Ecology -

It is a branch of science which studies the interactions among organisms and between the organisms and its physical ( abiotic) Environment.


Ecology deals with four levels of biological organization 

  • Organism  -a living entity which can function on its own. 
  • Populations  -sum of all living organisms of the same species living in a particular geographical area. 
  • Communities  -a group of people living together in one place. 
  • Biomes  – a large geographical area of various plants and animals.

Organisms and Population

  • Different organisms are adapted to their environment in teams of both survival and reproduction.
  • Rotation of earth around the sun and the tilt if its axis cause annual variations in the intensity and durations of temperature which results into distinct seasons.
  • Variations of temperature along with annual variations in precipitation such as rain and snow form major biomes such as desert, rain forest and tundra. 
  • Regional and local variations such as temperature water. Light and within each biome lead to the formation of a wide variety of habitats.
  • Both abiotic and biotic components characterise the environment

Major Abiotic Factors Of The Ecosystem


  • The average temperature on land varies seasonally, decreases progressively from the equator towards the poles and from plains to the mountain tops.
  • Thermal springs and deep-sea hydrothermal vents are unique habitats where average temperatures exceed 1000.
  • Temperature affects the kinetics of enzymes through it the basal metabolism, activity and other physiological functions of the organisms.
  • Organisms which can tolerate and thrive in a wide range of temperatures, they are called eurythermal organisms. Example – cat, dog, etc.
  • Organisms which can tolerate a narrow range of temperatures such organisms are called stenothermal organisms Example- fish, reptile etc.

Major Abiotic Factors Of The Ecosystem

    • Water 

      • It is the next important factor as life is unsustain without water.
      • The amount of water in an environment determines the productivity and distribution of plants.
      • For aquatic habitat, the quality of water becomes important like pH value, salinity and temperature of water.
      • Organisms which can tolerate and thrive in a wide range of salinities, they are called euryhaline and the organisms that tolerate only narrow range of salinities are called stenohaline.


      • Light is important because autotrophs make food with the help of light

       ( photosynthesis ) and 02 is evolved during this process.

      • The small plants like herbs and shrubs can perform photosynthesis under very low light condition as they are over shadowed by tall trees.
      • The plants depend on sunlight to meet their photoperiodic requirement for flowering.
      • Animals use the diurnal and seasonal variations in light intensity and duration 

       ( photoperiod) as cues for timing their foraging, reproduction and migration activities.


      • Soil composition, grain size and aggregation determine the percolation and water holding capacity of the soils, which along with parameters such as pH, mineral composition and topography determine the vegetation in any area.
      • In the aquatic environment, the sediment-characteristics often determine the type of benthic animals that can thrive there.

Responses To Abiotic Factor

    • During the course of million years of existence, many species would have evolved a relatively constant internal (within the body) environment that permits all biochemical reactions and physiological functions to proceed with maximum efficiency and thus, enhances the overall fitness of the species.

      The organisms try to constancy of its internal environment ( a process called homeostasis despite varying external environment conditions that tend to upset its homeostasis.

How Do Living Organisms Cope With Environment ?

    • Regulate 

      Some organisms maintain homeostasis by physiological and behavioural means.

      All birds and mammals and few lower vertebrate and invertebrate species maintain homeostasis by thermoregulation (can maintain their core internal temperature ) and osmoregulation.

      The success of mammal’s is largely due to their ability to maintain a constant body temperature.

      In summers, when temperature is lower we shiver, a kind of exercise that produces heat and raises the body temperature.

      Many regulators rely on altruistic social contact to keep warm in cold conditions.

      For example, rodents huddle together over newborn pups to keep them warm, penguins, in their extremely cold environments, also huddle together for warmth to protect themselves and their young.


      Majority of animals and nearby all plants cannot maintain a constant internal environment. Their body temperature is determined by ambient temperature.

      The osmotic concentration of the body fluids change with that of the ambient water osmotic convention, such animal and plants are simply called conformers.

