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Human-Induced Destruction of Coral Reefs
and Other Coastal Ecosystems

Volume 4, Number 38: 19 September 2001

It seems like almost everything bad that occurs nowadays -- such as extreme weather that brings floods, droughts, heat waves and hurricanes, or slight changes in longer-term climatic parameters that are claimed to have deleterious impacts on the health of plants, animals and humans -- is blamed on global warming, which is blamed on the ongoing rise in the air's CO2 content, which is blamed on the burning of fossil fuels.  In last week's Editorial (12 September 2001), we addressed the recent precipitous decline in the health of some of the planet's once-majestic coral reefs within this context, indicating that that unfortunate phenomenon is not a consequence of either the modest natural warming of the past century or the historical increase in the atmosphere's CO2 concentration.  We also promised we would return to the subject and tell you what we believe to be the true cause of this sad state of aquatic affairs.  In fulfilling this promise, however, we must acknowledge that our opinions are not totally original with us.  Rather, they are heavily influenced by a most important paper published in the 27 July 2001 issue of Science (Jackson et al., 2001), which addresses, not just the health of coral reefs, but the well-being of all coastal ecosystems.

The primary hypothesis advanced by the nineteen authors of this landmark study is that "humans have been disturbing marine ecosystems since they first learned how to fish."  Hence, to truly understand the reasons for ecosystem changes that have occurred over the past several decades, they believe -- and rightly so, it seems to us -- that it is necessary to know what occurred to them over the past several centuries.  The authors provide this historical background based on well-dated biological, biogeochemical, physical and historical proxies, including archaeological records from human coastal settlements spanning the last 10,000 years, historical records from documents, journals and charts from the 15th century onward, and ecological data from the scientific literature of the past hundred years.

Two examples of this approach to the problem, which Jackson et al. describe in some detail, are the near disappearance over the past 20 to 30 years of subtidal seagrasses in the offshore half of Moreton Bay near Brisbane, Australia, and the simultaneous die-off of turtlegrass in Florida Bay and the Gulf of Mexico.  Originally, i.e., before humans appeared on the scene, these grasses provided forage and habitat for enormous numbers of large dugongs (sea cows) and sea turtles, as well as many commercially-important fish and invertebrates.

Even before the arrival of Europeans, the aboriginal people of Australia had already harvested dugongs extensively; but as late as 1893, the authors note that herds comprising "tens of thousands of large individuals" were still observed in Moreton Bay.  Shortly thereafter, however, they report that "widespread colonial exploitation of dugongs for their flesh and oil resulted in the crash of the dugong fishery."  So drastic was the reduction in the dugong population that the report of a mere 300 individuals in Moreton Bay in 1978 was considered an important "discovery."

So what's the significance of these observations for seagrasses?  Jackson et al. note that moderate-sized herds of dugongs can remove over 90% of leafy seagrass blades and up to 70% of their roots.  This "systematic plowing " of the bay floor, say the authors, provides space for colonization by competitively inferior species of seagrasses, while producing massive amounts of floating debris and dung that are exported to -- and used by -- adjacent ecosystems.  It also provides for shorter grass blades, as do the grazing activities of sea turtles.  In the near absence of green turtles today, for example, Jackson et al. note that "turtlegrass beds grow longer blades that baffle currents, shade the bottom, start to decompose in situ, and provide suitable substrate for colonization by the slime molds that cause turtlegrass wasting disease."  They also report that "deposition within the beds of vastly more plant detritus also fuels microbial populations, increases the oxygen demand of sediments, and promotes hypoxia."  All of these deleterious consequences of massive human destruction of dugong and sea turtle populations tend to weaken seagrasses and make them more susceptible to other anthropogenic-induced assaults upon their environment, such as increased eutrophication, sedimentation and water turbidity, until there finally comes the proverbial "straw that breaks the camel's back," which can be any number of things of either human or natural origin.

