organisms and its environment


The large natural ecosystem which is distinct in its climatic conditions and has its specific group of climax plants and associated animals constitutes a biome.

Regional and local variations within each biome lead to the formation of a wide variety of habitats.

Rainfall, temperature range, nature of soil, barriers, latitude and altitude determine the nature and extent of biomes.

Major Biomes of India :

1. Tropical Rain forests:

In India, tropical rain forests are found mainly along western ghats and in North-Eastern Himalayas.

Dipterocarpus and Hopea are the most common tree species in Indian rain forests.

They show 30-40 m tall canopy with 4 -5 strata.

These possess highest standing crop among all biomes.

Buttress and leaf drip tip are common.

Woody climbers and epiphytes grow profusely in these forests.

The soil of such forests is highly leached and has low base content, so nutrient storage in soil is low.

They require mean annual temperature of 23 -27°C and 2000 -3500 mm rainfall.

2. Tropical Deciduous forests:

They occur in the northern and southern parts of our country in plains and low hilly areas.

Sal, teak, tendu, khair and chiraunji are common trees of these forests.

These forests show a short structure of 10-20 m.

During rainy season, the forest is lush green with dense foliage, whereas, in summer forest is largely leafless.

The soil of these forest is rich in nutrients due to less leaching.

They require a mean annual temperature of 22-32°C and mean annual rainfall of 900-1600mm.

Fig. : Biome distribution with respect to annual temperature and precipitation

3. Desert:

In these biomes the vegetation is very sparse due to extremes of temperature and very little rainfall (below 10 cm).

Hot deserts are characterised by high rate of evapotranspiration and albedo.

In cold deserts, the conditions are physiologically xeric and they exhibit permafrost while in hot deserts the conditions are physically xeric.

Important trees of Indian desert are Prosopis cineraria, Acacia sp., Salvadora sp. and Tamarix sp. common succulents are species of Euphorbia and many members of family Cactaceae. Cenchrus is an abundant grass of these biomes.

4. Coastal Biome :

Coastal areas are zones of transition between oceanic and terrestrial habitats, so are very sensitive.

These are detritus based biomes, where plants have to adapt for salinity and water logged conditions.

Mangrooves are the major types alongwith salt marshes or swamps.

Mangrooves are characterized by presence of pneumatophores and viviparous seed germination.

Common examples are Rhizophora, Sonneratia, Avicennia and Laguncularia. Besides this Phoenix, Pandanus and Casuarina are also found commonly in coastal areas.

5. Temperate Broad Leaf Forests:

Between 1500 m -2400 m altitude in western Himalayas predominated by oaks. e.g., Quercus floribunda , Q. lanuginose etc.

They require mean annual temperature of 6 -20°C and mean annual rainfall of 1000-2500 mm.

Show peak leaf fall during summer but never become leafless.

These have four strata with 25-30 m height and are rich in epiphytic flora.

Herbaceous layer is least developed and grasses are generally lacking.

6. Temperate Needle Leaf or Coniferous Forest:

Between 1700-3000 m altitude.

They require mean annual temperature of 6-15° C and mean annual rainfall of 500-1700 mm.

Taller trees (30-35 m) with evergreen canopy.

Predominated by economically valuable gymnospermous trees, like -Pine (Pinus wallichiana), Deodar (Cedrus deodara) , Silver fur (Abies pindrow), Spruce (Picea smithiana) and Cypress (Cupressus torulosa).

Some Important Biomes of the World -A Brief Account

A. Tundra :

It is located in the north of timber line or 60º N latitude below the polar ice.

It is absent in southern hemisphere.

It extends across North America, Europe and Asia.

Also called as arctic tundra.

Subsoil remains frozen except upper few inches in the summers.

The condition is called permafrost.

Vegetation is scanty, low growing and devoid of trees and thus, the region is termed arctic desert.

Common plants found here are grasses, sedges, mosses and lichens with occasional occurence of dwarf birches (Betula) and willows (Salix).

B. Taiga (North coniferous/temperate needle leaf forest) :

It stretches as an east west band just south of tundra across North America, Europe and Asia. It is also found in southern hemisphere.

Mean annual rainfall is 50-170 cm. In winter, average maximum temperature is 6°C and nights are long and chilly.

Summers are pleasant with average maximum temperature of 20°C and with long hours of day light.

The characteristic feature of this biome is the presence of numerous lakes.

Dominant climax vegetation of this biome comprises of tall evergreen conifers with needle-like leaves, capable of tolerating fluctuations in temperature and light intensity.

C. Chapparal (Mediterranean scrub forest) :

The biome extends along the Mediterranean, Pacific coast of North America, Chile, South Africa and South Australia.

Natural fires are characteristic of this biome.

Rainfall is very limited, occurs only in winter.

The climate remains dry in the rest of the year.

D. Grasslands:

(a) Savanna: Tropical gras.sland with well developed grass cover interspersed with scattered shrubs and small trees.

(i) Distributed in warmer parts of India, Africa and Australia.

(ii) Appear in areas with highly seasonal climate having distinct wet and dry periods.

(iii) Abundance of C4 photosynthetic grasses.

(iv) Common grasses of Indian savannas are -Dichanthium, Sehima, Phragmites, Saccharum, Cenchrus, Imperata and Lasiurus. Common trees and shrubs are -Zizyphus, Prosopis, Capparis, Acacia and Butea.

(b) Temperate grasslands : The temperate grasslands are present in North America (Canada and U.S.A.), South America, Eastern Europe, Central Asia, South Africa and Australia. These are of different types depending upon the constituent flora in different countries, such as Prairies (Canada and the USA), Pampas (South America), Steppes (Europe and Asia), Veldts (South Africa), Tussocks (New Zealand) and Dawns (Australia).

Aquatic Biomes

The aquatic ecosystems range from ocean to small ponds or lakes showing wide range of variations regarding salinity, depth and temperature.

