Principles of Ecology – UPSC (Environmental Geography)

In this article you will read Principles of Ecology for UPSC – Environmental Geography.

The word “ecology” (“Ökologie”) was coined in 1866 by the German scientist Ernst Haeckel. Ecology is the study of how living things interact with each other and with their environment. It is a major branch of biology but has areas of overlap with geography, geology, climatology, and other sciences.

Fundamental Concepts and Principles of Ecology

There are certain basic fundamental ecological principles which describe various aspects of living organisms e.g. evolution and distribution of plants and animals, extinction of species consump­tion and transfer of energy in different components of biological communities, cycling, and recycling of organic and inorganic sub­stances, interactions and inter-relationships among the organisms and between organisms and physical environment, etc.

The Following are the fundamental concepts and principles in ecology, beginning with organisms and the environment.

Organisms and the Environment

Organisms are individual living things. Despite their tremendous diversity, all organisms have the same basic needs: energy and matter. These must be obtained from the environment. Therefore, organisms are not closed systems. They depend on and are influenced by their environment. The environment includes two types of factors: abiotic and biotic.

1. Abiotic factors are the nonliving aspects of the environment. They include factors such as sunlight, soil, temperature, and water.
2. Biotic factors are the living aspects of the environment. They consist of other organisms, including members of the same and different species.

Niche

One of the most important concepts associated with the ecosystem is the niche. A niche refers to the role of a species in its ecosystem. It includes all the ways that the species interacts with the biotic and abiotic factors of the environment. Two important aspects of a species‘ niche are the food it eats and how the food is obtained. Each species eats a different type of food and obtains the food in a different way.

Habitat

Another aspect of a species‘ niche is its habitat. The habitat is the physical environment in which a species lives and to which it is adapted. A habitat‘s features are determined mainly by abiotic factors such as temperature and rainfall. These factors also influence the traits of the organisms that live there.

Competitive Exclusion Principle

A given habitat may contain many different species, but each species must have a different niche. Two different species cannot occupy the same niche in the same place for very long. This is known as the
competitive exclusion principle. If two species were to occupy the same niche, they would compete with one another for the same food and other resources in the environment. Eventually, one species would be likely to outcompete and replace the other.

The Ecosystem

An ecosystem is a unit of nature and the focus of study in ecology. It consists of all the biotic and abiotic factors in an area and their interactions. Ecosystems can vary in size. A lake could be considered an ecosystem. So could a dead log on a forest floor. Both the lake and log contain a variety of species that interact with each other and with abiotic factors.

When it comes to energy, ecosystems are not closed. They need constant inputs of energy. Most ecosystems get energy from sunlight. A small minority gets energy from chemical compounds. Unlike energy, the matter is not constantly added to ecosystems. Instead, it is recycled. Water and elements such as carbon and nitrogen are used over and over again.

The term ecosystem‘ was coined by A.G. Tansley in 1935. An ecosystem is a self-sustaining unit of nature. An ecosystem is a functional unit of nature encompassing complex interaction between its biotic (living) and abiotic (non-living) components. For example- a pond is a good example of an ecosystem.

Many ecologists regard the entire biosphere as a global ecosystem, as a composite of all local ecosystems on Earth.

In nature two major categories of ecosystems exist: terrestrial and aquatic.

• Forests, deserts, and grasslands are examples of the terrestrial ecosystem.
• Ponds, lakes, wet lands, and salt water are some examples of the aquatic ecosystem.
• Crop lands and aquarium are examples of manmade ecosystems.

The interaction between the living organisms and their environment can be studied in a puddle of water or a hole in a tree, which are very small ecosystems, or in large ecosystems such as a forest, river, or ocean. Irrespective of their sizes all ecosystems share many common characteristics.

