environmental factors
TRANSCRIPT
Chapter6
Every organism is expbsed, in the habitat where it lives, to thesimultaneous action of a wide range of abiotic and biotic factors. Environmentalfactors are those components of the environment capable of acting directly onliving organisms. This definition excludes components such as altitude' anddepth. In fact altitude acts as a result of temperature, exposure to sunlight, andatmospheric p,essure, and not directly. Similarly, depth affects aquatic organismsthrough increased pressure and by the action of reduced light. Those factors thatoperate on the population from outside (e.g. climate, predation. food supply,disease, etc) are known as Extrinsic. While ,those factors that are generatedwithin the population itself (e.g. home range, territoriality, genetic. etc.) are calledIntrinsic.
i. by eliminating some species from regions where climatic andphysicochemical conditions are unsuitable for them ann, in thisway, determining their geographical distribution;
ii. by modifying the rate of fecundity and mortality of various species,by acting on stages in development and also by causing dispersaland so influencing the population density;
iii. by encouraging the development of adaptive features, ofquantitative metabolic changes and also of qualitative changes,for example, diapause, hibernation, migl'ation, aestivation andphotoperiodic reactions.
I Q. How many people are thought to be infected with AIDS virus?
As early as 1840, Justus Liebig, a German biologist, proposed "The Lawof the minimum". It was shown in a study of growth of crop plants that growth isdependent or limited by any nutrient that is available in minimum quantity. Inother words, what he said was that each plant requires certain kinds andquantities of nutrients for its growth and survival. If one of these nutrient isabsent, the plant dies, and if it is present in minimal quantity, the growth will he
I minimal. For example, boron is an essential element, but is always scarce ill sui!.When it has been exhausted by crops, growth stops even if other elements ;irepresent in large quantities and environmental conditions are favourable.
Continued research through the years revealed that not only nutrients butother' environmental factors such as, light, temperature, salinity, humidity, etealso affect the growth of plants and animals. Eventually the Law of the Minimumwas incorporated with the "Law of Limiting Factor" deVeloped by a BritishPhysiologist, F. F. Blackman in 1905. Blackman observed that the rate ofphotosynthesis is influenced by five factors, namely, the amount bf carbondioxideavailable, the amount of water available, the intensity of solar radiation, amountof chlorophyll and the temperature of chloro~last. He further observed that tilerate of photosynthesis is governed by the amount of factor that is operating at alimited level. For example, the photosynthesis is affected by light intensity allclthe availability of carbondioxide. If all other factors are at optimum, a smallquantity of C02 would be a limiting factor and simply by increasing the lightintensity will not increase the rate of photosynthesis. This is known as "Leibig -Blackman Law of Limiting Factor".
Too much or too little of any abiotic factor canlimit or prevent growth of a population of a species in anecosystem even if all other factors are at or near theoptimum level.
The. idea of limiting factor can be applied, not only to those nutrlelltsessential for the survival of living organisms, as in Liebig's conception, but also toall environmental factors, and equally for both lower and upper limits. Thus eachliving organism shows limits of tolerance with respect to different environmentalfactors and the ecological optimum for eacll organism lies within these limits (Fig.6.1 ).
~~ cq;;J
~~
~ ~c:tJ qQ ":j;J ~
~~ c¥)~
d
130
For example, carbondioxide is necessary for the growth of all greenplants. Small increase in the concentration of C02 in the atmosphere willincrease the rate of plant growth, but very considerable increase becomes toxicLikewise, water and fertilizers are essential components for the growth andsurvival of crops, but plants may be killed by too much of water and by too muchof the fertilizer. Again, small additions of arsenic to the human diet have a tonicaffect, a further increase in the dosage, however, is fatal.
The Law of Tolerance may be summarized as:The existence, abundance and distribution of a
species in an ecosystem are determined by whether thelevels of one or more physical or chemical factors fallwithin the range tolerated by the species.
Not only organisms have a tolerance range, they also have a smellieroptimum range within which they thrive or function best. This optimum range melYvary from one season to another from one stage of an organism's developmtmtto another. Human infants, for example, have a much narrower tolerance rangefor temperature than adults. An organism's tolerance range can also vary atdifferent developmental stages. For example, adult blue crabs can tolerate freshor brackish water with a high chloride content and they are often seen in river'sfar from the ocean. The blue crab's larvae, however, can not live in freshwater sothe blue crab can never reproduce there.
To express the relative degree of tolerance, a series of terms have CC"leinto general use in ecology. The prefjx "steno" is used for these species thal c;=tntolerate only narrow or limited range in environmental factor. While term "eury" IS
used for those species that can tolerate a wide range of variation inenvironmerltal factors. The various terms are as follows:
Steno-thermalSteno-halineSteno-hydricSteno-phagicSteno-eciousSteno-bathic
Eury-thermalEury-halineEury-I-:ydr'icEury-phagicEu ry-eciousEury-bathic
refers to temperaturerefers to salinityrefers to moisturerefers to foodrefers to Ilabitatrefers to depth
1.\1
As an example. Let us compare the conditions under which trout egg"sand frog Rana pipiens eggs will hatch and develop. Trout eggs develop between00 C .and 120 C with optimum about 40 C. Frog eggs, however, will developbetween 00 C to 300 C with optimum about 220 C. thus, trout eggs (having a lowrange of tolerance for temperature) are steno-thermal, compared to frog's eggs(which have a wide range of tolerance for temperature) and thus are Eury-thermal. .
It is traditional in ecology to distinguish between abiotic factors and bioticfactors. The former includes water chemistry, climatic factors and soil factors,and the latter include predation, competition, parasitism and microbial diseases.
Water is the most important factor in the life of an organism as it is majorconstituent of the protoplasm. Water is necessary as a solvent for food and as anagent for the chemical transformation .of the materials within the body. The actionof water as an environmental factor is largely determined by its physical andchemical properties.
Water is the most abundant substance on earth (see Chapter 5). Innature, water is available in three forms: a) Atmospheric moisture: It is presentas invisible vapour called humidity and also as visible vapour called as fog orcloud. b) Precipitation: The amount of moisture falling on an area in liquid orfrozen form is called precipitation. Rainfall is the commonest type of precipitation.c) Soil moisture: When rain water penetrates the soil, it fills spaces between tilesoil particles. Soil moisture is the chief source of water for plants.
Water affects all life processes directly since it makes up a largeproportion of the bodies of plants and animals (e.g. cytoplasm holds 70 - 90%water in majority of organisms). For plants, as stated above, the chief source ofwater is the soil moisture, however, some plants (like lichens and epiphyticorchids) can absorb moisture directly from the air.
I Q. What percent world's population live in urban areas?
· U2
Animals obtain water by one of the fOllowing methods: a) by absorbing itthrough their skin from contact with some damp ground, b) by drinking andabsorbing water through the walls of digestive tract, c) directly from their food, forexample, many desert .animals never drink and rely on water contained in theirfood, or d) from water produced by metabolism, for example, few rodents andsome insects living in very dry stored food products make use of metabolic waterformed by the oxidation of fat. The animals lose water from their bodies throughurine, feces, respiration, and evaporation from the skin.
Plants and animals exhibit considerable variations in their requirements ufwater. On the basis of water requirement following three broad categories ofplants can be recognized.
terrestrial plants which can tolerate extremely dryconditions and pass long p'eriods without water.
terrestrial plants ~equiring moderate amount ofwater.
Likewise, animals can also be divided into three groups, depending onwater requirements, as under:
Aquatic animals which live in and require largeamount of water.
terrestrial animals which can tolerate extremely ciryconditions, and
terrestrial animals requiring moderate amount ofwater ..
Both plants and animals living in water or dry habitats exhibit a number ofspecial features in their morphology, anatomy, and physiology which enalJlethem to live under such extreme conditions. These features or adaptations are ofconsiderable interest and are described below briefly:
Aquatic plants or hydrophytes grow on extremely wet soil '.H1dwhere water IS available In abundance. According to their distribution In afreshwater body, they can be subdivided into the following five categories:
i. Free-floating: those that float freely on water surfacehaving no contact with soil e.g. Wolffia, Lemna, etc
ii. . Rooted with floating leaves: in certain hydrophytes, suchas Nelumbo, Trapa, the roots are fixed in the mud butleaves have long petioles which keep them floating on thewater surface.
iii. Submerged floating: hydrophytes in contact with wateronly, being completely submerged and not rooted in themud. e.g. Ceratophyllum
iv. Rooted submerged: hydrophytes, like Hydrilla,Potamogeton, Vallisenaria remain completely suspendedin water but rooted into the soil.
v. Rooted emergent: they grow in shallow waters at:Yrequire excess of water but their shoots are partly or whOilyexposed to air e.g. Sagittaria, Typha, etc.
Due to availability of water in plenty, the roots which areotherwise, the principal organs of water absorption in most plants, are oflesser importance in hydrophytes. The roots may be absent as in Wolff/a;or poorly developed as in Hydrilla; or without prominent root caps androot hairs as in Eichhornia. Usually root hairs and root caps are absent inhydrophytes. Emergent forms, however, are provided with root caps Infree floating forms, stem is slender, floating horizontally on water as inAzalia or thick, short and spongy as in Eichhornia. In submerged plants, itis longer, spongy and slender as in Hydrilla. The leaves are generallythin, long and ribbon shaped as in Vallisenaria or long and linear as inPotamogeton, or very much branched as in Trapa. In free floating forms,usually the leaves are large, thick and covered with waxy coating. The
. flowers are less common in submerged forms.
U~
Anatomically, the plants exhibit excessive developrnent otparenchyma and elaborate system of air spaces (aerechymCl) and usuallypoor development of vascular and mechanical tissues. Tt,c c~tlclt isabsent or poorly developed. The stomata are absent frolll the submergedorgans. The chlorophyll is found in all tissues throughout the piant body.Even in some plants (as in Azalia), chlorophyll may develop in roots aswell. Mucilage cells and mucilage canals are present. The reserve fooo isin the form of starch grains which occur abundantly in cortex and pith.
