Growth and Reproduction of an Organism | Ecology

The following points highlight the fourteen main factors affecting growth and reproduction of an organism. The factors are: 1. Troposphere 2. Stratosphere 3. Mesosphere 4. Thermosphere 5. Exosphere 6. Homosphere 7. Hetrosphere 8. Air Composition 9. Gases in Water 10. Light 11. Temperature 12. Water 13. Soil 14. Fire.

Factor # 1. Troposphere:

This represents the linear portion of the atmosphere that extends to about 8-16 km height from the surface of earth. It is thin in the polar regions i.e. about 10 km from earth surface. Only troposphere contains life.

It contains more than 90 per cent of gases in the atmosphere. Nitrogen and oxygen are the most aboundant gases in the troposphere, constituting 78.0 per cent and 20.9 per cent of the volume of total gases, respectively.

Generally temperature decreases with increasing height up-to tropopause, i.e., top of troposphere. The average temperature near the soil surface in about 15°C. The temperature lowers down to -57°C at the tropopause, which denote the transition to the stratosphere.

Factor # 2. Stratosphere:

The second layer of air mass above tropopause is called stratosphere. It extends upto 30 – 50 km. There is little mixing of gases between the troposphere and stratosphere. However a thin layer of ozone is present in the stratosphere at the height of 15 to 30 km.

The uppermost layer of stratosphere is stratosphere, which is the transition layer between the stratosphere and the mesosphere. In this zone, the temperature shows an increase in temperature from a minimum of about – 60°C to maximum of 5°C. The increase in temperature is due to ozone formation under the influence of UV (ultraviolet) rays of solar radiation.

Factor # 3. Mesosphere:

The third layer of atmosphere next to stratopause in called mesosphere. It extends up-to an altitude of 80 km and shows a decrease of temperature with height. The mesosphere is characterised by low atmospheric pressure and low temperature.

The temperature begins to drop down from stratopause, goes on decreasing with the increase in height, and seconds a minimum of about – 95°C at a level of 80 km or above. The upper limit of the mesosphere is called mesopause.

Factor # 4. Thermosphere:

Next to mesosphere is thermosphere, which extends up-to 500 km above the earth surface. Thermosphere is characterised by steady temperature increase with the height from mesopause.

The thermosphere includes the regions in which ultraviolet radiations and cosmic rays cause ionisation of molecules like oxygen and nitric oxide. This region is called ionosphere. In ionosphere, molecules of gases are so widely spaced that high frequency audible sound is not carried by the atmosphere.

Factor # 5. Exosphere:

The rest of the region of the atmosphere above the thermosphere is called exosphere or outer space, which lacks atoms, except those of hydrogen and helium. This extends up-to 32190 km above the earth surface. Exosphere has a very high temperature due to solar radiation. The earth’s magnetic field becomes more important than gravity in distribution in of atomic particles in the exosphere.

Factor # 6. Homosphere:

It extends up-to 100 km from earth’s surface. It is homogeneous due to air circulation in it.

Factor # 7. Hetrosphere:

It is the outer region of thermosphere, which bears gases mixed with radioactive substances.

Factor # 8. Air Composition:

Nitrogen and oxygen are major gases in the troposphere, where nitrogen is 78 per cent while oxygen 20.9 per cent. The remaining 1 per cent consists of Argon, water vapour, carbon dioxide, ozone and other gases.

Though water vapour, carbon dioxide and ozone (O₃) occur in minute quantities in the atmosphere, yet they are essential for maintaining life on earth.

Ozone occurs only in minute traces in the atmosphere and has a peak concentration around 25 km above the surface of the earth. At no level is its concentration more than a millionth of the main atmosphere. If the entire ozone content is compressed at normal temperature and pressure, it will constitute a layer only 3 mm thick.

But it has played a vital role in the evolution of life. The life sustaining role comes from the fact that radiations below 300 mm are biologically harmful, the UV radiation. Any large scale depletion of the ozone content can have devastatory effect on all biological systems, including plants.

Water vapour regulates the hydrological cycle, which sustain life in both terrestrial and aquatic systems. This also absorbs infrared radiation from the earth.

Carbon dioxide, water vapour and ozone play an important role in maintaining the heat balance of the earth. The composition of the atmosphere is a product of the activities of living organisms.