Types of ecosystems:

Ecosystems are classified as follows:

• (i) Natural ecosystems
• (ii) Manmade ecosystems

(i) Natural ecosystems

• Totally dependent on solar radiation e.g. forests, grasslands, oceans, lakes, rivers, and deserts. They provide food, fuel, fodder, and medicines.
• Ecosystems are dependent on solar radiation and energy subsidies (alternative sources) such as wind rain and tides. e.g. tropical rain forests, tidal estuaries, and coral reefs.

(ii) Manmade ecosystems

• Dependent on solar energy-e.g. agricultural fields and aquaculture ponds.
• Dependent on fossil fuel e.g. urban and industrial ecosystems.

Components of an Ecosystem

They are broadly grouped into:

(a) Abiotic and
(b) Biotic components

(a) Abiotic components (Nonliving):

The abiotic component can be grouped into the following three categories:

1. Physical factors: Sunlight, temperature, rainfall, humidity, and pressure. They sustain and limit the growth of organisms in an ecosystem.
2. Inorganic substances: Carbon dioxide, nitrogen, oxygen, phosphorus, Sulphur, water, rock, soil, and other minerals.
3. Organic compounds: Carbohydrates, proteins, lipids, and humic substances. They are the building blocks of living systems and therefore, make a link between the biotic and abiotic components.

(b) Biotic components (Living)

1. Producers: The green plants manufacture food for the entire ecosystem through the process of photosynthesis. Green plants are called autotrophs, as they absorb water and nutrients from the soil, carbon dioxide from the air, and capture solar energy for this process.
2. Consumers: They are called heterotrophs and they consume food synthesized by the autotrophs. Based on food preferences they can be grouped into three broad categories. Herbivores (e.g. cow, deer, and rabbit, etc.) feed directly on plants, carnivores are animals which eat other animals (e.g. lion, cat, dog, etc.) and omnivore’s organisms feeding upon plants and animals e.g. human, pigs, and sparrow.
3. Decomposers: Also called saprotrophs. These are mostly bacteria and fungi that feed on dead decomposed and the dead organic matter of plants and animals by secreting enzymes outside their body on the decaying matter. They play a very important role in recycling nutrients. They are also called detrivores or detritus feeders.

Ecosystem – Structure and Function

Interaction of biotic and abiotic components results in a physical structure that is characteristic of each type of ecosystem. Identification and enumeration of plant and animal species of an ecosystem give its species composition.

The important structural features are species composition (types of plants and animals) and stratification (vertical and horizontal distribution of various species occupying different levels). Another way of looking at the structural components is through the food relationships of producers and consumers. Several trophic levels exist in the ecosystem. For example, trees occupy the top vertical strata or layer of a forest, shrubs the second, and herbs and grasses occupy the bottom layers. These structural components function as a unit and produce certain functional aspects of an ecosystem.

Some of these aspects are: Productivity, energy flow, nutrient cycle

Species Composition:

A community is an assemblage of many populations that are living together at the same place and time. For example, a tropical forest community consists of trees, vines, herbs, and shrubs along with a large number of different species of animals. This is known as the species composition of a tropical forest ecosystem.

Each ecosystem has its own species composition depending upon the suitability of its habitat and climate. A forest ecosystem supports a much larger number of species of plants and animals than grassland. The total number and types of species in a community determine its stability and ecosystem balance (ecosystem equilibrium).

Stratification:

The vertical and horizontal distribution of plants in the ecosystem is called ecosystem stratification. The tallest trees make the top canopy. This is followed by short trees and shrubs and then the forest floor is covered with herbs and grasses. Some burrowing animals live underground in their tunnels or on the roots of the plants. Each layer from the treetop to the forest floor has its characteristic fauna and flora. This is termed as vertical stratification of forest ecosystems. On the other hand, the desert ecosystem shows low discontinuous layers of scant vegetation and animals with some bare patches of soil showing a type of horizontal stratification.

