Botanysemester-II
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Page 1 of 22 Unit 1 (Botany) B Sc 2nd Semester Ecology, Ecological Factors and Plant Communities Introduction to Ecology: The two components of the nature, organisms and their environment are not only much complex and dynamic but also interdependent, mutually reactive and interrelated. Ecology, relatively a new science, deals with the various principles which govern such relationships between organisms and their environment. The term ecology was coined by combining two Greek words, oikos (meaning house or dwelling place) and logos (meaning the study of) to denote such relationships between the organisms and their environment. Thus, literally ecology is the study of organisms at home. There is some controversy about the author who coined the term ecology and first included it in the literature , but there is consensus that the German biologist, Ernst Haeckel first gave substance to this term, Ernst Haeckel gave definition and substance to the term, which he first used in 1866, in the following statement written in 1870: ‘By ecology we mean the body of knowledge concerning the economy of nature the investigation of the total relations of the animal both to its inorganic and to its organic environment; including above all, its friendly and inimical relation with those animals the plants with which it comes directly or indirectly into contact-in a word’. Ecology is the study of all the complex interrelations referred to by Darwin as the conditions of the struggle for existence. More recently Kerbs (1985) has defined ecology as the scientific study of the interactions that determine the distribution and abundance of organisms. Soil origin, composition and formation Soil profile Soil: Soil is the surface layer of land. Soil is not a single factor but it is a complex of several soil factors, which together constitute the soil complex. With the exception of parasites and epiphytes all plants are dependent directly on the soil for mechanical anchorage, minerals and water supply. Type of land plants and animals are determined by grain size, porosity, pH and mineral composition of the soil. The principle components of the soil are the minerals and water which are responsible for the distribution of the plants. Formation (origin) of soil The process of soil formation is generally divided into two stages: a) Weathering and b) Soil development or Pedogenesis. a) Weathering: Breakdown of bigger rocks into fine, smaller mineral particles is known as weathering. The weathering processes are physical, chemical. (i). Physical weathering: Physical processes may be of following types. 1. Wetting-Drying: It is the disruption of layer lattice minerals which swell on wetting. 2. Heating-Cooling: It is disruption of heterogenous crystalline rocks in which inclusions have differential coefficients of thermal expansion. It occurs particularly in dry climates, where due to sun heating large boulders flake at surfaces. 3. Freezing: This is the disruption of porous, lamellar or vesicular rocks by frost shatter due to expansion of water during freezing. 4. Glaciation: Larger masses of snow and ice glaciers, while falling may cause physical erosion of rocks through grinding process. Page 2 of 22 5. Solution: Some more mobile components of rocks, such as calcium chlorides, sulphates etc., are simply removed by agents like water 6. Sand blast: In arid, desert conditions the rocks are disrupted by physical action of wind sand etc. (ii). Chemical weathering: Chemical processes include the following. 1. Hydration: As a result of taking water, due to reversible changes of haematite to limonite (Fe2O3 ← Fe2O3H2O), the rock swell. This swelling causes the disruption of sandstones etc. 2. Hydrolysis: In this process, components like alumino-silicates of rock breakdown during which elements such as potassium and surplus silicon are washed out which give rise to simpler mineral matter like clay alumino-silicates. For example, hydrolysis of orthoclase to kaolinite. 3. Oxidation-Reduction: Some oxidation-reduction chemical reactions such as reversible change of Fe3+ to Fe2+ cause disruption of rocks because Fe2+is more soluble than Fe3+. 4. Carbonation: Some chemicals produced in the atmosphere and those during the metabolism of microorganisms bring about carbonation. As for example reversible change of CaCO3 to Ca(HCO3)2 leads to solution loss of limestone or disruption of CaCo3 cemented rocks as the hydrogen carbonate is more soluble than the carbonate. 