Consequently the organisms show lot of diversity in their adaptations to the surroundings.

Aquatic biomes are of four main types:

1. Oceanic or marine biomes:

Oceanic biomes occupy more than two third of the earth's surface.

The marine environment is characterized by high concentration of salts (about 3.5% in open sea) and mineral ions (mostly Na+ and Cl ions followed by sulphur, magnesium and calcium).

The productivity of oceanic biome is less than that of most the terrestrial biomes.

The ocean basin is always like a wash basin or inverted hat and is differentiated into continental shelf, continental slope and ocean floor.

(i) Continental shelf: It extends from coastline to about 160 km in the sea, including a gradual sloping area with depth varying from 8-200m. It has high productivity.

(ii) Continental slope: It extends beyond continental shelf formed by abrupt steepening of angle of slope. It is characterized by presence of ridges, trenches and basins of mud and sand.

(iii) Ocean floor: It is nearly horizontal with deep trenches at places. It is bottom area of open sea.

Oceanic biome is divided into three major ecosystems-open sea, coastal region and estuary.

(i) Open sea: It includes the area of sea beyond continental shelf and is divided into 3 zoneseuphotic, disphotic and abyssal zone depending upon the degree of light penetration. On the basis of environment, it has two parts-pelagic (open water zone) and benthic (bottom zone).

Both producers and consumers occur in photic zone in abundance, whereas only few producers alongwith consumers occur in disphotic zone. The abyssal zone is characterized by the presence of consumers, scavengers and decomposers, while producers are absent.

(ii) Coastal region : It is the area of continental shelf and is usually divided into 3 zones: intertidal, littoral and neritic zones.

Intertidal zone is alternately exposed and covered with water. Beaches belong to this zone. Very few plants grow in sandy beaches. Crabs and few burrowing animals occur.

Littoral zone : It represents the floor area of continental shelf. This zone is characterized by strong wave action. Main producers of this area are brown and red algae like Laminaria, Macrocystis, Nereocystis and Gelidium.

Neritic zone : It comprises the coastal part (near shore area). It contains phytoplankton.

(iii) Estuary : Ecotone areas where river mouth meets the oceanic water. This area shows wide fluctuations in salinity due to mixing of fresh and sea-water. Estuary constitutes one of the most productive ecosystems. It includes both fresh water and marine organisms.

2. Ponds and lakes:

These are stationary fresh water bodies (Lentic ecosystems) on land occur in almost all biomes.

Ponds vary in size and may be natural or man-made depressions which get filled with rain or run off water.

These may be seasonal or permanent.

The lakes are much larger than ponds and have size of several hundred hectares with depth upto 100 meters.

Lakes develop in nature due to three reasons-(i) result of glaciation, (ii) natural or man-made depressions getting filled with water (iii) formed by cut off water from main stream of river and may be termed ox-bow or cut-off lakes.

3. Streams and Rivers:

These are flowing fresh water bodies (Lotic ecosystems) which differ in physical and chemical conditions, oxygen content, temperature, speed and volume of water.

River beds having sand are less in productivity than the ones having mud and stones.

Planktons are very rare in higher reaches due to fast moving water, while these appear in lower reaches when water flow slows down.

4. Marshes:

These are temporarily produced low lying areas, few cms in depth, containing turbid water.

These are common on the side of road, railway track, rivers, streams and inside forests.

Planktons are little due to turbidity of water and temporary nature.

Amphibious plants are common.


The most important key elements that lead to so much variation in the physical and chemical conditions of different habitats are temperature, light, water and soil.

(A) Temperature

Ecologically it is the most relevant factor, as temperature variation affects the enzyme kinetics, basal metabolic activities and the physiological functions of the organisms.

So thermal tolerance decides the geographical distribution of different species to a large extent as for example ; mango trees do not and cannot grow in temperate countries like Canada and Germany, snow leopards are not found in kerala forests and tuna fish are rarely caught beyond tropical latitudes in the ocean.

Based upon thermal tolerance, organisms are of two types:

(i) Stenothermal : Such organisms live in areas where the temperature is uniform throughout the year. The organisms cannot tolerate large temperature variation.

(ii) Eurythermal : Such organisms can tolerate large changes in temperature.

The organisms are classified into four temperature groups on the basis of their occurrence in different climatic zones:

(i) Megatherms : Organisms are adapted to high temperature throughout the year as found in tropical zone.

(ii) Mesotherms : They are adapted to mild winters and high summer temperature. The organisms live in subtropical zone.

(iii) Microtherms: They live in temperate areas where the winter temperature is low but the summer temperature is moderate.

(iv) Hekistotherms: The organisms are adapted to brief summer period of below 10°C and long snowy winter period. This condition occurs in arctic or alpine zone.

Some Rules Based Upon Effects of Temperature

(i) Bergman's Rule: Warm blooded animals (birds and mammals) have larger body size in cold climate than in hotter areas.

(ii) Allen's Rule: Extremities (legs, ears, tail and mouth) of warm blooded animals become smaller in colder areas as compared to animals of warmer areas.

(iii) Rensch's Rule: In colder climate, birds possess narrow and acuminate wings as compared to broader wings of birds found in warmer areas.

(iv) Jordan's Rule: As the temperature is lowered, some fishes possess larger size with larger number of vertebrae.


Thermoperiodicity or thermoperiodism is the response of living organisms to regular changes of temperature.

It is of two types, diurnal and seasonal.

(i) Diurnal Thermoperiodicity : It is response of organisms to daily changes of temperature. Generally, day time temperature is higher, while night time temperature is lower.

(ii) Seasonal Thermoperiodicity : It is response of organisms to seasonal changes in temperature. Alongwith photoperiodicity, it controls phenology of plants. Phenology is the occurrence of seasonal activities in relation to change in environmental conditions.

