Environment: The distribution of plants and animals is not haphazard. The differences in the vegetation and species of different places are mainly due to the difference in the environment. Plants and animals do not live in a vacuum. They stand or walk upon the earth, drift or swim in the water or float or fly in the air. Everything about them is their environment. Closely related to the concept of environment is habitat by which is meant the particular place or situation in which a plant or animal lives or grows. Habitat is more specific in meaning than environment as a whole.
Broad aspects of the landscape such as water, soil, desert or mountain, the physical influences such as moisture, temperature and light, and the total living population of any area, i.e. interdependent animals and plants, constitute the environment. The totality of the physical environment is called an ECOSYSTEM and the total world of life is called as biosphere. Every organism within the biosphere affects the life of every other, directly or otherwise. Man, for examples cannot continue to live without the bacteria in the soil, the green plants on the land and in the sea, and even the scavengers of the dead. And what man does to the environment matters greatly to all other living organisms around him.
Environmental factors fall into two categories
(1) Physical or abiotic (non living) and
(2) Living or biotic factors
Biotic Factors of Environment
The plants and animals live together and influence each other’s life and also modify the environment. In fact, living organisms are most potent factors in the growth and development of each other. Some biotic influences, such as pollution, dissemination of fruits and seeds, grazing browsing, parasitism, symbiosis etc. are direct. The other influences like burrowing of soil by worms and rodents, decay of the dead organic matters by soil organisms etc. are indirect.
Plants compete with one another for light, water and mineral supply. Large trees change the underneath climate. It is more moist, moderately warm but receives less light. As a result, forests have many layers of plants with different height (stratification). In moist tropics many woody lianas climb up the tree trunks for exposing their foliage to the sunlight and interfere in normal photosynthesis of the trees. They may deform or even kill the tree. Parasitic plants rob host plants of nutrition and may also cause diseases. Some plants growing in nitrogen deficient areas (i.e. insectivorous plants) trap insects to obtain supply of nitrogen. Certain fungi assist higher plants in obtaining water and minerals in return for food and water. Certain bacteria and cyanobacteria fix atmospheric nitrogen, thus, improve the fertility of soil.
Only some perennial grasses can withstand both excessive grazing and trampling. Trampled soil become compact and does not contain much air and water. Many insects and other animals help in pollination and dissemination of fruits and seeds in the plants. Earthworms and other burrowing animals improve the aeration of the soil and bring fertile soil over the surface. Animal excreta is an important source of organic matter to the soil. The super animal, the man has also been changing the flora and fauna by introducing new plants and animals. Fires are often man made. They destroy vegetation and volatilize the minerals, present in the soil.
Abiotic Factors of Environment
Of the many physical factors affecting living organisms, only few will be considered. Some factors are most important than others; and organisms vary with respect in now much they are controlled or limited by the various physical factors.
Substratum : The earth, water, and the air, or the bodies of other plants and animals upon which organisms live, constitute a surface or a medium and may be given the general title of “substratum”.
Animals may have structural adaptations to the particular substratum on which they live. The streamline form of fish, seals, and whales, the flattened body of lice, the long legs of the wading bird, the sharp claws that enable squirrels to run up the trunks of trees, the well protected hoofs of horses, pigs camels and zebras, the hooks that enable the tapeworm to cling to the intestinal walls instead of being out with the moving stream of intestinal contents : all are successful adaptations to a variety of type of substratum.
While some organisms have become structurally adapted to their substratum, other have changed the substratum to fit themselves. Examples are the nests of the ants, the tubes of many marine annelids, and most obvious of all the streets, ‘bridges, tunnels, airplanes, buildings, and homes of man.
Substratum may be a limiting factor. An organism that has become well adapted to a specific type of substratum is limited in its distribution by the availability of that substratum. It is true, however, that organisms live in an environment where many factors are acting. It is most frequently true that animals and plants are limited not by a single but by a whole complex of factors.
Temperature: Most living organisms can survive only in a narrow range of temperature (59 — 35°C). However, there are notable exceptions to it. Certain bacteria, cyanobacteria (blue green algae), seeds, spores and encysted protozoans can occur in hot springs (60°C) — 90°C) or ice (30°C to 50°C).At high temperature, most organisms are killed due to denaturation of enzymes. Very low temperature also kills the organisms by freezing their body fluid. Therefore many organisms have developed physiological and behavioral adaptations to avoid extremes of temperature. For example:
(i) Some animals can live in extremely cold regions (e.g. polar bear). They hide and undergo hibernation (winter sleep) in frost free shelters such as caves, burrows etc. during winter. Some animals undergo aestivation (summer sleep) in hot and dry season.
(ii) Some birds and animals migrate to warmer places in winter to avoid extreme cold.
(iii) Many desert animals live in deep burrows to avoid the intense heat of the desert. They become active only at night (nocturnal), early morning (auroral) or evening (vesperal).
(iv) Plants also have adaptation for protection against extremes of temperature. These include development of thick cuticle, corky bar, dense hairy coat, thick leaves, mucilage, high solute content etc.
Temperature varies in various quarters of the earth according to latitude and altitude. It is also influenced by plant cover, atmospheric humidity, water reservoirs, air current and snow. According to the change of temperature with the increase of latitude, various vegetation zones have been recognized. Similarly, on the basis of change in temperature due to altitude, many vegetation zones can be observed. Changes in both, the latitude and the altitude, show more or less similar effect upon the type of major vegetations of the world.
Light: Light may be a limiting factor. The penetration of light into the ocean determines the depth at which green plants may grow. Since CO2, H2O and salts are present in abundance, light is the most important factor. Algae are the most abundant at the surface of the sea and decrease with increasing depth until at 200 feet the plant life is very sparse. And since less than 0.1 % of the light reaches below 600 feet, green plants are absent from the ocean depths of course, the absence of the plant means that animals that depend directly upon plants for their food supply are also absent. Only carnivores and scavengers live on the ocean bottom, and these feed on each other and on organic stuff which settles out from above.
