How a Mangrove Cay Develops
A careful look at the pattern of coastal mangrove
species distribution in a mangrove cay suggests
zonation among the various species. Red Mangroves
line the outermost edges of a cay, and in the sea just
beyond the cay pioneer Red Mangrove saplings grow.
Black Mangroves lie inland of the Reds, and innermost
are White Mangroves and an occasional Buttonwood.
This zonation pattern was once thought to correlate
with the tolerance each species has for saltwater
immersion, but in fact Black Mangrove is the most salt
tolerant, so the pattern is not a simple case of response
to salt exposure. The cay appears to be expanding
outward. Red Mangroves continuously colonize the
outermost edges of the cay, but as sediment builds
and the cay rises, Black Mangroves in turn expand
their range outward, mixing with the Reds. Whites
and Buttonwoods likewise expand their ranges as
the sediment builds higher ground, as these species
are most sensitive to immersion in salt water. Such a
pattern has also been described for mainland coastal
mangrove communities: Reds are outermost, Blacks
intermediate, Whites and Buttonwoods innermost.
Mangrove cay ecology is a topic of debate. Some
ecologists dispute the idea that mangroves are sharply
zoned and represent a successional sequence of the
sort just described. Further, there is doubt about
how routinely mangroves build up cays by trapping
sediment (plate 12- 67). Geological evidence suggests
that mangrove cays originate from coral deposition,
not sediment accumulation by colonizing mangroves.
Not all mangrove areas seem to accumulate sediment,
and changing conditions caused by storms and
tides certainly influence the pattern of mangrove
distribution. Some mangrove cays have remained
stable for many years, without significantly expanding
or contracting. Disturbance history has a strong
influence on zonation patterns.
Mangrove and coral reef ecosystems bear the brunt
of tropical hurricanes and monsoons (plates 12- 68–
69). But these ecosystems rebound. It is clear that
coral reefs and mangrove forests provide a strong
degree of protection from coastal erosion and damage
from major storms. In addition, mangroves, by their
high productivity, act as key species in nutrient flux
in coastal tropical ecosystems. Mangrove forests are
deserving of protection throughout their range, as they
are essential assets.
Plate 12- 69. This male frigatebird has its throat pouch fully
expanded, an attempt to attract a female. Photo by John
Kricher.Plate 12- 68. This is Bird Caye off Dangriga, Belize, in 1978,
some 17 years after Hurricane Hattie, which had a major
impact on coastal Belize. Mangrove forests help buffer the
coast and inland areas from the ravages of hurricanes. The
birds are Magnificent Frigatebirds (Fregata magnificens),
attempting to nest on the decimated cay. Photo by John
Kricher.232 chapter 12 cruising the rivers to the sea