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Ecology Environmental Science UNIT-II Class 11th Full Chapter

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Table of Contents

Ecology (definition and types)


Ecology is the study of the relationships between living organisms, including humans, and their physical environment Seeks to understand the vital connections between plants and animals and the world around them. Ecology also provides information about the benefits of ecosystems and how we can use Earth’s resources in ways that leave the environment healthy for future generations. Ecology is the study of organisms and how they interact with the environment around them.


Ecologists study these relationships among organisms and habitats of many different sizes, ranging from the study of microscopic bacteria growing in a fish tank, to the complex interactions between the thousands of plant, animal, and other communities found in a desert. For example, ecologists may study microbes living in the soil under your feet or animals and plants in a rain forest or the ocean. an expert in or student of ecology.


In the 1860s, Ernst Haeckel, combined the term oikos —a place to live, home, habitat —with logia —discourse, study—to coin the word “ecology.” In the 1890s Ellen Richards included humans and harmony, quite a modern view. Variations over the years are :


A number of concepts and principles are basic to the study of ecology. They include the ecosystem, niche, habitat, and competitive exclusion principle.


The ecosystem is one of the most important concepts in ecology and often the focus of ecological studies. It consists of all the biotic and abiotic factors in an area and their interactions.


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 ecosystem. Two important aspects of any species’ niche are its sources of energy and nutrients and how it obtains them.


The habitat is the natural environment in which a species lives and to which it is adapted. A species’ habitat includes any factors of the environment — including both biotic and abiotic factors — that are related directly or indirectly to the use of the environment by the species.

Competitive Exclusion Principle

A given area 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, what would happen? The two species would compete with one another for the same food or other limiting resources in the environment. Eventually, one species might outcompete and replace the other. Alternatively, one species might evolve somewhat different adaptations to a similar but different niche so they could continue to live in the same area. Principle of ecology stating that two different species cannot occupy the same niche in the same place for very long


The different types of ecology include- molecular ecology, organismal ecology, population ecology, community ecology, global ecology, landscape ecology and ecosystem ecology. Molecular ecology is the use of molecular genetic tools to study ecological questions. Techniques such as microarrays and DNA markers are used to study the interactions and diversity of natural populations. (just like Molecular biology is the branch of biology that studies the molecular basis of biological activity.)

Global Ecology

Global Ecology is the study of the Earth's ecosystems among the land, oceans, and atmosphere. With four major environmental issues occurring on the global scale, habitat destruction, invasive species, decline of population densities and pollution, global ecology is needed to understand what is happening and why.Although many scientists study environmental problems at the population, community and ecosystem levels, there are some environmental problems that are even larger in scale and can influence the whole planet. For these types of problems, the field of global ecology would be used to investigate answers. Global Ecology is very important because it is used to understand large scale interactions and how they influence the behavior of the entire planet, including the earth's responses to future changes.

Landscape Ecology

Landscape Ecology is the study of the pattern and interaction between ecosystems within a region of interest, and the way the interactions affect ecological processes, especially the unique effects of spatial heterogeneity on these interactions.The work of beavers building a dam to flood an area is an example of a biological activity that can change landscape structure. Clearing of forest land for agriculture or the expansion of urban areas. What powers life? How do sunlight and nutrients affect the plants we depend on? How do greenhouse gases and other contaminants degrade the interactions among the plant, animal, and microbial populations that comprise ecosystems?

Ecosystem Ecology

Ecosystem Ecology is the study of these and other questions about the living and nonliving components within the environment, how these factors interact with each other, and how both natural and human-induced changes affect how they function.Understanding how ecosystems work begins with an understanding of how sunlight is converted into usable energy, the importance of nutrient cycling, and the impact mankind has on the environment. Plants convert sunlight into usable forms of energy that are carbon based. Primary and secondary production in populations can be used to determine energy flow in ecosystems. Studying the effects of atmospheric? CO2 will have future implications for agricultural production and food quality.

Concept and structure of ecosystem)


First Proposed by AG Tansley in 1935 “The System resulting from the integration of all the living and nonliving factors of the environment” Combination of two words “Eco” a Greek word means Environment and “System” means collection of parts and each parts having it’s own function to perform. An ecosystem is a geographic area where plants, animals, and other organisms, as well as weather and landscape, work together to form a bubble of life. Ecosystems contain biotic or living, parts, as well as abiotic factors, or nonliving parts.