5. Chelation: Some chemical exudates, produced through biochemical activity of microorganisms like lichens, Bacteria etc., are able to dissolve out mineral components of the rocks. these metals dissolved with organic products of microbial activity are known as chelates. For example, acids produced by lichens and bacteria have strong chelating properties. b) Pedogenesis: During weathering, the rocks are broken down into smaller particles. But this is not true soil and plants cannot grow in this matter. The weathered material undergoes further a number of changes which is a complex process, known as pedogenesis or soil development. It is a complete biological phenomenon. During this phenomenon, living organisms such as lichens, molluscs, bacteria, fungi, algae, microarthropods etc; as a result of secretion of organic acids, enzymes, CO2 production and addition of organic matter after their death, bring about geochemical, biochemical and biophysical processes. Due to all this, the crusts of weathered rock debris are converted to true soils consisting of complex mineral matrix in association with a variety of organic compounds, and a rich microorganism population. Composition of soil (Basic soil components): A soil is simply a porous medium consisting of minerals, water, gases, organic matter, and microorganisms. The traditional definition is: Soil is a dynamic natural body having properties derived from the combined effects of climate and biotic activities, as modified by topography, acting on parent materials over time. There are five basic components of soil that, when present in the proper amounts, are the backbone of all terrestrial plant ecosystems. 1. Mineral: The largest component of soil is the mineral portion, which makes up approximately 45% to 49% of the volume. Soil minerals are derived from two principal mineral types. Primary minerals, such as those found in sand and silt, are those soil materials that are similar to the parent material from which they formed. They are often round or irregular in shape. Secondary minerals, on the other hand, result from the weathering of the primary minerals, which releases important ions and forms more stable mineral forms such as silicate clay. Clays have a large surface area, which is important for soil chemistry and water-holding capacity. Additionally, negative and neutral charges found around soil minerals influences the soil's ability to retain important nutrients, such as cations, contributing to a soils cation exchange capacity (CEC). Page 3 of 22 The texture of a soil is based on the percentage of sand, silt, and clay e.g. if a soil contains 20% clay, 40% sand, and 40% silt then it is a loam. The identification of sand, silt, and clay is made on the basis of their size. Sand: 0.05 – 2.00 mm in diameter Silt: .002 - 0.05 mm in diameter Clay < 0.002 mm in diameter 2. Water: Water is the second basic component of soil. Water can make up approximately 2% to 50% of the soil volume. Water is important for transporting nutrients to growing plants and soil organisms and for facilitating both biological and chemical decomposition. Soil water availability is the capacity of a particular soil to hold water that is available for plant use. The capacity of a soil to hold water is largely dependent on soil texture. The smaller particles in soils, the more water the soil can retain. Thus, clay soils having the greatest water holding capacity and sands the least. Additionally, organic matter also influences the water holding capacity of soils because of organic matter's high affinity for water. When water is bound so tightly to soil particles, it is not available for most plants to extract, which limits the amount of water available for plant use. Although clay can hold the most water of all soil textures, very fine micropores on clay surfaces hold water so tightly that plants have great difficulty extracting all of it. Thus, loams and silt loams are considered some of the most productive soil textures because they hold large quantities of water that is available for plants to use. 3. Organic matter: Organic matter is the next basic component that is found in soils at levels of approximately 1% to 5%. Organic matter is derived from dead plants and animals and as such has a high capacity to hold onto and/or provide the essential elements and water for plant growth. The percentage of decomposed organic matter in or on soils is often used as an indicator of a productive and fertile soil. Over time, however, prolonged decomposition of organic materials can lead it to become unavailable for plant use, creating what are known as recalcitrant carbon stores in soils. 4. Gases: Gases or air is the next basic component of soil. Because air can occupy the same spaces as water, it can make up approximately 2% to 50% of the soil volume. Oxygen is essential for root and microbe respiration, which helps support plant growth. Carbon dioxide and nitrogen also are important for below ground plant functions such as for nitrogen-fixing bacteria. If soils remain waterlogged (where gas is displaced by excess water), it can prevent root gas exchange leading to plant death, which is a common concern after floods. 5. Microorganisms: Microorganisms are the final basic element of soils, and they are found in the soil in very high numbers but make up much less than 1% of the soil volume. A common estimate is that one thimble full of topsoil may hold more than 20,000 microbial organisms.