Factor # 9. Gases in Water:

In the aquatic systems, oxygen (O₂), carbon dioxide (CO₂), and other gases are partially dissolved in water.

In water bodies, oxygen may be a limiting factor for the growth of phytoplankton, and other aquatic organisms, generally in deep lakes, which receive heavy loads of organic materials.

The oxygen supply in water in regulated through diffusion from the air and from photosynthetic activity.

Carbon-dioxide is highly soluble in water and may be found in variable amounts. CO₂ combines with water and results in carbonic acid (H₂CO₃). The carbonic acid reacts with available limestone and form carbonates (CO₃^-2) and bicarbonates (HCO₃^-1).

Factor # 10. Light:

Light is the most essential abiotic factor without which no life can sustain on earth. It is non- lethal limiting factor both at the maximum and minimum levels. The main natural sources of light are-sunlight, moonlight, star light and light producing or luminescent organisms.

Light affects many physiological activities of the plants, e.g. photosynthesis, respiration, opening and closing of stomata, growth and flowering of plants, etc. Similarly, light as a complex physical factor, affects the colour vision, eye-size, skin pigmentation, reproduction and biological periodicity of the animals.

Electromagnetic Spectrum:

Sun is the ultimate source of energy for most organisms on earth, directly or indirectly. The radiant energy coming from the sun in the form of visible spectrum is called light or luminous energy. When the visible sub-light in passed through a prism, it is disposed into a series of wavelengths exhibiting seven different colors, i.e., violet, indigo, blue, green, yellow, orange and red (VIBGYOR).

Solar radiation, before entering the atmosphere, at 83 km above the earth surface carries energy at a constant rate of 2 cal cm^-2 min^-1 in known as solar constant.

Solar spectrum comprises short wave radiation, light and long wave radiation. The short wave radiations include cosmic rays. X-rays and ultraviolet (UV)rays, which have wavelengths shorter than 0.4 to 0.7 mm. This is also known as photosynthetically active radiation (PAR). However, the infrared wavelengths are longer than 0.740 mm.

The ultraviolet (UV) radiation wavelengths are 0.1 mm to 0.4 mm. They are strongly absorbed by ozone (03) layer present in the stratosphere, and only a small fraction of UV radiation reaches the surface of the earth.

As regards to their wavelength, these are three types of ultraviolet radiation.

These are:

(i) UV-C (0.100 to 0.280 mm),

(ii) UV-B (0.280 to 0.320 mm) and

(iii) UV-A (0.320 to 0.400 mm). Out of these three radiation types, UV-C radiation type is lethal, and UV-B is harmful to the organisms.

Earth is a very small object in solar system and if receives only 1-50 millionth part of the total solar radiation reaching the universe. The journey of solar energy from sun to earth surface is very much moderated while if passes through stratosphere and the atmospheric envelope surrounding the earth.

In the strasphere lies the ozone layer which absorbs the harmful solar radiations of short wavelengths. At any place on the earth surface the energy flux varies with season, time of day and atmospheric humidity and drought.

From the ecological point of view, the energy and its utilisation is most important in sustenance of life. Many of the environmental phenonmena, such as movement of wind currents, rainfall, chemical actions and organic behaviour are controlled by energy.

Effects of Light on Plants:

Light affects many physiological activities of the plants the quality, the intensity and duration of light are important to organisms. Here quality of light means, its wavelength, the intensity means, and energy measured in calories and the duration means, the length of day.

The intensity of light varies with latitude and time of the day.

Light affects the processes of:

(i) Photosynthesis,

(ii) Respiration,

(iii) Opening and closing of stomata,

(iv) Growth and flowering of plants,

(v) Movements,

(vi) Germination of seeds and

(vii) Reproduction.

Light is also an important factor in distribution of plants. Some plants grow and flourish in full sunlight, while others prefer shade and called shade loving plants.

The quality of light plays an important role in flower induction, seed germination and plant movements.

The duration of light regulates the phenological processes, such as flowering and fruiting in plants. The timing of seasonal activites of plants in relation to change in environmental conditions is called phenology.

On the other hand, in many animals, migration, hibernation and reproduction are controlled by the relative lengths of day and night. The photoperiod is known to affect breeding behaviour of insects, certain birds and mammals.