Other examples of the deleterious cumulative consequences of mankind's long history of harvesting sea life that are described by Jackson et al. deal with kelp forests, oyster beds, offshore benthic communities and, of course, coral reefs, the recent troubles of which are routinely blamed on global warming.  However, as Jackson et al. and we have both noted (see our Editorial of 1 January 1999 plus much of the material filed under Coral Reefs in our Subject Index) there are many much more direct anthropogenic assaults upon corals and their environment that have historically combined to place a truly heavy burden upon the backs of these fragile ecosystems.

In the case of Australia's Great Barrier Reef, these offending activities include (1) rising nutrient levels of coastal waters caused by runoff from agricultural activities on land, (2) increased sediment delivery to reefs, (3) tourists and the developers who build resorts and marinas for them, (4) huge catamarans and dive boats that take thousands of visitors to the Barrier Reef each day and dump their sewage in the sea on the way home, (5) sea life depleted to the point of exhaustion by over-fishing, (6) physical damage caused by the barbed hooks and scything nets used in fishing, (7) the nets of prawn trawlers stirring up the growing load of sediments, (8) the 6-10 tons of "bycatch" for each ton of prawn caught that are left to die, (9) outbreaks of the coral-devouring crown-of-thorns starfish caused by removal of its major predators, (10) the live reef-fish trade, (11) fishermen using dynamite and cyanide, and (12) various types of pollution.  Clearly, the time has definitely arrived where the further intensification of any one of these and a number of other contributing factors could well spell the end for many corals, as we indeed are seeing happen in many parts of the world.

Faced with a biological crisis that is so readily traced to the demonstrable local, as opposed to global, activities of man -- although transoceanic transport of fungi, viruses and chemical contaminants on windblown dust may well play a big role too (Dalton, 2001) -- it seems only logical that ameliorative measures should concentrate on these known local affronts to reef health that have known local antidotes, rather than on speculative global phenomena such as planetary warming, which may or may not be occurring, is likely not a consequence of human activity, and about which we can probably do nothing.

Just as we said about the purported deleterious consequences of PM10 in our Editorial of 5 September 2001, if it's truly that big a deal -- and we believe the coral reef problem is -- let's attack it directly, not via some roundabout, trickle-down, backdoor approach that will cost a fortune and likely not be effective in the end.  Efforts to save the world's coral reefs by attempting to change the planet's climate are akin to spitting into the wind; not only will they accomplish nothing positive, they may well come back to roost upon the doers of the deed -- that's us -- in a way we never intended but wished in retrospect we had more clearly foreseen.

This is the real world, folks, and if we're really serious, let's fight its real problems with proven programs that we know are capable of getting the job done.  And in this regard, anything designed to reduce the deleterious consequences of the offending activities listed above will work far better, far sooner and cost far less than what it would take to merely try to change earth's temperature.  Furthermore, since time is of the essence -- if we properly interpret the weeping, wailing and gnashing of teeth of those supposedly so concerned about the problem -- earth's coral reefs will be long gone before the anti-CO2 prescription for its salvation would have even the slightest hope of materializing.

Which makes us wonder.  Are the people who rant and rave about CO2-induced global warming killing earth's coral reefs really that concerned about them?  Or are they only using this provocative image as a means to leverage the introduction of a political agenda that cannot prevail on its own merits?  We know there are many people who are sincerely concerned about earth's many aquatic ecosystems and who sincerely believe they are threatened by rising temperatures.  With them we merely have a difference of opinion.  The others, however ... well, that's a story for another time.

Dr. Craig D. Idso
Dr. Keith E. Idso
Vice President

Dalton, R.  2001.  Riders on the storm provoke studies of Atlantic dust.  Nature 412: 754.

Jackson, J.B.C., Kirby, M.X., Berger, W.H., Bjorndal, K.A., Botsford, L.W., Bourque, B.J., Bradbury, R.H., Cooke, R., Erlandson, J., Estes, J.A., Hughes, T.P., Kidwell, S., Lange, C.B., Lenihan, H.S., Pandolfi, J.M., Peterson, C.H., Steneck, R.S., Tegner, M.J. and Warner, R.R.  2001.  Historical overfishing and the recent collapse of coastal ecosystems.  Science 293: 629-638.