These ecosystems are capable of operating and maintaining themselves without any major interference by man. Based upon the particular kind of habitat these are further divided as:

Terrestrial ecosystems:

Terrestrial ecosystems can be found anywhere apart from heavily saturated places, e.g. Forest, grassland, desert,etc.

Forest Ecosystem

A forest ecosystem is a natural woodland unit consisting of all plants animals and micro-organisms (Biotic component) in that area functioning together with all of the non- living physical (abiotic) factors of the environment. Trees are in dominance in this ecosystem. Forests are found in undisturbed areas receiving moderate to high rainfall and usually occur as stable climax communities. About 30 per cent of the land area of the earth is under forest cover, but due to man's intervention this area is gradually becoming smaller.

Grass land ecosystem

Grasslands are areas where the vegetation is dominated by grasses with a few sparsely distributed trees. Grasslands occupy about 12 per cent of the earth's area, which include tropical and temperate grasslands. This is a type of terrestrial ecosystem which are dominated by grass species but sometimes also allow the growth of a few trees and shrubs. Grass lands occur in regions, which receive annual rainfall between 25-85cms. Grasslands are found both in tropical as well as temperate regions of the world. The prairies, pampas, steppes, etc are more or less synonymous terms for grass lands which differ from each other in species composition and some conditions of the environment. However a common feature of all grasslands is intermittent and erratic rainfall which is not enough to support a forest but is more than that of a desert. The annual production of organic matter in grassland is high which accumulates rapidly and the soils develop a thick layer of humus. Roots of grasses and other herbs may penetrate as deep as 2 meters in the soil. That is why grassland soils are among the thickest and richest soils and have provided to humanity some of the richest agricultural areas of the world. Animal populations of grassland are also rich and diverse. Many mammals, ungulates, rodents and hares flourish with large populations of bison, antelopes, zebra, horses, donkeys, buffalo, giraffe, elephant, etc lion, tiger, foxes, wild dogs etc form the majority of carnivore populations.

Desert Ecosystems

Deserts make up about 18% of total land cover on earth and are characterized by little (less than 50cm/yr) or no rainfall. Desert biomes have very high temperatures because of the little vegetative cover, less cloud cover, low atmospheric moisture and the land's exposure to the sun. Humidity is very low, with a few events of very little rain in a year. These are arid ecosystems which usually receive less than 25cm of annual rainfall. The meager rainfall is not available to plants due to its fast run-off rate. The deserts are characterized by extremely hot days and cold nights.

Aquatic ecosystems

The aquatic ecosystem is the ecosystem found in a body of water. There are two main types of aquatic ecosystem - Marine and Freshwater.

The marine ecosystem

Marine ecosystems are the biggest ecosystems, which cover around 71% of Earth's surface and contain 97% of out planet's water. Water in Marine ecosystems features in high amounts minerals and salts dissolved in them. The different types of the marine ecosystem are: oceans, seas and estuaries.

The Freshwater Ecosystem:

Contrary to the Marine ecosystems, the freshwater ecosystem covers only 0.8% of Earth's surface and contains 0.009% of the total water. The basic kinds of freshwater ecosystems are: Lentic : Standing water ecosystems like pools, lakes or ponds. Lotic : Running water ecosystems such as streams and rivers.


Those ecosystems which rely on the human efforts to sustain are called artificial ecosystems. They have almost no diversity and have simple food webs. The cycling of nutrients is negligible. The inputs are provided by the human efforts. The man made ecosystems include the villages, towns, cities, rivers, orchids, dams, gardens, lakes and agriculture.


The structure of an ecosystem means the composition of biological community including species population (numbers), biomass, life history, distribution in space etc. the quantity and distribution of abiotic (non-living) materials such as water, soil, nutrients, etc and the sense of conditions of existence such as temperature light, humidity, wind, wave action etc. Ecosystem has two major components which are abiotic and biotic.

Abiotic components

It represents the non-living or physical environment of an ecosystem which has a strong influence on the life of living organisms. The physical and chemical components of an ecosystem constitute its abiotic component as under

Physical factors

These factors include soils, temperature, wind, sunlight, water, currents, humidity etc. these are very important and have strong influence on the ecosystem.

Chemical factors

These constitute inorganic and organic substances. The inorganic substances include Carbon, Hydrogen, Nitrogen, Potassium Phosphorus, and Sulphur etc these are involved in nutrient cycling and are present in an ecosystem at any given time. The organic substances finclude carbohydrates, lipids, and proteins and are present in the biomass or in the environment.

