Read this article to learn about the different Conditions Affecting the Organisms in Natural Ecosystems !
A number of environmental factors such as light, temperature, water, humidity, currents and pressures, general weather conditions, soil and fire (all physical factors), and atmospheric and dissolved gases, pH, nutrients and food (all chemical factors), and biotic factors (plants and animals of the same and other species ) effect the distribution and abundance of organisms in different habitats.
The two concepts combined form the Liebig- Blackman law. Any condition or factor that affects an organism by exceeding the limits of tolerance is called a limiting factor.
A combination of Liebig-Blackman law of minimum and Shelford’s law of limits of tolerance gives us a combined concept of limiting factors which is a better expression of the environmental conditions affecting the organisms in natural ecosystems.
1. Light as an Environmental Factor:
Light is defined as the radiation or brightness that enables things to be seen. The sun is the major source of light on earth. Solar radiation can be described in terms of its energy content and its wavelength. The energy comes in discrete packets called photons (Gk. photo, light).
So we can think of radiation as photons that travel in waves. The solar radiation (electromagnetic spectrum) can be divided on the basis of its wavelength. The energy content of photons is inversely proportional to the wavelength of the particular type of radiation, i.e., short wavelength radiation has photons of higher energy content than long wavelength radiation.
High-energy photons, such as those of the short wavelength ultraviolet radiation are dangerous to organisms because they can break down their cells and organic molecules. However, low energy photons, such as those of infrared radiation, do not damage cells. The visible light is considered most important because it is utilized in photosynthesis. Thus, visible light is only a small portion of the electromagnetic spectrum (380 n m to 750 n m). It is made up of a number of different wavelengths of radiation. When it is passed through a prism, we see these wavelengths as different colours of light.
Light and Plants:
Light affects plants of all habitats in many ways. The relationship of light intensity to photosynthesis in both terrestrial and aquatic plants follow the same general pattern of a linear increase up to an optimum or light saturation level. The intensity of light at which the rate of photosynthesis is just sufficient to meet the requirement of respiration is called compensation point.
The orientation of a plant towards light is called phototropism. Plants may be classified ecologically according to their requirements of light and shade; for example, long-day plants, short-day plants, and day-neutral plants. Sciophytes, plants growing in shady places, usually have large leaves, which are thin in texture and are sparsely distributed on the stem.
In heliophytes, plants that can grow well in the light, leaves are small, thick and crowded together on the stem. Variation in light and rate of transpiration produce marked differences in leaf structure. In sun exposed plants, leaves have thicker cuticle and cell walls. In sea, plants are restricted to euphotic zone due to their dependence upon light for energy.
Phytoplankton exhibit diurnal movement in response to light, moving down during bright light of the day. Thus, light as an environmental factor affects the pattern of distribution of plants through its effect on photosynthesis, chlorophyll production, leaf structure, stomatal movement, transpiration, and production and growth of flowers, seeds and fruits.
Light and Animals:
Light affects metabolism of animals. The absence of light results in the loss of colour among the cave animals. Formation of pigments depends on light. The role of pigmentation and protective colouration is well known in many terrestrial animals such as chameleon, leaf insect (Phyllium), stick insect (Caracisus), and deadleaf butterfly (Kallima paralecta). Many inhabitants of caves and deep sea, where light has no ecological significance, have vestigial eyes or are blind. In certain lower animals, the speed of locomotion is regulated by light.
This phenomenon is called photokinesis. The response of an organism to the duration of light and day-length is known as photoperiodism. It plays an important role in the life cycle of many animals. Metamorphosis in insects and wing production in aphids are affected by day-length. Salmon larvae undergo normal development only when they get sufficient light. Some fish can be induced to breed earlier by artificially changing day-length. Light also affects the movement and migration of honeybees, locusts and birds. The Phenomenon of diapause in insect eggs and pupae, which means spontaneous arrest of development at any stage, is generally triggered by changing photoperiod.
The fact that numerous species of insects interrupt the development of the young stages in summer, despite the apparently favourable temperature and abundance of food, and enter a resting phase (diapause) can definitely be atributed to the influence of light. In diapausing state insects have highly reduced respiratory rates, racous body fluids, and antifreeze substances for winter diapause. Some herbivorous insects may be in diapause for about 10 months of the year, only becoming active when their host plant is available. Thus, diapause serves to synchronise the whole life cycle of the insect with the environmental conditions.