Functions of ecosystem

Ecosystems are a complex dynamic systems. They perform certain functions. These are:-

• Energy flow through the food chain
• Nutrient cycling (biogeochemical cycles)
• Ecological succession or ecosystem development
• Homeostasis (or cybernetic) or feedback control mechanisms.

Ponds, lakes, meadows, marshlands, grasslands, deserts, and forests are examples of natural ecosystems. We have seen an aquarium; a garden or a lawn etc. in our neighborhood. These are a manmade ecosystem.

Energy Flow through Ecosystem:

Food chains and energy flow are the functional properties of ecosystems that make them dynamic. The biotic and abiotic components of an ecosystem are linked through them.

Charles Elton gave the concept of Food Chain, Food Web, and Ecological pyramid.

Food Chain:

The transfer of food energy from green plants (producers) through a series of organisms with repeated eating and being eaten is called a food chain. Each step in the food chain is called a trophic level.

E.g. Grasses → Grasshopper → Frog → Snake → Hawk/Eagle

During this process of transfer of energy some energy is lost into the system as heat energy and is not available to the next trophic level. Therefore, the number of steps is limited in a chain to 4 or 5. Following trophic levels can be identified in a food chain.

(i) Autotrophs:

They are the producers of food for all other organisms of the ecosystem. They are largely green plants and convert inorganic material in the presence of solar energy by the process of photosynthesis into chemical energy (food).

The total rate at which the radiant energy is stored by the process of photosynthesis in the green plants is called Gross Primary Production (GPP). This is also known as total photosynthesis or total assimilation. From the gross primary productivity, a part is utilized by the plants for its own metabolism. The remaining amount is stored by the plant as Net Primary Production (NPP) which is available to consumers.

(ii) Herbivores: The animals which eat the plants directly are called primary consumers or herbivores e.g. insects, birds, rodents, and ruminants.

(iii) Carnivores: They are secondary consumers if they feed on herbivores and tertiary consumers if they use carnivores as their food. E.g. frog, dog, cat, and tiger.

(iv) Omnivores: Animals that eat both plant and animals e.g. pig, bear and man.

(v) Decomposers: They take care of the dead remains of organisms at each trophic level and help in recycling the nutrients e.g. bacteria and fungi.

There are two types of food chains:

1. Grazing food chains: This starts from the green plants that make food for herbivores and herbivores in turn for the carnivores.

2. Detritus food chains: start from the dead organic matter to the detrivores organisms which in turn make food for protozoan to carnivores, etc.

Food web:

Trophic levels in an ecosystem are not linear rather they are interconnected and make a food web. Thus food web is a network of interconnected food chains existing in an ecosystem. One animal may be a member of several different food chains. Food webs are more realistic models of energy flow through an ecosystem.

The flow of energy in an ecosystem is always linear or one-way. The quantity of energy flowing through the successive trophic levels decreases. At every step in a food chain or web, the energy received by the organism is used to sustain itself and the leftover is passed on to the next trophic level.

Ecological pyramid:

Ecological pyramids are graphic representations of trophic levels in an ecosystem. They are pyramidal in shape and they are of three types: The producers make the base of the pyramid and the subsequent tiers of the pyramid represent herbivore, carnivore, and top carnivore levels.

Pyramid of number: This represents the number of organisms at each trophic level. For example in grassland, the number of grasses is more than the number of herbivores that feed on them and the number of herbivores is more than the number of carnivores. In some instances the pyramid of number may be inverted, i.e. herbivores are more than primary producers as you may observe that many caterpillars and insects feed on a single tree.

Pyramid of biomass: This represents the total standing crop biomass at each trophic level. Standing crop biomass is the amount of living matter at any given time. It is expressed as gm/unit area or kilo
Cal/unit area. In most of the terrestrial ecosystems, the pyramid of biomass is upright. However, in the case of aquatic ecosystems, the pyramid of biomass may be inverted.

Pyramid of energy: This pyramid represents the total amount of energy at each trophic level. Energy pyramids are never inverted.

