The environment is the aggregate of all those things and set of conditions which directly or indirectly influence not only the life of organisms but also the communities at a particular place. The environmental conditions which influence the life and development of plants are grouped into four main classes (ecological factors) which are as follows :
# (1) Climatic factors : The study of climatic factor is known as climatology. The chief climatic factors are :
- (i) Water : Rainfall is the chief source of soil moisture. Water exchange between earth surface and atmosphere is called hydrological cycle. Humidity of the air is expressed in terms of relative humidity. It is measured by hygrometer (Psychrometer). Epiphytes and cryoptogamic plants grow in those regions where relative humidity is high.
Annual rain fall determines the types of vegetation in any region such as :
(a) The area where rainfall is scanty are seen with deserts and xerophytic vegetation.
(b) The tropical area with heavy rainfall throughout the year consists of evergreen forests.
(c) The area with heavy rainfall during summer and low during winter consists of grasslands.
(d) The area with heavy rainfall during winter and low during summer consists of sclero-phyllous forests (The plants are shrubs stunted in height with leathery, thick evergreen leaves).
- (ii) Light : The radiant energy of sunlight carries out all important functions, without this life except few bacteria would disappear. On this basis of relative light requirements and the effect of light on the overall vegetative development, plants are classified ecologically into following categories :
(a) Heliophytes : These plants grow best in full sunlight. In these plant internodes are short, leaves are small narrow, thick and with cuticle and hair.
(b) Sciophytes : These plants grow best in lower light intensity. In these plant internodes are long, leaves are large broad and thin, leaf surface is dull.
The plants grow in total darkness are called etiolated (Long thin, weak and yellow in colours).
- (iii) Temperature : Temperature had a marked effect on the growth of plants. Due to high temperature plants suffer solarization and due to low temperature plants suffer freezing or frost injury.
On the basis of temperature the plants are classified as below :
(a) Megatherms or Climate or Tropical : The vegetation growing in the condition in which high temperature prevails throughout the year (30-40°). The dominant vegetation is tropical rain forest. They are in South America (on the side of Amazon river), Middle Africa (on the side of Congo River) and S.E. Asia.
The effect of altitude and latitude is similar upon plant vegetation. It is due to similar change of temperature is these two places.
The tropical rain forests are the most dense forests of the world.
(b) Mesotherms : Climate-subtropical, the high and low temperature alternates. The dominant vegetation is tropical decidous forest type. Those plants in which leaf fall takes place once in a year are called decidous plants e.g. Ficus religiosa (Sacred tree).
(c) Microtherms : The vegetation growing in the low temperature (10-20°C) condition. (The temperature remains low throughout the year). The vegetation is mixed coniferous forests type (Teiga).
(d) Hekistotherms : The vegetation growing in the very low temperature (0-10°C) conditions. The dominant vegation is Alpine vegetation (Tundra).
The plants growing at very low temperature are called cryophytes or psychrophytes.
- (iv) Wind : High wind velocity causes soil erosion, breakage and up rooting of trees. Most of the pollutants are dispersed through the medium of air. Wind do harm is blossom trees because it prevents working of insects.
Wind modifies the humidity. Dry winds cause dwarfing of plants. Wind helps in pollination, dispersal of fruits and seeds and prevents frost damage. If the areas subjected to strong winds the leaves of plants become small and rolled and these plants develop an overall shape that offer resistance to wind.
Sometimes shrubs and trees are planted to protect the field against wind. Such structures are known as wind breaks or shelter belts. These plants (Trees) are planted at 90° to the wind velocity.
Absolute and relative humidity vary with changes in temperature. Absolute humidity is maximum near equator and gradually decreases towards the poles. This indicates that relative humidity is affected by temperature as well as latitude. Temperature, rainfall and humidity are three major factors which effect and control the climate.
# (2) Topographic or Geographic factor : Topographic factors are concerned with the physical geography of the earth in an area. The chief topographic factors are as follows :
Micro climate refers to local combinations of factors such as wind, rate of evaporation, humidity, temperature which differ from regional climate.
