much more powerful perspective as well if you want
> to make predictions.
>
> Les
>
> Les Kaufman
> Professor of Biology
> Boston University Marine Program
> Faculty Fellow, Pardee Center for the Study of the Longer-Range Future
> and
> Conservation Fellow
> Betty and Gordon Moore Center for Science
> Conservation International
> lesk at bu.edu<mailto:lesk at bu.edu>
>
>
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