Biogeochemical Cycles

The movement of nutrient elements through the various components of an ecosystem is called nutrient cycling. Another name of nutrient cycling is biogeochemical cycles (bio: living organism, geo: rocks, air, and water). In ecosystems flow of energy is linear but that of nutrients is cyclical. The entire earth or biosphere is a closed system i.e. nutrients are neither imported nor exported from the biosphere.

Nutrient cycles are of two types: (a) gaseous and (b) sedimentary.

The reservoir for the gaseous type of nutrient cycle (e.g., nitrogen, carbon cycle) exists in the atmosphere and for the sedimentary cycle (e.g., Sulphur and phosphorus cycle); the reservoir is located in Earth‘s crust.

The Carbon Cycle

Of all the biogeochemical cycles, the carbon cycle is the most important. All life is composed of carbon compounds of one form or another. That is why it is of such grave concern today that human activities since the Industrial Revolution have modified the carbon cycle in significant ways.

The carbon cycle is a biogeochemical cycle in which carbon flows among storage pools in the atmosphere, ocean, and on the land. Human activity has affected the carbon cycle, causing carbon dioxide concentrations in the atmospheric storage pool to increase.

The source of all carbon is carbon dioxide present in the atmosphere. It is highly soluble in water; therefore, oceans also contain large quantities of dissolved carbon dioxide.

The global carbon cycle consists of following steps-

Photosynthesis:

Green plants in the presence of sunlight utilize CO2 in the process of photosynthesis and convert the inorganic carbon into organic matter (food) and release oxygen. Annually 4-9 x 10 13 kg of CO2 is fixed by green plants of the entire biosphere. Forests acts as reservoirs of CO2 as carbon fixed by the trees remain stored in them for a long due to their long life cycles. Avery large amount of CO2 is released through forest fires.

Respiration:

Respiration is carried out by all living organisms. It is a metabolic process where food is oxidized to liberate energy, CO2, and water. The energy released from respiration is used for carrying out life processes by living organisms (plants, animals, decomposers, etc.). Thus CO2 is released into the atmosphere through this process.

Decomposition:

All the food assimilated by animals or synthesized by plants is not metabolized by them completely. A major part is retained by them as their own biomass which becomes available to decomposers on their death. The dead organic matter is decomposed by microorganisms and CO2 is released into the atmosphere by decomposers.

Combustion:

The burning of biomass releases carbon dioxide into the atmosphere.

Impact of human activities

The global carbon cycle has been increasingly disturbed by human activities particularly since the beginning of the industrial era. Large-scale deforestation and ever-growing consumption of fossil fuels by growing numbers of industries, power plants, and automobiles are primarily responsible for increasing the emission of carbon dioxide.

Carbon dioxide has been continuously increasing in the atmosphere due to human activities such as industrialization, urbanization, and increasing use and number of automobiles. This is leading to an increasing concentration of CO2 in the atmosphere, which is a major cause of global warming.

Nitrogen cycle

Nitrogen is an essential component of protein and required by all living organisms including human beings.

Our atmosphere contains nearly 79% of nitrogen but it cannot be used directly by the majority of living organisms. Broadly like carbon dioxide, nitrogen also cycles from the gaseous phase to the solid phase then back to the gaseous phase through the activity of a wide variety of organisms. The cycling of nitrogen is vitally important for all living organisms.

There are five main processes which essential for nitrogen cycle are elaborated below.

(a) Nitrogen fixation: This process involves the conversion of gaseous nitrogen into Ammonia, a form in which it can be used by plants. Atmospheric nitrogen can be fixed by the following three methods:-

• Atmospheric fixation: Lightening, combustion, and volcanic activity help in the fixation of nitrogen.
• Industrial fixation: At high temperature (400oC) and high pressure (200 atm.), molecular nitrogen is broken into atomic nitrogen which then combines with hydrogen to form ammonia.
• Bacterial fixation: There are two types of bacteria-
  • Symbiotic bacteria e.g. Rhizobium in the root nodules of leguminous plants.
  • Free living or symbiotic e.g. 1. Nostoc 2. Azobacter 3. Cyanobacteria can combine atmospheric or dissolved nitrogen with hydrogen to form ammonia.