Light affects animals indirectly though the fact that animals depend upon plants for their food. Light also affects animals in variety of other mechanisms. Exposure to ultra-violet radiation enables some animals to manufacture vitamin D. Light affects animals directly as they perceive their prey or detect their enemies through sense of light. Light sensitive organs are present in nearly all animal groups from the protozoans to the highest forms. Many animals have evolved structures or forms which enable them to escape detection by their enemies which hunt by means of sight. Examples are the walking — stick insect, the dead — leaf butterfly, and the various colour patterns on insects, birds, snakes and mammals.
Precipitation : It occurs in the form of rain, dew, hail or snow. Rain is the major source of water to the• earth. The water retained in the soil depends on a number of factors, such as soil constitution, steepness of the slope and the periodicity of the rain. Seasonal distribution and the total rainfall determine the type of vegetation in an area. Areas with heavy rainfall support luxuriant vegetation and abundant animal life. Scanty rainfalls support sparse xerophytic vegetation and scarce animal life. Rainfall; throughout the year produces tropical evergreen forests. Good rainfall, which occurs in one or two seasons produces deciduous forests. Moderate rainfall occurring in 1- 2 seasons produces grasslands, while low seasonal rainfall gives rise to deserts. Dew is a very good source of water to shallow rooted plants of arid area. Hail occurs in the form of ice balls and causes mechanical injury to the plants. Snowfall occurs in the cold regions. Duration of snowfall determines the type of vegetation. In arcaic and alpine regions, trees are absent. Only shrubs, herbs, mosses and lichens can grow because of snowfall.
Water: Most aquatic animals cannot live without a large supply of constantly renewed water. Such are most fish and other truly aquatic forms, whose respiration is adjusted for utilizing oxygen dissolved in water. These must live in large bodies of water or in swift streams. But certain aquatic types like lungfish, Salamanders, and crayfish survive temporarily drying by burrowing into the damp mud. Frogs spend much of their adult lives on land but cannot live very far away from the water, to which they must return to lay their eggs.
The reptiles are the first vertebrates to have evolved forward eliminating water as a limiting factor in their distribution. They lay land eggs and have a heavy scaly skin through which water cannot escape. Some reptiles (turtles and crocodiles) have secondarily returned to the water, but the reptile group as a whole achieved its early success through adaptations to lack of water, and the group now has a large number of the most successfully adapted desert animals. Like the desert animals, plants which live on a low- water economy have a relatively low metabolic rate, inspite of the high temperature. In the desert, however, although sunlight, C02, 02, soil minerals, and all other factors are present in abundance, water acts as a limiting factor in excluding most plants and in determining the extremely wide spacing of those plants able to survive the arid conditions.
Humidity : Humidity refers to the amount of water vapour present in the atmosphere. It controls the formation of clouds, dew, fog etc. It directly regulates the rate of water loss from the body surfaces of plants and land animals by transpiration and perspiration, respectively. In arid areas, low humidity increases water loss. Therefore, both plants and animals develop modification to reduce water loss in such areas. The high humidity decreases transpiration and therefore, more water becomes available for metabolism. This result in the production of luxuriant growth as is found in tropical rain forests. Epiphytic plants like orchids grow only in areas with high humidity such as tropical rain forests, because they get their water supply directly from air. Stratification or distribution of plants in the different strata in the forest is also controlled by the humidity in the different layers of the atmosphere. The plants which require the least atmospheric humidity grow into tall trees while those requiring high humidity constitute the ground flora.
Wind : The wind is a powerful agent influencing the growth and distribution of many organisms. Drying action of high, warm winds creates physiological draught. The wind removes humid air about the leaves and increases transpiration and ultimately the foliage turns brown. Strong and persistent winds blowing from a constant direction bend the branches of trees. Hurricanes and other violent windstorms sweeping over forested areas may uproot and break the trees. The hurricanes and strong storms cause deaths among animals also, and often carry individuals far from their normal environment and set them down elsewhere. Winds are an important means of dispersal for seeds and small animals such as spiders, mites, and even snails.
Soil: The soil is a radically different environment for life than the one above the surface, yet the essential requirements do not differ. Like animals that live outside the soil, soil fauna require living space, oxygen, food and water.
Soil may be defined as mineral material that may exist in solid or unbroken form, such as boulders, gravels, large outcrops, solid sills of stones, or as finely divided particles of mineral matter referred to as sands, silts, or clays, depending upon the texture.
The texture of a soil is determined by particle size and the proportional amount of these sized particles within any one soil sample. Particle size is determined by the diameter of individual soil fragments. According to the international system of soil classification, soil particles with a diameter larger than 2.0 mm are gravel soils, those with diameter ranging from 0.02 mm to 2.0 mm are sands, soil particles with diameter of 0.002 to 0.02 mm are silts. The finest textured soils, with particle diameter of less than 0.002 mm are CLAYS.
To the soil fauna the soil in general possesses several outstanding characteristics as a medium of life. It is relatively stable, both chemically and structurally. The atmosphere remains saturated, unit soil moisture drops below a critical point. The soil affords a refuge from high and low extremes in temperature, wind, evaporation, light and dryness. This permits soil fauna to make relatively easy adjustments to unfavorable conditions. The number of different species, representing practically every invertebrate phylum found in the soil is enormous. There are 250 species of protozoa alone in English soil. In the soil of beech woods in Austria live at least 110 species of beetles, 229 species of mites, 46 species of snails and slugs.