Thermal Stratification in Lakes

The occurrence of temperature variations in different horizontal layers as in a deep water body is called thermal stratification.

A deep water body like lake has three temperature strata -epilimnion, metalimnion and hypolimnion.

(a) Epilimnion: Upper stratum, with highest dissolved oxygen concentration. This area is warmer during summers.

(b) Hypolimnion: Lower stratum of water characterised by a temperature gradient of less than 1°C per meter. It contains more dense, cooler and relatively quite water.

(c) Metalimnion: It is transitional stratum of marked thermal fluctuations between hypolimnion and epilimnion. Its middle layer is characterised by temperature gradient of more than 1°C per meter of depth called as thermocline. The term thermocline refers to plane or surface of maximum rate of fluctuations in temperature of metalimnion.

Fig. : Thermal stratification in a temperate lake

(B) Water

Next to temperature, water is another important factor influencing the life of organisms.

It is an important component of protoplasm which is a general solvent.

Water is also present over more than 71% surface of earth as oceans, lakes, rivers, ice caps and glaciers.

Sea water has high percentage of salt content (3.5%).

Water present on land is called fresh water. Its salt content is low i.e., less than 0.5%.

The salt concentration (measured as salinity in parts per thousand) is less than 5 percent in inland water, 30 -35 percent for sea and more than 100 percent in some hypersaline lagoons.

Some organisms are tolerant to a wide range of salinities (euryhaline), but others are restricted to a narrow range (stenohaline).

Regular movement of water amongst various regions and components of biosphere viz. aquatic systems, air and land constitutes water cycle.

Water comes over land or water body as precipitation or rainfall.

The total global rainfall is equal to 4.46 G.

Precipitation comes from water vapours present in air.

At any time atmosphere contains only 0.13 G of water vapours (1 G or geogram = 1020 gram).

(C) Light

Light has a wide range of spectrum.

Electromagnetic spectrum is a complete range of oscillating waves that travel together through space at a speed of 3 × 105 km/sec.

At 83 km above Earth's surface, solar radiation carries energy equivalent to 2 cal/cm2/min. This value is called as solar constant.

Short wave radiations are Cosmic Rays (with wavelength less than 10–5 nm), gamma rays (10–3 to 10–5 nm), X-rays (10–1 to 10–2 nm) and UV rays (100 to 400 nm).

All the short wave radiations are extremely harmful. Most of them are trapped in ionosphere and mesosphere.

UV rays are also harmful. They are of three types :

UV-C and about half of UV-B radiations are absorbed by ozone layer of stratosphere.

A large amount of the rest is dissipated by particles of troposphere, only a small amount reaches on the Earth.

Light affects photosynthesis, growth, reproduction, movements, stratification, photoperiodism and phenology in plants, whereas, it affects migration, reproduction, development, pigmentation, locomotion and periodic activity in animals.

Light Zonation of Lakes :

Littoral zone Exposed to wave action and is highly productive

Limnetic zone Open water body, rich in planktons.

Euphotic zone Receives maximum light above light compensation point.

Disphotic zone Receive diffuse light at or below light compensation point. Also known as twilight zone.

Profundal (Dark) zone No light

Benthic zone It is the bottom zone of perpetual darkness.

Fig. : Zonation in deep lake showing gradient of light and oxygen

Soil Composition

Soil consists of four components, two solid and two non-solid.

The solid components are mineral particles and organic matter.

The two non-solid components are air and water.

A fifth component of variable nature is soil organisms.

The proportion of different components is

Mineral Particles 40%
Organic Matter 10%
Air 25%
Water 25%

Chief characteristics of the soil are studied with the help of soil profile.

Type of soil profile depends upon climate and vegetation of the area.

The smallest three dimensional volume of soil required to study its profile is called pedon.

Most soils possess 3-4 horizons and a number of sub horizons.

A soil horizon is a horizontal layer approximately parallel to soil surface that possesses distinctive properties which are unlike the ones present in adjoining regions. In general, a profile consists of O, A, B, C & R horizons.

Fig. : Hypothetical diagram of the soil profile to show principal horizons


It is the breaking of rocks into fine particles as present on the soil.

Weathering occurs due to the following methods:

(i) Physical Weathering: It is caused by alternate heating and cooling, alternate wetting and drying, action of frost, snow, rain and wind.

(ii) Chemical Weathering: Oxidation, reduction, carbonation and solubilization reactions to break the rock.

(iii) Biological Weathering : By Lichens, Mosses.


It is addition of organic matter or humus into weathered rock.

Humification is essential for starting biological activity and nutritional cycling.

Humus is dark coloured amorphous substance, is slightly acidic, colloidal and acts as reservoir of nutrients.

Main functions of humus are biogeochemical cycling, preventing soil from compaction, helping in formation of soil crumbs, improving aeration and water holding capacity of soil.

It also makes soil spongy, therefore, easy for penetration by the roots.

Eluviation and Illuviation

These two processes bring about transport and deposition of materials in the soil.

Eluviation is washing down of materials from upper strata.

Eluviation helps in enriching the different layers of soil with minerals.

Illuviation is deposition of washed down minerals in lower strata.

Mineral Matter

It consists of inorganic substances present as particles of different sizes and composition.

(i) Gravel: It is made of fine pebbles with a size of 2-10 mm.

(ii) Sand: It consists of grains of quartz or silicon dioxide (Si02). Size varies from 0.02-2.0 mm. Sand is chemically inert. It allows quick percolation of rain or irrigation water. Aeration is good.

(iii) Silt: It is formed of fine grains of quartz. The size is 0.002-0.02 mm. It is chemically inert.

(iv) Clay: It is made of Al, Fe and Si. The size is below 0.002 mm. Clay particles are chemically active and have fine interspaces that can hold abundant water but aeration is poor.

