NAG 301:
SOIL AND VEGETATION ECOLOGY
Dr. K. Chatterjea
LECTURE TOPICS: THE PLANT SPECIES, THEIR
ECOLOGICAL PROPERTIES, AND THEIR INTERACTIONS
(The lectures under this title are very inter-related. So
they are outlined together. There will be several
lectures on the following sub-topics)
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2.
3.
4.
5.
6.
Range of Tolerance
Competition
Symbiosis
Population controls
Ecological Niche
Migration and Range
The distribution of a species population is a function of its resource requirements for growth
and reproduction and of its ability to exploit available environmental resources.
Range of tolerance: Organisms vary in their environmental requirements as well as in their
tolerance of unfavourable conditions.
Tolerance of physical conditions vary between species, between individuals in a species
population and between stages of maturity.
Beyond certain upper and lower threshold values of tolerance, the species is unable to survive.
This is called the tolerance limits of the species and the intervening range is the range of
tolerance.
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Ecological optimum
Tolerance Range
Vigour
Ecological optimum
Environmental gradient
Interaction between plants
Green plants are usually considered independent organisms. But, in reality, they are not.
Competition between plants:
During the establishment of the community competition between plants trying to occupy the
same niche occurs. The physical suitability of an area for a species does not ensure that the
species will be growing there. This depends on the nature of the other species trying to colonize
the area.
Intraspecific competition
Interspecific competition
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Factors plants compete for:
Light
nutrients
soil moisture
presence of pollinators
oxygen
soil conditions
carbon dioxide
humidity, wind conditions
Parameters change as colonization takes place.
Light
The effects of competition on growth
of plants
Competition for light
Leaves shaded, reduced photosynthesis
Reduced shoot growth
Reduced root growth,
Reduced absorption of nutrients
Competetion for water, oxygen,
and mineral nutrients
Because a habitat contains insufficient resources to support all species capable of living there,
competition is inevitable, and the success of a particular plant depends on its ability to compete
for space, light, water, and soil nutrients.
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Competition for light is a very important aspect as light cannot be stored: must be used at
maximum efficiency: large range of light requirements by the different species.
Competition for light is the reason for the complicated structure of the tropical rain forest, an
ecosystem in which the maximum use of light occurs.
Plants also compete for water, air, nutrients. The ability to obtain these depend on the rate of
root growth, photosynthetic ability.
An example of plants/
trees which do not
compete with each other
for water:
Trees have taproots to
access water from deeper
layers
Shallow rooted grass
obtains water from the
upper layers
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Plants vary in their optimum requirements for successful growth. The more 'demanding' or
'aggressive' may, because of their greater reproductive capacity, vigour of growth and size, so
modify climate and edaphic conditions as either to depress or to exclude the less demanding and
aggressive from a particular habitat.
Competition is most intense between individuals of the same or different species which makes
similar demands on the same supply of light etc.
A plant will have its greatest competitive capacity when growing under optimum physical
conditions.
Plants may be excluded from their optimum habitats by more vigorous species. The rampant
growth of weeds on cultivated land is an example.
The annual plants in the deserts may be due to absence of competition from other plants.
Halophytes grow best in saline soils where other plants cannot survive.
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Competition is the principal biotic factor which, under natural conditions, determines those
plants which will be able to co-exist in a given habitat. The physical factors of soil and climate
are permissive, that of competition is selective.
Symbiosis among plants
"Life together" - includes all effects of one plant on another.
Commensalism: the plant benefits without actually harming the other.
Mutualism: both plants benefit.
Parasitism: one plant benefits at the expense of the other.
Lianas and epiphytes offer examples of commensalism: They use the plant for support/
anchorage.
Mutualism occurs when two plants growing in close proximity benefit each other. The
partnership of algae and fungi is an example.
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Parasitic plants obtain all or part of their nutrients and water from other plants.
POPULATION CONTROLS IN ECOSYSTEMS
The population of a species = birth rate - mortality rate
If left unchecked, the number of individuals in a species population can reach astronomical
levels. In most ecosystems, checks come into play.
THE ECOLOGICAL NICHE
It is the status of an organism within its ecosystem resulting from the species adaptations,
physiological responses, and behaviour. The total requirements of a species for all resources and
physical conditions determine where it can live and how abundant it can be at any place within
its range. These requirements, therefore, may be termed 'the ecological niche'. The 'niche' of a
plant, for example, might include temperatures it can tolerate, the soil water potentials at which
its roots can survive and take up water, the intensity of sunlight required for photosynthesis, the
required concentrations of various nutrients, densities of pollinators, herbivores, etc. --obviously difficult to quantify, but an useful concept --> indicates the interrelationships that
must be considered to understand the determinants of distribution and abundance.
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The fundamental niche: the total range of physical environmental conditions that are suitable for
its existence.
The realized niche: that part of the fundamental niche actually occupied by the species.
PLANT MIGRATION AND RANGE DEVELOPMENT
The geographical limits of the area within which a particular population of organisms occurs
are known as its range. While its presence suggests that the current ecological conditions are
suitable for a species’ successful growth and reproduction, its absence does not necessarily
mean the reverse.
The degree to which a species realizes its potential range, as set by environmental conditions
and biological interactions, is largely a reflection of its ability to migrate. Migration from
one area to another is dependent on (1) the dispersion of individuals and, (2) their successful
establishment in the new habitat.
The existing world biotic distributions are the result of the dispersal of organisms from their
initial centres of origin in the course of which organisms are brought into contact with new
habitats and with each other. Migration is an important process in evolution.
The dispersal of an organism from one ecologically suitable habitat to another may be
prevented by intervening ecological barriers. One of the most effective barriers is salt water.
Very few land or freshwater plants have seeds adapted to long periods of immersion in salt
water. High mountain ranges and extensive deserts also cause hindrance.The efficiency
(effectiveness) of any barrier depends on (1) the scale of the barrier, (2) migrational ability of
the organism involved. Successful migration depends on (1) the establishment and survival of
sufficient individuals to form or initiate a breeding population, (2) the presence of a vacant
niche in the new territory, (3) the presence of immigrants with a competitive ability to
displace existing organisms.
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