Biotic component

The live component of an ecosystem comprises plants, animals, and microorganisms (Bacteria and Fungi). They carry out different functions and based on their role they are classified into three main groups. They are:


Producers are mainly green plants having chlorophyll. They produce carbohydrates by photosynthesis process. In effect the plants convert solar energy into chemical energy using water and carbon dioxide. These are called Autotrophs (self feeder) since they produce their own food, Part of the food produced by the autotrophs is utilized for their own consumption for survival and growth while the remaining is stored in the plant parts for future consumption. This becomes the food for other biotic components in the environment.


Consumers are living things, which do not have chlorophyll, and hence they are unable to produce their own food. They rely on the producers for their food requirements. Consumers are called Heterotrophs. Consumers are classified into four categories. They are

Primary Consumers or Herbivores

They are also called first order consumers. They eat the producers or plants. Examples are cattle like cow and goat, deer, rabbit etc.

Secondary Consumers or Primary Carnivores

They are also called second order consumers. They eat herbivores Examples are snakes, cats foxes etc.

Tertiary Consumers

They are also called third order consumers. They feed on secondary consumers. They are large Carnivores. Example is Wolf.

Quaternary Consumers

They are also called fourth order consumers. They feed on secondary consumers. They are very large Carnivores and feed on tertiary consumers and are not consumed by other animals. Examples are lions and tigers


Decomposers called, as Sapotrophs are mainly microorganisms like Bacteria and Fungi. The dead organic materials of producers and consumers are their food. They break down the organic matter into simple compounds during their metabolic process. These simple compounds are nutrients, which are absorbed by the producers thus completing a cyclic exchange matter between the biotic and abiotic components of the ecosystem.

Tropic relationship (food chain, food web, ecological pyramids)


The study of the structure of feeding relationships among organisms in an ecosystem. Trophic ecology investigates how various organisms at different feeding (trophic) levels interact within an ecosystem. In general, feeding or trophic relationships are represented as a food web or as a food chain. Living organisms are made up of organic matter. Organic compounds are made up of carbon. Methane is the simplest form CH,. Living organisms are made up of larger organic molecules (biomolecules) such as: proteins, carbohydrates, nucleic acids, lipids...etc... They therefore need organic matter to grow and to obtain energy.a) FOOD CHAIN b) FOOD WEB c) ECOLOGICAL PYRAMIDS


A food chain is the sequence of who eats whom in a biological community (an ecosystem) to obtain nutrition. The transfer of food energy from the source i.e. food chain. A food chain is a sequence of feeding relationships between organisms living within the same community. plants (producers) through a series of organisms (herbivores to carnivores to decomposers) with separated stages of eating and being eaten is known as the The term food chain was first of all introduced by a German zoologist Karl Samper as early as 1891. In a food chain energy in the form of food flows from primary producers to primary consumers (herbivores) from primary consumers to secondary consumers (carnivores) from secondary consumers to tertiary consumers and so on.

Grazing or Predator food chain

The grazing food chain begins with green plants at its base as producers. Plants act as the source of energy for the primary consumers. This type of food chain starts from the living green plants goes to grazing herbivores, and on to carnivores Ecosystems with such type of food chain are directly dependent on an influx of solar radiation. This type of chain thus depends on autotrophic energy capture and the movement of this captured energy to herbivores. Most of the ecosystems in nature follow this type of food chain. Examples of grazing food chains are as under:

Detritus food chain

This type of food chain goes from dead organic matter into microorganisms (decomposers) or organisms feeding on detritus (detrivores) and then goes to their predators. The detritus food chains start from dead and decaying organic matter of animals and plants known as detritus. Such ecosystems are thus indirectly dependent on direct solar energy. Here the detritus act as the source of energy for the primary consumers termed as detritus feeders for example bacteria, soil mites, worms, fungi. For example, such type of food chain operates in the decomposing accumulated litter in a temperate forest. This food chain depends upon dead organic matter either in the form of fallen leaves or dead animals and is indirectly dependent on solar energy. Examples.