Soil Porosity

It is percentage of interspaces present per unit dry weight of soil.

The value of soil porosity is 30% in sandy soils, 45% in loam soil and 50% in clay soil.

There are two types of soil pores, micropores and macropores.

Micropores are small sized interspaces having a diameter of 20 µm or below.

They hold water by capillarity. Macropores are interspaces with a size of more than 20 µm.

Soil Air

It is air present in macropores with a size between 20-50 µm.

A good soil should have 25% air by volume.

Soil air is required for respiration of roots and several microorganisms.

Soil air is richer in CO2 and poorer in O2.

Soil Types

(a) Red Soils: These are acidic laterite soils which are deficient in lime, magnesium, phosphorus and potassium, but rich in organic matter, iron and aluminium. Such soils support tea, coffee, rubber, cardamom, areca nut and p'addy cultivation.

(b) Black soils : Also called black cotton soils/regurs with dark brown or black colour from organic matter, clay/hydrated iron and aluminium silicate and have undifferentiated B-horizon (A-C soil).

(c) Terai/Babar soils: Mostly colluvial and highly productive.

Residual soils develop in situ. Transported soils are brought from other places through gravity (colluvial), running water (deposited at flood plains and called alluvial), wind (eolian = aeolian) and glacier (glacial soil).

Soil Texture

Three main types.

(a) Sandy soils:

The soils contain about 80% or more of sand, the remaining being silt and clay. Sandy soils are porous and loose. Water holding capacity is poor. Chemical nutrition is little.

(b) Clay soils: They are soils having 40-50% of clay, the rest being silt. Sand is little. Clay soils have abundant capillary pores. Therefore, water holding capacity is high. Inorganic nutrients are available in good quantity. However, aeration is poor.

(c) Loam soils: The soils contain 20% clay, 40% sand and 40% silt. These have good mineral nutrition, aeration and hydration. Therefore, loam soils are the best for plant growth.

Soil pH

It determines the type of soil microorganisms, solubility of different minerals and type of plants which can grow.

In alkaline soils (pH above 7), there is reduced availability of Zn, Mn, Cu and Fe.

In acidic soils there is abundance of iron, Mn and Al, but deficiency of Ca, Mg and K.

Certain soils possess excess of salts especially those of Na and Mg.

They are called Saline soils. Salinity increases with excessive irrigation.

Soil Organism

A number of organisms live inside soil.

It includes bacteria, actinomycetes, fungi, algae, parts of higher plants, protozoa, rotifers, nematodes, insects, earthworms, molluscs and burrowing vertebrates. They form the living components of soil.

(E) Topography

Topography i.e., surface configuration of an area (physical features like hills, plains or slopes) also influences the distribution of organisms. For example,

(i) The centre and edge of a pond or a stream

(ii) Exposed side and underside of a rock

(iii) North and South face of a ridge or a mountain are usually inhabited by different species of organisms.


Change in one environmental factor leads to change in others also i.e., all factors are integrated.

An organism would have evolved various mechanisms to maintain its internal environment at homeostasis to perform its physiological and biochemical functions in response to changing external factors of environment.

This constancy is necessary for its overall fitness or maximum performance. This may be maintained naturally or artificially.

There appears various possibilities of responses, such as :

(i) Regulate: It is maintenance of homeostasis by physiological or behavioural means like thermoregulation and osmoregulation. e.g., all birds and mammals and a few lower vertebrate and invertebrates; but plants do not have such mechanisms.

Evolutionary success of mammals is believed largely due to their regulation ability.

(ii) Conform: These organisms cannot maintain thermal and osmotic balance with environment. e.g., approximately 99% of animals and nearly all plants. Thermoregulation is energetically expensive especially for small animals having large surface area relative to their volume, due to this, very small animals are rare in polar regions.

Some species have ability to regulate upto a limited range, beyond which they simply conform (partial regulators).

For localised or short outburst of stressful conditions the organisms have two alternatives, like migration or suspended growth.

(iii) Migrate: Temporary movement of organisms from stressful area to more favourable area in terms of food, shelter, spawning or climate.

e.g., Siberian crane migration from Siberia to Keoladeo National Park (Bharatpur, Rajasthan). Locust migrates for food and salmon fish migrates for egg spawning. Ungulate migration in Africa is for food .

(iv) Suspend: It is the stage in life cycle where an organism changes its developmental, physiological, structural, biochemical behaviour to pass through unfavourable conditions. e.g., Thick walled spores in bacteria, fungi and lower plants.

Dormancy in seeds and other vegetative parts in higher plants.

Hibernation (winter sleep) is shown by organisms which are unable to migrate, like Bears.

Aestivation: It is metabolic inactivity of organism during hot dessicating summers. e.g., snails and fishes.

Diapause : Stage of temporary suspension of development under unfavourable conditions. e.g., Zooplanktons in lakes and ponds.


It is an attribute of the organism that enables it to survive and reproduce in its habitat. Adaptations may be morphological, physiological or behavioural and are either fixed genetically or remain epigenetic.

Some adaptations are given below :

(i) Seals have a thick layer of fat (blubber) below the skin to reduce loss of body heat.

(ii) Altitude sickness can be expressed at high altitude where body does not get enough oxygen due to low atmospheric pressure and causes nausea, fatigue and heart palpitations. Under these conditions, body increases RBC production, decreases binding capacity of Hb and increases breathing rate. These physiological adaptations allow organisms to respond quickly to stressful conditions.

(iii) Archaebacteria can flourish at temperature exceeding 100°C while humans can perform the metabolism in a narrow range (37ºC).

(iv) Antarctic fishes can survive below 0°C and a variety of invertebrates and fishes are adapted biochemically to survive great depths with crushing pressure. In Antarctic fishes, body fluid contain antifreeze solutes.

(v) Desert lizards lack the physiological ability to cope with extreme temperature but manage the body temperature by behavioural means.