      Loss or gain of heat is a function of surface area. The small animals have large surface are relative to their volume. They loss body heat very fast in low temperature.

      So they expend energy to generate body heat through metabolism for adjusting.therefore, very small animals are rarely found in polar regions. 

       Conformers include fish, reptiles, insects, amphibians and worms.


      The temporary movement of organisms from the stressful habitat to a more hospitable area and return when favourable conditions reappear ,is called migration . 

    • The long distance migration is very common in birds . In winter, famous keolado National park ( bharatpur ) in rajasthan hosto thousands of migratory birds coming from Siberia and other extremely cold northern region . 


    • Some bacteria, fungi and lower plants, uber unfavourable conditions slow down metabolic rate and form a thick -walled spore to overcome stressful conditions.

      These spores germinate under onset of suitable environment . 

      The animals that fail to migrate might avoid the stress by escaping in time ,e.g, bear goes into hibernation during winter . 

      Snail and fish go into aestivation to avoid summers 

      Zooplanktons under unfavourable conditions enter  diapause, a stage of suspended development .


        1. Any morphological ,physiological and behavioural attribute of the organisms that enables it to survive and reproduce in its habitat is called adaptation .

          Over a long period of time, many adaptations have evolved and are incorporated in the gene, thus becoming heritable . 

          1. Adaptations in kangaroo rat ( Dipodomys merriami )

          In the absence of an external source of water, the kangaroo rat in North American deserts is capable of meeting all its water requirements through its internal fat oxidation.

          Kangaroorat has the ability to concentrate its urine so that minimal volume of water is used to remove excretory products.

          Adaptation of plants in desert

          • Thick cuticle on the leaf surfaces of the desert plants reduces transpiration.
          • Stomata is sunken to reduce transpiration.
          • Desert plants follow a special photosynthetic pathway called crassulacean acid metabolism (CAM) in which stomata remains closed during day time and open during night time.
          • In some plants like opuntia leaves are modifies into spines to reduce transpiration and photosynthesis takes place in flat green stem called as phylloclade 


          Adaptation of animal in cold climate 

          • Mammals from colder climates generally have shorter ears and limbs to minimize heat loss, called Allen’s Rule. 
          • Aquatic mammals like seals have a thick layer of fat ( blubber) below their skin that acts as an insulator and reduces loss of body heat.


          Adaptation in desert lizards 

          • Desert lizards deal with the high temperatures of their habitat, by managing to keep body temperature fairly constant by behavioural adaptation.
          • Desert lizards bask in the sun and absorb heat when their body temperature drops below the comfort zone and move into shade when the ambient temperature starts increasing.
          • Some species are capable of burrowing into the soil to hide and escape from the above-ground heat.


          Adaptation in high altitude 

          • When a person moves to high altitude place, develops altitude sickness because in the low atmospheric pressure of high altitudes, the body does not get enough oxygen.
          • Symptoms include nausea, fatigue and heart palpitations.
          •  The body compensates low oxygen availability by increasing red blood cell production, decreasing the binding capacity of hemoglobin and by increasing breathing.


A group of individuals living in a geographical area who can interbreed and share or compete for similar resources is called a population.

Characteristic of a population

Population size and density :- of a species is the number of individuals of species per unit area or volume.

     Population density (PD)=  No. of individuals in a region (N)

                                                    No. of unit area in a region (S)

Birth or natality rate :- it is expressed as the number of births per 1,000 individuals of a populations per year.

Death or mortality rate :- it is expressed as the number of deaths per 1,000 individual of a population per year.

Age Pyramid

    1. Expanding ( triangular shaped pyramid )
    2. Stable ( Bell shaped pyramid )
    3. Declining (urn shaped pyramid)

      It the age distribution is plotted for the population, the resulting structure is called an age pyramid.

      The shape of the pyramids reflects the growth status of the population and is of three types.