- (i) Altitude : Height of mountain chains. With the increase in altitude climate changed as decrease in temperature, increase in humidity, increase in precipitation and increase in wind velocity.
Earth’s vegetation can be divided into different zones based on altitude :
(a) Upto 1800 feet : Tropical rain forest.
Tropical moist deciduous with 1000-1500 mm rainfall.
Tropical moist evergreen with 2500 mm rainfall.
(b) 1800-4000 feet : Grassland or desert, savanna.
(c) 4000-7500 feet : Temperate deciduous forest. Oak is common.
(d) 7500-12000 feet : Coniferous forest (Temperate evergreen forests) e.g., Pinus, Abies, Picea.
12000 feet is regarded as tree or timber line. Above this height of plants decreases.
(e) 12500-14500 feet : Alpine vegetation (tundras) Rhododendron, Betulla. Cushion shaped dwarf shruby vegetation.
(f) 145000 upward : Snow line.
Generally the vegetation that develops on base of mountain to top is Tropical Temperate Taiga Tundra. Species diversity generally increase as one proceeds from high altitude to low altitude and from high latitude to low latitude.
- (ii) Steepness of the slopes : Steep slopes cause fast running of water which result in erosion and do not permit the accumulation of humus so the soil becomes denuded. In such soil plants can not grow properly and vegetation changes to xerophytic plants.
- (iii) Exposure of slopes : Exposure of slope to sun and wind affects very much the kind of plants growing there. Generally the slopes exposed to sun and wind supports vegetation. That’s why green houses and hot beds are always built in a way to face sun or southern slopes which receive greater amount of solar energy.
- (iv) Direction of mountains chain : Mountains steer or deflect winds into different dierctions. Outer Himalayas show frequent rains with luxurient vegetations while the middle and inner Himalayas are dry with poor vegetation. The southern slopes of Himalayas e.g. Kullu valley are directly exposed to sunlight and has luxurient mesophilous vegetation due to mansoon wind. Where as Northern slopes of Himalayas e.g. Lahul valley exposed to weak light and strong dry wind, thus they have xerophilous vegetation.
# (3) Edaphic factor : The study of soil is called edaphology or pedology. The soil can be defined as “the upper crust of earth surface in which plants roots are anchored.” The term soil is derived from the Greek work solum.
- (i) Soil formation : It is derived from rocks by weathering which is of three types :
(a) Chemical weathering : It is caused by oxidation, hydrolysis or carbonation.
(b) Mechanical weathering : It is caused by living organisms, e.g. lichens, grazing animals or earthworm.
Weathering results into conversion of rocks to small fragments. Humus accumulated and now this can be called as soil. The development of soil is called pedogenesis. Soil is of two types :
Residual soil : If the soil remain at the same place where it is formed.
Transported soil : This soil brought from their place of origin to other place by some agents. It may be :
Alluvial soil : Carried by running water (rivers).
Colluvial soil : Carried by gravity.
Eolian soil : Carried by wind.
Glacial soil : Carried by glacier.
- The soils of planes of India is mainly alluvial. In India the principal residual soil types are :
Reddish soil of Vindhyas and South.
Black soils of South West India.
Calcareous soil : With 20% CaCO3.
Laterite soil : Oxides of iron and aluminium.
Peat soil : With high percentage of humus 90%.
Black soil : Predominantly with clay and humus (very fertile because most of minerals are present in it).
- (ii) Soil profile : A fully formed soil shows different layers called horizons. The sequence and nature of these layers is called soil profile (Cross section of soil) which consist of following horizons.
(a) Horizon ‘O’ : It is uppermost horizon made of organic matter. It has both fresh or nondecomposed as well as partially decomposed matter. It consist of following two sub-layers :
O1 region (Aoo) : It is uppermost layer which consists of freshy added organic matter such as dead leaves, branches, flowers and fruits.
O2 region (Ao) : It is present below O1 region. It consists of organic matter which is in different stages of decomposition.
(b) Horizon ‘A’ : It is rich in mineral elements. A large amount of completely decomposed organic matter is present in this region. It shows downward loss of soluble salts clay, aluminium and iron. So this region is called zone of leaching or zone of eluviation.
