Biology Reference
In-Depth Information
below carrying capacity, and, according to Sale (
1991
), on a small scale,
i.e., that of a few meters, reef fish assemblages appear to be nonequilibrial
showing great variability over time and space. Territorial fish are con-
tinuously competing for space, defending their small territories which
serve as sources of food, shelter, and nest sites. Territories are dead rock
surfaces covered by algae. In one study, three species of pomacentrid
fishes were studied. They inhabited territories that were contiguous but
did not overlap. Over 40 months, the total habitat area remained more or
less the same. Pelagic juveniles or older juveniles from other sites occupied
territories that had become vacant by mortality. However, the species
occupying particular territories changed, indicating that recruitment was
by chance or ''lottery'' (who comes first, wins). Species involved in such
lotteries must be iteroparous. Sale referred to such lotteries as ''non-
equilibrial,'' other authors as ''equilibrial.'' In another study, Sale et al.
(
1984
) made a 2-year visual census of young of the year recruits to
2 habitats each, on 7 reefs. Selected sites were very similar. Nevertheless,
there was significant variation in abundance between reefs, between
years, or both. Recruitment variability was also documented in several
subsequent studies reviewed by Doherty and Williams (
1988
). Sale et al.
(
1984
) reported results of a 9-year study of assemblage structure of fish
from 20 coral patch reefs, based on 20 non-manipulative censuses. The
total number of fish species found was 141, belonging to 34 families. The
average reef was about 8.5m
2
in surface area and had about 125 fish
of 20 species. Variation in total fish numbers among censuses was at least
2-fold, but reached 10-fold in 12 reefs. Variation in composition and
relative abundance of species was also great. This variability could not be
attributed to changing physical structure of the reefs, or to the effect of rare
species leaving or entering the reefs.
Sale and Tolimieri (
2000
) questioned the necessity of density depen-
dence for long-term persistence in reef fish populations. Like Sale above,
they emphasize the nonequilibrium aspects of reef fish populations. There
are situations in which demographic rates change in relation to density
without negative feedback. Thus, per capita recruitment will decrease
with increasing population size although absolute numbers of recruited
larvae do not change. Furthermore, there are many density-independent
processes, and many existing populations do not persist. Also, populations
do not automatically bounce back from a low population size.
Several recent studies appear to support the view of great variability in
reef fish assemblages. Chittaro and Sale (
2003
) compared reef fish assem-
blages at small patch reefs in the Caribbean (St. Croix, US Virgin Islands)