The term "xerophytes" is loosely defined as plants of dry habitats.They mostly inhabit deserts and other arid regions. Xerophytes aregenerally classified into three categories:
Ephemerals: They are short-lived annual plants· whichcomplete their life cycles within a very small period (6-8weeks) when sufficient moisture is available. Since theseplants avoid dry season, they are also called as 'droughtevaders". The seed germination is soon followed byflowering without undergoing a phase of vegetation gmwtll.The dry period is passed in the form of seeds.
Succulents: Many xerophytes possess mechanism forstorage of water in their tissues. The succulence i.edevelopment of water storing tissues may occur in roots(e.g. Asparagus), or leaves (e.g. Aloe), or stems (e.g.Opuntia). The rest of the plant body other than succulentorgans is generally very mucll reduced, the roots al·eusually shallow and fibrous, and help in absorption oftraces of moisture from the soil surface. Among Cacti andEuphorbia, the leaves are absent or present in the form ofmodified spines. The vascular system is usually ill-developed.
iii. True xerophytes or drought resistants: The plantswhich have the ability to maintain growth under conditionsof water stress and high temperature are true xerophytes.The plants are woody trees, shrubs and herbs. The leaves
. are generally small or absent. Among grasses, the leavesroll inwards to protect stomata and help in reducing waterloss. The resin and latex cells are present. The cuticle isthick, and the hypodermis is sclerenchymatous or maypossess some chlorophyllus tissues. The stomata aresunken. Osmotic pressure of cell sap is very high.
Mesophytes are those plants which grow in moderately moisthabitats and need well aerated soils. In some respects they stand'between the hydrophytes and xerophytes. Broad leaved trees growing inwet depressions, along lakes, rivers are mesophytes, They generally lackspecial structural and physiological adaptations found in hydrophytes andxerophytes. Some of. the significant morpho-anatomical features ofmesophytes are as follows:
Roots are fairly branched and contain root caps and root hairs;stems are generally aerial, ,solid and fairly branched; leaves are generallylarge and broad, thin and varied in shape; leaves are green and lack hairyor waxy coating; cuticle is moderately developed; epidermis is welldeveloped; stomata present on both surfaces of leaves; vascular andmechanical tissues fairly developed; exhibit wilting during noon hours.
The body of aquatic animals is flexible and generally provided withstructure like flagella, cilia, antennae or fins, which help in movement.They generally show two types of aquatic adaptations.
'I -.------------------------------~- - -
I lJ Wllat is the most dangerous indoor air pollutant?
Such adaptations occur only in those animals such as fish,which had never lived on land and their ancestors were alsoaquatic. They live permanently in water and exhibit followingadaptations:
a. Head, body and tail compressed into an elongatedstreamlined form so as to offer least resistance whilemoving through water.
b. Presence of various types of fins; the pectorals andpelvics act. as stabilizers; median fins as the keel of aboat (preventing rolling in water) and the caudal fin isthe principal means of propulsion and steering.
c. Respiration by gills thus making use of oxygendissolved in water.
\d .. Presence of air bladder that act as hydrostatic organ in
many teleost fishes; in some it acts as assessorybreathing organ. ~
e. Presence of .lateral line organs, which function as rheo-receptors.
f. Skin with mucous glands and mostly covered withscales.
g. Fishes generally face the problem of osmo-regulation.The cause of the problem is the difference ofconcentration of soluble salts in the body fluids and thesurrounding water. In freshwater fishes, water tends toenter the body through all the possible surfaces andthus tends to dilute the body fluids. In freshwaterfishes, the skin (which is covered with scales and.mucous) is not permeable.
1.\7
Furthermore, to cope with the steady inflow of water,freshwater fishes produce highly dilute urine. Tile mainwork of the kidney in these fishes, therefore, lies inwater excretion. Salt losses, which occur duringexcretion, are made up by selective absorption of saltsthrough the gills. In contrast, marine fishes live in. amedium that is hypertonic to the body fluids and thusthey tend to lose water continuously by diffusion tomore saline environment and gain salts through theirosmotic membranes. To counteract, the water losses,marine fishes drink seawater and thus increase the saltcontent of the body fluids. Whereas dehydration isprevented by this process, excess slats must beeliminated. For this purpose, marine fishes havespecial cells in gill epithelium, which eliminate excesssalts.
Such adaptations occur in those animals which are lungbreathers and whose ancestors were terrestrial but ',i' to thestress of circumstances such as inhospitable rand, foo...J~LcHcity orcompetition with others for space, tlleyre forced to return towater. Secondary aquatic adaptations of some large animals,particularly mammals are as follows:
a. Body streamlined; neck constriction absent; tail present(as in Cetacea, Pinnipeda, Sirenia, etc).
b. Water borne animals become large in size (whales,hippopotamus, etc).
c. Shortened dorso-ventrally flattened cranium and thefacial portion becoming an elongated, slender snout orrostrum as in porpoises.
d. Neck becomes short and immobile.e. External ear or pinna disappears so as to avoid
hindrances in locomotion.
f. Nostrils and eyes move towards the apex of face.g. Limbs become fleshy and fin like expansion paddles for
swimming.h. Power of m'astication is lost hence coronoid of
mandible is reduced.i. Skin becomes smooth and naked.
Water economy is one of the major problerr]s confrontingterrestrial and semi-terrestrial animals. The amount of water necessary tomaintain life varies from species to species. It also varies with stage in thelife cycle of a single species. Many mechanisms have evolved whichenable to maintain the critical level of water balance, thereby permittingsome species to live in hot deserts, some in water and others in habitatsintermediate in moisture content.
Adaptations of animals to meet the problem of dryness ordesiccation are behavioral anatomical and physiological. Two of the maineffects of desiccation are loss of the body water and drying. out ofrespiratory epithelium. The body walls. of some animals such as,planarian, earthworms, amphibians which also function as respirn.torymembrane: :; ~~;;::;Ii 1<;llilJrdJle IS subJecte' 0 ll, y, 1';::>5, 1I1lJ Cell::. ell c;
permanently injured. Therefore, to survive these animals must alwaysremain in moist hUr;'id conditions .. To maintain a moist condition, manyanimals secr'ete mucous coating and in unusually dry ?ituation they mayresort to behavioral adaptations such as burrowing, coiling or formingaggregates.
Arthropods have an external covering over the respiratorysurfaces. Insects and myriapods have a well-developed covering over tilerespiratory surface and tracheal system. Vertebrates skin is manylayered, in addition to a well-protected respiratory surface. There aremany structur'al modifications that help in preventing loss of water. Forexample, there is a operculum in gastropods; in insects a wax layermakes the integument impermeable to water and so on.
F. Adaptations in Xerocoeles
Like plants, the animals in extremely dry habitats are also adaptedto conserve water. They fall in two categories namely, drought evadersand drought resistors. .
a. Drought Evaders: ~Drought evading animals usually remaindormant during the most unfavourable conditions (for example,when the climate is very hot and dry). The animals mayundergo aestivation, hibernation or encystment. The eggs orpupal stages, of some insects for example, many lie indormant condition for 8 - 9 months or perhaps several years.When the rains arrive, a variety of insects may be seen inswarms in deserts. These include grasshoppers, ants, beetles,wasps, crickets; moths, etc. Some amphibians, for example,spade-footed toad may undergo aestivation for about 8 - 10months. They appear on the ground when rainfall saturatesthe earth surface. Likewise, birds nest during the rainy seasonwhen food is most abundant for the young ones. If droughtdevelops during breeding season, some birds do notreproduce.
b. Drought resistors: The animals, - which remain activethroughout the year, have several morpho-physiologicaladaptations to counter aridity and heat. Drought· resistinganimals either stores water or conserve it by reducingevaporation. Loss of water is prevented 'in varying ways:
1. Some animals simply avoid heat by adopting nocturnalhabits and remaining underground or in shade during thedaytime.
2. Some rodents that are active in the daytime periodicallyvisit their burrows and passively loose heat throughconduction. by pressing their bodies against the coolburrow-walls.
3. Jome birds and bats undergo a daily torpor.'-0-. W-h-a-t -p-er-c-e-nt-o-f-n-1Lltorvehicles are found in USA?
I-HI
4. The animals like jack-rabbit and desert fox have large earsthat reduce the need of water evaporation to regulate thebody temperature. Their ears function as efficient radiatorsto the cooler desert sky, which on clear days may have aradiation temperature of 25° C below that of animal.
5. The kangaroo-rat and pocket-mouse can live indefinitelyon dry seeds and do not require drinking water. Theyobtain water from their own metabolic processes and fromhygroscopic water in their food.
6. Sweat glands in most of the mammals living in desert areeither greatly reduced or absent.
7. Scorpions, snakes and lizards have impermeable skin toprevent loss of water.
8. In camels, water is lost from the tissues and not from theblood. The camels store fat in thei'r humps which preventthe flow of heat inwards to the body and also providemetabolic water.
9. Excretion of uric acid in birds, reptiles, kangaroo rats incrystalline form is also an adaptation to conserve water.
Light is among the most important factors of life, and in fact, life could notexist without light. Because on light depends the photosynthesis by green plantswhich, in turn, support all animals (including humans) and most microbial life onearth. Light also influences the daily and seasonal activities of plants and animalsin many ways.
The main sources of light are sunlight, moonlight and star-light. However,only sunlight is important from ecological point of view. Light is defined as thevisible part of the spectrum of solar radiant energy.
I Ans: Of total world's motor vehicles, 35% are found in USA. IIt comprises of wavelengths ranging from 390 u to 700 u, and constitutes
about 44% of the total radiations received on the earth's surface. The radiation
1-11
consisting of wavelengths below 390 u is called .ultraviolet and constitutes 4oio ofthe total radiations. The radiation consi$ting of wavelength above 760 u is calledinfrared or thermal radiation ,and constitutes 52% of the total solar radiations.Only visible light is used in photosynthesis.