The animals are divided into three categories:

(i) Short-day animals, e.g. sheep, deer, etc.,

(ii) Long-day animals, e.g., turkeys, starlings, etc., and

(iii) Day-neutral animals, rabbit, ground squirrel, etc.

Effects of Light on Aquatic Systems:

Life activity under water is often controlled by the availability of light.

Light makes a limiting factor for plants in deep waters, e.g., in oceans and deep lakes. In aquatic systems, the pressure of light determines where producers and consumers are to live in water for example, there phytoplanktons (phyto=plants, plankton = small) live in illuminated surface layer of water, whereas benthic organisms are found in the sediments of a lake.

According to penetration of light, a deep lake can be divided into three zones:

1. The Littoral Zone:

This is shallow water around the edge of the lake that supports rooted vegetation. The light penetrates through shallow water.

2. The Limnetic Zone:

The open water zone found beyond the littoral zone in called limnetic zone. In this zone, phytoplankton grows in abundance,. Here light can penetrate, 20 to 40 m depth depending upon clarity of water.

3. The Profundal Zone:

The dark zone, where light does not reach, is called profundal zone. The sediments at the bottom of lakes and ponds form the benthic region which makes a habitat for benthic organisms, such as snails, slugs and micro-organisms.

Factor # 11. Temperature:

Temperature is a variable factor which is influenced by time, season, latitude, altitude, slope, direction, soil texture, plant cover, and human activities, such as urbanisation and industrialization.

It penetrates energy region of the biosphere and profoundly influences all for and of life by exerting its action through increasing and decreasing some of the vital activities such as the metabolism, reproductive behaviour, embryonic development and growth.

Temperature is a measure of intensity of heat. In terms of standard unit its is commonly expressed as degrees either in Fahrehenit or Celcius scale. Heat is a form of energy, called thermal energy.

Thermal energy is exchanged between animal and environment by radiation, conduction, convection and evaporation. These four basic needs of transfer of heat energy occur within the organisms and in the interface between the organisms and their environment.

Temperature is the degree of hotness or coldness of a substance. The vertical temperature gradient over earth’s surface is called lapse rate, the value of which is 6.5°C per 1000 m elevation.

Temperature has an emphatic effect on the climatic conditions, growth responses of plants and activities of organisms.

Temperature also acts as a stimulus for plants, determining the timing of their development, for example, thermoperodism (i.e., a day-night temperature differential) is essential for optimum growth of some plants.

Exposure to cold, (i.e., vermalisation) enhances seed germination, as well as induces flouering in some plants.

Thermoregulation and Homeostasis:

Temperature also controls the rate of processes inside an organism, and other activities.

In cold-blooded animals, also known as ectotherm, the body temperature tends to match with the temperature of the environment, in which they live.

Many active ectotherms, e.g., frogs and snakes, control their body temperature by moving around or by seeking shade. While some ectotherms are nocturnal in habit and they feed during the night.

On the other hand, the endotherms, i.e., warm-blooded animals, such as birds and mammals, regulate their body temperature by physiological processes and maintain a more or less constant temperature within their bodies even when the temperature of the environment fluctuates.

The warm-blooded animals (endotherms) have certain physiological mechanisms to keep their body temperature constant or within tolerance limits.

The maintenance of relatively constant internal environment is called homeostasis. To maintain a constant body temperature in the case of endotherm or warm-blooded animals provides them a metabolic advantage over other organisms.

This is an established fact that biochemical pathways and enzymes often function at their maximum at a temperature about 37°C in endotherms, and therefore, these animals may remain active even under cold conditions by exhibiting different types of adaptations to minimise the loss & heat.

Thermal Stratification in Lakes:

Differences in temperature of at different depths in deep lakes, result in thermal stratification.

During summer, temperature is higher in the surface water, which is separated from the deeper water mass by a thermocline, i.e., a zone of gradual change in temperature. During summer, surface water bears a higher temperature, which is separated by deep cold water by thermocline zone.

