Organism requires essential materials which are necessary for growth and reproduction ere governed by following rule
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1. Lie big’s “Law” of the minimum:
Under “steady state” conditions essential material available in amounts most closely approaching the critical minimum needed will tend to be the limiting one. This law of minimum is less applicable under “transient state” conditions when the amounts, and hence the effects of many constituents are rapidly changing.
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2. Shelford’s Law of tolerance:
The presence and success of an organism depend upon the completeness of a complex of conditions. Absence or failure of an organism can be controlled by the qualitative or quantitative deficiency of excess with respect to any-one of several factors which may approach the limit of tolerance for that organism.
3. Combined concept of Limiting Factors:
The presence and success of an organism or a group of organisms depends upon a complex of conditions. Any condition which approaches or exceeds the limits of tolerance is said to be a limiting conditions or a limiting factors.
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4. Conditions of existence as regulatory Factors:
Light, temperature and water are ecological important Environmental factors on land; light, temperature and salinity are the big three in the sea. In fresh water other factors such as oxygen may be of major importance.
All these physical conditions of existence may not only be limiting factors in the detrimental sense but also regulatory factors in the beneficial sense – that adopted organism respond to these factors in such a way that the community of organisms achieves the maximum homeostasis possible under the conditions.
Blackman (1912) says that a process is affected by many factors and rate is controlled by the slowest movement and this is known as limiting factors.
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Ecological succession: The gradual replacement of one type of plant community by the other is referred as plant succession. E. P, Odum says “Plant succession is an orderly process of community change in a unit area”. Clements gave a simple definition “Succession is a natural process by which the same locality becomes successively colonized by different groups of communities.
Succession is a complex universal process which begins, develops and finally stabilizes at the climax stage.
Causes of Succession:
1. Climatic causes e.g. Drought, Heavy rainfall, hails and lighting.
2. Topographic causes e.g. Erosion, soil deposition.
3. Biotic causes grazing, harvesting, deforestation. Kinds of succession: Depending upon the nature of base area on which it develops the succession is of two kinds:
1. Primary succession:
When succession starts on extreme base area on which these was no previous existence of vegetation it is called primary succession.
2. Secondary Succession:
This succession starts on the secondary base area which was once occupied by original vegetation but later became completely cleared off vegetation by the process called denuation. These primary and secondary successions may be of the following types:
(a) Hydrosere: aquatic
(b) Halosere: Salty soil or saline water
(c) Xerosere: xeric habitat, two type
(i) Pasmmosere: Sandy habitat.
(ii) Lithosere: rock surface.
(d) Serule: succession of microorganisms.
In xerophytic succession rock particular is occupied by lichens first than mosses, later herbs shrubs and trees follows them Odum (1971) classify succession into autotrophic and heterotrophic successions.
Plant adaptations:
Warning (1895) classified plants into several ecological groups on the basis of their requirements of water and also on the basic of substratum on which they grow.
(1) Plants of acidic soils – Oxylophytes
(2) Plants of saline soils – Haloplytes
(3) Plants growing on the sand – Psammoplytes
(4) Plants growing on the surface of rocks – Lithophytes
(5) Plants growing in the crevices of rocks – Chaemophytes
(6) Plants of water – Hydrophytes
(7) Plants of xeric – Xerophytes
(8) Plants of moist soil – Mesophytes.