Light is extremely variable. Variation in intensities, quality of light and theduration of light are known to influence different ecosystems in various ways. Themovement of earth around its inclined axis and around the sun results in anuneven distribution of energy in. time and space. The seasonal changes in thelight duration in the daily cycle results from this movement. Water vapours furthermodify the quality and quantity of light, dust particles, and vegetation. In forests,major proportion, of light intensity is absorbed by large trees while the groundvegetation (like shrubs) receive a very diffused illumination. Light penetration inwater depends on the turbidity, solute content and planktonic growth of water.The intensity of light decreases with increasing depth successively. As the lightpasses through water, the longer wavelength radiations are absorbed and thedeepe'r water layers receive light rich in blue radiation. This is why deep lakesand ocecfns appear blue in colour. In general, light intensities are weaker atdawn, sunset and in winter. At equator, daylight prevails for about 12 hours, inboth summer and winter. Towards, poles in summer day lengths become largerthan 12 hours.
A. Effect of light on plants: Some of the well known effects of light onplants, directly or indirectly are as follows:
i. Chlorophyll production: Light is an essential factor in theformation of chlorophyll pigment in chloropr.yllus plants. Only fewplants like seedlings of conifers, young fronds of ferns and fewspecies of algae may become green even without light. Light alsoinfluences the number and position of chloroplasts. The upper partoff leaf which receives full light has larger number of chloroplastsas compared to the leaves which grow under shade.
ii. Stomatal movement: Opening and closing of stomata isregulated by sunlight. This in turn, affects transpiration andabsorption of water.
Q. How much money is made each year from illegal trade in wildlife and wildlifeproducts?
1-12
iii. Effects on transpiration rate: Indirectly light affects thetranspiration rates through increase in temperature. Transpirationrates correspondingly affect water absorption from soil. Thus, lightintensities are always associated with dry habitats and hightranspiration rates.
iv. Distribution of plants: Light conditions at poles are different fromother parts of the earth. Th.us, total amount of radiation receivedfr'l":l earth's surface differs with latitude (distance from equator).This may be one of the reasons for the difference in the quality ofvegetation in different parts of earth's surface.
v. Photoperiodism: Total length of the daily light r riod to which anorganism is exposed is called Photoperiodism. Based onphotoperiodic response, plants can be classified as short-dayplants, long-day plants and neutral-day plants. Short-day plantsdevelop norm'ally when the photoperiod is less than a criticalmaximum (between 12 - 14 hours), for example, Salvia, Datura,Cannabis, etc. Long-day plants are those whose flowering isstimulated by day length longer than a particular value (16 - 18hours). These usually bloom fn late spring or summer, forexample, Brassica, Nigella, etc. Neutral-day plants are thosewhose flowering is not affected by day length, but rather iscontrolled by age, number of nodes, rrp iOLis cold -reatrnpnt andlike. Examples are Poa, Cucumis, Nicotiana, etc. Thus, light asphotopu: ioc.i f.llays an important role in local distribution in plantsby affecting stem elongation, flowering, fruit development andother processes.
vi. Overall vegetation development: Plants may be classifiedaccording to their tolerance to light intensities into Heliophytes(which grow best in full sunlight), and Sciophytes (which grow bestat lower light intensities or under shade). The sciophytes maintaina high rate of photosynthesis in low intensities of light whileheliophytes are adversely affected by the shade.
I.:lJ
In short, light is an important ecological factor that influences thedistribution of plants through its effects on photosynthesis, vegetative$hape, tissue differentiation, number and position of chloroplasts,chlorophyll production, leaf. structures, stomatal movement,transportation, growth and development of flowers, seeds and fruits.
B. Effects of light on Animals: Light has far reaching eHects onanimals as well. These are as under:
i. Metabolism: The metabolic rate of different animals is g-reatlyinfluenced by light through its heating effect on tissues and byionisation. of the protoplasm. The increased intensity of lightresults in an increase in the enzymatic activity, general metabolicrate and degree of solubility of salts in the protoplasm. Animalsreceiving poor light, for example, in caves show slow rate ofmetabolism and are .therefore sluggish in their movement.Similarly, normal development of Salmon larvae occurs only in thepresence of sufficient sunlight. In the absence of light, theirdevelopment does not remain normal and mostly they die. Incontrast, the larvae of Mytilus grow well in darkness than in light.
ii. Pigmentation: It has been observed that the production andconcentration of pigment in skin is directly proportional to t!leexposure of animals to the duration of light. The process ofpigmentation in animals is influenced in number of ways, as a) inthe development of skin colour. For example, animals living incaves lack pigmentation. If they are exposed to light for longerperiods, they regain pigmentation. Human beings living in tropicsare darker in colour as compared to those who live in temperateand cold regions, b)in protective coloration: Some animalsdevelop coloration in their skin which more or less resemble withthe background in which they live and thus afford a good means ofprotection from their enemies. For example, polar bears, polarfoxes, and arctic hares are white in colour and are thus invisibleagainst white snowy background. c) colour change: Some insects,fishes, crustaceans, lizards and amphibians are able to changetheir colour or pattern rapidly according to the surroundings wherethey live. This helps then in protection from enemies. Forexample, the white crab Cryptolithode perfectly blend the whitepebbles on the beech and hence very.difficult to recognize it.
[i[8ow much of agricultural land has been destroyed by salinization?
l-l-l
iii. Movement: The influence of light on the movement of animals isvery much in lower animals. The phenomenon of movement ofanimals in respect to sunlight is called Phototaxis. Animals likeEuglena, Ranatra, etc. are positively phototactic i.e. they movetowards the source of light. Whereas, bats, slugs, earthworms,copepods, planarians, etc. are negatively phototactic i.e. theymove away from light. Light also controls the locomotory activity orvelocity of movement of certain lower organisms. Thisphenomenon is known as Photokinesis. For example, the blindlarvae of the mussel crab Pinnothres move faster if exposed toincreased. sunlight. In some animals only a part of their bodyshows movement in response to light and this has been called asPhototropism. For example, the movement of flagellum in Euglenatowards light.
iv. Reproduction: In many animals like birds, light intensities initiatethe breeding activities. The gonad of birds are found to becomeactive with increased illumination during summer and to regres.sduring winter with shorter periods of illumination. Some animal'slike sheep, deers and goat are short-day animals. They may bebrought to sexual act by decreasing the length of exposure to daylength.
v. Photoperiodism: Each daily cycle of a period of illuminationfollowed by a period of darkness is called Photoperiodism. Thelength ot" day has been found to be a important factor to time theactivities of organisms, such as, development of gonads, molting,migration, deposition of fat etc. The timing of different activitiesduring the various parts of an annual or daily cycle has beencalled Biological Clock. The photoperiod affects the sensoryreceptors in animals which leads to the synthesis of certainenzymes and hormones that ultimately initiate a change inphysiological activities. It has been assumed that the photoperiodacts on hypothalamus through sensory receptors.
I AilS: 50 percent. _ --.J
I-IS
Seasonal changes in hair coloration of some mammals aildfeather coloration in some birds are related to photoperiod. Inbirds, there is definite relationshi'p between length' of the day andegg laying. Gonads of birds become more active with an increasein photoperiod in summer. Duration of photoperiod has beenfound to affect the activity of gonads in some fishes as well. Trout,for example, that ordinarily spawn in December can be induced tolay their eggs in August by artificially changing the day length. Thefreshwater snail Lymnaea will not lay eggs on an 11-hour day butlays abundantly when the days are 13 Y2 hours long or longer.Migration of birds is known to be affected by photoperiodism assome birds are seen migrating towards south during winter whendays are shorter and towards north during summer when days arelonger. Many other life processes among animals, for example,wing production in aphids, metamorphosis in mosquitoes, moltingof feathers in birds, development of furs in mammals, storage offat in the bodies and son are known to be influenced by thephotoperiodism.
"'v,i: Periodic rhythms: In many organisms, both plants and animalscertain physiological processes are observed to follow a definiterhythmic pattern. Rhythms with a periodicity of about 24 hours aregenerally called Circadian rhythms (from the. Latin "circa" meaningabout and "dies" meaning day) whereas the annual rhythms arecalled Circannual rhythms. They are maintained by anendogenous mechanism, which is not fully undefStood. Theserhythms are composed of repeated sequence of events whichoccur in a given order and at definite time interval. For example,wilen Drosophila are kept under constant conditions from thelarval stage on" they will still emerge from pause with a regularcircadian rhythms. Bats emerge from their roosting places at dusk,and they retain this rhythm even when kept in complete darknessin the laboratory or even when they receive a different pattern ofillumination from that which corresponds to the succession of daysand nights. Another striking example of daily movement is to befound in the "vertical migration" of zooplankton which regularlyoccur both in lakes and oceans. These organisms tend to swimdownwards in deeper waters during day time and move towardsthe surface at night as a result of their light sensitivity.
). How manY'plants feed most of the world's estimated 30,000 edible soecies?
Temperature is a measure of heat energy content in an object, and iscommonly expressed as degrees either in the Fahrenheit or Centigrade scale. It .is one of the important environmental factor that influences all forms of life byexerting its action through increasing and decreasi'ng some of the vital activitiesof organisms, such as, behaviour, growth and deatr~. It also acts as a limitirlgfactor for the growth and distribution of plants and animals.
