Ecology | HSC Part-II – Biology Notes

CHAPTER – 11

Ecology

Definition

Ecology is the scientific study of different relationships with each other and with their non-living environment which supports them.

Levels of Organisation of Ecology

Starting with individual organism, there are several levels at which interactions between the organism and their environment can be studied. These levels of study are

1. Population

2. Community

3. Ecosystem

4. Biomes

5. Biosphere

1. Population

A group of organisms of same species living and interacting in the same area is known as population. In other words members of the same species in an ecosystem constitutes a population. Population ecology mainly deals with factors that affect population size and composition.

2. Community

The next level of ecological organization is a community. It consists of a group of organisms (plants and animals) belonging to different species which interact in a particular area. In other words members of different species in an ecosystem form a community. Community ecology studies various interaction such as predation and competition among different species.

3. Ecosystem

It is higher level of ecological study and includes both abiotic factors and community of species of a particular area. Ecosystem is the basic structural and functional unit of ecology. It consists of a community of living organisms which interact with each other and also with their non-living environment to maintain equilibrium of life. Ecosystem ecology deals with study of the flow of energy and cycling of material between biotic and abiotic components of a particular area. The ultimate source of energy for any ecosystem is the sun. Examples of a ecosystem are a pond, a lake, ocean, forests, desert etc.

4. Biomes

Biomes are assemblage of large and similar ecosystem which cover vast geographical areas of the earth. For example coniferous forest biome includes all the comferous forest ecosystems which extend across the Europe, Asia and North American continents. The desert biome includes an extensive desert area scattered over many regions of the earth. Biome can also be described as a large easily recognizable community unit which is spread over vast geographical areas. Although a biome includes many species of plants and animals, it is primarily identified by its plant life and is named after a predominant vegetation, known as the population. For example forest biome or grass land biome. A biome is also named after some environmental factor. For example ram forest or temperate grass land.

5. Biosphere

All the ecosystem and biomes collectively form the biosphere. In other words, the entire portion of planet earth where life is present is called biosphere. Biosphere consists of the earth from its surface to about few meter depth, all the water and the gaseous capsule surrounding the earth. The surrounding air envelop extends to about two mile above the surface of earth. Beyond the biosphere, no living organism can survive.

Ecosystem

Ecosystem is the basic structural and functional unit of ecology. It consists of a community of living organisms which interact with each other and also with their non-living environment, to maintain equilibrium of life. An ecosystem regulates the flow of energy and cycling of material between its living (biotic) and non-living (abiotic) components. The ultimate source of energy for any ecosystem is the sun. There is no limits or boundaries on the size of an ecosystem. It may be as small as a bottle containing some Protozoa and Algae or as large as an ocean. Examples of an ecosystem are a pond, a lake, ocean, forest, desert etc. The term ecosystem was first coined by Tanslley in 1935.

Components of Ecosystem

An ecosystem is made up of two types of components. Non-living or abiotic and living or biotic.

Abiotic Components of Ecosystem

Abiotic components consists of non-living material and energy. These components include every thing that surrounds and individual organism except other living organism.

Abiotic components are basically of two types; Material and Energy. The material may be both organic and inorganic and include carbon, nitrogen, water mineral, carbohydrates, proteins etc. The energy is in the form of light, heat or chemical energy obtained by breakdown of chemical bonds.
The abiotic components are controlled by abiotic factors which can be divided into three categories

1. Climate Factors

2. Topographic Factors

3. Edaphic Factors

1. Climate Factors

Light
Light is the ultimate source of energy for an ecosystem and plays an important role in the lives of both plants and animals. Plants use light for photosynthesis. In terrestrial ecosystems, light is not a limiting factor because only about 3% of available light is used for photosynthesis. However, in deep and turbid waters, light can be a limiting factor for photosynthesis. Light affects organisms in three ways; Intensity of light, its duration or photoperiod and its quality (wave length).

