Subject: Environment and Ecology
Topic: Environmental Factors

We know that the physical environment consist of factors like air, light, heat, water, soil and wind. These abiotic factors determine the success of an organism through their effect of structure, life-history, physiology and behaviour. Growth and reproduction of an organism are also affected by biotic factors, which include all other organisms in the habitat. A brief account and role various factors in physical and biological processes is given below.

Atmosphere

Atmospheric layer: The atmosphere is divided into a series of concentric shells or spheres, due to variations in temperature and pressure at various altitudes. These spheres are: troposphere, stratosphere, mesosphere and thermosphere.

Troposphere: The lower portion of the atmosphere, extending to about 8-16 km height from the earth surface, is known as troposphere. It contains more than 90 per cent of gases in the atmosphere. Generally, temperature decreases with increasing height up to tropopause (top of troposphere).

Ecological Adaptations

The special characteristics of plants and animals that enable them to be successful under prevailing set of environmental conditions, are called adaptations. The organisms in natural world exhibit various types of morphological, physiological and behavioural adoptions. These special features have evolved over a long period of time, through the process of natural selection. The ultimate aim is to seek food and space the organisms need for their survival. The adaptive traits provide mechanisms for many organisms to live and thrive in different types of ecosystems and habitats.

Phenotypic Plasticity and Ecotypes

The phenotype is the physical expression of the interaction between genotype of an organism and its environment. The differences in the environmental conditions within the local habitat. Such variation among indivi-duals, produced by the influence of the local conditions of the habitat, is known as pheno-typic plasticity. Usually, species having a wide range of distribution evolve genetically adapted local populations, called ecotypes. Ecotypes differ from each other on the basis of morpho-logical and physiological characters. Although ecotypes of a species differ genetically, they are interfertile.

Strategies of Adaptation in Plants

Plant have special traits that help them to enhance their tolerance limits to light regimes, dry conditions, high temperature, water-saturated condition and saline environments. In plants, flower have evolved special struc-tures to ensure pollination by insects or other animals. Plants have developed various mechanisms to deal with strees condition of the environment.

Adaptations to light regime: Individual plants as well as plant communities, adapt to different light intensities by becoming shade to learn sciophytes or sun adapted (heliophytes). Heliophytes are adapted to high intensity of light, and have higher temperature optima for photosynthesis, as well as have high rates ore respiration. The shade adapted plants generally have low photosynthetic, respiratory, and metabolic activities. Plants such as ferns and several herbaceous plants growing on the ground under the dense canopy of trees, are shade tolerant plants.

Adoptions to water scarcity and heat: Plants of hot deserts are adapted to survive in dry conditions of soil and high temperature in dry conditions of soil and high temperatures. The plants which have a short life span are known as ephemerals. For example, in desert areas of Rajasthan, many annual plants germinate forms seeds, complete their life cycle quickly during the rainy season. Some plants have deep tap roots, which can reach even up to water table, in arid climates, and therefore, are capable of absorbing water from deep soil. Some prominent example of plants having deep root systems are Prosopis (mesquite), palms an some species of Acacia.

As in the case of cacti and succulents, the presence of fleshy leaves and stems to store water (succulence) is an adaptation to dry environments. In cacti, leaves are reduced to spines, whereas stems are modified into fleshy and spongy structures. Some cacti have expandable stems for storing water, and have spreading root systems in the surface layer of the soil. Many tropical plants, particularly grasses which grow in hot and arid climates, possess C4 pathway of photosynthesis. As you have learnt in Chapter 3, plants having this pathway perform better in low soil water environments. Such plants, therefore, use less water to achieve higher rates of photosynthesis, particularly at higher temperatures. Many desert plants, such as cacti and succulents, close their stomata during the day and open them in the night to reduce transpiration. You know from Chapter 3 that such plants possess CAM pathway of photosynthesis. Many xerophytes may accumulate proline (an amino acid) in response to stress. The heat shock proteins (chaperonins) provide physiological adaptations to plants to high temperatures. These proteins help other proteins to maintain their structure and avoid denaturisation at high temperatures.