(c) Horizon ‘B’ : It is dark in colour due to accumulation of leached substances like clay, iron and aluminium from horizon. So it is called as zone of accumulation or zone of illuviation.
Horizon ‘O’, A and B are together called as top soil.
(d) Horizon ‘C’ : It consists of partially weathered parental rock material. It is called as sub soil.
(e) Horizon ‘R’ : It is the lowermost layer of soil which consist of bed rocks (unweathered).
- (iii) Composition of soil : The garden soil is made up of :
(a) Mineral matter (40%) : They are derived from rocks (by disintegration). The soil, derived from lime stone, is called chalky soil.
Size of mineral particles : Depending upon their size, the mineral particles are of following types :
Coarse gravel : More than 5.0 mm.
Fine gravel : 5.0 to 2.0 mm.
Coarse sand : 2.0 to 0.02 mm.
Slit : 0.02 to .002 mm.
Clay : Less than 0.002 mm.
Sandy soils have more coarser particles and lower water holding capacity and better aeration. Sand is most porous. Clayey soils have fine particles which have high water holding capacity and very poor aeration. Clay is least porous (water logged). Loam (50% sand + 25% clay + 25% slit) are best for plant growth.
The best apparatus used to analyse the soil is sieving.
(b) Organic matter : Humus is total organic matter in the soils. It is rich in N P K. The humus is formed from decay and decomposition of dead plant and animal matter. It is in colloidal state and increase water holding capacity of the soil. The formation of humus is called humification which is caused by microbial activity. The humus soil is the best soil as it has got high water holding capacity, high porocity, aeration and high organic matter content. The complete decomposition of humus forms minerals. The formation of minerals is called mineralization. So, humus is the secondary source of minerals in the soil.
The three distinct layers of humus in soil of forests are :
Litter : All dead fresh organic matter fallen (undecomposed) recently to the ground is called litter.
Duff : The layer, where decomposition is just started, is called as duff as duff layer. Partially decomposed litter is called duff.
Leaf mold or Real humus : When the litter is modified into dark, finely divided, amorphous organic matter by the activities of micro-organisms living in soil is called humus. Humus is maximum in peat soil (90%).
(c) Soil solution : The soil solution is the primary source of inorganic nutrients for plants. Soil solution helps in exchange of ions. pH of fertile soil is 6 to 7. pH below 5 inhibits bacterial activity. The plants prefer to grow in acidic soil are called oxylophytes e.g. Drosera. The plants prefer to grow in alkaline soil are called halophytes.
The soil rich in nutrients is called eutrophic and soil with less amount of minerals is called as oligotrophic.
(d) Soil air : 20-25% air or O2 is necessary for proper growth of plants. The well aerated soil support the plant growth well because :
Root respiration increases.
The capillary potential of the soil increases.
The accumulation of CO2 could not take place.
The root growth increases.
Poor soil aeration supresses root hair development and may reduce the rates of absorption of water and minerals.
(e) Soil micro-organims : Soil has its own distinctive flora and fauna (bacteria, algae, fungi, protozoans, nematods, earth worms, molluscs, insects etc.) which make the biological system of the soil complex.
These organisms play following important roles in the soil :
Nitrogen fixation
Mycorrhizal association
Soil borne diseases
Decomposition of organic matter.
# (4) Biotic factor : Biotic factor means the effect of living organism upon other living organism.
In natural conditions organisms live together and influence each others life i.e. show interactions. Interactions may be :
(i) Positive interactions
(a) Mutualism or Symbiosis : An association of two organism in which both partners are benefitted, (but can not live separately) e.g. lichens, mycorrhiza, symbiotic nitrogen fixers, pollination, Zoochlorella and Zooxanthallae. Ruminant mammals have flagellates and bacteria for cellulose digestion which obtain food and shelter.
(b) Commensalism : It is the relationship between two living individuals of different species in which one is benefitted while the other is neither harmed nor benefitted except to negligible extent. e.g. epizoic algae, epiphytes and parasitic vascular plants. Jackals follow a lion or tiger while arotic fox follows a seal for obtaining food from pieces or bits left by the predators.