(vi) Kangaroo rat of North American desert fulfil water demands by internal oxidation of fats and it also has the ability to concentrate its urine.

Water Based Adaptations (Plant Specific)

On the basis of dependence of plants on water and relations of plants to water, Warming (1909) recognised three kinds of plant communities:

(1) Hydrophytes, (2) Mesophytes and (3) Xerophytes

1. Hydrophytes

Plants growing in water or in water saturated soil are called hydrophytes. They require abundance of water to complete their life cycle. These are of basic three types:

A. Submerged: Submerged plants are those in which the leaves are entirely beneath the water e.g., Hydrilla, Vallisneria, Potamogeton and Ceratopyllum.

Fig. : Submerged plants

B. Floating: In floating plants leaves float on the water surface, but roots and stem may remain in water, or float on water like the leaves. On the basis of root fixation, these are classified into two groups.

(i) Free floating: They change their position due to water current, because their roots are not embedded in soil. Leaves float on the water surface and stem and roots are also free from substratum e.g., Wolffia, Lemna, Eichhornia, Pistia.

Fig. : Free floating plants

(ii) Rooted floating: In these plants leaves float but the roots adhere to bottom soil particles e.g., Nelumbium speciosum, Nymphaea stel/ata, Jussiaea, Trapa etc.


Fig. : Fixed floating plants

C. Amphibious plants: The basal part of the body (roots and lower portion of stem) is embedded in water saturated soil. The remaining body (upper part of stem and leaves) lie straight up, emerging in the air. Occasionally due to rain, leaves also may be immersed into water e.g., Typha, Ranunculus, Polygonum, Cyperus etc.

Fig. : Amphibious Plants

Morphological adaptations


1. Roots are either completely absent (e.g., Salvinia, Wolffia, Ceratophyllum) or poorly developed (e.g., Hydrilla).

2. Root pockets are present as balancing organs in Azolla, Eichhornia, Lemna, Pistia etc. instead of root cap.

3. Some hydrophytes have floating roots in addition to normal adventitious roots which provide buoyancy e.g., Jussiaea repens.


1. Long, slender, spongy and flexible e.g., Potamogeton and Hydrilla.

2. It may float horizontally on the surface of water e.g., Azolla or may form offset as in Eichhornia or may be a rhizome e.g., Nymphaea.


1. Long, slender, delicate petioles with floating leaves e.g., Nymphaea.

2. Petiole may be swollen and spongy e.g., Eichhornia.

3. Submered hydrophytes have thin, long ribbon shaped leaves (e.g., Vallisneria) or finely dissected leaves e.g., Ceratophylum.

4. Leaves of floating hydrophytes are large, entire and flat. These are often coated with wax e.g., Nymphaea or covered with hairs e.g., Salvinia.

5. Emergent hydrophytes show heterophylly i.e. leaves below the water are long, narrow, and dissected and those outside the water are entire and broad e.g., Ranunculus aquatilis, Umnophilla heterophylla, Sagittaria sagitifolia.

Anatomical adaptations

1. Presence of large air spaces and aerenchyma.

2. Mechanical tissue i.e. sclerenchyma is either poorly developed or absent. In Typha, pith is sclerenchymatous.

3. Vascular tissue specially xylem is poorly developed.

4. Cuticle is absent.

5. Stomata are absent or vestigeal in submerged hydrophytes. Floating leaves are epistomatic and emergent leaves are isostomatic.

6. Epidermis is always single layered.

7. Mesophyll is uniform.

2. Mesophytes

Plants growing in places of moderate water supply. These plants cannot live for a long time either in water saturated or in moisture deficient soil. e.g., garden plants and crops.

3. Xerophytes

Plants growing in places of deficient water supply. These plants grow in deserts or on rocks, e.g., Opuntia, Aloe, Agave, Casuarina, Calotropis, Muehlenbeckia, etc.

Types of xerophytes :

(A) On the basis of nature of soil and cause of unavailability of water:

(a) Physical xerophytes grow in soils which are physically dry (due to shortage of water) e.g. Opuntia, Casuarina, Ruscus, Muehlenbeckia (Cocoloba) etc.

(b) Physiological xerophytes grow in soils having sufficient water which is not available due to high salt concentration (salinity) or very low temperature.

(B) On the basis of life cycle and water storage:

(a) Ephemerals: Short living, brief life span (6-8 weeks), escape dry season by disappearing leaving their seeds; hence, not true xerophytes, so are called drought evaders and drought escapers e.g. Cassiatora, Tribulus.

(b) Succulents (fleshy xerophytes) : Absorb large quantities of water during rainy season and store it in different body parts; suffer only externally; hence drought avoiding or drought resistant xerophytes.

(i) Stem succulents (chylocauly) : e.g., Opuntia, Euphorbia antiquorum, E. splendens,

E. tirucalli, Cereus.

(ii) Leaf succulents (chylophyllous) : e.g., Aloe, Agave, Begonia, Bryophyllum.

(iii) Root succulents (chylorhizous) : e.g., Asparagus, Ceiba parviflora.

(c) Non-succulents : Drought endurers, true xerophytes; can withstand long drought periods (perennial non succulents) e.g., Casuarina, Zizyphus, Nerium, Acacia, Capparis.

Xerophytic plants
Fig. : A. Phylloclade of Opuntia, B. Phylloclade of Muehlenbeckia , C. Casuarina

Morphological adaptations


1. Roots are well developed, profusely branched and extensively spread.

2. Roots are deep (phreatophytes).


1. It is generally hard and woody with thick bark.

2. Mostly covered with hairs, wax, silica.

3. Some plants may show modification of stem into leaf like structure called phylloclade e.g. Opuntia; while in Ruscus and Asparagus the stem is modified into cladode.

Leaves: are modified in the following ways.

