CHARACTERISTIC
OF ECOLOGY
Ecologists have given variety of
meanings to the term community (Schoener, 1986; Fauth et al., 1996).
The term has been used by many of
them for associations of plants and animals occurring in a particular locality
and dominated by one or more prominent species or by some physical
characteristic (Daubermiire, 1968).
A community,
technically often referred to as biota or biotic community is a
local association of several species populations. According to Krebs (1994), a
community is an assemblage of the populations of living organisms in a
prescribed area or habitat.
According to Clarke (1954), a group
of mutually adjusted plants and animals inhabiting a natural area is known as a
community. He prefers to use the term biocenose (a shortened form of the word
“biocoenosis” coined by Mobius in 1880) for community. When territorial ranges
of several species populations overlap, they may coexist within a given limited
area.
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The organisms in such an area
represent a community (Fig. 4.1). A community always contains plants as well as
animals, because both are very necessary for the survival of the community.
Just as populations possess characteristics above and beyond those of their
component organisms, the community also exhibits characteristics above and
beyond those of its constituent populations.
A major community is the smallest
ecological unit that is self-sustaining and self-regulating. It is made up of a
large number of smaller minor communities that are not altogether
self-sustaining. For example, forests and ponds are major communities; decaying
logs and ant hills are minor communities. Members of a major community are
relatively independent of other communities, provided they receive radiant
energy from the sun. These members normally show a similarity in their
physiological make-up, behaviour, and mode of life.
A number of parameters can be
estimated in a community, but their interpretation depends on nature of the
community. In plant ecology, two schools have developed over the question of
nature of the community. The organismic school holds that communities are
integrated units with discrete boundaries. The individualistic school holds
that communities are not integrated units but collections of populations that
require the same environmental conditions. However, the individualistic
interpretation of the community is favoured more.
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All communities have certain general
characteristics in common, and their maintenance is governed by similar forces.
Communities do not have exact limits but tend to overlap each other. Animals
frequently shift from one community to another because of seasonal or other
variations. Communities may be widely separated, but if the environmental
factors are the same, similar kind animals may be found in them.
Some
characteristics of the community are as follows:
(a)
Stratification:
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Communities that possess a large
biomass usually exhibit stratification, that is, the populations they contain are
spaced out or distributed into definite horizontal or vertical strata.
Five
vertical subdivisions of the forest community are:
(a) Subterranean,
(b) Forest floor,
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(c) Herbaceous,
(d) Shrub, and
(e) Free strata.
Similarly, there are animals that
live on the forest floor, others on shrubbery and low vegetation, and still
others in the treetops. Many forms shift from one substratum to another,
especially in a diurnal manner. Many of the adjustments and requirements of a
particular stratum are very similar in forests widely separated from each other
in many parts of the world. The animals that occupy such similar strata, or
ecological niches, although geographically separated, are called Ecological
equivalents.
(b) Ecotone:
There may be an intermediate
transitional zone between two distinct communities. This is called an Ecotone,
or a tensional zone. An ecotone can be defined as a zone of transition between
adjacent ecological systems having a set of characteristics uniquely defined by
space and time scales and by the strength of interactions between adjacent
ecological systems (Holland, 1988).
An example may be the marginal
region between a forest and a pasture or open land. Several examples of
aquatic- terrestrial ecotones may be given, such as wetland ecotones (ecotones
between wetlands and other types of ecosystems), lotic ecotones (fluvial
boundaries of rivers and stream ecotones). As a rule, the ecotone contains more
species and often a denser population than either of the neighbouring
communities and this is known as the Principle of edges.
Recently is has been recognised that
ecotones are not simply static zones where two communities join but are dynamic
and have unique properties (Risser, 1990). They provide unique habitats for
biota; they regulate inter-patch dynamics; they may serve as early indicators
of hydro-climatic change; and they have strong visual quality. These
characteristics of ecotones are significant for aquaitc landscape management
(Petts, 1990).
(C) Ecological
Dominance:
In every community some plants and
animals exert a dominant influence, in determining the nature and function of
the whole community, because of their numbers, activities, or other important
reasons. They are called Ecological dominants. An ecological dominant may be
defined as a species population that exercises a major controlling effect on
the nature of the community.
Thus, each community usually
contains one or two species populations at the producer, herbivore, carnivore,
and reducer levels that are recognizable by the controlling influence they
exert on the community. These dominant species populations are the members of
the community through which a major portion of the energy transfer is affected.
