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How
the type of atoll affect the structure of the
communities
The
structure
To
describe the structure of the fish communities
inhabiting the atoll waters, several approches have been
used, which focus on the following aspects:
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the
species structure : the communities
composition in terms of the species, genera and
families of which they consist, and the parameters
liable to affect this composition.
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the
trophic structure: a description of the
community in terms of trophic categories.
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the
size structure: the distribution of the
groups of species or individual species encountered,
in terms of their size.
-
“functional
groups” : in addition to size
groups and trophic groups, definitions of another
kind can be used, based on the behaviour of the fish
species (diurnal versus nocturnal activity, their
mobility, gregariousness, etc.) can be used to
classify fish into groups and to combine some of the
groups obtained into what have been called
“functional groups”, such as the
gregarious carnivorous fish inhabiting the reefs or
the large micro-herbivorous fish. These functional
groups are artificial constructs which help us to
understand how these communities function by
arranging groups of species in a small number of
classes showing some degree of biological and
ecological homogeneity.
The
structure of the atoll fish communities depends on many
factors and it is difficult to describe an average
pattern. Here will deal only with the trophic structure
and attempt to show how various factors affect this
structure.
Factors
which affect the trophic structure
For
this purpose, we will focus on three main parameters:
the range of species (for the sake of simplification, we
will call this parameter “the diversity”), and
the density and biomass of the fish communities. We will
also restrict this description to the parts of the reefs
fringing the motus.
At
a given atoll, the fish community inhabiting the reef
fringing the motus can be subdivided into a number of
trophic groups. We have defined 8 such groups:
fish-eaters, macro-carnivores (fish feeding on large
invertebrates), micro-carnivores (fish feeding on small
benthic invertebrates), plankton-eaters,
macro-herbivores (fish feeding on algae and phanerogamia),
micro-herbivores (fish feeding on epyphytic algae and
algal turf), coral-eaters and scavengers.
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Each
of these groups can be
schematically represented by a cube
(fig). The sizes
and shapes of the cubes vary depending on the
diversity, the density and the biomass of the
trophic group in question. |
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After
classifying the atolls in three size classes (small,
medium and large) (fig11)
and the reefs in terms of their hard bottom
covering (10%, 20% and 40% of the total covering),
we have shown in fig12
how the trophic structure of the fish
communities varies depending on these two
factors (the size of the atoll and the
proportion of hard bottom covering).
The
cubes sizes are proportional to the maximal
value observed on the whole atolls. |
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The
variations observed using statistical modelling
procedures (see Table
3) made it possible to clearly identify the
main features of this trophic structural
pattern:
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regardless
of the size of atoll involved, the largest
group of fish was that consisting of micro-herbivores
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the
coral-eaters, scavengers and
macro-herbivores form relatively minor
groups
-
the
plankton-eaters, piscivores and
macro-carnivores had diversity /density /biomass
ratios which depended on the type of atoll
and the nature of the sea-bed. Large-sized
species belonging to these categories were
found to occur at the larger atolls and the
biomass was found to increase with the rate
of hard bottom covering.
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A
single trophic group is not necessarily
homogeneous and the species of which it is
composed can differ from one atoll to
another, resulting in differences in the
densities and the biomasses of the trophic
groups, as can be seen in fig13.
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Production
rate of the trophic groups of an atoll
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The
production rate of an atoll has probably a
long-term stability, and several groups with
variable production rates probably make a
relatively stable contribution over a long
period of time.
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To
know more
References Kulbicki
M;, Galzin R., Harmelin-Vivien M., Mou Tham G., Andréfouët
S. 2000 Les communautés de poissons lagonaires dans les
atolls des Tuamotu, principaux résultats du programme
TYPATOLL (1995-1996). Nouméa, IRD, Doc.Sci. Tech. II3:
26-125
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