  • The pyramids also indicate the ratio of pre-reproductive, reproductive, and post-reproductive individuals in a population.

Population Growth

    1. The size of a population depends on the food available, prediction pressure, and weather. Therefore the size of the population is not a static parameter.

      The population density on a few basic processes :

      1. Natality: it is the number of births during a given period of time. It increases the population density.
      2. Mortality: it is the number of deaths in a given time period. It decreases the population density.
      3. Immigration: it is the number of individuals of the same species added to a habitat in a given time period. It increases the population density.
      4. Emigration: it is the number of individuals of the same species that move to a different habitat in a given time period. It decreases the population density.

      The population density is given by the following equation :-

      Nt = No + {B+l}- (D+E)}

      Where, N,=Population density at time t 

      B = birth rate

      I = Immigration 

      D = Death rate 

      E = Emigration

      N = Population in the beginning

      This equation shows that the population density will increase, if the number of births plus the number of immigrants (B+l) is more that the number of death plus the number of i,e, (D+E), otherwise it will decrease.

Population Growth Models

There are two models of population growth:-

1. The exponential growth 

2. Logistic growth 


Exponential growth 

Exponential or geometric growth is common where the resources ( food + space ) are unlimited.

The equation for exponential growth can be derived as follows:

Where N = population size, N = population density after time t, 

No = population density at time Zero 

r  = intrinsic rate of natural increase 

e = the base of natural logarithms ,b = birth rate ,D = Death rate 

r is an important parameter assessing impact of biotic and abiotic factors on population growth .

In exponential growth, when N in relation to time is plotted on graph, the curve become J shaped. 


Logistic growth 

The resources become limited at certain point of time , so no population can grow exponentially. 

This growth model is more realistic .

Every ecosystem or environment or habitat has limited resources to support a particular maximum number of individual called its carrying capacity (k) .

When N is plotted in relation to time t, the logistic growth show sigmoid curve and is called 

Verhulst – Pearl Logistic Growth .

Population Interaction

Interspecific interactions arise from the interaction of populations of two different species. 

Interspecific interactions could be 

  1. beneficial (+)
  2. detrimental/ harmful / (-)
  3. Neutral (0) to one of the species or both.

Predation :- 

It is an interspecific interaction where organisms of higher trophic levels feed on organisms of lower trophic level.


Herbivores are predators as they feed on grass.


Exotic species have no predators hence they grow very rapidly.


example – the prickly pear  cactus introduced into Australia and the invasive cactus was brought under control only after a cactus-feeding moth was introduced in the country.


Role of predation in an ecosystem are:-

  • Predator keeps prey populations under control.
  • Predator acts as a passage for the transfer of energy across trophic levels.
  • Predator help in maintaining species diversity in a community by reducing the intensity of competition among competing prey species.
  • Some species of insects and frogs are cryptically-colored (camouflaged) to avoid being detected easily by the predator.
  • The monarch butterfly is highly distasteful to its predator (bird) because of a special chemical present in its body which is acquired by feeding a poisonous weed during the caterpillar stage.
  • All insects are known to be phytophagous as these feed on plant sap and other parts of plants.
  • Thorns in acacia, and cactus are morphological means of defense against insects.
  • Many plants produce and store some chemicals that make the herbivores sick if eaten and even kill the predators.

Calotropis produce poisonous cardiac glycosides against herbivores.

  • Nicotine, caffeine, quinine, and opium are produced by plants as defenses against grazers and browsers.


The type of interaction in which one species benefits and the other is neither harmed nor benefited is called commensalism.

Example –

  • An orchid growing as an epiphyte on a mango branch.
  • Barnacles growing on the back of a whale
  • Clownfishes live among the stinging tentacles of sea anemones.
  • The egrets are in close association with grazing cattle. The cattle egrets are benefited from the cattle detection insects because cattle stir up the bushes and insects are flushed out from the vegetation. To be detected by cattle egrets.


The competition is between two organisms belonging to same species, this is called intraspecific competition. 