In an ecosystem there are a very large number of interlinked chains. This forms a food web. A food web (or food cycle) is the natural interconnection of food chains and generally a graphical representation of what-eats-what in an ecological community. A food web is made up of interconnected food chains. Food web is a network of food chains were different types of organisms are: interconnected at different trophic levels so that there are a number of options of eating and being eaten at each trophic level. In an ecosystem most consumers have multiple food sources and most species are consumed by several types of predators. As a result individual food chain become interconnected to form a food web


Ecological pyramids are diagrams that illustrate how ecologically important factors, such as energy, biomass, and population size, vary between trophic levels in an ecosystem. Graphic representation of trophic structure and function of an ecosystem, starting with producers at the base and successive trophic levels forming the apex is known as ecological pyramid. The concept of ecological pyramid was developed by Charles Elton in 1927 and after his name these pyramids are also called as Eltonian pyramids. Based on the parameters selected to depict the trophic relationship, an ecological pyramid may be of the following types.

Pyramid of number

The pyramids of numbers show the relationship between producers, herbivores and carnivores at successive trophic levels in terms or their numbers. This pyramid shows the relationships between producers, herbivores and carnivores at successive trophic levels in terms of their numbers. When plotted the relationships among the number of producers, primary consumers, secondary consumers, tertiary consumers and so on in any ecosystem it forms a pyramid structure called the pyramid of number.
(a) In an aquatic and a grassland ecosystem numerous small autotrophs support lesser number of herbivores which support further smaller number of carnivores and hence the pyramid structure is upright.
(b) In forest ecosystem lesser number of producers support greater number of herbivores who in turn support a fewer number of carnivores.
(c) In parasite food chains one primary producer support many herbivores on which numerous parasites are feeding which support still more hyperparasites

Pyramid of Biomass

When we plot the biomass (net dry weight) of producers, herbivores, carnivores and so on we have a pyramid of biomass. The pyramids of biomass are comparatively more fundamentalism; as the reason is they instead of geometric factor; show the quantitative relationships of the standing crops. The pyramids of biomass in different types of ecosystem are of different types:
a) When larger weight of producers supports a smaller weight of consumers an upright pyramid results e.g. forest ecosystem.
b) When smaller weight of producers supports larger weight of consumers an inverted pyramid of biomass is formed e.g. aquatic ecosystem (pond ecosystem)

Pyramid of energy

The energy pyramid gives the best picture of overall nature of the ecosystem. The energy pyramid shows how the amount of energy entering each level varies across trophic levels. In general, only about 10% of the energy entering a trophic level is transferred to the trophic level above it, so the energy pyramid always has a distinct step-like pattern with less energy entering each trophic level up the food chain. As the quantity of energy available for utilization in successive trophic levels is always less because there is loss of energy in each transfer thus energy pyramid is always upright.

Functions of ecosystem (energy flow in an ecosystem)

The functions of the ecosystem are as follows:
It regulates the essential ecological processes, supports life systems and renders stability. It is also responsible for the cycling of nutrients between biotic and abiotic components. It maintains a balance among the various trophic levels in the ecosystem. It cycles the minerals through the biosphere. The abiotic components help in the synthesis of organic components that involve the exchange of energy.


Energy flow, also called the calorific flow, refers to the flow of energy through a food chain. Energy is the capacity to do work. Solar energy is transformed into chemical energy by Chlorophyll bearing plants convert this energy from the sun into carbohydrates and sugars using carbon dioxide and water. This process is known as Photosynthesis. The generalized form of the photosynthetic reaction is

The sun's energy thus enters the living beings through photosynthetic reactions and is passed from one organism to another in the form of food. On average about 10 percent of net energy production at one trophic level is passed on to the next level. Processes that reduce the energy transfer between trophic levels include respiration, growth and reproduction, defecation, and nonpredatory death (organisms that die but are not eaten by consumers).

Energy flow is the key function in an ecosystem and it is unidirectional or one way flow. It is one of the most fundamental processes that are common to all the ecosystems. In every ecosystem the energy flow provides a foundation for life and thus imposes a limit on the abundance and richness of life. The flow of energy is unidirectional and is governed by the two laws of thermodynamics. First law of thermodynamics states that energy can neither be created nor be destroyed but it can be transformed from one form to another. The solar energy captured by the green plants (producers) gets converted into biochemical energy of plants and later into that of consumers.