(c) Protoco-operation : It is interaction between two living organism of different species in which both are mutually benefitted but they can live without each other. e.g. tick bird ox pecker and Rhinoceros.
(ii) Negative interactions
(a) Deforestation : By deforestation the land is exposed to erosion and desertification. Deforestation in catchment areas causes floods in plains. Deforestation reduces the chances of rainfall.
(b) Competition : Competition can be defined as the rivalry between two or more organism for obtaining the same resources. Competition is greatest between the individual of the same species (intraspecific) which make similar demands upon the same supply at the same time in same area (niche).
Gause’s hypothesis or Principle of competitive exclusion : Gause noted that out of two species of Paramecium grown together, one is eliminated. It is called Gause’s hypothesis.
(c) Grazing : Constant grazing does not permit the plants to grow. Heavy grazing results in soil erosion (sheet erosion). Selective grazing and browsing a responsible for marked changes in vegetation. The only way grazing animals help vegetation is that they add their excrete to the soil.
(d) Fire : Most of the fires are of biological origin. Fires are mostly man caused. Sometimes fire develops due to mutual friction between tree surfaces in forests.
Fire destroy plant communities. A number of grasses are stimulated by a fire or in a burnt forest the grasses first and luxuriently.
(e) Exploitation : One species harm another species by making its direct or indirect use.
Parasitism : The parasite grow on other living organism for food and support called host e.g. Cuscutta, Orobanche.
Predation : It is an association between members of two species in which members of one species capture, kill and eat up members of other species. The former is called predator and later is called prey. A predator is a free living animal which catches and kills another species for food. Most of the predators are animals but sometimes some plants such as insectivorous plants and some fungi (predaceous fungi or animal traping fungi) e.g. Datylella, Arthrobotrys. They feed on small insects, protozoans and nematodes.
(f) Antibiosis or Amensalism : One organism inhibits the growth of other organisms through the secretion of antibiotics. It is common in micro-organism. This is based on biological antagonism. It is also called allelopathy. Smoother crops (e.g. Barley, rye, millets, alfalfa sun flower) are those which do not allow weeds to grow near by.
The environment is the aggregate of all those things and set of conditions which directly or indirectly influence not only the life of organisms but also the communities at a particular place. The environmental conditions which influence the life and development of plants are grouped into four main classes (ecological factors) which are as follows :
# (1) Climatic factors : The study of climatic factor is known as climatology. The chief climatic factors are :
- (i) Water : Rainfall is the chief source of soil moisture. Water exchange between earth surface and atmosphere is called hydrological cycle. Humidity of the air is expressed in terms of relative humidity. It is measured by hygrometer (Psychrometer). Epiphytes and cryoptogamic plants grow in those regions where relative humidity is high.
Annual rain fall determines the types of vegetation in any region such as :
(a) The area where rainfall is scanty are seen with deserts and xerophytic vegetation.
(b) The tropical area with heavy rainfall throughout the year consists of evergreen forests.
(c) The area with heavy rainfall during summer and low during winter consists of grasslands.
(d) The area with heavy rainfall during winter and low during summer consists of sclero-phyllous forests (The plants are shrubs stunted in height with leathery, thick evergreen leaves).
- (ii) Light : The radiant energy of sunlight carries out all important functions, without this life except few bacteria would disappear. On this basis of relative light requirements and the effect of light on the overall vegetative development, plants are classified ecologically into following categories :
(a) Heliophytes : These plants grow best in full sunlight. In these plant internodes are short, leaves are small narrow, thick and with cuticle and hair.
(b) Sciophytes : These plants grow best in lower light intensity. In these plant internodes are long, leaves are large broad and thin, leaf surface is dull.
The plants grow in total darkness are called etiolated (Long thin, weak and yellow in colours).
- (iii) Temperature : Temperature had a marked effect on the growth of plants. Due to high temperature plants suffer solarization and due to low temperature plants suffer freezing or frost injury.
On the basis of temperature the plants are classified as below :
(a) Megatherms or Climate or Tropical : The vegetation growing in the condition in which high temperature prevails throughout the year (30-40°). The dominant vegetation is tropical rain forest. They are in South America (on the side of Amazon river), Middle Africa (on the side of Congo River) and S.E. Asia.
