Topic: Population, Biotic Community and
Physiognomy and stratification: A community is first noticed
by its physiognomy. Physiognomy refers to the external appearance or “look” of
the community. The external appurtenance is the total effect created by the
combination of vertical structure and architec-ture of dominant species of
vegetation. For instance, the high physiognomy of forest differs markedly from a
low physiognomy of a grassland. However, several communities may have similar
physiognomy, yet they differ sharply on the basis of species composition and
dominants (e.g., different forests types).
Stratification of a community depicts vertical layering of
the vegetation. Different layers are occurred by different species. The vertical
stratification provides physical structure to the plant community, in which many
forms of plants and animal life are adapted to live in a well developed forest
eco-system exhibits a highly stratified structure, consisting of several layers
of vegetation. These layers include the canopy, the understory tree layer, the
shrub layer and the herb layer. Similarly, a pond community has surface dwellers
and bottom dwellers. Vertical stratification lead to increase in number of
species and to efficient use to resources of a habitat by different types of
plants. In aquatic ecosystems, stratification from surface to bottom is
determined by light penetration from temperature profile and oxygen profile.
Species diversity: Some communities, such as tropical
rain forest and coral reef community, show high species diversity with many
different kinds of species living at each trophic level. In other communities,
like a desert, there may be relatively few species in the entire community.
Species diversity includes the total number of species present in a community
and the relative abundance of these species. Diversity is recognised as an
important functional attribute of biotic community. You will study several
diversity-related aspects in Chapter 20.
Keystone and link species: The species having much
greater influence on community characteristics, relative to their low abundance
or biomass, are called keystone species. These species play a vital role in
controlling the relative abundance of other species. Removal of keystone species
causes serious disruption in the functioning of the community. For example, in
the tropical rain forests, the different species of figs are the keystone
species as the produce large quantity of fruits. During the time of food
scarcity, these fruits are eaten by monkeys, birds, bats and other vertebrates.
Thus, by protecting the fit trees, the animals dependent on them are also
Only a few species work as keystone species, and several
other work as critical link species. Mycorrhizal fungi in soil are critical link
species as they establish essential links in the absorption of residues. Some
critical link species may also provide food for the network species; other play
important roles as pollinators of flowers or dispersal agents of seeds and
fruits. Tropical rain forests are rich in critical link species due to high
degree of animal-dependent pollination and dispersal.
Ecotones and edge effect: The transition zone between
two communities is known as ecotone. For example, the ecotone between grassland
and forest. Ecotone contains few species from both communities. The total number
of species is often greater in the ecotone than in the adjoining communities.
The tendency of increased variety and density of some organisms at the community
border is known as edge effect. The organisms which occur primarily, or most
abundantly, or spend the greatest amount of their time in junctions between
communities, are called edge species.
Analysis of Plant Communities
Analysis of community characters is generally done for: (a)
recording variation within and between communities, and (b) naming and
classifying communities. Community analysis involves measurements of various
characters in sample plots (also called quadrates) located randomly within the
community. Measure-ments made in sample plots are appropriately processed to
reflect the characteristics of the entire community. Various community
characters can be categorised as:
Analytic characters, which are directly observed or
measured in sample plots.
Synthetic characters, which are derived from the measurements
of analytic characters.
The analytic characters may be either qualitative or
quantitative. Qualitative analytic characters are based on non-quantitative
observation; for example, the species composition and stratification of
vegetation. On the other hand, quantitative analytic charac-ters, as the name
suggests, are measured. The major quantitative analytic characters, as the same
suggests, are measured. The major quantitative analytic characters are:
Frequency (based on percentage of plots in which a
species is present, indicating its dispersion in space).
Density (number of individuals per unit area, indicating the
relative abundance of a species).
Diversity (Total number of species in a unit area, including
plants, animals and microbes).
Cover (percentage land area occupied by a species, indicating
the influence zone of a species; cover is expressed as basal cover, area
occupied by stem bases, or crown cover, the area covered by canopy).
Biomass (quantity of living materials per unit area,
indicating the growth of a species; see more details on biomass and productivity
in Chapter 18).
Leaf size (percentage of species having different leaf sizes,
indicating the adaptation of the vegetation to the prevailing environment).
Synthetic characters (e.g., presence and constance) reflect
the pattern of distribution and performance of difference species through all
the locations where the community occurs.