(a) Sclerophyllous: Stiff and hard leaves e.g., Banksia, Dasilirion.

(b) Trichophyllous: Leaves covered with hair e.g., Nerium, Calotropis.

(c) Microphyllous : Small, fleshy leaves e.g., Capparis.

Leaves of xerophytes are generally caducous e.g., Euphorbia or may be completely absent e.g., Capparis aphylla. Leaves of grasses get rolled to reduce transpiration e.g., Agropyron, Ammophila.

Anatomical adaptations

1. Presence of thick cuticle on leaf and stem epidermis.

2. Presence of waxy layer on the epidermis of leaves.

3. Stomatal frequency is reduced.

4. Sunken stomata are present.

5. Hairs are present on the leaf epidermis and substomatal chamber.

6. Intercellular spaces are only few and small.

7. Mechanical tissue i.e., collenchyma and sclerenchyma are well developed.

8. Epidermis may be multiple.

9. Water storage tissue is present.

10. Vascular tissues are present in large amount.

4. Halophytes

Plants growing in saline soil.

Morphological characters

Root: Mangrooves have specialized roots called pneumatophores which are negatively geotropic. These are modified tap roots which have pneumathodes for gaseous exchange.

Stem : Mostly succulent or fleshy.

Leaves: Evergreen, thin, leathery.

Anatomical characters

1. Presence of thick cuticle on stem.

2. Stem hypodermis is multilayered, thick walled.

3. H-shaped specules are present in the stem cortex to provide mechanical support.

4. Pericycle is sclerenchymatous, 3-4 layered.

5. Vascular tissue is well developed

6. Upper and lower leaf epidermis is thickly cuticularized.

7. Sunken stomata are present only in the lower leaf epidermis.



(i) Deme: Local population (population living in a specific area).

(ii) Metapopulation : Whole set of local populations connected by dispersing individuals.

(iii) For the purpose of ecological studies, a group of individuals resulting from asexual reproduction is also considered as population.

Population attributes / group attributes

Some characters are unique to the group and are not characteristic of the individuals forming it, like an organism born and dies, and has age, but it does not have a birth rate, death rate and age ratio.

These population characters can be best expressed by statistical methods, some important characters are:

1. Population Density

The number of individuals per unit area, like millions of Spirogyra filaments in a pond, or 200 plants of Parthenium in an area.

This can also be expressed as "The population biomass per unit area or volume" when we have to count a large number of organisms (like grasses) or to find out the role of a single huge banyan tree in an area.

Relative density is a good measure of finding out the total density of fishes in a lake by counting the number of fishes caught per trap.

Tiger census in India is based upon pug marks and fecal pellets which indirectly estimates population size.

2. Age Ratio Pyramids

Age Pyramlds : An age pyramid is a graphic representation of proportion of various age groups of a population. There are three types of age pyramids -triangular, bell-shaped and urn-shaped.

(a) Triangular : It is graphic representation of a young or growing population and has a very high proportion of pre-reproductive individuals.

(b) Bell-Shaped: The pyramid is bell-like with pre-reproductive individuals being only marginally more than the reproductive individuals. Population is said to be mature or stable.

(c) Urn-Shaped : It has small number of pre-reproductive individuals followed by a large number of reproductive individuals. Such a population shows negative growth.

Fig. : Representation of age pyramids for human population

3. Population Growth

Some attributes of population are used to estimate its growth, as population size may fluctuate in a given habitat during a given period due to change in four basic processes, namely

(i) Natality: Birth rate, inherent ability of a population to increase and refers to number of births during a given period in the population that are added to initial density.

The per individual change in a population due to natality can be estimated by using

Where, Nn = New individuals produced
N = Initial population

t = Change in time

(ii) Mortality: Death rate, number of individuals dying in a population in a given period.

(iii) Immigration : One way permanent inward movement of the individuals of same species into a habitat with existing population. This may help to speed up the growth or prevents extinction of a smaller population. In plants, it is equivalent to settlement of disseminules.

(iv) Emigration : One way permanent outward movement of number of individuals from a population to other habitat area, hence reducing the size of local population. Plants are fixed, so do not show emigration.

By these population characters the density of a population (N) at time t can be expressed after a period of time t + 1 as

N(t + 1) = Nt + [(B + I) -(D + E)]

Where; B = Number of birth, I = Number of immigrants, D = Number of deaths and

E = Number of emigrants

So, it can be concluded that births and deaths are most important factors influencing population density and other two are specialised cases.

4. Growth Models

Biotic Potential and Environmental Resistance: Biotic potential is the maximum or potential natality.

The sum of environmental factors that limits the population size is called environmental resistance.

Environmental resistance rises with the rise in population size.

The influence of environmental resistance over the biotic potential is denoted by (K-N/K).

Carrying Capacity (K) : The maximum number of individuals of a population which can be supported with optimum resources for their survival is called carrying capacity of the environment.

Growth of a population depends upon its biotic potential, death rate and birth rate. Depending upon the amplitude of these three, a population may show :

(a) Exponential growth, and (b) Logistic growth

(a) Exponential growth :

Darwin believed the geometric growth of a population when the resources are unlimited, as each species realises its inherent power to grow.

This intrinsic rate of natural increase is called r.

The value of r is an important parameter to assess impact of environmental factors on population growth.

(i) Any increase or decrease in a population N during time t will be dN/dt = (b - d) × N, where (b = per capita birth rate) and d = per capita death rate. If (b - d) = r, then dN/dt = rN

(ii) The integral form of exponential growth equation will be Nt = N0ert where; Nt = Population density after time t, N0 = population density at time zero, e = the base of natural logarithms (2.71828).

(iii) The magnitude of r was 0.0205 in 1981 for human population in India while it reached 0.0176 in 2001. For Norway rat it is 0.015 and for flour beetles it is 0.12.