The temperature is highly variable in time and space. The atmospllerictemperature falls to - 88° C in Arctic regior] while rises as high as 58° C in dry hotdeserts. The soil temperature in the deserts may reach beyond 65° C and thereare thermal springs where temperature. exceeds 85° C. Again at any given place,there are large diurnal and seasonal variations. The temperature of air near thesurface of land is sometimes 17° C higher in the daytime than at night, and indesert localities this difference may be as much as 40° C. Similarly, in ceric:'tinparts of North America, the yearly temperature fluctuates between - 40° C toabove 45° C in summer. Nearer the equator the diurnal and seasonaltemperature changes are negligible. Temperature fluctuations are comparativelyless in aquatic environments. Minimum temperature in a pond or lake do notusually fall below 0° C and in oceans the minimum temperature has been foundto be - 2.5° C - 3.5° C. In oceans, the difference between a day and nighttemperature is of 4° C. however, the difference is reduced with an increase indepth. The maximum temperature in marine environment is about 36° C. Thelowest temperature recorded for any landmass is - 70° C (Siberia).
The most important feature of freshwater' lakes and deep ponds,particularly in temperate regions, is the occurrence of thermal stratification (SeeChapter 3).
A. Temperature tolerance: All organisms live within a range oftemperature tolerance. Few species can exist for great lengths of timein temperature below 0° C and above 55° C. As temperaturedecreases, the speed of all chemical reactions, including lifefunctions, slow down. Hence plants are not productive and certainanimals hibernate in winter temperatures
147
Animals can not live at very high temperatures, because whentemperature increases beyond 550 C, it destroys the enzymesessential for life. Certain bacteria,however, can tolerate temperatureupto 900 C, as found in thermal springs. A few blue green algae alsooccur in hot springs at about 700 C. Some bacteria, algae and lichensare among those which can survive in subfreezing temperature aswell. Living organisms can survive very low temperatures in resting ordiapause state; for example, myriapods to -500 C; rotifers to - 1920 Cand nematodes to - 2700 C without harm. Few insects in diapauseare able to tolerate temperature as low as - 800 C.Plants have been classified into the following four groups on the basis
of their heat tolerating capacity.
i. Megatherms: Plants growing in regions where. high temperature.prevails throughout the year, for example, desert vegetation andtropical forests.
ii. Mesotherms: Plants of the regions where high temperaturealternates with low temperatures, for example, tropical deciduousforests and aquatic plants.
iii. Microtherms: Plants of the region where low temperature prevailsthroughout the year, for example, coniferous forests.
iv. Hekistotherms: Plants in regions with very low temperatures, forexample, alpine vegetation.
Animals on the other hand, are grouped into two categories on thebasis of their ability to regulate body temperatures namely,
i. Poikilothermic or Ectothermic: Usually referred as "coldblooded animals", these include animals like reptiles, amphibians,fishes and most of the invertebrates, in which the bodytemperature fluctuates with temperature outside. They have nointernal temperature regulating mechanism. These cold bloodedanimals become inactive below 80 C and above 400 C. Most of thecold blooded animals become dormant during extreme cold, calledhibernation or during extreme hot condition called aestivation.
I O. What percentage of world's potential food supplies is lost to pests? . __ :
I IX
ii. Homeothermic or Endothermic: Usually referred to as "Wi::W1 I
blooded animals"" these include animals like birds and mammals.The warm blooded animals maintain their body temperature at aconstant level irrespective of the temperature outside. The warmblooded animals·are generally active throughout all seasons.
B. Thermo-regulation in cold-blooded animals: Some cold bloodedanimals avoid both heat and cold by undergoing into a state ofdormancy during a period of environmental stress. Many insects,some crustaceans, mites, and snails enter Diapause. Eggs, larvae,and pupal stages may be involved. Amphibians and turtles burythemselves in the pond mud;. snakes and lizards seek burrows. Therethey remain in a suspended animation until the temperature warms orcools again. Winter dormancy is called Hibernation while summerdormancy is called Aestivation.
Oiapause is a stage in the life history of somecold blooded animals when the development andgrowth is suspended or is greatly retarded, metabolicactivity is highly reduced to resist or passunfavourable extremes of the environment forexample, coldness, heat or non-availability of food.
A few animals, (especially insects, certain amphibians and reptiles)exercise some degree of thermo-regulation by physiological orbehavioral mechanism. For example, hawk-moths can raise thetemperature of their flight muscles to 32° C - 36° C by vibrating thewings rapidly before take off; gregarious larvae of butterily may raisetheir temperature upto 2° C by clustering together; locusts andgrasshoppers may increase the temperature by basking sideways inthe sun; ants move their larvae·to warm or cool places within the nest;bees maintain temperature within their hives between 13° - 25° C byfanning with their wings to evaporate water droplets when it is too hot,or releasing body heat through the increased metabolic activity, whentoo cold. Some animals like frogs and lizards lower their bodytemperature slightly by evaporating or cooling through skin or viarespiratory tract by panting.
I Ans: 55% (35% before harvest; 20% after harvest).
14<)
C. Thermo-regulation in warm blooded animals: Though severalfactors may operate, the most important role in thermo-regulation isplayed by skin.
When a mammal, a typical warm-blooded animal, is subjected tosevere cold, following adaptations help from excessive cooling:
i. The subcutaneous fat serves as insulator and reduces he$t lossfrom the body e.g. polar bears, seals, whales, etc. have t11icklayerof subcutaneous fat.
ii. The hairs are raised and brought into more or less vertical positionby the concentration of hair muscles. The air is, thus, get trappedin the spaces between hairs. This air is warmed by the body andbeing a poor conductor of heat, the hairy coat serves as ainsulatory layer. In birds, the same function is perfol'med byfeathers.
iii. The superficial blood vessels in the skin contract so that blood isdirected from the surface layer to deeper layers. This reduces theloss of heat to surrounding atmosphere from the general surfaceof skin.
The response to excessive heat involves the reverse of the aboveprocesses i.e. heat production is cut down and heat loss from thebody is encouraged by following means:
i. Animals living in hot climate have relatively less subcutaneous tat.Furthermore, fat deposition tends to be localized so as to facilitateloss from other parts of the body. For example, fat in camel isstored in hump and in buffaloes, it is located on top of the neck.
ii. The hairs are lowered by relaxation of hair muscles so that they lieflat against the body surface. With no spaces between hairs, noair can be trapped against the skin. The insulation is, thus,reduced and convention or radiation from the body encouragesheat loss.
1:'0iii. The su'perficial blood vessels are dilated so that blood is brougllt
up near the surface from which it can loose heat more rapidly.
iv. Sweating by sweat glands and evaporation of sweat from the bodysurface cools the skin and blood flowing through it. In dogs, cats,etc. there are no sweat glands and heat is lost by panting.
v. Metabolic rate falls in hot conditions so that less heat is generatedwithin the body.
D. Effects of temperature on plant and animals: Temperatul'e 11a5been found to affect the living organisms in various ways, forexample, it has significant roles on cells, morphology, physiology,behaviour, growth and distribution of plants and animals. Some of tilewell studied effects of temperature are as under:
i. Temperature and Cell: The minimum and maximqm temperaturehas lethal effects on the cells and their components. For example,if it is too hot, the proteins in cells coagulate; if it is too cold. ce,1proteins are destroyed as ice crystals are formed.
ii. Temperature and Metabolism: All metabolic processes areinfluenced by temperature. Since temperature regulates theactivity of enzymes, all chemical reactions in the body 0:organisms are controlled by temperature. It affects the rate oftranspiration and photosynthesi~ in plants; respiration and othermetabolic processes in plants as well as animals; in plants, it alsoaffects the germination of seeds.
iii. Temperature and Reproduction: The maturation of gonads,gametogenesis and liberation of g'ametes takes plc1ce at specifictemperatures and it varies from species to species. Breeding insome animals remain unaffected by' temperature, whereas insome breeding occurs only in summer or winter. Temperature alsoaffects fecundity of an animal. For example, in grasshoppers ofgenus Melanophu5 and Camnula when reared at 32° C produce20 to 30 times as many eggs than those rea.red at 22° C.Flowering in plants is also affected by temperature throughthermo-periodism.
,IA 3 '11' ---II ns: nil Ion. ---I
131
iv. Temperature and Sex Ratio: In some animals, such as, rotitelsand daphnids, sex ratio is affected by temperature. Under normalconditions, daptlnids give parthogenic ,eggs that develop intofemales only whereas with increase in temperature,. they giveeggs that develop either into males or females. .
v. Temperature and Growth: The growth rate of different plants andanimals is very much influenced by temperature. for example, theadult trouts do not feed and thus do not grow until the water iswalmer than 10° C Likewise in oyster, Ostrea verginea, the lengthof body increases from 1.5 mm to 10.3 mm with an increase intenJperature from 10° C to 20° C. Sea urchin, Echinus esculentus,show maximum size in warmer waters. Coral reefs do not flourishwell if water temperature drops below 21° C.
Both extremely low and high temperatures have adverse effectson the growth of plants as well. Low temperatures bring aboutmuch cold injuries as desiccation, chilling injury, an~ freezinginjury among plants. In desiccation, tissues are dehydi-ated andinjuries are due to rapid transpiration and slow absorption of waterduring winter. Chilling injury is the killing of plants of hot climatewhen exposed to low temperature for some time. In some plantsof temperate climate, if exposed to low temperature, the water isfrozen into ice crystals causing injury to cells. Extremely hightemperatures cause stunting and final death of plants, which isdue to adverse effects on physiological processes liketranspiration, protein metabolism, etc. this is called "heat death".
vi. Temperature and Coloration: Some insects, birds and mammalsin warm and humid conditions bear darker pigmentation than theraces of same species in cool and dry climates. This phenomenonis called "Gloger's rule". In the frog, Hyla species and the hornedtoad, Phrynosoma low temperatures induce darkening. Someprawns turn light coloured with increase in temperature.