A thermocline often creates two different layers:

(i) Epilimnion, and

(ii) Hypolimnion.

i. Epilimnion:

The epilimnion represents the upper layers of water. During winter, in a temperate lake water is at a freezing temperature on the surface. The surface water is cool during autumn and warm in spring. This results in a free mixing of autumn and spring turnover. This turnover redistributes oxygen and nutrients, resulting in a bloom of shytoplankton growth.

ii. Hypolimnion:

The hypolimnion represents the lower layer of water. During winter, the temperature of lower layer remains about 4°C in temperate lakes.

Due to stratified conditions, both in winter and summer, growth of phytoplankton in low due to low nutrients and oxygen supply.

In terrestrial environment temperature fluctuations are varied and marked. Lowest temperature recorded for any land mass is -70⁰C (in Siberia). Higher temperature, may often reach up-to 85°C in deserts at noon. In Rajasthan, the highest temperature exceeds 50°C. Water in hot-springs and geysers may be as high as 100°C.

The temperature varies from place to place and likewise the vegetations of different areas also differ considerably. Desert plants grow in extreme heat; aquatic plants grow in low temperature range, and grasses prefer to grow in the area of moderate temperature.

Factor # 12. Water:

Water is one of the most important climatic factors. It affects the vital processes of all the living beings. This is the sole agent that sets in motion the nutrients of the soil and makes them available to plants. It affects the morphology and physiology of the plants. It in combination with other factors regulates the structure and distribution of plant communities.

In nature, water may be found in vapour, liquid and snow or ice states. In the atmosphere, water is found in the form of vapour. The quantity of water retained in the atmosphere depends on temperature and wind. Vapour increases in the atmosphere if the temperature rises and pressure decreases.

At certain temperature and pressure, the maximum water-laden air is called saturated atmosphere. At saturation point, if the temperature is lowered the water holding capacity of atmosphere is reduced which causes the condensation of water vapour in the form of rain drop, dew, front, sleet, snow, etc. This is called precipitation.

The total amount of water on earth remains the same. However, it moves from one place to another and regulates the climate through its role in rainfall distribution and temperature modification. It has significant effect on vegetation type and its composition.

Hydrological Cycle:

Water of atmosphere reaches to the earth surface through precipitation, and from these again its reaches to the atmosphere through evaporation and transpiration. Thus, a continuous circulation of water from earth to atmosphere and vice versa is maintained in nature. This is called hydrologic cycle. The hydrological cycle is the movement of water between aquatic systems, air and land.

Water vapour, an important constituent of the atmosphere, can condense to form clouds, and eventually rainfall. Plants play an important role in the hydrological cycle through the process of transpiration. In tropical forests as much as 75 per cent of annual rainfall is returned to the atmosphere by plants, which recharges the atmospheric water vapour.

Rainfall is uneven on earth’s surface. For example, deserts receive very low rainfall, (i.e., less than 100 mm year^-1) each year, while same areas receive high rainfall, e.g., in Cherrapunji, average rainfall is more than 11,000 mm per year.

Atmospheric Humidity:

The water vapour present in unit volume of air is called absolute humidity. This is expressed in terms of percentage of water vapour present in unit volume of air at certain temperature. The amount of water vapour required to saturate the same unit volume of air under constant physical conditions is called relative humidity.

This is affected by light intensity, temperature, wind velocity, vegetation and soil water. Relative humidity (RH) is measured by an instrument called hygrometer. It plays an important role both in life of plants and animals.

Plant-Water Relations:

For land plants, the main source of water to soil is rainfall and melting of snow.

The potential energy of water is referred to as water potential. This is the source with which soils hold water and it is quantified in terms of pressure.

At sea level pressure exerted by the atmosphere is one bar which equals about 0.1 megapascals (MPa).

The upper limit of water availability of a soil is called field capacity. The water potential of a soil at field capacity is about − 0.01 MPa.

The lower limit of water availability of a soil is called wilting point. The water potential of a soil at wilting point is about − 1.5 MPa.

While the osmotic potential in the roots of plants is responsible for the entrance of water in the roots from the soil.

Plants have adapted to variations in water availability by habitat selection (e.g. xerophytes, mesophytes, hydrophytes, etc.) phenological adjustments, or by having higher water use efficiency.

Factor # 13. Soil:

The earth’s crust is complex in composition and its surface is covered with the soil which supports rich and highly diversified biotic communities.