Intensity of Light

The intensity of light depends upon season, latitude, angle of incidence and time of day. Plants are classified according to their requirement of light and shade into heliophytes and sciophytes. Intensity of light plays important role in plants. In plants intense light may destroy the chlorophyll. It also affects opening and closing of the stomata, permeability of the cell membrane, height and surface area of plant etc. In animals light affects development of pigments.

Photoperiod
Duration of light (photoperiod) affects flowering, leaf fall and other aspects of plant growth. In animals biological rhythms such as reproductive cycles sleep cycles, hibernation, migration, are affected by light duration.

Quality of Light

Out of seven colours of visible light (electromagnetic spectrum), chlorophyll absorbs only red and blue light for photosynthesis. Light of short wave length such as ultraviolet rays is harmful for life. Most of the light reaching the earth is converted to heat and thus light also affects the temperature of the ecosystem.

Temperature
Environmental temperature affects the structure and distribution of organisms because of its effects on biological processes. The living organism can function only between temperatures of 0⁰C and 50⁰C and beyond that range they die. Temperature of water usually does not show vast variations therefore aquatic life possess very few adaptions related to temperature. However, most land animals have adaptions to face sudden changes in temperature.

Water
Water is most important ecological factor because it is essential for life and directly effects the physiology distribution and structural adaptations of both animal and plants. It acts as a limiting factor in terrestrial ecosystems. The availability of water in an ecosystem depends on factors such as rain, snow, type of the soil, kind of vegetation etc. Water is the raw material for photosynthesis and also plays part in many vital biochemical reactions, It determines the quality and quantity of flora and fauna of an ecosystem. In areas where there is more rain forest are formed and where less water is available deserts are found. Therefore, both plants and animals show many adaptations with regard to water. The water is continuously recycled in nature in the form of hydrological cycle.

Atmosphere and Wind

Atmosphere is a gaseous cover around the earth. The main gaseous component of the atmosphere are CO₂, oxygen, nitrogen and water vapour. The CO₂ is essential for photosynthesis; oxygen is essential for respiration; while nitrogen is necessary for protein synthesis. Therefore, most of the components of atmosphere are linked with bio-geo-chemical cycles.
Wind is the circulation an the moving atmosphere. The velocity of the wind can affect the growth and distribution of vegetation, particularly trees. Plants in exposed areas with high winds show adaptations like presence of strong stem and roots. The dispersal of seeds and spores in plants and migration of birds is affected by the winds. Strong winds increase the rate of transpiration and can create xerophyic conditions.

Fire
Natural fires are mainly caused by lightning, volcanic activity, mutual friction between tree branches etc. It brings about sudden changes in an ecosystem by destroying many species of plants and animals. Fire also have some positive effects. For example they release and recycle many nutrients. In many areas of the world forests have been changed to grass lands due to effect frequent fires. Fire also favours growth of some fungi. These fungi are called pyrophilus fungi.

2. Edaphic Factors

The factors relating to condition and composition of soil are known as edaphic factors. The scientific study of soil is called pedology. The soil is a layer of materials lying above the rocks of earth’s crust. It forms a link between the non-living and living components. The type of soil determines the kind of plants which can grow well in a particular ecosystem. The structural components of the soil are inorganic material, organic material and soil water. Formation of soil depends upon two processes
a. Weathering: It is the breakdown of rocks into fine particles of minerals.

3. Topographic Factors

Topography refers to the physical features of land of an area and includes such components as shape of the surface, altitude, slope exposure to wind etc. Topographic can influence many climatic factors and thus plays an important role in ecosystem.

i. Altitude

Higher altitude generally have lower temperature higher rainfall, increased wind speed, more light or exposure to sun etc. Therefore vegetation at different altitude is different and show distinct zonation.

ii. Slope

Steep slope have fast drainage and therefore the soil becomes thinner and cannot retain water. Consequently, mountains usually show xerophylic vegetation. In northern hemisphere, south facing slopes get more sunlight than north facing slopes and therefore, there is marked difference in the vegetation.

iii. Direction of Mountains

Mountains and their direction affect the climate by changing the direction of the rain laden wind. The windward side of mountain have more rainfall than the other side.