The effect of altitude and latitude is similar upon plant vegetation. It is due to similar change of temperature is these two places.
The tropical rain forests are the most dense forests of the world.
(b) Mesotherms : Climate-subtropical, the high and low temperature alternates. The dominant vegetation is tropical decidous forest type. Those plants in which leaf fall takes place once in a year are called decidous plants e.g. Ficus religiosa (Sacred tree).
(c) Microtherms : The vegetation growing in the low temperature (10-20°C) condition. (The temperature remains low throughout the year). The vegetation is mixed coniferous forests type (Teiga).
(d) Hekistotherms : The vegetation growing in the very low temperature (0-10°C) conditions. The dominant vegation is Alpine vegetation (Tundra).
The plants growing at very low temperature are called cryophytes or psychrophytes.
- (iv) Wind : High wind velocity causes soil erosion, breakage and up rooting of trees. Most of the pollutants are dispersed through the medium of air. Wind do harm is blossom trees because it prevents working of insects.
Wind modifies the humidity. Dry winds cause dwarfing of plants. Wind helps in pollination, dispersal of fruits and seeds and prevents frost damage. If the areas subjected to strong winds the leaves of plants become small and rolled and these plants develop an overall shape that offer resistance to wind.
Sometimes shrubs and trees are planted to protect the field against wind. Such structures are known as wind breaks or shelter belts. These plants (Trees) are planted at 90° to the wind velocity.
Absolute and relative humidity vary with changes in temperature. Absolute humidity is maximum near equator and gradually decreases towards the poles. This indicates that relative humidity is affected by temperature as well as latitude. Temperature, rainfall and humidity are three major factors which effect and control the climate.
# (2) Topographic or Geographic factor : Topographic factors are concerned with the physical geography of the earth in an area. The chief topographic factors are as follows :
Micro climate refers to local combinations of factors such as wind, rate of evaporation, humidity, temperature which differ from regional climate.
- (i) Altitude : Height of mountain chains. With the increase in altitude climate changed as decrease in temperature, increase in humidity, increase in precipitation and increase in wind velocity.
Earth’s vegetation can be divided into different zones based on altitude :
(a) Upto 1800 feet : Tropical rain forest.
Tropical moist deciduous with 1000-1500 mm rainfall.
Tropical moist evergreen with 2500 mm rainfall.
(b) 1800-4000 feet : Grassland or desert, savanna.
(c) 4000-7500 feet : Temperate deciduous forest. Oak is common.
(d) 7500-12000 feet : Coniferous forest (Temperate evergreen forests) e.g., Pinus, Abies, Picea.
12000 feet is regarded as tree or timber line. Above this height of plants decreases.
(e) 12500-14500 feet : Alpine vegetation (tundras) Rhododendron, Betulla. Cushion shaped dwarf shruby vegetation.
(f) 145000 upward : Snow line.
Generally the vegetation that develops on base of mountain to top is Tropical Temperate Taiga Tundra. Species diversity generally increase as one proceeds from high altitude to low altitude and from high latitude to low latitude.
- (ii) Steepness of the slopes : Steep slopes cause fast running of water which result in erosion and do not permit the accumulation of humus so the soil becomes denuded. In such soil plants can not grow properly and vegetation changes to xerophytic plants.
- (iii) Exposure of slopes : Exposure of slope to sun and wind affects very much the kind of plants growing there. Generally the slopes exposed to sun and wind supports vegetation. That’s why green houses and hot beds are always built in a way to face sun or southern slopes which receive greater amount of solar energy.
- (iv) Direction of mountains chain : Mountains steer or deflect winds into different dierctions. Outer Himalayas show frequent rains with luxurient vegetations while the middle and inner Himalayas are dry with poor vegetation. The southern slopes of Himalayas e.g. Kullu valley are directly exposed to sunlight and has luxurient mesophilous vegetation due to mansoon wind. Where as Northern slopes of Himalayas e.g. Lahul valley exposed to weak light and strong dry wind, thus they have xerophilous vegetation.