(b) Nitrification: It is a process by which ammonia is converted into nitrates or nitrites by Nitrosomonas and Nitrococcus bacteria respectively. Another soil bacterium Nitrobacter can convert nitrate into nitrite.

(c) Assimilation: In this process nitrogen fixed by plants is converted into organic molecules such as proteins, DNA, RNA, etc. These molecules make the plant and animal tissue.

(d) Ammonification: Living organisms produce nitrogenous waste products such as urea and uric acid. These waste products as well as dead remains of organisms are converted back into inorganic ammonia by the bacteria. This process is called ammonification. Ammonifying bacteria help in this process.

(e) Denitrification: Conversion of nitrates back into gaseous nitrogen is called denitrification. Denitrifying bacteria live deep in the soil near the water table as they like to live in the oxygen-free medium. Denitrification is the reverse of nitrogen fixation.

Water Cycle

Water is essential for life. No organism can survive without water. Precipitation (rain, snow, slush dew etc.) is the only source of water on the earth. Water received from the atmosphere on the earth returns back to the atmosphere as water vapour resulting from direct evaporation and through evapotranspiration the continuous movement of water in the biosphere is called water cycle (hydrological cycle).

Water is not evenly distributed throughout the surface of the earth. Almost 95 % of the total water on the earth is chemically bound to rocks and does not cycle. Out of the remaining 5%, nearly 97.3% is in the oceans and 2.1% exists as polar ice caps. Thus only 0.6% is present as fresh water in the form of atmospheric water vapors, ground, and soil water.

The driving forces for water cycle are( 1) solar radiation (2) gravity.

Evaporation and precipitation are the two main processes involved in the water cycle. These two processes alternate with each other Water from oceans, lakes, ponds, rivers, and streams evaporates by the sun‘s heat energy. Plants also transpire huge amounts of water. Water remains in the vapor state in the air and forms clouds that drift with the wind. Clouds meet with the cold air in the mountainous regions above the forests and condense to form rain precipitate which comes down due to gravity.

On average 84% of the water is lost from the surface of the oceans by evaporation. While 77% is gained by it from precipitation. Water runoff from lands through rivers to oceans makes up 7% which balances the evaporation deficit of the ocean. On land, evaporation is 16% and precipitation is 23%.

Phosphorus Cycle

Phosphorus is a major constituent of biological membranes, nucleic acids, and cellular energy transfer systems. Many animals also need large quantities of this element to make shells, bones, and teeth. The natural reservoir of phosphorus is rock, which contains phosphorus in the form of phosphates.

When rocks are weathered, minute amounts of these phosphates dissolve in soil solution and are absorbed by the roots of the plants. Herbivores and other animals obtain this element from plants. The waste products and the dead organisms are decomposed by phosphate solubilizing bacteria releasing phosphorus. Unlike the carbon cycle, there is no respiratory release of phosphorus into the atmosphere.

Atmospheric inputs of phosphorus through rainfall are much smaller than carbon inputs, and gaseous exchanges of phosphorus between organism and environment are negligible.

Ecological Succession

Hult used the term first-time “Ecological Succession” for the ‘Orderly changes in communities’. Odum called it Ecosystem development. Ragnar Hult was the first (1881) to publish a comprehensive study of ecological succession as it is taking place in a given region. He was the first to recognize that a relatively large number of pioneer plant communities give way to a comparatively small number of relatively stable communities.

F.E. Clements (1916) defined succession as a natural process by which same locality becomes successively colonized by different groups of plants or communities thus communities are never stable.