Biotic Components of Ecosystem

The living organisms in an ecosystem constitute its biotic component and are collectively known as the community. On trophic or nutritional basis, biotic components are divided into two types

i. Autotrophs

Which can synthesis their own complex organic foed from simple inorganic molecules. Green plants and some bacteria fall in this category.

ii. Heterotrophs

Which cannot synthesis complex organic food and dependent on autotrophs or other organisms for their nourishment. Animals, fungi and many bacteria are heterotrophs.

Ecological Succession

Succession is an orderly and progressive change in a community structure of an ecosystem over a long period of time. Such changes mostly take place due to change in environment or due to some disturbance such as fire, volcanic eruption, flood, movement of glaciers etc. These disturbances remove the existing community and the disturbed area may be colonized by a variety of new species. The new species, in turn may gradually be replaced by still other species over a period of time. The replacement process goes on till a relatively stable community is developed. Thus during succession the community passes through many predictable transitional stages called seres. The first established community is called pioneers or primary community and the final relatively stable and mature community is known as the climax community. The word succession was used by Hult for the first time for orderly changes in community. Two main types of successions are

1. Primary Succession

2. Secondary Succession

1. Primary Succession

If the succession begins in a lifeless area where no living organism is present and where soil has not yet been formed, it is called primary succession. In other words the community is formed from a scratch. For example succession on a new volcanic island or on newly formed sand done, is a primary succession. Primary succession require thousands of venus to form a climax community.

2. Secondary Succession

It occurs in places where an existing community has been renowned by some disturbance such as fire or other climatic factors. In such a place there may not be any living organisms but the soil remains intact. As a result the secondary succession is more rapid than primary succession. The new communities developed are different from the pressure ones. Depending on the habitat, where the succession begins, the succession is divided into

i. Hydrosere: Succession occurring in water

ii. Xerosere: Succession occurring on dry soil or dry terrestrial habitats such as rocks, sand dunes, etc.

iii. Mesosere: Succession on land but where adequate moisture is available.

i. Hydrosere OR Hydrach

Primary succession starting in open water such as ponds, lakes etc. which are ultimately converted to land community is called hydrosere. There are several seral stages based on dominant plant types. The seral stages of hydrosere in a pond are

a. Phytoplankton Stage

The first invaders are phytoplanktons such as green algae (spirogyra), blue green algae (eynobacteria), diatoms etc. Since the pioneer community is phytoplankton, this seral stage is called the phytoplankton stage. Later on Protozoans such as Amoeba, Euglena etc also invade the aren.

b. Submerged Stage

The death and decay of planktons make the soil suitable for growth of rooted hydrophytes such as Hydrilla, Vallisnaria etc. Animal life at this stage is blue fish, sun fish.

c. Floating Stage

The death and decay of submerged hydrophytes and accumulation of sediments from surrounding area decrease the water level. Therefore pond becomes suitable for growth of rooted plants with floating leaves. These plants include Nymphea, Nelumbium etc. Some free floating plants are also seen. These plants lose water through transpiration and consequently the water level is very much reduced. Animal life is represented by Hydra, snails, frogs.

d. Reed Swamp Stage

This stage is also called amphibious stage. due to death and decay of plants, pond becomes more and more shallow. Plants are rooted but part of their bodies is above water level. Examples of such plants are Typha, Sagittaria. Animals are insects, water spiders, giant bugs.

e. Sedge Meadow Stage

The water level continuously decreases and the substratum becomes more suitable for growth. The stem is usually in the form of rhizome. Transpiration decreases the water level more and more and finally the marshy soil is exposed. The oxygen of the air oxidises the nutrients of the wet soil to form nitrates and sulphates. Thus the soil fertility is increased plants of this stage are Juneus.

f. Woodland Stage

The water of the soil evaporates rapidly and the soil becomes more and more drier. The marshy vegetation is replaced by land plants such as Salix, a shrub and populs, a tree. Animals are terrestrial like Zebra, Deer etc.