# (3) Edaphic factor : The study of soil is called edaphology or pedology. The soil can be defined as “the upper crust of earth surface in which plants roots are anchored.” The term soil is derived from the Greek work solum.
- (i) Soil formation : It is derived from rocks by weathering which is of three types :
(a) Chemical weathering : It is caused by oxidation, hydrolysis or carbonation.
(b) Mechanical weathering : It is caused by living organisms, e.g. lichens, grazing animals or earthworm.
Weathering results into conversion of rocks to small fragments. Humus accumulated and now this can be called as soil. The development of soil is called pedogenesis. Soil is of two types :
Residual soil : If the soil remain at the same place where it is formed.
Transported soil : This soil brought from their place of origin to other place by some agents. It may be :
Alluvial soil : Carried by running water (rivers).
Colluvial soil : Carried by gravity.
Eolian soil : Carried by wind.
Glacial soil : Carried by glacier.
- The soils of planes of India is mainly alluvial. In India the principal residual soil types are :
Reddish soil of Vindhyas and South.
Black soils of South West India.
Calcareous soil : With 20% CaCO3.
Laterite soil : Oxides of iron and aluminium.
Peat soil : With high percentage of humus 90%.
Black soil : Predominantly with clay and humus (very fertile because most of minerals are present in it).
- (ii) Soil profile : A fully formed soil shows different layers called horizons. The sequence and nature of these layers is called soil profile (Cross section of soil) which consist of following horizons.
(a) Horizon ‘O’ : It is uppermost horizon made of organic matter. It has both fresh or nondecomposed as well as partially decomposed matter. It consist of following two sub-layers :
O1 region (Aoo) : It is uppermost layer which consists of freshy added organic matter such as dead leaves, branches, flowers and fruits.
O2 region (Ao) : It is present below O1 region. It consists of organic matter which is in different stages of decomposition.
(b) Horizon ‘A’ : It is rich in mineral elements. A large amount of completely decomposed organic matter is present in this region. It shows downward loss of soluble salts clay, aluminium and iron. So this region is called zone of leaching or zone of eluviation.
(c) Horizon ‘B’ : It is dark in colour due to accumulation of leached substances like clay, iron and aluminium from horizon. So it is called as zone of accumulation or zone of illuviation.
Horizon ‘O’, A and B are together called as top soil.
(d) Horizon ‘C’ : It consists of partially weathered parental rock material. It is called as sub soil.
(e) Horizon ‘R’ : It is the lowermost layer of soil which consist of bed rocks (unweathered).
- (iii) Composition of soil : The garden soil is made up of :
(a) Mineral matter (40%) : They are derived from rocks (by disintegration). The soil, derived from lime stone, is called chalky soil.
Size of mineral particles : Depending upon their size, the mineral particles are of following types :
Coarse gravel : More than 5.0 mm.
Fine gravel : 5.0 to 2.0 mm.
Coarse sand : 2.0 to 0.02 mm.
Slit : 0.02 to .002 mm.
Clay : Less than 0.002 mm.
Sandy soils have more coarser particles and lower water holding capacity and better aeration. Sand is most porous. Clayey soils have fine particles which have high water holding capacity and very poor aeration. Clay is least porous (water logged). Loam (50% sand + 25% clay + 25% slit) are best for plant growth.
The best apparatus used to analyse the soil is sieving.
(b) Organic matter : Humus is total organic matter in the soils. It is rich in N P K. The humus is formed from decay and decomposition of dead plant and animal matter. It is in colloidal state and increase water holding capacity of the soil. The formation of humus is called humification which is caused by microbial activity. The humus soil is the best soil as it has got high water holding capacity, high porocity, aeration and high organic matter content. The complete decomposition of humus forms minerals. The formation of minerals is called mineralization. So, humus is the secondary source of minerals in the soil.
The three distinct layers of humus in soil of forests are :
Litter : All dead fresh organic matter fallen (undecomposed) recently to the ground is called litter.
Duff : The layer, where decomposition is just started, is called as duff as duff layer. Partially decomposed litter is called duff.
Leaf mold or Real humus : When the litter is modified into dark, finely divided, amorphous organic matter by the activities of micro-organisms living in soil is called humus. Humus is maximum in peat soil (90%).