I Q. What is the best way to control pests? j
/cOvli :"'Temperamre'.land :Morphdlogy::Temperature' also" affe'cts the,." ?;',Z): Ie absolute' siZE!' of thEd:5bdy arYdthefelative pr6portions'of\Va~iOLJS
'.'; >~':'f!:C-body;parts of·)an'anima.'1: (Berg'man's:rtJle). For>1example,"de'er.of;;' " ',iii i),')" (Himalayas "has ;shOrter legs,' tail arid ears; ;'whereas,i:the related
:t ~,<'.;\ : : i ~species living'iri plain's have" relatively largEr extreriiiti'esi'! Eskimos;1:-i :i;:c',:l ':':'l)iWnich';live'ih1'cblde-r'le'giOn have short limbs' 'Birds'ihd;mammals
attain a greater size in colder parts than in warmer regions: lY,',", ,
Tl' ,,,I r, ("Y;;:fh~:':~~trkm'it1~~f brffi'~rt1Hlcirs';iike':t~ir;'; silc1U(iei:lW,~hll~gs are;'; ;ii: >Y;',',; 'Jr~ ~eIMiVJ'iy $M'9'a~nnf?tdlci~?'pt~rts';{8gn;jli\'va;rmEirb'bHf;(Alr~(~'{fLJi'e)::,.,~,'~;:,';::":f1~~i:~~~~B::~~~~~~~~;,:¥~~,~(g1iq~;~':f"~~,:~::~:;~:;~r~j:c'i";~;
, " ~ , ' -,- ." r " : 11 'f1 ~\ I \ _ : ' ;' r: ~ . • I • ',' ) t ,1 I - " ,
,I "I viii. Temp'eratur'e'and Phenotypic'Changes: Temperatuce IS known" ; \ -, •... ., - "<', (j I , ' r ~.' ' , I I, "., " " ' ,\'! .' (l I ~ t " ~. I •
. to' affect .the 'phenotype of' som,e 'anlma.ls. for. Instahc'e In: ":'.':) " :; f ~ .H,"I·' \ :.l' ":.1 : ~'" ~i; :-~ .:~ 'j-~ -. i: :",'-:!.:.c. :.;:..•.. \ "/(',: ': :' 'i ,., (,',;''';'! ~~.:.~,
Drosdphilla;the'temperature a:ffects ·the 'eye' size and the humber, , .. ' .,. of. legs,M9r.e,over"it;l.sollle animals. like ,PE\P~ni?,th~. body form
" " 'un~~yrg§es<.,differe~t~a~ioni·in ,rel~tio'r1,~R<~'e~sofla,l,shanges"i(l,'\ " ;ite,rnp~ri?-tu,rl?:", ,?fld),~is:! Rhtnpme,n0rJ.ls ,pa,lleQj Cyclomorph9sis,
'.: During,winter,t,h~l:1ea.9.of paphnia i;;, round,While,in springth;ere"",,~,e\!~'16p~'a ':.h~lrn·~t !N~'~)rQ~~'~:S;:PR'"the';~e,~9·w5!i~~.~!tain$ ~i!s
maximum size during sur:nmer"'1nQ:t~e ,Ile~t:winte,r\ :,the"he,adbecomes round again. Thus, degree of warmth of water and sizeof helmet are in a way co-related, V ~·.:'::..·~Ujh.n~ .\!
.Theamourifof moisture fallingorian area, regardlesscifits'physical farm(i.e'.whether'lt is liquid orfro'zen Or vaporOus) is 'known ks PRjdpit~tion.! Thus:precipitatidh 'may'be'in fhefohti ofdrizzlerain,:snow;deW;J slee(o(hAil. 'DrizzleinvOlve' nlinute "drops Which 'flOat i inai(lt is'of' rittle' importance"as very'littlemoisture penetrates the' soil because much of it eVClpO'rates·rapidly.Rainfalli\:"themost common formofp'recipitation: it consists' of drops of liquidwi:lterwhich'arelarger and .heavier. 'Gentle and steady rain is most effective and good for 'plantsa-ssoilwill'absorb J./ater effectively'whichwO'uld beth'gH a\~~ilaole to pla'llts;>';':
'(:';.'{ -;\,·1 ~ ..."i·:·"l !(~:~.'(.";:"1'~:::; c ["",;t)-~ ! .:j-,~-('
ATMOSPHERE-. \ .
Atmosphere' may' be;'defined as; mLilti-'layeredgaseous I envelopesurrounding our planet, the Earth, The atmosphere extends upto the height of480 km above the earth's surface. The rest above the atmosphere is calledExosphere or Outer space. In atmosphere,' aho'ut 95%'of the total air'is"present'intheregfon uptc> the' height of "aboUt '20 km' above the' earth's 'surface whileremainirig 5% in rest of ~he height above: 1;< ,i . '.' . :' ,.,; ,
I. -) ;.;. "j 1 ' .;:, " '.!' Ij ~.-:." -(':' _ ; , .' '; .'; ,; !-l J '-' 'r , ~; ~ : ;' i -1;'
;,The ~modern. atmosph~re can be looked I upon ' in: three.' ways; by'cOfTlP?sitiQn, ,by.,t~mperature and ~y function (Fig. 6.2). I)' '"j:', )( I'
By chemical composition the atmosphere may be divided into tworegion~, the Horposphere, and Heterosphere. The altitudinal zone from thesurface of the' earth to a height of about 80 km is called homosphere, The blendof various 'gases is uniform throughout, in this region: The only exceptions aretheconcentration -of' 6'zo'n'e in the "olone layer'" (at the height of about 18 ~ 50 km),and the variations in water vapours and pollutants. Heterosphere is definec asthe outer' atmosphere, which is not unifo'rrriin its gaseous content. It begins at
\about 80 km and extends onwards to the <;>uterspace.
By temperature, the atmosphere may be divided into four distinct zones,the Troposphere, Stratosphere, Mesosphere and Thermosphere, Each zone hasthe following features: . ", ; , .
i. Troposphere: The lowest layer of the atmosphere in which man with'other animals and plants live, is called troposphere, This is the onlylayer capable of supporting life on earth, It varies in thickness fromabout 8 km (towards the poles) to about 17 km (in equatorial region).On average, it is about 10 km thick, The' important events such as,cloud formation, precipitation, thunder, light.ning, etc. all take place inthis zone. The air present in this is composed of 78% nitrogen, 21 %oxygen, 0,9% argon, 0.036% carbondioxide, 0.001 % neon and0.0005% helium by volume. The air temperature in this zone graduallydecreases with height at a rate of 6.50 C per kilometer, Towards tileupper layers of troposphere, the temperature decreases upto - 57u C
K~'480' ;;,
, ... ,. !;":.,
OJ•...OJ..c .
.0.. .. -1/) ,oco
80 ,<])o '-(f) <1.'Q)-C
50 ~5}'(I)
OJ o '-•... ~ 0.'OJ ~£ OJ..c ~ "IJ) if, •...
20 0.. OJ\11' . '..c0 ~
0..
10 E I/)(]) 00 s::. cI Q.u-, 00 NQ. 00'-f-
(/
/
I(It\,
\\ ,. ,
----- . Stratopause
Ozone layer
. Fig. 6.2 An integral 'chart of our modern atmosphere by composition,temperature and function. The plot of temperature and scale tells thetemperature of any altitude.
157
'I' Stffltosphere: The second zone which is 30 km thick is known asI. ~
Strt\ll)sphere. In this zone. the temperature from a minimum of - 57°C rlt,E.lS to - 2° C.· the stratosphere exhibits several significantdirltlrflilces from troposphere such as, a) water vapours are virtuallyt\ll~\i-)Ilt.b) ozone is present in significant amount, c) thin vispy crystals01 h~ll present, and d) in fact, the reason the stratosphere becomeswt\rlnor with the increasing height is that ultraviolet rays absorbed bytl\{} Mone is transformed into heat.
\ ...' MOllosphere: Next to the stratosphere,the third zone of about 30 kmIII,
tllld\ is called Mesosphere. In this zone, the temperature shows again~,d\~nease upto - 90° C.
iv. ThMtnOsphere: Above the mesosphere is the thermosphere. Itt)xtr>nds upto the height of 480 km. It is characterized by a gradualri~~\lin temperature upto 1200° C and hence called thermosphere.
011 I~\tl \),,'1sisof function, we identify two zones, an Ozonosphere thatfilters harmful \\,~\Velengths of ultraviolet rays protecting earth's surface, and an
t f .,\'111\.\1 laver called Ionosphere. The ionosphere, which extendsou er UI1l· I' J
h hOlll tl\t1lt1\osphere, includes the region in which cosmic rays causet roug l'k d .. 'd I' h h I I. . t' \)111\,,!t)cules I e oxygen an nltnc OXI e. n lonosp ere, t e mo ecu es10nlza Ion . . .of air are so \\ i\kl)~spaced that h.lgh frequen~y audible sounds are not carned bythe atmospllt*), Knowledge of Ionosphere IS Important for radiO transmiSSion
and spactl tl;t\'1)l.
Tlll~ It):'\ Jt'o\'e 480 km height upt032, 190 km from the surface of our
I t'~ E\\".l\!lare or Outer space. Exosphere lacks atoms except that ofpane I::" "h d 1
'I' i 11t11iullland has a very high temperature due to solar radiations.y rogel h ,I
All !lIt) l;,yers of atmosphere are of interest to the ecologists sinceto ether tht1\' h'i111the total blanket of qirin the biosphere. Air is an important
g I . al \"','h"i Rnd also serves a medium for life. The importance of itseco ogle' ,I· . . , .
t't ell1 .~.\,~.~,like, oxygen, nitrogen, carbondloxlde IS. very well known.cons I U ~",
I o. How mut~~;I.~'~Planet's land is threatened by desertification? J
Air in motion is called Wind. It is an important ecological factor that affectslife of living organisms mainly, on flat plains, alohg sea coasts and at high altitudein mountains. For estimating wind speeds and their effects on land surface, theBeaufort scale is used as given in Table 6.1.