Biotic communities are dynamic in nature and change over a period of time. The process by which communities of plant and animal species in an area are replaced or changed into another over a period of time is known as ecological succession.

Both the biotic and abiotic components are involved in this change. This change is brought about both by the activities of the communities as well as by the physical environment in that particular area. The physical environment often influences the nature, direction, rate, and optimal limit of changes.

During succession both the plant and animal communities undergo change. During succession, some species colonize an area and their populations become more numerous, whereas populations of other species decline and even disappear.

The entire sequence of communities that successively change in a given area is called sere(s). The individual transitional communities are termed seral stages or seral communities. In the successive seral stages, there is a change in the diversity of species of organisms, an increase in the number of species and organisms as well as an increase in the total biomass.

There are two types of successions (i) Primary succession and (ii) Secondary succession.

Primary succession

Primary succession takes place over bare or unoccupied areas such as rock outcrop, newly formed deltas, and sand dunes, emerging Volcano Islands and lava flows as well as glacial moraines (muddy area exposed by a retreating glacier) where no community has existed previously.

The plants that invade first bare land, where the soil is initially absent are called pioneer species. The assemblage of pioneer plants is collectively called the pioneer community. A pioneer species generally show a high growth rate but a short life span.

The community that initially inhabits a bare area is called the pioneer community. The pioneer community after some time gets replaced by another community with different species combinations. This second community gets replaced by a third community. This process continues sequence-wise in which a community replaced previously by another community.

The terminal (final) stage of succession forms the community which is called a climax community. A climax community is stable, mature, more complex, and long-lasting. The animals of such a community also exhibit succession which to a great extent is determined by plant succession. A climax community as long as it is undisturbed, remains relatively stable in dynamic equilibrium with the prevailing climate and habitat factors.

Succession that occurs on land where moisture content is low for e.g. on bare rock is known as xerarch. Succession that takes place in a water body, like ponds or lakes is called hydrarch.

Secondary succession

Secondary succession is the development of a community which forms after the existing natural vegetation that constitutes a community is removed, disturbed, or destroyed by a natural event like a hurricane or forest fire or by human-related events like tilling or harvesting the land.

Secondary succession is relatively fast as the soil has the necessary nutrients as well as a large pool of seeds and other dormant stages of organisms.

Homeostasis of Ecosystem

Ecosystems are capable of maintaining their state of equilibrium. They can regulate their own species structure and functional processes. This capacity of the ecosystem of self-regulation is known as homeostasis. In ecology, the term applies to the tendency for a biological system to resist changes.

For example, in a pond ecosystem, if the population of zooplankton increased, they would consume a large number of phytoplankton and as a result, soon zooplankton would be a short supply of food for them. As the number of zooplankton is reduced because of starvation, the phytoplankton population starts increasing. After some time the population size of zooplankton also increases and this process continues at all the trophic levels of the food chain.

Note that in a homeostatic system, the negative feedback mechanism is responsible for maintaining stability in an ecosystem. However, the homeostatic capacity of ecosystems is not unlimited as well as not everything in an ecosystem is always well regulated. Humans are the greatest source of disturbance to ecosystems.

Productivity of Ecosystem

The productivity of an ecosystem refers to the rate of production, i.e., the amount of organic matter accumulated in any unit of time.

Productivity is of the following types:

1. Primary productivity
2. Secondary productivity
3. Net Productivity

1. Primary productivity:

It is defined as the rate at which radiant energy is stored by the producers, most of which are photosynthetic, and to a much lesser extent the chemosynthetic microorganisms. Primary productivity is of the following types:

(a) Gross primary productivity:

It refers to the total rate of photosynthesis including the organic matter used up in respiration during the measurement period. It depends on the chlo­rophyll content. The rate of primary productivity is estimated in terms of either chlorophyll content as chl/g dry weight/unit area, or photosynthetic number, i.e., amount of CO₂ fixed/g chl/hour.