Competition is struggling by the organisms for the same resources like food, shelter and sex. 

The competition between organisms belonging to different species is collide interspecific competition. 

  For example-  In some shallow south American lakes visiting flamingoes and resident fishes compete for their common food, the zooplankton in the lake 

Resources need not be limited for competition to occur. 

For example- Abingdon tortoise in Galapagos islands became extinct within a decade after ghosts were introduced on the island, apparently due to the greater browsing efficiency of the goats. Lost their natural habitats. 

  • Competitive release occurs when one of two species competing for the same resource disappears, thereby allowing the remaining competitor to utilize the resource more fully than it could in the presence of the first species. 
  • Conneli’s elegant field experiments showed that on the rocky sea coasts of Scotland, the larger and competitively superior barnacle balanus dominates the intertidal area, and excludes the smaller barnacle chthamalus from that zone.
  • Gause’s competitive exclusion principle’ states that two closely related species competing for the same resources cannot coexist indefinitely and the competitively inferior one will be eliminated eventually.
  • Species facing competing might evolve mechanisms like resource partitioning that promote co-existence rather than exclusion.
  • If two species compete for the same resource, they could avoid competition by choosing different times for feeding or different foraging patterns, this is called resource partitioning.
  • Macarthur showed that five closely related species of warblers living on the same tree were able to avoid competition.


Parasitism is the association between two organisms in which one organism is benefited and another organism is harmed.

Some of the parasitic adaptations are –

  •  Loss of unnecessary sense organs 
  • Presence of adhesive organs or suckers to cling on to the host.
  • Loss of digestive system.
  •  High reproductive capacity.
  • The life cycles of parasites are often complex involving one or two intermediate hosts or vectors to facilitate parasitization of its primary host.
  •  The human liver fluke depends on two intermediate hosts, a snail, and a fish to complete its life cycle.

Effects of parasites on the host –

  • The majority of the parasites harm the host.
  • They may reduce the survival, growth, and reproduction of the host and reduce its population density.
  • They might render the host more vulnerable to predation by making it physically weak.




These are the parasites which live inside the host’s body at a different site like kidney, liver, lungs etc, for food and shelter.

Examples – tapeworm, liver fluke, plasmodium

These are the parasites which feed on the external surface of the host organisms for food and shelter. 

Example – lice on humans, ticks on dogs, 


The phenomenon in which one organism ( parasite ) lays its egg in the nest of another organism is called blood parasitism.


The interaction in which one species is harmed while the other is neither benefited nor harmed is called amensalism.

Example- Penicillium inhibits the growth of various bacteria by secreting penicillin. 


Mutualism is the interaction between two living organisms where both the organisms are equally benefited and no one is harmed. 

Examples-  Lichens represent an intimate mutualistic relationship between a fungus and photosynthetic algae or cyanobacteria. 

  • Mycorrhizae are associations between fungi and the roots of higher plants, where the fungi help the plant in the absorption of essential nutrients from the soil while the plant in turn provides the fungi with energy-yielding carbohydrates.
  • Plants provide nectar and pollen for pollinating agents and the pollinating agents in turn pollinate the flowers of plants.
  • Animals disperse the seeds of plants and plants provide juicy fruits for seed dispersers.
  • Fig tree is pollinated by a species of wasp only where the wasp pollinates the fig inflorescence while searching for suitable egg-laying sites and in return for the favor of pollination the fig offers the wasp some of its developing seeds, as food for the developing wasp larvae.

Sexual deceit 

  • Sexual deceit is the process in which petal of its flower bears an uncanny resemblance to the bee in size,colour and markings.

Example – the mediterranean orchid ophrys.

  • The male bee is attracted to what it perceives as a female, ‘pseudocopulates’ with the flower, and during that process is dusted with pollen from the flower.
  • When this same bee ‘pseudocopulates’ with another flower, it transfers pollen to it and thus, pollinates the flower.