S. Beaufort Speed Land effectsNo. description .0 Calm. .Under 1 Smoke rises straiqht up1 Liqht air 1 - 3 Direction shown by smoke dirt2 Liqht breeze 4-7 Wind felt on face; leaves rustle3 Gentle breeze 8 -12 Leaves in constal1t motion; wind
extends liqht flaq4 Moderate breeze 13-18 Raise dust and loose paper5 Fresh breeze 19 - 24 Small trees and leaves begin to
sway6 Strong breeze 25 - 31 Large branches in motion;
whistlinq of teleqraph wires7 Moderate qale 32 - 38 Difficulty in walking8 Fresh qale 39 - 46 Break twiqs of trees9 Strong gale 47 - 54 'Damage to vegetation and
structures10 Whole qale 55 - 63 Trees uprooted11 Storm 64 - 75 Widespread damage12 Hurricane or Cy~lone 75 &.above Total damage
The velocity of wind is affected by such factors as geographic situations,topography, vegetation masses and position with respect to sea shores. Airmoves a region of high to tliat of low pressure. The pressure differences aremainly due to differential heating of atmospheric air.
Wind brings about a number of physical, anatomical and. physiologicaleffects on plants and animals. Some of the damages caused by the wind are asfollows: .
I Ans: 33 million sguare kilometers. \
i. The wind of high v610city may cause the breaking of liveCpr,q.'7'.ches.qlj. tree and sometimes complete uprooting.
~<)..';~~~~lr;rir 'tl:i):JEt Ih~~:r]O;o~:)if;t.lhe,qfY!! ;~i.: ;~l~i,bf11\:\/ bc)l~r~ (.~,ri~"):;U,;,j;; :i/, .':.l;,. :;fLli, !~;if!~HfQfl9j.YYI.D.P.~~tE9lTl\!I9t'\:9.~QSjt?Rtj,diPil,c;~.ti9[';)?f~f?IJ,iY-?!-!~t~,.~~errnan.emic' i, .~·,:)F,li :;<:a,lt.~[:aiip.Jl)n.,j~?fprm 8nq QQIJIP8.si1ipnjo!i~h9qt~.rf)hLC~'t.")D9:f.9(m~O,
deformations are commoll 10 !f,~,eJ~gro,'{Vjr)Q:a.1.9Qg..tQ,e.;se.g:C;gg~J~u:,;';,:)
.", • iii.' '., VioJentwinps"often''C8.({se flatteI1ing' or'lbdgir:lO I'of.'nlantslikewhe9t,~;:·,>"i!9~~.t-lo·)i'AR :. l i ••••.";\tf· '1,'1 t~, ••..:t"l'''··i r. ('.. 'maize;"oa( sugarcan~, etc'-'against the gr~u[l'~.: ..~"'._~~, ,. -;.,
.', . '." ._',':';;:::.:';~:"~~~";~,'''~~':~';;~;,:,;..)~"...;";';~r'i' . -:: ,~",_;.~:.'l\Y.~:~:':,~·r.:;.,.:;.•\) ,.~~\:.,l
.,.IV. q~Part(cle,s\of _solt~br,;lce,.canl.e.a'oy,"strongwi nds.may actas':abraslve,!1,i,~;,.,?/~~)r!:tQt.GJl'l:fY~0J)1~J:J.:l!:i'$.~Q~~?,J~(?Y~~,",9rJIQYY,e(~~C\rtd,f,wit~~i:hM[.b~e r.od'ed.
'~';;;',;~,~;';.:f.i~f;~;,~;~,~~~~~;;~~t~'a2gt~~;~~+~,~\'..r~l ." 'f"'<"' . "~"8<;:~~:~:~J~:1!j~~~~~'~ ;'v. Strorl.g'winds3.oause :~oil erosion ~nd make a land unsuitable for
~'-:'0:~~~~;·~~~:~~~I~~~1.~~!3f*!~~~~Jt~~~L'.:.'~~~~;',~"~~',~.:~~'=.":~~'~~J~:';:,.l:n:·i;·',;:i/.::~~··'. . ,:'vi. Along sea coast~.,.tbe salt water are carried along by strong winds
·andJ>,t,Jc.Qsc;l.lt~R~~V~,rave·.iQN~iouil·e;fects ·on.-some plant,s'growing.. fluik,:" r !!in. tll'Ei \lfCiHh' of;bc'eans. i t, L_' :; ..',. ,1.1 ,""-,
.w~·:~·:~·~:.~.\~.l~~.:U~::~:;~f".:\:~_.Y)!}dl-::~fl_~'.:_.!i~ . ',' ."... -c_ •• _ , ~ t
r.',~ .~i;~\~,f"P \:-': ,., ;j'. ....-1;' >:~' _ f'(-":' ":'" .', :·)J·r .. ·I ..·..:" ;n, .. vii....:rhe;Win\d-tlefi:)(matjpn~o'ften '.l:::auses the ,development,' oJ,dense,
, ") ".. - ~~ .• " .~I' \ 'I! 1 ,1 " \ -. r_.: • ~ I I . ' ,-' " 1''-'.''. '. 1...redOish':::type:.oU<Ylem"called'compressed wood. ,,':,'~; .'::" ':
;: !):-:(~ I !0;~i.~r:,}r~::.!\/ oj ()G~;';lfJl;(}:1 ;:.(; -- \-;. ! ~1;>:P ~;nc/~~::~ 'j
, ...._.viji,_. SJL9ng.w.irig.ii,'._c.~ij~~,:"~n,tf1c.rec;l.se. in "the. (i;l.te, of. ,evaporatibn ...and.' _.tr§.n~pirati~ni~l),n·d~rsuc,~·;t.irCl'l't'n~t~Q~.E3,S"pJ?Qt§'!~).U9:r.tftMi.hrai.ri;an
Jnt~!!1i:l!~.~';l~~r:-?~I9.:~~~CJ.~9~~us~S.uff,~c.!~omdesiccation.>' ";:~' ..80~;ir!i~t)ii"-;~(1-7- 'j""UuJ.;Zt~t :", -1· ."f.".";'_'; ~.~
"),~):,j ri:,Pl\~r~~GJ.r.fiWi,n.,9;:~-'l:cj;e,O~;$1;}gHY,ni')Hi ?f,,9)'ying )vj0.d,?: ge~E@lly: su'ffe rfrom dehydration and 'consequent loss of turgidity. Under these
ier IO,if)IJ!2' J~Rn>90L9~T·2.W~)LR~@~r,l,~,;9f'~~[T(fl'IR,v\{f:I1~9·~:"~(, \",;u;·j'. _ "
For centuries people h1i\je)icCf~ij'8ENM81iifeniH:f6~r~fS1SJRtfgmssl'ahds!i~~;destructive but research during past 30 years has revealed that properly used orIi'ght!Jil'E)s;(;:iq bEFB0(i)16£:j!(<ili.jitGlO IfGficg,Y,~t j QaIa~rrF-!Fe ils j\M~l (r6lb"dr'~al'1{liiWfo res tsand'J~rass;lan!Itst ~ lNontln.:Am§ricajb Indbnesia\o;Australi2l.'ra'ndr, South 1and' iE'a'~t,Africa. ,2vvo!loi
r·--··-·--·---·-·~·_-·--·--·--··-~-· ....~...~_._-----------~__ --t_M .'--....__._~-;.:~~~ii7;:~.:::...;.;::;;-~.::::';-i~~-GZ;~.~'(}:-How'm an -peopre-die-each e'arbe-cause"of-wate,cbom-ctisecrses? . ...L-._ --'------- "=~J
J': };;;i i"Th~E;~,condi)i9nsare f)e:ces~?ry)or setting Qfafire, ,;' : "l:!' c.
'-ll' i~:':;\"V ,-,:;':r I,'; .:" •. -,;; l '-~':I ·.....,n .. -,:,>;,' 1,>. ;,~:,"'j : .~: • ,
'" i,. anaccumulatioll of dry organic,matter sutficient to burn,:):.::''''~:!".;.: " ..",;.' ,,:-. "., ' .. d" .,,' .• ' ':1 f .. : ' .. ' . , I '
ii, "'dry w'a her COri<;Jitionstorender the material combustible, andiii, ' . a'sburce'of,ignHion:'" ", ','".~·,..;:::':rl::t.:.:.~ t:I:" ti;;);n~~_~'{1;'_~"/ ::"".~ n, :.. (, ...:: .. I .~The three important source of ignition are:
• ~t-, I •• :. : t • ~
\';, "': 'C), '/Ligh'tning:'Lighfning'lias been"the mostcom'm6ri source of igni'lon,,'., ," l~i': 'I .A.n n'Ljally '16'!'rrililloil" th'U'nderstorms 'occur on ear II, When lightning(,1 ~!1,1 ii)l'st~bk'es hit'the' ground, the dry material 'is kindled a:ndfires are set off.