g. Climax Stage

The climax community is represented by trees and thus forests are formed. If there is heavy rainfall in the area, as in tropical regions, rain forests are developed. In areas with moderate rain fall, mixed forests are developed.

ii. Xerosere

This type of succession starts on a bare rock on land where there is deficiency of water and organic matter. The serial stages xerosere are

a. Crustose Lichen Stage

A bare rock does not possess moisture and organic matter, but some special types of lichens the crustose lichens, can survive on such bare surfaces and form the pioneer species. These lichens can produces acids which help in breaking the rocks and therefore start formation of the soil on other species. The lichens of this stage are Rhizo-carpon etc.

b. Foliage Lichen Stage

These lichens are large and leaf like in structure. They have ability to absorb moisture from the air and retain it. They can bind the loose soil particles together and thus the soil becomes more rich. Lichen of this stage are Pasmelia etc.

c. Mass Stage

Due to death and decay of the lichens, the organic matter is deposited in the soil which becomes richer in nutrients and can hold water also. As result plants like mosses develop. They can absorb water from the soil through rhizoids. Plants of this stage are polytrichum, selaginella etc.

d. Herb Stage

Due to more availability of moisture, humus and better anchorage, small herbaccous plant develop. In the beginning annual herbs are formed but later on perennial herbs develop. Some larger animals such as nematodes, ants and other insects also invade the area.

e. Shrub Stage

The conditions become suitable for growth of larger plants (shrubs). They shadow the herbs which die off adding more humus to the soil. Large number of animals such as snails, millipedes, some amphibians, reptiles and mammals like shrews, squirrels , foxes some birds are also found.

f. Climax Stage

More and more humus is deposited in the soil due to death and decay of plants and animals. This allows formation of woody trees. Thus final climax community is formed. The type of forest will depend on the climate conditions.

Approaches to Ecology

Autecology (Population Approach)

It is the ecological study related to a single species (population). For example if you study effects of pollution on a single mango tree or all the mango trees in an area, it will be autecology.

Synecology (Community Approach)

It is the ecological study related to different species (community). In synecology you have to consider together the individual and the community. The type of ecological study has development the concept of succession and climax that is the progressive replacement of one community by more stable community in a particular area.

Ecosystem Approach

This approach takes into consideration the whole ecosystem as a unit. Ecosystem approach studies the biotic and the abiotic components their interaction, the flow of energy and the cycling of materials between the living and the non living and the non living components. This approach may take into consideration a small ecosystem such as a pond or a large ecosystem such as a desert.

Habitat Approach

Habitat is a particular place or area with a specific set of physical conditions in which a community of living organisms live. These organisms possess special adaptations which help them to survive in that habitat. The flora and launa of different habitat are different because of different physical conditions. Since defining habitat is relatively easy, some scientists use this approach for ecological studies. Some ecologists divide the ecology on the basis of type of habitat in which the organisms live. Habitat is divided in two main types; aquatic and terrestrial. Aquatic habitat is of three types; fresh water habitat, marine habitat and estuarine habitat. Estuaries are places where rivers and streams meet the sea. Terrestrial ecology can be divided into forests, desert, grass land habitat.

Evolutionary Approach

The distribution of organisms is result of long term evolutionary changes and their interaction with environment. Evolutionary approach tells us about changes since life originated. This knowledge comes from study of fossil record. Various evolutionary theories are also part of this approach.

Historical Approach

It deals with study of various stages and periods of time through which humans have passed during their evolution. One aspect of this approach is to study the development of tools in human history. The first period is called stone age or Neolithic period (1500 to 3000 B.C), during which humans used tools made up of stones.

Interdependence of Organism

(Interactions in an ecosystem)

The organisms in an ecosystem not only interact with their physical environment but also with each other. These interactions may be beneficial or harmful.

Positive Interactions

In this type of interaction organisms form associations for mutual benefit or for benefit of one species without harming the other. Mutualism and commensalisms belong to this type.

