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.
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.
The characteristic of community are:
1. SPECIES DIVERSITY
In an ecosystem ecology each community is composed of
taxonomically different species. Species diversity refers to number of
different species in community both abundant and rare species. Species
diversity has two components.
·
Species richness:
it refers different types of species and their numerical strength. Technically
it refers to ratio between different species (s) and total number of species
(n).
·
Species evenness: it refers to a measure which qualifies as to how even
species are in terms of their number.
Species diversity can be measured by
using various diversity indices. the mathematical expression based on
species abundance data. species diversity can be measured separately
either as species richness or evenness or diversity as a whole.
2.
RESILIENCE AND RESISTANCE
For
a community to be stable, it requires two components-resilience and resistance.
·
Resilience:
resilience is a ability of a community to recover after facing a disturbance or
displacement.
· Resistance:
it is the ability to avoid disturbance(any event that alter the structure of a
community) or displacement(shifting of the community to some other place)
3. DOMINANCE
Usually one community has one or more species which occur in
large number. such species are called dominant and the community is often named
after them.
4. DIVERSITY
The community consists of different group of plants and
animals of different species, may be large or small, may belong to one life
form or another but are essentially growing in a uniform environment.
5. PERIODICITY
This includes study of various life processes (respiration,
growth, reproduction etc.) In the various seasons of the year in the dominant
species in the community. The recurrence of these important life processes at
regular intervals in a year and their manifestation in nature is called
periodicity.
6. STRATIFICATION
Natural forest communities possess a number of layers or
stores related to the high of plant. for example ,tall trees, smaller trees,
shrubs and herbaceous layers from different strata. this phenomenon in a plant
community is called stratification.
7. ECOTONE AND EDGE EFFECT:
·
Ecotone: each
community has spatial limits or boundaries. the boundaries between communities
may be very sharp. such as, boundaries between a forest and a lake or the
boundary may be less sharp ,e.g., boundary between two types of forest or a
forest and a grassland community. often there is some transitional area in
between two communities that is known as ecotone. Where species of both
adjacent communities are found. Ecotone is
a marginal zone and easily recognizable.
·
Edge effect:
usually in one ecotone the variety of one species is larger than in any of the
adjacent communities. a phenomenon of increased variety and intensity of plants
at the common junction is called edge effect and essentially due to wider
range of suitable environmental conditions.
8. ECOLOGICAL NICHE
Different species of animals and plants fulfill different
functions in the ecosystem ecology. the role of each is spoken of as its
ecological niche. that is the role that a species plays in its ecosystem. The
total range of its interaction with other species of its environment is known
as ecological niche.
We
can also say that ecological niche is a small habitat within a habitat, in
which only a single species can survive. E.P. Odum has
differentiated habitat and ecological niche by saying that the
habitat is an organism’s address and the ecological niche is its
profession.
9. KEYSTONE SPECIES
In an ecological community, there are some little players
and some big players. the biggest players of all are referred to as keystone
species. Keystone species is first coined by by Paine in 1966.
A keystone species may be defined as one whose presence or
absence, decrease or increase in abundance, strongly affects other
species in the community.
Paine through his classic experiments showed that predators and herbivores can
manipulate relationships among species at lower trophic level s and thereby,
control the structure of the community. such predator species are called
keystone species or keystone predator.
10. INTERSPECIFIC
ASSOCIATION
Interspecific
association: species interact with various way. Such as: competition,
predation, parasitism, mutualisms, commensalisms etc.
·
Competition: species can compete with each other for finite resources.
it is considered to be an important limiting factor of population size, biomass
and species richness. Direct competition has been observed between individuals,
population and species. Example-a lion chasing a hyena from a kill, or a plant
releasing allelopathic chemicals to impede the growth of a competing
species.
·
Predation:
predation is hunting another species for food. there is a positive negative
interaction. Some predators kill their prey before eating them(e.g., a hawk
killing a mouse),herbivores feed on plants(e.g., a cow grazing). Predation may
effect the population size of predators and prey and the number of
species coexisting in a community.
·
Mutualism:
mutualism is an interaction between species in which both are
benefited. Example-rhizobium bacteria growing in nodules on the roots of
legumes and insects pollinating the flower of angiosperms.
·
Commensalism:
commensalism is a type of relationship among organisms benefits while the other
organism is nither benefited nor hermed.the organism that benefited is called
the commensalism. While the other organism that is neither benefited nor
harmed is called host.
Example-an epiphytic orchid attached to the tree for support neither benefits
the tree for support benefits the orchid but neither harms nor benefits the
tree.