The endogenous rhythms of animals are also affected by light. In many countries now light therapy is being used to cure endogenous depression and the use of light therapy may even prove useful in controlling reproduction.
2. Temperature as an Environmental Factor:
Temperature is defined as the degree or measure of heat or cold. Temperature is the most important physical factor of the environment and exerts its influence on plants and animals in various ways. In poikilothermic (Gk. poikilos, changeable) animals, the body temperature (Tb) depends on the temperature of environment. They have a variable Tb, whereas in homeothermic animals, such as birds and mammals, a constant Tb is maintained (37 to 40°C).
Ectotherms have a Tb, which is mainly dependent on extemal heat sources, whereas endotherms have a Tb, which depends on their internally generated metabolic heat. However, ectotherms can be thermoregulators or thermoconformers, but endotherms are always thermoregulators and use both behavioural and physiological mechanisms to regulate their Tb. The terms stenothermal and eurythermal are used to describe narrow and wide range of temperature tolerance respectively.
Temperature and Metabolism:
All metabolic processes are controlled by temperature. In many organisms, especially poikilothermic animals, the rate of metabolism increases with increasing temperature up to a certain limit. Each animal has a lower and upper limit of temperature tolerance. An ‘optimum’ temperature is required for the activity of most enzymes. Sudden changes in temperature affect the rate of respiration and other activities in crustacea and other animals.
Temperature and Development:
An optimum temperature is necessary for the growth and development to proceed at a normal rate. The life history of the dragonfly, Tetragoneuria, is so regulated by temperature that at one place two years are required to complete the development, whereas at another place only one year. The ambient temperature also affects the absolute size as well as the relative proportions of certain body parts in many animals. For example, variation in the size of ears of arctic fox (Alopex lagopus), red fox (Vulpes vulpes) and desert fox (Megalotis zerda) is indicative of the role of temperature in different climatic conditions. Since the heat is lost through the surface, the small ears of arctic fox help to conserve heat, whereas the large ears of desert fox help in heat loss and evaporation.
Temperature and Reproduction:
In many animals, the sex ratio is affected by the temperature of environment. In Daphina, under normal condition parthenogenetic eggs are produced, which develop into females. But when the temperature of environment is raised, they give rise to sexual eggs, which after fertilization may develop either into males or females. In certain cases, the temperature exerts a more severe limiting effect on organisms when the moisture is either very high or very low, than when it is moderate.
Similarly, moisture plays more critical role in the extremes of temperature. For example, the cotton boll weevil cannot develop if the relative humidity is less than 40% or more than 88%, no matter how favourable the temperature may be. Similarly, the animal remains dormant, regardless of the humidity, if the temperature is lower than 10° C or higher than 39° C.
Other Effects of Temperature:
Temperature affects animal behaviour and structure. Experiments show that Drosophila may undergo structural modifications at high temperatures. The vestigial wings of one of the mutants will develop into normal wings at high temperatures. In Drosophila, the temperature may affect the mechanism of heredity by affecting the position of genes and the behaviour of chromosmes. In higher plants, the development of cold tolerance is triggered by environmental signals such as temperatures of 0-5° C usually accompanied by decreasing day length.
Acclimatization may also increase the freezing tolerance in plants. During frost hardening many species of plants and animals accumulate compounds such as free amino acids, glycerol, sorbitol, that protect their membranes from the effects of dehydration. Insects also have strategies that allow them to survive during the low temperatures of winter. However, the ability of plants to acclimatize to higher temperatures appears to be less than to low temperatures.
3. Soil (Edaphic) Factor:
Soil as an ecological factor is of great significance, as it affords a medium for anchorage of plants and a depot of water and minerals. Soil also serves as habitat for numerous animals. It can be defined as the weathered layer of the earth’s crust with living organisms and their products of decay intermingled. Thus, soil is not only a factor of the environment of organisms but they produce it as well. Soil of any area is affected by climate as also by the vegetation growing on it. As a result, soils are distributed on a latitudinal pattern corresponding approximately to vegetation zones (Chapman and Reiss, 1995).
Soil is formed by disintegration and decomposition of rocks due to weathering (action of rain water, running streams, glaciers, wind, and temperature) and the action of soil organisms such as bacteria, fungi, protozoa, and earthworms, and also interactions of various chemical substances present in the soil.