The biotic communities are dynamic in nature and change with
the passage of time. The successive replacement of communities in an area over a
period of time is known as ecological succession. Both abiotic and biotic
components are involved in such change. Succession is a community-controlled
phenomenon, which results due to the action and co-action on living organisms.
Physical environment often determines the nature direction, rate and optimal
limit of change. During succession, change occur both in plant and animal
communities. The plant succe-ssion, however, is easily visible. Two basic types
of succession can be distinguished. Succession occurring on previously
unoccupied sites, such as a rock outcrop or glacial moraine, is called primary
succession. The more common type of succession is the secondary succession,
which occurs in an area where the natural vegetation has been destroyed or
removed. For example, the forests destroyed by fire and excessive lumbering may
be reoccupied by herbs in the initial stages. The reappearance and establishment
of communities in such areas is called secondary succession. The plant that
invade the bare land initially, are called pioneer species. The assemblage of
pioneer species forms the pioneer community. Generally, the pioneer species show
high rate of growth but short life span. In time, the pioneer community is
replaced by another community with different species combination. This second
community is replaced by a third community, and so on. The different communi-ties
or stages represented by combinations of mosses, herbs, shrub and trees
replacing one another during succession are refereed to as seral stages or seral
communities. The plant species which get established later, during plants
species are low growing and long lived. The terminal stage of succession is
represen-ted by the climax community. The climax community is stable and does
not show change in species compositions remain the same. The sequence of
communities succeeding each other during the course of succession represents the
sere. The succession occurring in water bodies like ponds and lakes is called
hydrach succession, and the succession taking place in terrestrial areas with
low moisture (for example, rock, sand) is known xerarch. These two types or
succession are described below.
Succession on a Barr Rock (Xerarch)
Lower plants, like lichens, from a crust over the bare rocks
and begin to form soil from their organic remains and by stimulating chemical
breakdown of the rocks. Lichens are normally followed by mosses, which speed up
the process of soil accumulation by trapping wind-blown particles. Mosses grow
in bunch, and together with lichens, make a mat over the substratum. Lichens and
mosses, which get established on barren rock, are the pioneer species forming
the pioneer community. The accumulation of soil particles in the lichen-moss
carpet provides suitable substratum for the germination of seeds of herbaceous
plants which are dispersed in it. The seeds of higher plants germinate and grow
successfully in pockets of newly formed soil on the rick (Figure). Gradually,
more soil is accumulated and herbaceous species make way for the invasion of
shrubs followed by trees. The dead shoots and fallen leaves accumulate and
enrich the soil. Passing through the seral stages in course of time. Climax
community gets established. Depending upon the climate condition and extent of
soil formation, the climax community is generally dominated by trees. The
changes in biotic community from the pioneer to the climax stage may take
hundreds of years.
Succession in Aquatic Environment (Hydrach)
Ecological succession also occurs in water bodies like ponds
and lakes. Water bodies are prone to silting as a result of soil erosion from
surroundings area. Blockage of rivers by land-slides and construction dams lead
to formation of new lakes on land where hydrach succession sets in due to
invasion of aquatic species (Figure). In a pond, the phytoplankton and
zooplankton constitute in pioneer community. Submerged aquatic plants, with
their roots anchored in the mud, are next to colonise the pond. The dead remains
of these organism settle at the pond bottom. Beside, floating plants species
invade the pond. With the continued siltation, the pond bottom is gradually
raised and water layer becomes shallow and rich in nutrients. As a result,
rooted, emergent plants with aerial leaves, such as reeds, are able to colonise
the pond. This is accompanied by the invasion of Dragon files, crustaceans and
more rooted species of plants. Thus, the species composition of the pond keeps
changing with time. With increased setting of silt and deposition of dead
organic matter derived from floating and rooted species, the pond becomes
shallower until it gets trans-formed into a terrestrial habitat. Ultimately,
terrestrial species, like grasses, bushes and trees, colonise the pond area and
a climax community is established. The colonization by land plants usually
progresses from margins toward the centre of the pond area.
Change in Community Characteristic and Climax
The exact sequence of species and communities that appear
during primary of secondary succession, varies with the habitat conditions. The
seral stages differ from late successional or climax stages with reference to
structure and functions (Table). The average size of individuals generally
increase and the community organisation becomes more complex in the climax
community as compared to the seral community. The food webs become complex
during successional stages. The efficiency of energy use and nutrient
conservation increase as the community progresses towards the climax stage.
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