LETTERS TO THE EDITORS
The Role of Algae
To the Editors:
"Algae-Dominated Reefs" by Peter S. Vroom, Kimberly N.
Page, Jean C. Kenyon and Russell E. Brainard (September-October)
extols the virtues of "healthy" coral reefs dominated by
seaweed but ignores over 150 years of research achievement that
began with Charles Darwin's pathbreaking 1842 book Coral
Reefs. As Darwin hypothesized, and uncountable drill cores
confirmed, coral reefs and atolls are built from growth of corals
and coralline algae. The authors are struck by the predominance of
macroalgae and lack of corals on what appear to be otherwise healthy
reefs from a variety of Pacific reef sites. But fleshy macroalgae
are incapable of constructing the very reef structures they now so
boldly dominate.
This is a classic example of the "shifting baseline
syndrome" whereby our perceptions continually shift to later
and later time periods when degradation has already worked its
turmoil, and scientists of today are happy to believe that what they
are seeing is pristine, no matter what the history, ecology or
underlying geological structure might tell them. If corals were
always so unimportant in the reefs the authors examined, how did the
atolls and coral reefs get there in the first place?
The authors also make the mistake of equating all macroalgae, but
many studies have shown that coralline algae are useful in
facilitating coral recruitment and growth on reefs, whereas fleshy
macroalgae are harmful in the extreme because they are capable of
overgrowing corals, of inhibiting their growth and of inducing
disease. A more defensible interpretation of the data they represent
would have treated the different algal groups separately because of
their varied ecosystem functions.
John M. Pandolfi
University of Queensland
Australia
Jeremy B. C. Jackson
Scripps Institution of
Oceanography
University of California, San
Diego
Smithsonian Tropical Research Institute, Balboa, Republic
of Panamá
Drs. Vroom, Page and Kenyon respond:
Drs. Pandolfi and Jackson are confusing "healthy" with
"reef accretion." Reef accretion is just one parameter out
of many that must be combined to assess reef health. We do not
ignore the fact that islands and atolls are built from coral and
coralline algae, but rather, as we state in our article, many reefs
are in different successional states. For instance, the northwestern
Hawaiian Islands, which represent healthy, anthropogenically
unimpacted ecosystems dominated by top predators, may have been
dominated by corals in past millennia. However, as tectonic plate
movements shift these islands farther north, cooler sea-surface
temperatures are currently not as conducive to extensive coral growth.
This does not mean these reefs aren't healthy, but merely that they
represent a different type of reef than one that might be found
right on the equator. This is natural and is what would be expected.
Similarly, even reefs situated near the equator possess different
geomorphic zones characterized by heterogeneity and differing
amounts of coral cover. At many reefs, severe wave motion in fore
reef regions hinders coral growth while promoting algal growth.
Therefore, dense coral thickets and reef accretion likely occur in
more sheltered parts of the ecosystem.
We are confused as to why Drs. Pandolfi and Jackson say we equate
all algae. We clearly separated coralline algae, turf algae and
macroalgae in the article and discussed how each group is important
to the reef ecosystem. We acknowledge that coralline algae are
essential for coral recruitment and growth. As scientists, we know
that biological organisms play many roles in ecosystems, and to
stereotype all macroalgae as harmful without understanding the roles
they may play in healthy ecosystems seems shortsighted and
scientifically irresponsible.
For instance, Caulerpa taxifolia, a noxious invader that
overgrows acres of substrate in the anthropogenically disturbed
Mediterranean, is a natural, but minor, component of most tropical
Pacific reefs we survey, yet we have never observed it to overgrow
living coral in these ecosystems. Instead, it often grows
harmoniously with coral, forming small patches on the underside of
thriving coral heads and tables, well away from living coral polyps.
If past millennia were not conducive to algal growth in reef
systems, then what allowed for the thousands of currently known
tropical reef algae to evolve? Because many corals are not suited to
harsh water environments, what besides algae would have occurred in
fore reef areas in reef ecosystems thousands of years ago?