(b) Net primary productivity:

Also known as apparent photosynthesis or net assimilation, it refers to the rate of storage of organic matter in plant tissues in excess of the respiratory utilisation by plants during the measurement period.

2. Secondary productivity:

It is the rate of energy storage at consumer’s levels-herbivores, carnivores, and decomposers. Consumers tend to utilize already produced food materials in their respiration and also converts the food matter to different tissues by an overall process. Some ecologists such as Odum (1971) prefer to use the term assimilation rather than ‘production’ at this level-the consumer’s level. It actually remains mobile (i.e., keeps on moving from one organism to another) and does not live in situ like the primary productivity.

3. Net Productivity:

It refers to the rate of storage of organic matter not used by the heterotrophs or consumers, i.e., equivalent to net primary production minus consumption by the heterotrophs during the unit period, as a season or year, etc. It is thus the rate of increase of biomass of the primary producers which has been left over by the consumers.

Principles of Ecology

Some im­portant principles of ecology in terms of eco-system may be outlined as follows:

1. Eco-system is a fundamental well structured and organized unit that brings the physical environment and living organisms together in a single framework which facilitates the study of interactions between biotic and abiotic components. Eco­systems are also functional units wherein two biotic com­ponents, namely autotrophic and heterotrophic compo­nents are of major significance.

2. The biotic and abiotic components of the biosphere eco­system are intimately related through a series of large scale cyclic mechanisms that help in the transfer of energy, wa­ter, chemicals, and sediments in various components of the biosphere.

3. Sustained life on the earth is a characteristic of the eco-system, not of individual organisms or population (D.B. Botkin and E. A. Keller 1982).

4. In 1974, M. J. Holliman suggested four environmental prin­ciples to describe the holistic nature of the natural environment which largely influence the biological communities in a biosphere eco-system.

The different principles are as follows:

• (i) Nothing actually disappears when we throw it away be­cause all the materials are rearranged and cycled and recycled through a series of cyclic pathways in the natu­ral environment.
• (ii) All systems and problems are ultimate if not intimately, inter-related. It does not make squabble over which cri­sis is most urgent. We cannot afford the luxury of solv­ing problems one by one that is both obsolete and eco­logically unsound anyway.
• (iii) We live on planet earth whose resources are finite.
• (iv) Nature has spent literally millions of years refining a sta­ble eco-system.

5. According to D. B. Botkin and E.A. Keller (1982) the physical and biological processes follow the principle of uniformitarianism. This principle states that the same physical (right from the origin of the planet, earth, and its atmosphere) and biological (since the origin of the first organism) processes that operate today, operated in the past not necessarily with constant magnitude and frequency with time and will operate in future but at rates that will vary as the environ­ment influenced by human activity.

6. Natural hazards affect adversely the biological communi­ties in general and man in particular when biological pro­cesses are associated with natural hazards, yet severe haz­ards are created.

7. All living organisms and the physical environment are mutually reactive. The varying degrees of interactions among organ­isms, at both inter and intraspecific levels, are positive, negative, and sometimes neutral.

8. Solar radiation is the main driving force of the eco-system and it is trapped by green plants through the process of photosynthesis. Energy flow in the eco-system is unidirectional and non-cyclic. Eco-system energy flow (energetics) helps the eco-system. The energy pattern and energy flow are governed by the laws of thermodynamics.

9. The energy is transferred from one trophic level to the next higher trophic level but organisms at higher trophic levels receive energy from more than one trophic level.

10. R. L. Linderuan (1942) suggested some principles about the relationships between the trophic levels within a natural eco­system.