In land communities, plants are
usually the dominants and some communities are named according to their
dominant vegetation. However, in plant communities abundance is rarely
correlated with dominance. For example, in a forest, an herbaceous species may
be very abundant while a free species is hardly so, in terms of the numbers of
individuals present; but it is the tree species thus determines the nature of
such a community and is therefore the dominant.
In plant communities the dominants
are simply those plants that overtop all others in the community. In doing
this, they modify the amount of light the subordinate species receive, the
humidity, the amount of precipitation, the extent of air movement, and the
composition and temperature of the air. Some plant communities have only a
single dominant; for example, pine trees in a pine forest, or saal trees in a
saal forest. But in a mixed forest there may be several codominants. In the
ecological cycle of a community, the removal of a dominant species causes
disturbances and changes in the character of the community.
(D) Seasonal
and Diurnal Fluctuation:
The component populations of a
community may succeed one another in time as well as in space. The most
conspicuous temporal variation communities are seasonal. E.J. Salisbury (1925)
was actually the first to demonstrate such temporal separation of plant
communities in temperate woodland. Local fluctuation of populations in
communities, similar to seasonal migrations but over much more restricted
distance, result from diurnal responses.
The active day time mammalian
population of a wooded area may include squirrels, but by night there may
appear reccoons, opoossums and wood rats. These temporal relations between
populations in a community, like the spatial ones, must be considered in terms
of the niche concept.
(E) Pattern
Diversity:
Pattern diversity is the basic
characteristic of community organization. There are several patterns. Such as
stratification patterns (vertical layering), zonation pattern (horizontal
segregation), activity patterns (periodicity), food-web patterns (network
organization in food chains), reproductive patterns (parent- offspring
association and plant clones), social patterns (flocks and herds), coactive
patterns (resulting from competition, antibiosis, mutualism), and stochastic patterns
(resulting from random forces).
(f)
Periodicity:
Periodicity (activity patterns)
refers to the rhythmic patterns of organisms in search for mates, food, and
shelter. Some community periodicities are correlated with the daily rhythms of
day and night, some are seasonal, and others represent tidal or lunar events.
(g)
Turnover:
In a community, as on all other
levels of living organization, turnover occurs continuously. Individuals of the
various populations emigrate or die out and are replaced by others. The
important point is that this flux is automatically self-adjusting. As a result
of this, the community- remains internally balanced and exhibits a numerical
steady-state, i.e., in all populations making the community, the numbers of
individuals remain relatively constant.
For example, in a large permanent
pond the number of algae, frogs, fish and any other plants or animals, will be
more or less the same from decade to decade. Annual fluctuations are common;
but over longer periods of time constancies of numbers are characteristic to
most natural communities.
(h)
Interdependence:
Various kinds of interactions bring
about a certain degree of integration within the members of a community. Food,
reproduction and protection are the principal links which make the members of a
community interdependent.
According
to Krebs (1994), there are five characteristics of communities that can be
studied:
(i)
Species Diversity:
This means what species of animals
and plants live in a particular community. An important characteristic of the
community is its diversity, which is a function of the number of different
species it contains and their abundance. Diversity actually depends on Species
richness and on the evenness (equitability) of species abundance. Two
hypothetical communities comprising the same species may differ in their
structure and diversity, depending on their relative abundance distributions.
Communities in which the species are
all more or less equal in abundance exhibit evenness, whereas communities with
one or a few abundant species and many rare ones show dominance. However,
communities possess both taxonomic diversity (diversity of species) and
ecological diversity, which reflects the variation in ecological roles of
species in the community.
(ii) Growth
Form and Structure:
A community may be described by
major categories of growth forms such as trees, shrubs, herbs and mosses. These
different growth forms determine the ratification of the community.
(iii) Dominance:
This refers to the fact that all
species in the community are not equally important. Of the hundreds of species,
relatively few exert a major controlling influence by virtue of their size,
numbers, or activities. Thus, dominant species are those which are highly
successful ecologically and largely determine the conditions under which the
associated species have to grow or live.
(iv)Relative
Abundance:
This emphasizes the relative
proportions of different species in Ac community.
(v)
Trophic Structure:
The feeding relations of the species
in the community determine the low of energy and materials from plants to
herbivores to carnivores. Thus, “who eats whom” decides many things in the
community. These characteristics can be studied in all the communities that are
in equilibrium and also in communities that are changing. The changes may be
spatial along environmental gradients such as altitude, temperature and
moisture. The changes may also be temporal, which are called succession and may
lead to a stable community or the so-called climax community.