Second law of thermodynamics states that energy dissipates as it is used or in other words it gets transferred from a more concentrated to dispersed form. As energy flows through the food chain there occurs dissipation of energy at every trophic level. The loss of energy takes place through respiration, loss of energy in locomotion, running, hunting and other activities. At every level there is 90% loss of energy and the energy transferred from one trophic level to the other is only about 10%. Lindeman (1942) put forth ten percent law for the transfer of energy from one trophic level to the next. According to the law during the transfer of organic food from one trophic level to the next, only about ten present of the organic matter is available for next trophic level. The remaining is lost during transfer or broken down in respiration. The energy flow in an ecosystem through various trophic levels can be explained with the help of various energy flow models, one model is discussed as under.

Single channel energy flow model: The principle of food chain and working of thermodynamic can be understood by simple single channel model. Lindemann (1942) explained one such model for lake (fresh water ecosystem), as depicted in the fig

Ecological Succession (types and stages)

● Ecological succession is the process by which natural communities replace (or “succeed”) one another over time. For example, when an old farm field in the midwestern U.S. is abandoned and left alone for many years, it gradually becomes a meadow, then a few bushes grow, and eventually, trees completely fill in the field, producing a forest. 

● Each plant community creates conditions that subsequently allow different plant communities to thrive. For example, early colonizers like grasses might add nutrients to the soil, whereas later ones like shrubs and trees might create cover and shade. Succession stops temporarily when a “climax” community forms; such communities remain in relative equilibrium until a disturbance restarts the succession process.

● Understanding how succession happens in a variety of ecosystems—and what kinds of disturbances and time spans lead to the formation of different plant and animal communities —is important for scientists who want to understand ecosystem dynamics and effectively protect or restore natural communities. 

● For example, many natural communities in North America have adapted to periodic disturbances from wildfires: This can help maintain prairie or savanna communities that depend on open habitat and nutrient cycling that might occur as a result of fire. There are two major types of ecological succession: primary succession and secondary succession.

Primary succession

● Primary succession happens when a new patch of land is created or exposed for the first time. This can happen, for example, when lava cools and creates new rocks, or when a glacier retreats and exposes rocks without any soil. During primary succession, organisms must start from scratch. First, lichens might attach themselves to rocks, and a few small plants able to live without much soil might appear. These are known as “pioneer species.” 

● Primary succession begins when no plant life is present on the landscape, such as after a lava flow or glacial retreat. Over centuries, soil forms and deepens and successive communities of plants grow. 

● Gradually, the decomposition of those plants contributes to soil formation, and more and larger plants begin to colonize the area. Eventually, enough soil forms and enough nutrients become available such that a climax community, like a forest, is formed. If the site is disturbed after this point, secondary succession occurs.

Secondary succession

● Secondary succession happens when a climax community or intermediate community is impacted by a disturbance. This restarts the cycle of succession, but not back to the beginning —soil and nutrients are still present. 

● For example, after a forest fire that kills all the mature trees on a particular landscape, grasses might grow, followed by shrubs and a variety of tree species, until eventually the community that existed before the fire is present again. 

● Secondary succession begins after a disturbance, like a fire. Crucially, some soil and nutrients remain present—fire, in fact, may help recycle those nutrients.


● A climax community is the “endpoint” of succession within the context of a particular climate and geography. In the midwestern U.S., for example, such a community might be a hardwood forest with oaks and hickories as the dominant tree species. 

● A climax community will persist in a given location until a disturbance occurs. However, in many ecosystems, disturbance occurs frequently enough that a matrix of community types may be consistently present on the landscape

● For example, in an area prone to wildfires like the western U.S., mature forests may exist near grassy meadows with fewer, scattered trees. Consistent disturbance and variation in factors like water and nutrient availability over the course of decades thus allows many plant and animal communities to thrive within a particular climatic and geographic niche—not just those adapted to the absence of disturbance seen in climax communities. 

● In "climax communities, " like redwood forests on the Pacific coast of North America, the species composition might change very little for decades or even centuries, with ancient trees dominating the canopy and infrequent disturbances creating few opportunities for new plants to establish themselves.


Ecological succession can occur in many contexts and over many time spans. In Hawaii and Iceland, primary succession occurs on lava flows where new land has formed; in Canada’s Athabasca Dunes, it happens when new sand is deposited along a lakeshore; in the Andes, it occurs when glaciers retreat. In many regions, secondary succession occurs where wildfires have destroyed conifer forests, or where former agricultural land is reverting to meadow or scrubland. What these examples have in common is that the climax community is not the first one present on the landscape after succession begins: First, intermediate communities occupy the space, sometimes for many years, creating ideal conditions for the communities that follow

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