(iv) Equation dN/dt = rN describes geometric growth resulting in a J-shape curve. Such population stops abruptly due to environmental resistance, which becomes effective suddenly, or a resource may become depleted. Decline in J-shape population is density triggered e.g., Algal blooms, insect population.

Fig. : Population Growth Curve

(b) Logistic Growth

This growth form is characterised by function of carrying capacity (K) for a given population, giving it a more realistic form.

Such forms are represented under limited resource conditions where a population finally reaches an asymptote.

This growth form can be described as Verhulst Pearl Logistic Growth and is expressed as .

Since resources for growth for most animal populations are finite and become limiting sooner or later, this plot is more realistic.

Life History Variation : Populations evolve to maximise their reproductive fitness in a given habitat. It includes variation in life history, evolved in relation to the selection pressure imposed by environmental factors in order to achieve the most efficient reproductive strategy.

Some of the strategies are listed below:

(i) Small number of large sized individuals are produced (mammals and birds).

(ii) Larger number of small sized individuals are produced (Oysters and pelagic fishes).

(iii) Some organisms breed once in life time (Bambusa and Pacific salmon fish).

(iv) Some organisms breed many times during their life (mammals and many birds).

5. Population Interactions

The interactions between members of different populations are based upon 3 factors:

(i) Requirement and mode of obtaining food.

(ii) Nature of shelter or space required.

(iii) Habits of the species like aggregation, breeding etc.

Different population interactions (+, -, 0 for beneficial,

detrimental and neutral respectively).

The various important types of interactions between members of biotic community are described below:

A. Competition

It is a process in which the fitness of one species (measured in terms of its 'r' the intrinsic rate of increase) is significantly lower in the presence of another species.

It is the struggle between two or more organisms for obtaining various requirements for their survival.

It is both intraspecific, i.e., between organisms of the same species and interspecific, i.e., between organisms of different species.

Intraspecific competition is more acute because all organisms of the same species have similar requirements for food, space, light, water, shelter, mate etc.

The interspecific competition occurs when organisms of different species belong to same trophic level or have similar feeding habit e.g., in grassland, severe competition occurs between herbivores like rabbits, deer, bisons etc., as all feed upon grass.

Competition for zoopalnktons between visiting flamingoes and resident fishes in South American lakes.

It may be emphasized that competition is noticed only when required commodity is in short supply.

If grass is in plenty and fulfill the needs of all herbivores of that area, there will be no competition or only little, if at all it occurs.

Carnivorous animals like tigers and leopards compete for the common prey.

In forest, shrubs, herbs and trees compete with one another for water, inorganic nutrients, sunlight and for insects that bring about pollination and for animals that bring about dispersal.

In competition superiority of individual (intraspecific) or superiority of species (interspecific) plays an important role.

No two species can occupy the same ecological niche and live together in a biotic community, this can be further understood by Gause's competitive exclusion principle (1934).

Gause found that if two species of Paramecium namely P caudatum and P aurelia, were grown together in same culture medium, initially both increase in number, but eventually P caudatum population declines and is eliminated by superior species P aurelia.

This shows that if two species are occupying same ecological niche and competing for common resources, then superior type will exclude or eliminate the inferior type of species.

There are some circumstancial evidences which supports exclusion of species due to competition e.g.,

(i) Introduction of goats resulted in exclusion of Abingdon tortoise from Galapagos islands.

(ii) Same interaction occurs between Balanus and Chathamalus on rocky coasts of Scotland (Connell, 1961).

Coexistence :

Species facing competition might evolve mechanism to live in the same niche by changing the feeding time or foraging patterns, i.e., resource partitioning.

If different species coexist inspite of being competitors, it is because they are specialised or adapted differently (different feeding habits) to obtain same resources.

That is why Darwin found that fourteen species of finches coexist in Galapagos islands due to development of different feeding habits.

Several plants grow together by sending roots to various depths.

More examples are cited below to explain coexistence aspect of competition.

(i) Five closely related species of warblers avoid competition by changing foraging pattern (MacArthur).

(ii) "Tribolium-Trifolium" model is best to explain both exclusion and coexistence.

(iii) Habitat diversification can also reduce competition e.g., Tribolium and Oryzaephilum (Crombic, 1947).

Competitive release: There occurs a dramatical increase in population of a less distributed species in a geographical area when its superior competitor is removed experimentally from that area.

Plant and herbivores are more affected than carnivores.

Resource need not to be limiting for competition to occur as feeding efficiency of one species might be reduced due to inhibitory presence of other species. This is called "interference competition".

B. Predation

It is a type of interaction in which the members of one species capture, kill and eat up members of other species.

The species that captures is called predator and the other that is captured is called prey.

Most of the animals except the scavengers (animals eating the dead animals only) are predators.

Even certain plants (e.g., Nepenthes, Utricularia, Dionaea, Drosera) are predators, catching and digesting insects in addition to their autotrophic mode of nutrition.

They are called insectivorous or carnivorous plants.

Prey-predator relationship, like competition, is an interaction of ecological importance.

Prey-predator relationship is utilized by man in biological control of pests.

Opuntia was weeded out in Australia with the help of its natural herbivore called Cactoblastis (Cochineal insect).

Red locust menace was brought under control in Mauritius by Mynah.

Mosquito larvae are eaten by larvicidal fish such as Gambusia (top minnow).

In the rocky intertidal communities of the American pacific coast the starfish Pisaster is an important predator.

Role of predation :

(a) Transfer of energy (in ecological sense herbivores are not very different from predators).

(b) Keeping prey population under control.

(c) Rabbit population in Australia increased tremendously because the land does not have its natural predators. Red faxes in Newzealand became top carnivores due to absence of a natural top carnivore.

(d) Predators help to maintain species diversity in a community as they can reduce the intensity of competition among prey species e.g., Experimental removal of Pisaster (star fish) resulted in extinction of more than 10 species of invertebrates in American Pacific coast.

