Factors which have some effect on the life of an organism at some stage in its development are called environmental factors. These can be divided into groups as follows: first, a climatic group, which includes conditions of light, temperature, water availability and wind; second, topographic influences of slope angle, orientation and altitude; third, edaphic(soil) factors, especially pH and fertility; and fourth, biotic controls, such as a competition between species and the effects of grazing.
These groups are themselves interrelated so that it is extremely difficult to isolate the influence of individual factors. For example, topography and climate will influence soil development; and climate and soil will influence the pattern of biotic controls by determining the species which may inhabit a particular place and compete there for survival.
Lightis extremely important environmental factor because it is the vital source of energy for ecosystems and it can also act as a control of functions such as reproduction and migration. Excess light can be a limiting factor in ecosystem development by damaging plant tissues and decreasing productivity.
The influence of light varies with its three main aspects: its quality (that is, wavelength composition), its intensity and its duration (day length).
Temperature is a universally important environmental factor both for its direct effects on organisms and for its indirect effects in modifying other factors such as relative humidity and water availability. Each species has its own minimum, maximum and optimum temperatures for life but the actual limits at any time vary with such things as the age of the individual and water balances in the body. Generally, aquatic plants and animals have narrower tolerance ranges for temperature than those which live on land. This is mainly because there is far more temperature variation in terrestrial ecosystems.
Water availability may often restrict ecosystem development because most organisms need large amounts of water to survive. It not only forms a large percentage of the tissues in plant and animal bodies but it is also essential for transport and cooling. In plants, water provides support and is essential for photosynthesis.
Distributions of plants may largely depend on the effectiveness of precipitation; this will be a function of the kind of precipitation, the type of vegetation present and the rate of evaporation. In many areas fog or dew is important in providing essential moisture for plant growth and thus extending the distribution ranges of species.
In the case of animals, water usually only acts as a limiting factor when it is in short supply. There is a great variation in the amounts of water needed different species but usually cold-blooded animals require less than warm-blooded ones, which use it for heat regulation. Some animals display specific adaptations for survival in arid habitats. Desert animals may avoid the hottest and driest season by becoming inactive – that is, aestivating.
Wind can act as an environmental factor either directly by causing mechanical damage to plants or indirectly by affecting relative humidity and evaporation rates. High wind velocities can cause an appreciable increase in the rate of transpiration and limit plant growth. In very exposed situations such as mountain summits, coasts and open plains vegetation may be dwarfed as a result of wind action.
Topography can influence ecosystem development in three major ways. First, by the direct effects of altitude on temperature. Temperature decreases as altitude increases either at the dry adiabatic lapse rate (10°C/km) or, more usually, at a lower rate than this, approximately 6°C/km. Second, topography can act indirectly, since temperature changes affect relative humidity. The combination of changes in temperature and relative humidity leads to the development of an altitudinal zonation of ecosystems. At a low level, desert merges into pine forests, which are succeeded by fir and spruce, and then by alpine communities at the highest altitude.
The third way in which topography can influence ecosystem development is by local variation in slope orientation and angle. South-facing sides of valleys receive strong incident light (in the northern hemisphere) and are therefore warmer and drier than north-facing slopes which are in the shadow for a lot of the time. This leads to great contrasts in species structure and productivity between sides of valleys. Angle of slope will be a critical factor in soil formation and drainage.
The soil is a vital component of terrestrial ecosystems, particularly in cycling nutrients without which all life would cease. Soil and the rest of the ecosystem are closely related; one will influence the workings of the other. Particular attributes of soils, such as texture, pH, soil climate and organic content operate in a closely interrelated fashion to exert control on rates of decomposition, nutrient cycling and plant distribution and productivity.
Soil texture is very important in determining the soil climate, since it affects aeration, drainage and ease of root penetration.
Biotic factors are the interactions that occur between living things. Biotic factors are usually far more diverse and intricate than other environmental controls because they rely on the activities of a wide variety of organisms.
Most habitats can be occupied by many different types of plants and animals. The success of a particular species will depend on its ability to obtain its requirements for life. Competition arises if the resources of a habitat are insufficient to meet the demands of all the organisms living there. Generally competition is most intense between individuals of the same species or of different species that have similar ecological niches, especially at young stages in the life cycle.
Man is by far the most important biotic factor. He has caused fundamental modifications of ecosystems by fire, hunting and agriculture, man has obliterated large areas of natural systems and caused pollution of both terrestrial and aquatic habitats.
6. Read the text Modification of the Atmosphere through and make the review.