Soil is defined as the unconsolidated top or superficial layer of earth crust lying below any aerial vegetation and undecomposed dead organic remains and extending down to the limits to which if affects the plants growing above its surface. Beneath the soil, lie the subsoil and un-weathered rocks.

Soil is a stratified mixture of inorganic and organic materials. The inorganic or mineral constituents of soil are derived from some parent materials, the soil forming rocks by fragmentation or break-down of weathering and the organic constituents of the soil are formed either by decomposition of dead remains of plants and animals or through metabolic activities of living organisms present in the soil.

Weathering of rocks is the initial step of soil forming process which is brought about by:

(i) Physical factors, such as temperature, water, ice, gravity and wind,

(ii) Chemical factors, such as solvent action, hydrolysis, oxidation, reduction, carbonation and hydration, and

(iii) Biological factors.

Soil plays an important role in plant growth by providing water, nutrients and anchorage.

Soils support the growth of crops, grassland and forests on which we depend for food, fiber, wood and building materials.

Edaphic factors are those which are dependent on the soil, such as soil constitution, soil water, soil air, soil organisms, etc.

Soils at different places vary considerably in their structure, components and properties. These differences in the soils are often largely responsible for differences in vegetation within the same climatic region, and consequently of great significance in the distribution of plant communities.

The mineral composition of soils depends on the minerals in the parent material and the extent of weathering.

Soil Profile:

The vertical layered structure of soil is called the soil profile. It develops due to weathering process, accumulation of organic matter and the leaching of mineral matter.

These are four main horizons in a soil profile: the O-horizon is the organic layer composed of lead organic residues, found above the A-horizon.

The A-horizon, also called the top soil, is the uppermost mineral layer, which also contains roots and partially decomposed organic matter. The A-horizon designates the top stratum which is subjected to marked leaching.

The B-horizon is the subsoil, lying under A-horizon has little organic matter, very few plant roots and a sparse micro-flora and fauna. In it, iron and aluminium compounds are often accumulated. A and B-horizons collectively represent the true soil.

The C-horizon represent, the less weathered parent malarial. In this layer the organic matters are present in small amounts and little or no life is present.

The development of soil profile is mainly governed by the climate and the kind of vegetation.

Here, in the following figures, the soil profiles of a temperate deciduous forest and tropical rain forest are given:

Characteristics of Soil Profile:

Soils in forest, grasslands and deserts differ markedly in colour, clay, organic matter content and depth.

In desert soils, the top soil (A-horizon) has little or no humus, while sub-soils (B-horizon) make a mixture of sand, clay, minerals and salts.

However, in the grasslands the roots of native plants penetrate deep in the ground, forming a dense sod that holds moisture and prevents erosion. This adds large amount of organic material to soil each year.

While in temperate forest soils, the top soil (A-horizon) is a rich mixture of humus and inorganic soil components as exhibited in (Fig. 16.9 A).

In tropical rain forest, dense clay subsoil (B-horizon), heavy rainfall, and high temperatures result in nutrient-poor and shallow soils, as exhibited in (Fig. 16.9 B).

Properties of Soils:

Soils are made up of four components:

The two solid substances are:

(i) Small mineral particles and

(ii) Organic matter.

The other components are:

(iii) Air (gaseous), and

(iv) Water (liquid).

(i) The small mineral particles come from weathering of rocks. They include about 25 elements and of which 16 elements are essential. These are divided into two categories, i.e., macronutrients, such as C, H, O, N, P, S,Ca⁺⁺ , K⁺, Mg⁺⁺ and micronutirents, such as Fe, Mn, B, Mo, Cu, Zn, and CI.

(ii) The organic matter is of plant and animal origin, both from dead and living organisms.

In an average soil, about 45 percent of the volume is filled with mineral particles and about 5 percent ley organic matter.

The remaining 50 percent of the volume is filled with soil (iii) air and (iv) water. The amount of air or water varies greatly, depending on how wet soil is at the time.

The proportion of mineral particles of different sizes in soil represents its texture. According to varying soil particles they are divided into following categories. The particles vary in sizes, such as gravel (more than 2 mm); coarse sand (between 0.2 to 2 mm); fine sand (0.02 to 0.2 mm); silt (0.002 to 0.02); clay (less than 0.002 mm).