According to one estima'te';7001cl'of ~iHE;'forestfires are caused by
,..1\:._i!' .. r",!.::.,\li'~"~it1~.ir~~·r~:;\· ~(~. ~C,,);t(")l::. ..•.:t~: ~l;"I:-:'II'l'il il: (/.; ~,-.. :')1'\'; :;:1')
;,' '""~.','ii,'" M~n\: lv1j~noft~,n,~,~t~ fi~~ 9¢lib~ra,tyly t9,ri:'9djfy tre el')vIronlllent for histil.,.:' ,.i; .,._ .(J. ,.I '}'1' 1 "., ,. ',' 'I.' -..... . ,,,1 .. ,' . l I, ) ...l" • . •
own ends such as" to clear ground for agriculture, to Improve.j ~:';ij!.;; (",t (~'d:~)'f : •.' ."~J:~! e.:.' ,1 ;'y';'.'~If: " :i '.'<t, ;.- ' ..~:... :'!"'<' :.: .'.. :
-, conditions for huntlllg, to Improve forage .for,;grl3.~lng.,t9 develop grassand shrub vegetation, to open up the coun'tryside, to build roads orhousing scheme?, ,to JedL!ce ,ambush, from e,n~mies, and to maketravel easier, Sometime fires result due to carelessness of man (eg,throwing off a cigarette butt), ' ,', '
iii, Sometimes, mainly in fore,sts, fires develop due to mutual friction\I : between trees (bamboo, etc,)
Fires are generally classIfied as:", ';; ,',a. Surface Fire: Surface fires are low-level fires that usually burn only
undergrowth and leaf litter on the forest floor, These are most'common type of fires and sweep the ground' surface rapidly, These
,,I. fires ki!'ls8~dlin'g's"cirid small' t'rees b'utdo not ki'll rDost mature trees,Wildlife can usually escape from'these fairly slow burning fires,
b. Crown Fire,: 'ThE)$e.,9-r~ ,extrer;n~ly hot fires. that burn groundvegetation and tree tops, They occur in forests where all fire has beenprevented,tor "several:years,:'aljowing the build up deadwood, leavesand other flammable ground litter, In such forests an intense surface
C,'.:;; 'il i:' ;',vfixe ,9[jy.en py str'()llg}b'irLd~(mqy,i1eap"up into the;can0l:W These rapidlyburning fires can destroy all, vegetation,killwildlife and lead toaccelerated soil erosion,
161
c. Ground Fire: These fires develop in such conditions where organicmatter accumulates richly as heaps. Thus, these fires which arecommon in bogs are flame less and burn particularly decayed leavesor peat below the ground surface. These fires may smoulder forweeks before being detected and are very difficult to extinguish.
The harmful effects of fires are:i.' In general, fire destroys an ecosystem by destroying primary
. producers (i.e. plants) and killing or driving out .consumers (i.e.animals). It must be noted that temperature that prevail when fire inprogress may reach upto 1300° F.
ii. Fire brings about marked alterations of environmental charactersSUcil as, light, nutrient cycles, rainfall, soil pH, fertility of soil and soilfauna. Sometimes habitats. need thousands of years to return tonormal conditions.
1. In forests where ground litter accumulates r.apidly, a surface fireevery 4 - 5 years is essential, because it burns away flammablematerial and thereby helps prevent more destructive crown and
. ground fires.
2. It reduces dead organic matter to soluble ash and releasesimportant nutrients such as, calcium, phosphorus, potassium,etc.
5. It stimulates the germination of certain tree seeds, several kindsof legumes and grasses.
I O. How long does it take to renew one inch of top soil?
162
6. Some wildlife species, such as deer, elks, moose, muskrat,quail depends on occasional surface fires to maintain theirhabitats and to provide food in the form Of vegetation thatsprouts immediately after fire.
7. Surface fires can be used to control outbreaks of tree diseasesand pests.
8. It, thus, helps in regulating ecosystem by elimination of certainspecies and favouring the survival and growth of desiredspecies.
A. Definition and importance: The soil is one of the most importantfactor called Edaphic factor. Soil may be defined as the upper layers ofearth's crust to which most plants are rooted and from where they derivetheir water and nutrient supply. In addition, soil is a means of support andshelter for terre'$trial animals. Most of them like nematodes, annelids,arachnids, insects, rodents, etc., live under the soil. A close examinationreveals that soil consists of numerous inorganic particles of various sizes,organic substances and a large variety of ·mi~'morganisms.
Thus, as most people think, soil is not a dead matter of mineralparticles or dirt but a healthy soil is alive and dynamic consisting ofmicroorganisms a~ bacteria, fungi, .nematodes, protozoans and insects .
.soil is as important to living organisms ,as water and air. The nutrients inthe food we eat come from the soil. It also provides us with wood, paper,cotton and many other vital materials and helps purify the water we drink.
B. Soil components: The components of mature soil are arrangedin a series of zones called Soil Horizons (Fig. 6.3). Each zone has adistinct texture and \ distinct composition that vary with different types "ofsoils. A cross sectionaj view of the horizons in a soil is called Soil Profile.
In general, most\mature soils have following four horizons, anorganic (the "0" horizon) and three mineral horizons (A, B & C).
i. 0 - Horizon: This ,is the upp~rmost horizon of the soil also. .: . 1'" J • 'I' j' . !
.' called Surface litter layer. This may aga,in be differentiated intotwo layers'~" ':, '.' :. ': "J ,,':) . :"); !.>:: ..'.' .
. '.: ',. 1 ~.,. ;"'" ! (.
01: It is top l'aye~O'f.sbii\c6'ri'sistrrig of freshly fallen litter(i.e. dead leaves, twigs, barks, flowers, fruits, and animal
i " ... : 'remains) which', ha's' not "yet :started decomposition. It ispresent in soils of forests, 'but, I absent in deserts,grasslands and cultivated fields.
-' ':''-.~ '. '.< ,1< >. ,,_<I ~~';;; ..;:~;; "..11 j, \.,.i:
r02: The layer' below litter ,Iaye(where the litter is brokeninto sma)1 fragments and is partly 'decomposed. In thislayer are present billions of microorganisms and animalslike insects, nematodes, worms, etc. are abundant. It isalso referred to as Duff-layer. , . ,
~ ., '/I.;~· !;. ,:Ij'(;., :,.:l./j ~.~l~r~'ijl·~'I(:·"':'1f
Fig 6.3: Hypothetical diagram of, the soil profile to show main
, " horizons.
1· I': "'1. In" 1\(1_ ';."\(\,"\( '_I/f)l i·"ll"j.-" •..J I' 'Ti (11 .;-·"'i: ... <.-.) r ~j
i: J ~i"'i:A,~HRr,i~9n:Try,,A}:Rri~prof Top-soil I,~yer",i?:LJ~uallyaporous mixture of partially decomposed organic matter
. ,_., - (humus), 'Iiving organisms, and some inorganlt~-mlnerai',
Ih-t. ":., )' , .' '. . \ .,' : . \ . . .... ~-'
particles.- Normally it is darker and looser than deeper layers,Thi~m~y be divided' intc/ two sUb-lay~rs, ' ':, ,
II! 'I
'. " A 1: In this' laye'r'. the' 'plant and animal remains are'; '~om'pl~;tely dec6rnposed' forming'" light amorphous
subst'ance,"grey t'6 black in colour,'called 'humus, Humus,'being sticky, coat~ and binds·''t'he . ;:lqd, . ;:: r''1Q Clay
p~i1:icle's-in' t'op~'oii''together j 110 clumps. A fertile soil, has a; '" mid< top~bii lay~r with high cdnteiit'of hurhLi~. This region,
," having a 'mixture ot 'h'umus "cindmineral particles is also: '\. 1 ", - ,: r I '\
" termed as humid region: ' .1. '. I"!: \ .. I. : ("]" ':-::,:.I, ',I I'
'A2: T'his regiOn :j'sof lighY colour, The mineral particles inthis'reglon' are of larger size a~d is wit'hv'ery little humus,Most of the minerals like silicates, clay, oxides of iron andaluminum move or leach down at a faster rate and hencethis region is called Leaching zone.
, ' .',
iii. B - Horizon: In this horizon, the mineral cotnponents of soilbecomes hard due to cementing action of iron. Calciumcarbonate deposits may be f6und in some areas, Tile organicmaHer is very poor. The roots of shrubs and big trees usuallyreach uplo this region. This region, also called Sub-soil layer,is poorly developed in dry area~, '
iv. C - Horizon: It is light coloured and completely lack organicmaterial. It is composed of partially broken, down inorganicmaterial of parent bedrock,
, ,
v. R - Horizon: This, the parent, un-weathered bedrock. It isimpenetrable layer, except for fractures.
C. Process of soil formation: How soils are formed? The pmcess ofsoil formation may be divided into two phases, namely a) weathP: ,n9 dnOb) maturation.
I. Weathering: The weathering is a process by which large rocks arebroken down to small particles or fragments, called Regolith or·Fragmentation. This is a long-term process occurring mostly under theinfluence of the climatic conditions of the area, and hence calledWeathering. The mechanical or physical process of weatheringincludes action of water, temperature, glaciers, gravity, etc. Thesecause weathering of rock through such processes as wetting-drying,heating~cooling, freezing, glaciation, sandblast, etc. The chemicalprocesses of weathering include hydration, oxidation, and carbonationof mineral compounds in the rocks by the action of weak acids, forexample, carbonic acid. Traces of sulphuric acid and nitric acid alsooccur . in certain regions and' influence weathering. Biologicalweathering includes the action of various, particularly, lower plantslike lichens and mosses which secrete various organic acids andproduce humic acid after death and decay. These acids help inweathering of rOcks.
ii. Maturation: During weathering, the rocks are broken down into smallparticles but this is not enough and plants can not grow in this matter.The weathered material undergoes a number of changes, which is avery complex process called Pedogenesis. Whereas in weatheringmostly physical and chemical factors are involved, maturation islargely a biological phenomenon. During this phenomenon, livingorganisms, such as, lichens, mosses, fungi, bact~ria, algae, micro-arthropods, molluscs, ~tc. as a result of secretion of organic acids,enzymes, addition of organic matter after death, bring aboutgeochemical, biochemical and bib-physical processes. Due to allthese processes, the crust of weathered rock is converted into truesoil consisting of a complex mineral matrix in association with avariety of organic compounds, and a rich microorganism population.
Hifi
D. Soil types: There are several systems of soil classification on thebasis of the mode of their formation particularly the nature of the origin ofmineral matter, sqil are sometimes classified as a) Residual and b)Transported soils ..
Residual soils are those in which the whole process of soilformation i.e. weathering and maturation both occurs at the same place.Thus, the soil formation occurs at a place where parent or bedrockpresent.
Transported soils are those where the weathered material isshifted to different places by various agencies. The maturatio(l of soiloccurs at new place. Depending" upon the nature of agents, thetransported soils may be named as Alluvial (if carried by rivers andstreams), Moraine (by glaciers), Aeolian (by wind), Colluvial (bygravitational forces as landslides)Lacustrine (by wave action of standingwaters as lakes), and Marine (by oceanic waves).