Negative Interactions

In this type of interaction one organism gets the benefit and the other is harmed. Parasitism, predation and grazing are considered to be harmful interactions.
The interactions between the organisms lead to close associations between them. There are two basic types of associations:

Intraspecific Association

Associations between members of the same species. These associations are found in social animals for social organizations or for formation of social groups.

Interspecific Associations

Associations between different species. In these associations, members of one species feed on members of another species and may also use them as habitat. Following are the types of interspecific associations

1. Parasitism

Parasitism is an association between two living organisms in which one lives in or on the body of the other and derives nourishment from it. In other words, one gets the benefit and the other is harmed. The organism getting the benefit is called the parasite and the one which is harmed is called the host. There are following types of parasites

Ectoparasites
They live on the surface of the body of the host. Examples, leech, lice, etc.

Endoparasites
They live in the tissues or cavities of the host. Examples Plasmodium, Tnenia, Entamoeba.

2. Symbiosis

Generally the symbiosis is defined as an association between two living organisms of different species for mutual benefit or in which one partner gets the benefit and the other is neither benefited nor harmed. In other words none of the two partners is harmed. There are two varieties of symbiosis

a. Commensalism

b. Mutualism

a. Commensalism

It is an association between two living organism of different species in which one is benefited and the other is not affected, that is it is neither benefited nor harmed. Following are some examples

Epiphytes are plants that grow on trunks and branches of trees for support only but can synthesis their own food. In htis way the epiphyte is benefited without harming the tree. Epiphytes are common in tropical rain forests. Orchids and mosses are common exampels.

Small Fishes live in the cloaca of sea cucumber for protection only. No harm is done to sea cucumber.

b. Mutualism

It is an association between two species of living organisms for mutual benefit. The two partners are dependent on each other for their survival and growth. The organisms in this association may be two animals, two plants or a plant and an animal. Example of mutualism are

Nitrogen Fixing Bacteria

They live in the nodules of leguminous plants. The bacteria fix atmospheric nitrogen for the plant and in return the plant provides shelter for the bacteria.

Lichens
It is an association between green algae and fungus. The algae cells are protected by the fungus and derive moisture from it, while the fungus obtains oxygen and carbohydrates from the algae.

Sea Anemone and Hermit Crab

The sea anemone attaches itself to shell of hermit crab. The anemone gets free transport while the crab is camouflaged by the anemone and is protected from its predators.

3. Predation

In this relationship one population called predators feeds on the members of another population known as prey. Normally predators are larger than the prey and quickly capture and kill the prey. Predators are usually animals bet a few like pitcher plant and other carnivorous plants are plant predators. Mostly the predators are secondary or tertiary consumers of an ecosystem.
The prey-predators interaction keeps the population of both the prey and the predator in equilibrium. Any species of a prey without its natural predator, disturbs the balance of the ecosystem due to explosive increase of their population. The degree of harm caused by this interaction, depends upon the relative population of the prey and predators. For example Ospreys (a kind of hawk) fed on fishes of the ocean. The supply of fishes is so large that this interaction does not cause much harm. However any disturbance in the relative number of prey and predator may result in extinction of any one or both.

4. Grazing

Grazing is a feeding association between herbivores and plants. Natural herbivores include rabbits, deer, zebras, wild horses, elephants etc. Man has also introduced cattle and sheep in the grass land both direct and indirect effects on the ecosystem. Moderate grazing is not harmful, but the over grazing greatly surfaces the growth capacity of grasses and other plants. Consequently all the producers are eliminated and green pastures are turned into barren lands. The indirect effect of overgrazing is hardening of the soil and soil erosion. The plant roots and under ground stem of grasses bind the soil together. Their destruction by grazing makes the soil loose which erodes easily. Too many animals trampling the soil makes it hard and poorly aerated. This may lead to conversion grass lands into deserts. Grasses have many adaptation to save them from effects of grazing.