Fine texture soils have a predominance of clay and silt particles; while the coarse textured soils have a predominance of sand particles.

Sandy Soils:

It is formed of 85 percent sand and 15 percent clay and silt, also known as light soil. This soil is not rich in nutrients and is less fertile. It is characterised by xerophytic vegetation.

Clayey Soil:

It is formed of 50 per cent clay and 50 per cent silt or sand or both also known as cold or heavy soil. It has high water holding capacity and less water logging.

Silt Soil:

It has 90 percent silt and 10 percent sand. It has good soil porosity and water holding capacity. This is poor in nutrient supply.

Loamy Soil:

It is composed of 70 per cent sand and 30 per cent clay or silt or both. This is considered best soil for plant growth. It has good water holding capacity, water infiltration and adequate aeration. Roots penetrate deep in loamy soils.

Soil Moisture:

Plants absorb a small quantity of rain water and dew directly, but they take a large quantity of water from the soil.

Water held in the soil is of following types:

(i) Gravitational Water:

This is free water which percolates downwardly through the pore spaces between soil particles and is accumulated in the pore spaces in the form of ground water.

(ii) Capillary Water:

The amount of water present around the soil particles and held by surface tension and attraction force of water molecules. Capillary water remains readily available to the roots up-to a certain soil moisture tension.

(iii) Hygroscopic Water:

Water, which is adsorbed on the soil particles and held on the surface of particles by forces of attraction and cohesion of its molecules. This is the moisture, remaining in air-dry soil, and cannot be used by the plants.

Soil Organic Matter:

The organic matter stored in soil may be divided into two categories:

(i) Detritus or Litter:

This is freshly dead and partially decomposed plant and animal material.

(ii) Humus:

This is colloidal, amorphous and dark coloured substance that constitutes the organic component of soil. It is formed by the decomposition of plant and animal remains and has a complex and variable chemical composition. Being a colloid, it can hold water and therefore improves the water retaining properties of soil; it also enhances soil fertilely.

Acidic humus (mor) is found in region of coniferous forest, where the decay is brought about mainly by fungi. Alkaline humus (mull) is typically found in grassland and deciduous forest it supports an abundance of micro-organisms and small annelids, e.g., earthworms.

A part from the dead organic matter, soil harbours very large number of organisms which include detritus feeders, such as termites, earthworms, carpenter ants and crabs; bacteria and fungi.

The soil organisms act as decomposers and play an important role in decomposition of plant and animal residues.

Factor # 14. Fire:

Fire is a biological factor rather than a physical factor as it is mostly caused by man’s activity. Lightning initiated fires have destroyed plants and animals since their early appearance on earth.

A large body of information has been developed on the effects of fire on grassland and forests as well as on the use of fire in land management. Native people in various lands have used fire to enhance hunting, improve visibility and provide forage, etc.

The branch of ecology which deals with the effects of the fire an ecosystem is called ‘fire ecology’ or ‘ecopyrology’ plants having ability to withstand fire with little or no damage are referred to as pyrophytes. Many pyrophytes are known to occur in Siwalik hills of India.

The examples of pyrophytes are Cochlospermum religiosa, Combretum nanum, Grewia sapida, etc. These small plants are supposed to have become permanently dwarf by annual forest fires. Pyrophytes are generally woody plants with thick bark.

Fires caused by man’s activity are responsible for complete destruction of vegetation at certain places resulting in temporary or permanent alterations in the characters of vegetations. Fire has marked effects on the physical environment through removal of the plant cover, burning of litter mass present on the soil surface, and loss of nutrients due to volatilisation.

There is mortality among a variety of animal groups due to occasional or recurrent fires.

Generally in fires the aerial parts of plants are destroyed completely but their roots, rhizomes or other underground parts may sometimes remain unaffected which under favourable conditions may grow and produce new plants.

Fire generally makes the area suitable for the growth of grasses and thus imporves the quality of forage. Post-bum plants are preferred by herbivores. Animals grazing on burnt grasslands are found to gain in weight more rapidly than these grazing on unburnt grasslands. Fire removes harmful plant parasites and pests.

Fire is an important factor in temperate forest and grassland regions, as well as in tropical areas having dry seasons.