(c) Soil solution : The soil solution is the primary source of inorganic nutrients for plants. Soil solution helps in exchange of ions. pH of fertile soil is 6 to 7. pH below 5 inhibits bacterial activity. The plants prefer to grow in acidic soil are called oxylophytes e.g. Drosera. The plants prefer to grow in alkaline soil are called halophytes.
The soil rich in nutrients is called eutrophic and soil with less amount of minerals is called as oligotrophic.
(d) Soil air : 20-25% air or O2 is necessary for proper growth of plants. The well aerated soil support the plant growth well because :
Root respiration increases.
The capillary potential of the soil increases.
The accumulation of CO2 could not take place.
The root growth increases.
Poor soil aeration supresses root hair development and may reduce the rates of absorption of water and minerals.
(e) Soil micro-organims : Soil has its own distinctive flora and fauna (bacteria, algae, fungi, protozoans, nematods, earth worms, molluscs, insects etc.) which make the biological system of the soil complex.
These organisms play following important roles in the soil :
Nitrogen fixation
Mycorrhizal association
Soil borne diseases
Decomposition of organic matter.
# (4) Biotic factor : Biotic factor means the effect of living organism upon other living organism.
In natural conditions organisms live together and influence each others life i.e. show interactions. Interactions may be :
(i) Positive interactions
(a) Mutualism or Symbiosis : An association of two organism in which both partners are benefitted, (but can not live separately) e.g. lichens, mycorrhiza, symbiotic nitrogen fixers, pollination, Zoochlorella and Zooxanthallae. Ruminant mammals have flagellates and bacteria for cellulose digestion which obtain food and shelter.
(b) Commensalism : It is the relationship between two living individuals of different species in which one is benefitted while the other is neither harmed nor benefitted except to negligible extent. e.g. epizoic algae, epiphytes and parasitic vascular plants. Jackals follow a lion or tiger while arotic fox follows a seal for obtaining food from pieces or bits left by the predators.
(c) Protoco-operation : It is interaction between two living organism of different species in which both are mutually benefitted but they can live without each other. e.g. tick bird ox pecker and Rhinoceros.
(ii) Negative interactions
(a) Deforestation : By deforestation the land is exposed to erosion and desertification. Deforestation in catchment areas causes floods in plains. Deforestation reduces the chances of rainfall.
(b) Competition : Competition can be defined as the rivalry between two or more organism for obtaining the same resources. Competition is greatest between the individual of the same species (intraspecific) which make similar demands upon the same supply at the same time in same area (niche).
Gause’s hypothesis or Principle of competitive exclusion : Gause noted that out of two species of Paramecium grown together, one is eliminated. It is called Gause’s hypothesis.
(c) Grazing : Constant grazing does not permit the plants to grow. Heavy grazing results in soil erosion (sheet erosion). Selective grazing and browsing a responsible for marked changes in vegetation. The only way grazing animals help vegetation is that they add their excrete to the soil.
(d) Fire : Most of the fires are of biological origin. Fires are mostly man caused. Sometimes fire develops due to mutual friction between tree surfaces in forests.
Fire destroy plant communities. A number of grasses are stimulated by a fire or in a burnt forest the grasses first and luxuriently.
(e) Exploitation : One species harm another species by making its direct or indirect use.
Parasitism : The parasite grow on other living organism for food and support called host e.g. Cuscutta, Orobanche.
Predation : It is an association between members of two species in which members of one species capture, kill and eat up members of other species. The former is called predator and later is called prey. A predator is a free living animal which catches and kills another species for food. Most of the predators are animals but sometimes some plants such as insectivorous plants and some fungi (predaceous fungi or animal traping fungi) e.g. Datylella, Arthrobotrys. They feed on small insects, protozoans and nematodes.
(f) Antibiosis or Amensalism : One organism inhibits the growth of other organisms through the secretion of antibiotics. It is common in micro-organism. This is based on biological antagonism. It is also called allelopathy. Smoother crops (e.g. Barley, rye, millets, alfalfa sun flower) are those which do not allow weeds to grow near by.