• Principle-1: With an increase in distance between the organisms of a given trophic level and the initial source of energy, the probability of the organisms to depend exclusively on the preceding trophic level for energy decreases.
• Principle-2: The relative loss of energy due to respira­tion is progressively greater to higher trophic levels because the species at higher trophic levels being rela­tively larger in size have to move and work for getting food and therefore more energy is lost due to respira­tion.
• Principle-3: Species at progressively higher trophic lev­els appear to be progressively more efficient in using their available food supply, because increased activity by predators increases their chances of encountering suit­able prey species, and in general predators are less spe­cific than their prey in food preference.
• Principle-4: Higher trophic levels tend to be less discrete than the lower ones because the organisms at progres­sively higher trophic levels receive energy from more than one source and are generalists in their feeding habit and they are more efficient in using their available food.
• Principle-5: Food-chains tend to be reasonably short. Four vertical links is a common maximum because loss of energy is progressively higher for higher trophic levels and species at higher levels tend to be less discrete.

11. The inorganic and organic substances are circulated among the various components of the biosphere through a series of the closed systems of cycles collectively known as biogeochemical cycles.

12. The eco-system productivity depends on two factors:

• (i) The availability of the amount of solar radiation to the primary producers at trophic level-I.
• (ii) The efficiency of the plants to convert solar energy into chemical energy.

There is marked positive correlation between primary produc­tivity and solar radiation.

13. There is an inbuilt self-regulating mechanism in natural ecosys­tem, known as homeostatic mechanisms, through which any change caused by external factors in the eco-system is coun­ter balanced by the responses of the system to the change in such a way that ultimately eco-system or ecological stability is restored. The ecological diversity and complexity enhance ecological or eco-system stability.

The ecological stability can be attained by the following manners:

• (i) According to C. S. Elton (1958), increase in the diversity of food webs promotes ecosystem stability.
• (ii) According to P.H. MacArthur (1955), the ecosystem sta­bility increases with increase of number of links in the food web.
• (iii) According to E.P. Odum (1971), high species diversity of a mature ecosystem representing a climax community is related to more stability of natural eco-system.

14. Eco-system instability results when an eco-system becomes unable to adjust to environmental changes.

15. According to Charles Darwin (1859), the evolution of species epitomizes the inherently dynamic nature of the ecosystem.

16. Darwin’s concept of the progressive evolution of species was subsequently challenged by Devries and a new concept of mutation was proposed. The mutation is a process of spontane­ous evolutionary change which introduces inheritable varia­tions in species.

T. Dobzhansky (1950) suggested the following ideas regard­ing mutation:

• (i) The mutation process furnishes the raw materials for evolution.
• (ii) During sexual reproduction, numerous gene patterns are produced.
• (iii) The possessors of some gene patterns have greater fit­ness than the possessors of other patterns in available environment.
• (iv) The frequency of superior gene patterns is increased by the process of natural selection while the inferior gene patterns are suppressed.
• (v) Groups of some combinations of proven adaptive worth become segregated into closed genetic system, called species.

17. The transition stages of sequential changes from one vegetation community to another vegetation community are called ‘sere’. The sere is complete when the succession of vegetation community after passing through different phases, culminates into equilibrium condition. The vegeta­tion community developed at the end of succession is called ‘Climax vegetation’, ‘Climax community’ or ‘Climax climax.’

18. Besides community succession, the eco-system also under­goes the process of successional changes. There are two fundamental ideas regarding the process of successional changes.

• According to E.P. Odum (1962), ecological succession is one of the most important processes which results from the community modifying the environment, (ii) According to R. H. Whittaker (1953), the successional development of the eco­system is characterized by four major changes in the eco­system viz.
  • (a) Progressive increase in the complexity and diversity of the community;
  • (b) Progressive increase in the structure and productivity of the eco-system;
  • (c) Increase in soil maturity;
  • (d) Increase in relative stability and regularity of populations within the eco-system and stability of the eco-system itself.

19. The eco-system is mainly modified by man through the exploitation of natural resources. Man reduces ecological diversity and complexity by removing a host of biotic com­munications.

20. Preserving diversity in a world of rapidly shrinking resources will require a prompt and universal response on an appro­priate application of ecological knowledge.