Soil profile consists of several distinct horizontal layers often called soil horizons. The upper or A- horizon is the topsoil. It contains most of the organic matter consisting of litter and humus. It also has a zone of leaching through which dissolved materials move downward. The roots of small plants are embedded in topsoil. The next or B- horizon is composed of mineral soil (sub soil). The third or C- horizon represents the unconsolidated parent material. The last or D- horizon represents the rock or unmodified parent material. (Fig. 2.1).
The exact nature of the soil profiles depends upon the interaction of climatic and biotic factors, contour of the land, and the type of parent rock in the given area. Hence, the relative thickness of horizons varies from soil to soil. For example, soils formed in grasslands have a deep A- horizon made up from decaying grasses over many years. In forest soils, both the A and B horizons have enough nutrients to allow root growth. In tropical rain forests, the A- horizon is shallower and the B- horizon is deeper, signifying more extensive leaching.
Physico-Chemical Nature of Soil:
The physico-chemical composition of soil is an important factor, which determines the presence or absence of particular plants and animals. Physically, soils may be classified into (a) clay soil, containing more than 30 per cent of clay (size of particles less than 0.002 mm in diameter); (b) sandy soil, containing 10 per cent each of clay and silt (particle size 0.05-0.002 mm) with a large amount of sand (particle size 0.05-1.00 mm); and (c) loam, containing about 30 per cent each of clay and silt, the rest being sand. Loam is considered the best soil for plant growth and is exploited for agriculture.
Humus of the soil is of considerable importance to both plants and animals. It consists of organic matter in various stages of decomposition, usually present in the topsoil. Soil microorganisms act upon the nitrogenous organic compounds of humus, converting them into nitrates, which are absorbed by plants. Thus, humus is the source of plant food and the seat of most of the bacteria in the soil. Humus combined with finest clay particles forms the colloidal complex of soil.
Chemically, the soil contains a variety of inorganic salts or soil nutrients such as chlorides, nitrates, sulphates, phosphates and carbonates of Na, Ca, Mg, K, and Fe. Soil also contains trace elements like Mn, Cu, Zn, Al, Mo, and B. The minuteness of amounts of these trace elements involved in plant growth may be illustrated by the fact that 1 part in 200 million parts of Zn will have a detectable effect. Because the quantity of some elements like P, N, and K is the limiting factor in agricultural soils, considerable success has been obtained by adding the mixtures of these elements in the form of potassium phosphate and ammonium sulphate.
Besides these nutrient salts, a certain amount of organic compounds such as proteins and their decomposition products of plant or animal origin are present in the soil. Acidity or alkalinity of the soil is also an important limiting factor for organisms. For example, land snails do not occur in acidic soil; they are more common on soils rich in calcium because they need this material for their shells.
Peaty soils are not inhabited by animals for they are poorly aerated and more acidic. Earthworms prefer soil rich in humus. The amount of soil water is no less important. It determines the type of flora in a particular habitat, which in turn determines the type of that habitat. The soil air present in the interspaces of soil particles is very important as it helps in normal respiration of soil organisms.
Effect of Soil on Organism:
Soil types affect the distribution and abundance of organisms both directly and indirectly. The suitability of soil for agriculture depends on its physico-chemical nature. For example, soils containing a high proportion of sand are easily cultivated but they do not have water-holding capacity. As a result, water quickly drains from them into groundwater. Clay particles can hold water well, so there is little drainage from clay soil. Productivity is generally high in soils rich in humus and nutrients.
In desert soils, productivity is low due to lack of water and organic material. In wetlands, waterlogging of the soil may sometime cause problems as anaerobic conditions are produced. Mangroves get rid of this problem with the help of pneiunatophores and aerial roots, which can take in oxygen from the air.
The nature of soil in any habitat also affects the distribution of animals. Animals such as worms and termites mix up the different horizons. Many soil organisms like bacteria, fungi, protozoa, earthworms and rodents are useful agents in altering the soil. Some burrowing animals make the soil loose for better aeration and movement of water.
Soils are also affected by the vegetation growing on them as also by soil pollution. Pesticide pollution of agricultural soils kills many non-target organisms and disturbs the ecological balance. Persistent chemicals may remain in the soil for a long time and in turn several food chains may be adversely affected. Soil erosion also affects productivity adversely.