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is sometimes used for group of populations that
occur in the same area without regard to their
interactions; community however denotes an
association of interacting population (Ricklefs
1990
). Begon et al. (
1995
) considered commu-
nity as an assemblage of species population
which occurs together in space and time. Odum
(
1996
) defi ned community as any assemblage
area of populations living in a prescribed area or
physical habitat; it is an organised unit to the
extent that it has characteristics additional to its
individual and population components and func-
tions as a unit through coupled metabolic trans-
formation. Communities not only have a defi nite
functional unity with characteristic trophic
structures and patterns of energy fl ow but they
also have compositional unity in that there is cer-
tain probability that certain species will occur
together. However species are to a large extent
replaceable in time and space so that function-
ally similar communities may have different spe-
cies composition.
Distribution is the geographical extent of pop-
ulation or other ecological units. Primarily the
presence or absence of suitable habitat condition
determines its range. Individuals in a population
may be distributed according to three broad pat-
terns - random, uniform and clumped. Random
distribution is relatively rare in nature, occurring
where the environment is very uniform and there
is no tendency to aggregate. Uniform distribution
may occur where competition between individu-
als is severe or where there is positive antago-
nism, which promotes even spacing. Clumping or
aggregation results from social tendencies of
individuals to form groups, the clumped distribu-
tions of resources, and the tendency of progeny to
remain in the vicinity of their parents (Ricklefs
1990
). Odum (
1996
) stated that determination of
the type of distribution, of the degree of clumping
(if any) and of the size and permanence of groups
is necessary if a real understanding of the nature
of a population is to be obtained and especially if
density is to be correctly measured. Thus, sampling
methods and statistical analyses which would be
quite sound for random or uniform distribution
might be entirely inadequate or misleading when
applied to strongly clumped distribution.
Considerable interest has been generated on
distribution of communities in a variety of water
bodies. A fundamental task of community ecol-
ogy is to identify factors that determine the
relative abundance and distribution of species so
that factors that structure one community may be
useful in predicting another, given similar envi-
ronmental components (Seifert
1984
). The distri-
butional patterns of organisms are controlled by
dispersal mechanisms, historical factors (con-
necting pathways, dispersal barriers) and tolerance
to environmental factors (Carter et al.
1980
).
Wilbur and Travis (
1984
) group the description
of species patterns into two study types: species
association within replicated physical environ-
ments along environmental gradients (Whittaker
1967
) or along derived multifactorial gradients
(Gauch
1989
).
Research efforts have focused on macro distri-
butional patterns of benthic invertebrate commu-
nities within drainage basin (Hawkes
1975
) and
in larger biogeographical studies within and
among drainage basins (Wright et al.
1984
;
Corkum and Curries
1987
; Moss et al.
1987
). In
developing the River Continuum Concept,
Vannote et al. (
1980
) used drainage basin as a
framework within which a continuously inte-
grated series of physical gradients along a river
are associated with changes in functional feeding
mechanisms of invertebrates. However, diffi cul-
ties occurred and modifi cations are necessary
when applying this scheme to the lotic systems
located in different biomes (Wiggins and Mackay
1978
; Minshall et al.
1985
) and to rivers in other
parts of the world (Winterbourn et al.
1981
). A
patchy (contagious) distribution of insects is a
common feature of aquatic habitats (Minshall
and Minshall
1977
) and probably is frequently a
result of the patchiness of the substrate. Little
effort has been made to document this for either
plant or mineral substrates, but the works by
Jonasson (
1948
), Ulfstrand (
1967
), Resh (
1976
,
1979
) and Lamberti and Resh (
1979
) are notable
exceptions and generally support this causal
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