Biogeochemical Cycles

(Cycles in Ecosystem)

There is a constant recycling of essential nutrients within the biosphere from environment to organism and back to environment. Since the path of recycling involves living organism (bio), soil (geo) and chemical reaction these cycles or circular paths are called biogeochemical cycles. The elements which are recycled include Carbon, Hydrogen, Oxygen, Nitrogen, Phosphate and many others. Carbon, Hydrogen and Oxygen are closely associated with each other. Therefore, they form a single carbo, hydrogen, oxygen cycle, but the nitrogen and phosphorus form independent cycles.

Nitrogen Cycle

The nitrogen is an essential part of protoplasm, proteins, nucleic acids, chlorophyll etc and is therefore, necessary for living organisms. The exchange of nitrogen between soil and atmosphere and its circulation through bodies of living organisms takes place through inter-related processes known as nitrogen cycle.
The atmosphere contains about 78 percent nitrogen gas. It is an inert gas and the organisms cannot use it directly. Therefore, nitrogen is first changed to soluble nitrogen compounds such as nitrates (NO₃) which the plants can absorb from the soil. Conversion of atmospheric nitrogen to soluble nitrogen compounds is called nitrogen fixation.

Source of Nitrates

There are three sources of nitrates for living organisms

i. Nitrogen Fixation by Thunderstorm

ii. Nitrogen Fixation by Living Organisms

iii. Decomposition of Dead Organic Material

i. Nitrogen Fixation By Thunderstorm

The first source of nitrates is thunderstorm and lightening. Nitrogen can combine with atmospheric oxygen at very high energy level. The lightening during rains provides this energy which is used to combine atmospheric nitrogen and oxygen to form nitrogen oxides. These oxides dissolve in water to form nitric acid which falls down into the soil and react with mineral ions there to forms nitrates.

ii. Nitrogen Fixation by Living Organisms

The second source of nitrates are bacteria and cynobacteria (blue, green, algae) which can covert free atmospheric nitrogen into soluble nitrates and nitrates. There are two types

a. Symbiotic Nitrogen Fixation

Symbiotic bacteria, such as Rhizobium, live in the nodulated roots of legumes such as bean, pea etc. These bacteria have the ability to absorb free atmospheric nitrogen and convert it to organic compounds such as nitrates which enter the soil.

b. Non-Symbiotic Nitrogen Fixation

It takes place by the activity of free living soil bacteria like clostridium and Azobaeter and blue green algae like Nostoc etc. They absorb free nitrogen from air and convert it to nitrates of the soil.

Nitrates from Decomposition

The third source of nitrogen for organisms is soil nitrates from decomposition of dead organisms and excretory wastes. This process occurs in two steps

i. Ammonification

ii. Nitrification

i. Ammonification

The dead animals, plants and excretory waste is decomposed by the saprophytic bacteria and fungi to form simple compounds like amino acids, water and carbondioxide. The amino acids are then changed to ammonia and ammonium ions. This process is called ammonification.

ii. Nitrification

Ammonia and ammonium, formed as a result of ammonification is converted to nitrites and nitrates by a process known as nitrification. It takes place by the activity of two groups of bacteria together called the nitrifying bacteria. Nitrite bacteria, such as Nitrosomonas, first convert ammonia to nitrites. The second group the nitrate bacteria such as Nitrobacter, then convert nitrites into nitrates.

The nitrates from all the above sources are taken up from the soil by the plants and are used for formation of plant proteins. These proteins are then taken up by animals as food and converted to animals protein. The dead plants, animals, the excretory waste is decomposed and the nitrates are recycled as described a above.

Denitrification
The loss of soil nitrogen is called denitrification and mostly takes place through the activity of denitrifying bacteria such as Pseudomonas. They convert nitrites and nitrates back to molecular nitrogen and return it to atmosphere. These bacteria are anaerobic and use nitrates as oxidizing agent instead of oxygen. Therefore, in well acrated soils their activity is restricted and soil nitrogen is not lost. The lost nitrogen of the soil is made up by fixation of nitrogen from atmosphere.

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