(e) Term prudent predator (Slobodkin, 1962) explains that predator does not exterminate its prey by overexploitation.

For their defence, prey species have evolved various adaptations, viz

(i) Camouflage e.g., Insects, frogs

(ii) Monarch butterfly is well known for its general unpalatability to its predator birds. This insect is able to sequester the highly toxic glycosides present in milkweeds on which its caterpillar stages feed (Brower et.a/., 1968). There larvae develop on milkweed, providing the protection to plant against herbivory.

(iii) Highly poisonous cardiac glycosides are produced by Calotropis and nicotine, caffeine, quinine, strychnine, opium are other means of chemical defence in plants.

(iv) Association of bull horn Acacia cornigera with Pseudomyrmex ferrugenea (ant) is also against herbivory.

Association of Acacia -Pseudomyrmex and Monarch butterfly -milkweed are examples of coevolution also.

C. Parasitism

It is a relationship between two organisms of different species usually differing in size in which one organism spends a part or whole of its life, on or in the body of other organisms and gets nourishment and shelter from that.

The former organism is termed parasite and the latter as host.

This also depresses the growth rate of host population or may reduce the total size of host population.

Parasites are smaller generally, majority of them are host specific.

High reproductive potential, loss of digestive system and unnecessary sense organs, presence of specific sucking or adhesive organs are some of their characters, but they have poor means of dispersal.

D. Mutualism

It is an obligate association of two organisms in which each derives benefit from the other.

In mutualism, two organisms often live together and can't live separately, the two organisms may be plants, animals or one plant and other animal.

(i) Mutualism between plant and plant e.g., lichen (Alga and Fungus), mycorrhiza (Fungus and roots of higher plants), Rhizobium (N2-fixing bacteria in root nodules of legumes).

(ii) Mutualism between plant and animal e.g. , Green algae Chlorella vulgaris (endosymbiont) in gastrodermal cells of Hydra. Plant pollinator relation sometimes is a one-to-one coevolutionary relation like fig and wasp relation, Ophrys and Colpa relation, Yucca and Pronuba relation.

(iii) Mutualism between animal and animal e.g., Protozoan Trichonympha in gut of termites. Protozoan helps in cellulose digestion and in return gets shelter.

E. Protocooperation

It is a non-obligatory interaction between two organisms of different species, in which both are mutually beneficial to each other, but can easily live separately.

Two birds, namely red-billed ox pecker and yellow-billed ox pecker feed on ticks and other parasites sticking to the skin of black rhinoceros and relieve him of the parasites.

The birds sitting on Rhinoceros for feeding on ticks, etc., also warn the animal of approaching danger.

The birds are not only benefitting the animal, but themselves are benefitted too as the animal is providing food (ticks, etc.) to the birds.

This is an example of protocooperation as the birds have no close association with animal, it just visits him occasionally for feeding.

Fig. : Protocooperation between tick bird Ox Pecker and Rhinoceros.

Another example of protocooperation is the relationship between plover bird and the crocodile.

The bird enters the open mouth of the crocodile, feeding on the leeches which are attached to the lining of buccal cavity.

The bird not only is getting benefit from the animal (getting food) but helps the animal in getting rid of leeches which are sucking its blood.

The association between sea-anemone and hermit crab may also be taken as an example of protoco-operation in which sea-anemone is attached to snail's shell.

Sea-anemone is sedentary and can move from one place to another using snail's shell as portable home and is able to procure more food.

The hermit crab is protected from its enemies by the sea-anemone having nematocysts.

F. Commensalism

It is an interaction between two organisms of different species, benefitting only one species, the other species is neither benefitted nor harmed.

The species which is benefitted is termed commensal and the other species is called host.

Examples of commensalism are observed in diverse types of animals and even in plants.

(i) The pilot fish (Remora) always accompanies shark with a purpose of feeding upon small pieces of food falling off when shark is tearing its prey. The fish is not attached to shark at any time.

(ii) Sucker fish is attached to underside of shark, getting a free ride and occasionally detaching itself when the latter is feed upon small pieces of food.

Fig. : Commensalism between Sucker fish and Shark

(iii) Jackals and arctic foxes follow lions and seals, respectively for feeding upon left out food pieces by the predators.

(iv) Barnacles (Balanus) live attached to whale's body, not getting any benefit from it except shelter.

(v) Epiphytes are attached to other plants but not getting anything from the host. They are green, thus nutritionally independent and have special hygroscopic roots which can absorb water present in the atmosphere in the form of vapours. They get shelter only from the host. They are able to get proper sunlight too for photosynthesis by growing on higher branches of trees in thick forests.

(vi) Cattle egrets (birds) forage close to where cattles are grazing because the cattles as they move, stir up and flush out from the vegetation insects that otherwise might be difficult for the egrets to find and catch.

(vii) Sea anemone has stinging tentacles that protect clown fish living among them.

G. Amensalism

It is an interaction between two organisms of different species in which one species inhibits the growth of other species by secreting certain chemicals.

This phenomenon of inhibition of growth of one species by the other species through secretion of certain chemicals is also termed allelopathy (in plants), or antibiosis or biological antagonism.

Examples of amensalism are evident in micro-organisms.

Penicillium secretes penicillin, that inhibits the growth of large number of bacteria. Similarly, different species of Streptomyces, an actinomycete, produce wide range of chemicals which inhibit the growth of other bacteria, some of which cause various diseases in human beings.

Such chemicals isolated from these microorganisms are thus, used as antibiotics for curing various diseases caused by bacteria.

Inhibition of growth of one species by organism of other species is observed in higher plants as well.

Roots of black walnut (Juglans nigra), secrete a chemical juglone which is toxic to other plants like apple, alfalfa, etc. Convolvulus arvensis inhibits the growth of wheat.