On the basis of texture Of mineral particles, the soils may beclassified as,
i. Sandy soils: It mainly consists ~ofsand particles. Sandy soil isloose and dry. Its water holding capacity is poor. Rainwater israpidly absorbed by it but also rapidly pulled very deep in earthby the force of gravity. The nutrients are also very poor. It isnot suitable for plant growth.
ii. Clayey soil: It mainly consists of clay particles. Such particlesremain closely fused together and have a very fine inter-spaces. Therefore, water and air can hardly penetrate throughit. Such a soil is not good for plant growth.
iii. Lqam soil: If a soil contains equal quantity of sand, clay andsilt, such soils are good for plant growth.
iv. Sandy Loz:m soils: If Sdlid is relatively more in a loam soil. itis also good for plPontgrowth.
v. Silt Loam soil: Ifsiltis'relatively diore in aiiba'm soil:::'lt is not',,': suitable for plant, growth,.as::itl bec6mes"water Ilogged on
' .. ,',-..;:...; getting water.>,'". ';';'''' I,;, ,,, ,';'- ;j"H.. !::;> l' )j;: ,:;I ;, ,'1 : '.j,
.?li;:I.":, l>~··!·l;··:(i~"·:';·'l
E. Properties of Soil: As pointed out earlier, soil is not merely• i composed .oJ mineral particles but!iUs,a: ver,ycomplex ,structure and is
90fl]posed.: of ,s!?yeral, :compo.n,entpL'!J.h~ls9iIJ;.influences. plants more.. ".<:Jir~ct!y Jt~qr t.~.~.:anim_~I~;Ih~,n9t.~r,E?co(·P,9-r~rJt-ge~\()ck;geterrT)ines the
availability of nutrients to plants and also the physical properties of thesoil. For example, alluvial soils are more fertile as minerals and 'organic
.' ',,;' matter ?re! C:.~cjed,toth,e;lJl,a,loJ;19riye!.lc:;ou.rs~.qlJjth~,Dt8!3r hand, aeolian.'.1. ~. ~oil~~r~,.~nsui~able ~1nq:pqqr I!n.~u~r!en..ts:?;~d,;r~nYle\suPf?,~,~t,l.very little
vegetation. The various Important properties that Influence plant growth'are'as und·e~.' .' .' "C," ,'-' .. ' .' ::,.,' .':, "''"'' '. ", .," .:~",.' :.~). ~',; ,,~"j L'.; ";i' ,":j~f~': ~ll.:: '} '" "'I :.l.: \.!....i"~. II' ' ':1.,' ~,
I ~,,", r 1 " : , '" I ' '~ • ( , • '
i. Soil Texture: The soil texture' refers to the proportion of mineral. .: particles 6fvarious'sizes.· Depending upon tHeir size (diameter)
basis, different names have been given to these particles:
\Clay ".................... less than 0.002 mmSilt 0.002 - 0.02 mmSand (fine) 0.02 -: 0.20 mmSand (coarse) 0.20-2.0 mmStones and gravel : · above 2.0 mm
./ ,., , . i"
Soil texture helps determine. soil, porosity: a measure of thevolume of pores per volume of soil. and the average distancesbetween those spaces. The soils may be called clayey, sandy
. loamy (soils containing a mixture of clay, sand, silt and humus arecalled loams) or by similar names according to the proportions ofvarious particles. The larger pacticles are inert and the amount ofwater that can be held. in .the. spac;es. between these particles isvery little. Thus, the sandy soils can not support a good vegetationthough the roots grow ea?ily afld branc~profusely. The clayparticles are colloidal in nature and being small in size leave thincapillary spaces between the particles. The clayey soils, therefore,have a high water holding capacity. The actions of mineralsubstances get easily absorbed on the clay pElItdes and becomeavailable to plants.' .
IMj
However, the clayey soil has a disadvantage. When wet the clayparticles imbibes water, swell and become sticky. On drying theparticles remain tightly stucK together and large cracks develop inthe soil, feading to injury to the' roots. Th~ loamy soils 1"'''''::' ~mixture of sand and clay comhinp. lrC' r-:,v,·~'''2:- "'; tWU types 01particles and ~::' L,,:;~' r·~;:t",G .')1 piant growth.
ii. Soil Atmosphere: Gases found i~ pore spates of soil profile formthe soil atmosphere. The soil atmosphere contain the main gasesnamely, oxygen, carbondioxide and nitrogen. Soil air differs fromatmospheric air in having more of moisture and carbondioxide andless oxygen. The soil atmosphere is affected by temperature,atmospheric pressure, wind, rainfall, etc. Loam soils with humuscontain a normal proportion of air and water (about 34% air and66% water) and therefore are good for majority of crops.
iii. Soil Water: The main source of soil water is precipitation (i.e.rainfall, snow, dew, sleet, etc). On the basis of retention capability,the soil waters have been classified as under:
a. Gravitational water: In a well saturated soil, the extra amountof water displaces air from the pore spaces between soilparticles and fill:':'::y it :s ;:.. I (Jil"IIi'ltnd in rore spaces. Thisaccumulated excess water of large SOil spaces IS caileuGravitational water. When this gravitational water furtherpercolates down and reaclles to t11r' levels 01 bedrock, it iscalled Ground water. The upper surface of ground water iscalled Water-table. Th~se waters are ecologically important inthe leaching of nutrients. '
b. Capillary water: The water which is held by capillary force(i.e. surface tension and attraction of forces of watermolecules) in smaller soil channels, when the gravitational andground waters have been drained, is called Capillary water.Capillary water occurs as a thin film around soil particles in thecapillary spaces and represents the normal available water toplants. It remains in soil for long periods and carries with itnutrients in solution.
I Ans: 83 % (78% from fossil fuels; 5% from nuclear plants). I
H)t>
c. Hygroscopic Water: This water is held tightly by the smallparticles of the soil as a result of cohesive and adhesive forcesand hence can not be easily absorbed by plants.
d. Water vapour: Some water in soil is present in vapourousstate, in the pore spaces between soil particles.
e. Combined water: This is the form of water present ashydrated oxide of aluminum, iron, silicon, etc. in the soil. .
All the water present in soil is not available to plants. The totalamount of water present in the soil is called Holard. The amount ofwater that plant roots can absorb out of holard is called Chresard.While the amount of soil water which can not be absorbed by plantroots is called Echard.
iv. Inorganic elements of the soil: The main inorganic elements ofthe soil are compounds of following elements such as, aluminum,silic;on, calcium, magnesium, potassium, iron and sodium. Soilalso contains smaller amounts of compounds of the followingelements, Iike_ boron, manganese, cobalt, copper, zinc,molybdenum, etc. Most of these inorganic salts exist in the soil inthe form of weak solutions. Soil solution may contain complexmixture of minerals as carbonates, nitrates, sulphates, chlorides,etc. The chemical nature of soil solution depends 'on the nature ofthe parent matter through which water has percolated and Climaticcpnditions of the region.
v. Organic matter ot soil: The chief organic component of soil isHumus. It is black coloured, odourless ana homogenous complexsubstance. Chemically, it contains aminoacids, proteins, purines,aromatic cOr1!pounds, sugar, fats, oils, resins, tanning, etc. It isderived from the decomposition of dead plant and animal remains.The colloidal properties of humus help to improve the structure ofsoil, by increasing the water holding capacity and reducing theaggregation of mineral colloids and also by providing anions whichget abs9rbed on the humus colloids. The insulating prQperty of theorganic matter reduces the .temperature iluctuations witTiin the soiland helps in better root growth.
170
vi. Soil organisms: The large variety of organisms inhabiting the soilsuch as, bacteria, fungi, ascomycetes, algae, protozoans,nematodes, earthworms, molluscs, insects, etc. help in thedecomposition of organic matter and release of nutrients. Amongthese,
a. Several bacteria, like Azobacter, Clostridium and blue greenalgae like Nostoc, Anabaena, etc. help in soil fertility throughnitrogen fixation.
b. Some bacteria and fungi are known to secrete growthhormones, thus promoting the plant growth.
c .. Some blue green algae and bacteria produce mucilaginoussubstances, which help binding particles into largeraggregates.
d. BurrO\ying worms like, earthworms and others besides helpingin humus formation, also improve aeration of soil. \
e. Some fungi, like Actinomycetes prefer saline soils and bringabout the decomposition of cellulose. They produce antibioticsof great economic value. .
f. Some fungi and bacteria, as well as animals like nematodes,are serious pathogens of many root plants. .
vii. Soil pH: Many chemical properties of soil centre on soil reaction.As regards, their nature, ~ome soils are neutral, some are p,cidicand others basic. Again, different plants have different toleranceranges towards soil pH. Typically, soils range between a very acidpH of 3.0 and a pH of' 8.0 very alkaline. Soil acidity has apronounced effect on nutrient availability. As soil acidity increases,the proportion of exchangeable AI+ increases and Ca+, K+, Na+,and others decreases. Such changes bring about not only nutrientdeprivation but also aluminum toxicity ..
I Ans: 5 - 20 percent (average 10%); 80 - 95 percent lost as heat. I
171
Soil pH strongly affects the microbial activities, as below pH 5.0, .baCterial as well as fungal activities are reduced. Highly alkalinesoils i.e. those with excess of carbonates and bicarbonates ofcalcium, potassium, and magnesium are unfit for plant growth.Some plants grow on soil with high salt concentrations. Suchplants are called as halophytes. Neutral or slightly acidic soils,however, are best for the growth of majority of plants.
Beside abiotic, climatic, and edaphic factors discussed abovethere are a number of biotic env~ronmental factors that affect thedistribution and abundance of plants and animals and regulate thedensity of population from time to time which take place betweenthe different organisms in a habitat. The various types of bioticinteractions are given in next chapter.
I O. How much of the earth's supply of freshwater is available to us for our u~? I