[Coral-List] parrotfishes and coral reef health
Kaufman, Leslie S
lesk at bu.edu
Tue Feb 14 13:25:10 EST 2017
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.
Professor of Biology
Boston University Marine Program
Faculty Fellow, Pardee Center for the Study of the Longer-Range Future
Betty and Gordon Moore Center for Science
lesk at bu.edu<mailto:lesk at bu.edu>
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