Adaptations in aquatic environments: Plants which remain permanently immersed in water are called hydrophytes. They may by submerged, or partly submerged and show the presence of aerenchyma large air spaces) in the leaves and petioles. Aerenchyma helps to transport oxygen produced during photosyn-thesis and permits its free diffusion to other parts. These tissues also impart buoyancy to the plants. Presence of inflated petioles in Eichornia (water hyacinth) keeps the plants floating on the surface of water. Roots are poorly developed or absent in free floating hydrophytes like Wolffia, Salvinia, Ceratophyllum and Hydrilla.

Adaptation in saline environments: Halophytes are plants of saline environments, which are adapted to grown in high concentration of salt in soil of water. Halophytes occur in tidal marshes and coastal dunes, mangroves and saline soil. The halophytic plants, under hot and dry conditions, may become succulent and dilute the ion concentration of salts with water they store in cells of stems and leaves.

Mangroves are found in marshy conditions of tropical deltas and along ocean edges. Some species of mangroves can excrete salts through the salt glands on the leaves. Some mangroves can exclude salts from the roots by pumping excess salts back into soil. For coping with conditions of high salt concentration and osmotic potential, many mangrove plants have high levels of organic solutes, such as proline, and sorbitol. Dunaliella species (green and halophytic algae found in hyper saline lakes) can tolerate saline conditions by accumulating glycerol in the cells, which helps in osmore-gulation.

Avicennia and Rhizophora (red mangrove) are dominant species in mangrove forests. Since halphhytes are exposed to saline and anaerobic conditions in wetlands, they have developed special adaptations, like pneumato-phores, prop and stilt roots, and vivipary (seed germinate while on the tree), etc. The presence of pneumatophores (the respiratory roots) helps to take up oxygen from the atmosphere and transport it to the main roots. Prop and stilt roots, in many species of mangroves, give support to the plants in wet substratum.

Adaptations to oligotrophic soils: The oligotrophic soils contain low amounts of nutrients. These soils generally develop in old and geologically stable areas, such as soils found in much of the tropical rain forest region. Due to intense weathering and high rates of leaching, these soils have a poor nutrient retention capacity. In nutrient-poor soils, nutrient accumulation in vegetation is high. Many plants growing in nutrient-poor soils possess mycorrhizae, which have mutualistic association of roots with fungi. Mycorrhizae help in efficient absorption of nutrients (e.g., phosphorus). Mycorrhizae are to two types, endomycorrhizae and ectomycorrhizae. In endomycorrhizae, the fungal hyphae dwell inside roots. These types of Mycorrhizae are found in many vascular plants. In ectomy-corrhizae, the fungal mycelium forms a mat outside the root. Ectomycorrhizae occur in several tree and shrub species in temperature regions.

Strategies of Adaptations in Animals

Like plants, animals also adjust to different environmental conditions to survive and flourish. Carnivores and herbivores have flourish. Carnivores and herbivores have adap-tations to eat a certain kind of food. Some animals have adaptations to avoid being eaten by the predators; others have behavioral adaptations to attract a mate. The males of some animals (particularly the plumage of the birds) have bright colouration, which give advantage in sexual selection and mate attra-ction. However, a majority of animals adaptation to environmental variations and strees conditions are physiological and behavi-oural, a summarised in Table. Some of these adaptation are describe below.

Migration: Migration involves long-distance or short distance movement of animals from one region to another. Many organisms that fly or swim, undertake extensive migration. Activities of migration exhibited by the some animals are given in Table. Arctic terns are sea birds that make a round trip of thousands of miles between their North Atlantic and the Arctic breeding grounds to the Antarctica every year. In Africa, wild beasts migrate long distances, following a geographical pattern of seasonal rainfall and availability of fresh vegetation. Locust migrates is search of new feeding grounds from the food depleted areas, in large number in the arid regions.

Answer of Question 1: D

Answer of Question 2: A