Biology Notes for Class 12

Chapter 13. Organism and Populations

Chapter Summary

          As a branch of biology, Ecology is the study of the relationships of living organisms with the abiotic (physico-chemical factors) and biotic components (other species) of their environment. It is concerned with four levels of biological organisation-organisms, populations, communities and biomes.

          Temperature, light, water and soil are the most important physical factors of the environment to which the organisms are adapted in various ways. Maintenance of a constant internal environment (homeostasis) by the organisms contributes to optimal performance, but only some organisms (regulators) are capable of homeostasis in the face of changing external environment. Others either partially regulate their internal environment or simply conform. A few other species have evolved adaptations to avoid unfavourable conditions in space (migration) or in time (aestivation, hibernation, and diapause).

              Evolutionary changes through natural selection take place at the population level and hence, population ecology is an important area of ecology. A population is a group of individuals of a given species sharing or competing for similar resources in a defined geographical area. Populations have attributes that individual organisms do not- birth rates and death rates, sex ratio and age distribution. The proportion of different age groups of males and females in a population is often presented graphically as age pyramid; its shape indicates whether a population is stationary, growing or declining.

           Ecological effects of any factors on a population are generally reflected in its size (population density), which may be expressed in different ways (numbers, biomass, per cent cover, etc.,) depending on the species. Populations grow through births and immigration and decline through deaths and emigration. When resources are unlimited, the growth is usually exponential but when resources become progressively limiting, the growth pattern turns logistic. In either case, growth is ultimately limited by the carrying capacity of the environment. The intrinsic rate of natural increase (r) is a measure of the inherent potential of a population to grow.

           In nature populations of different species in a habitat do not live in isolation but interact in many ways. Depending on the outcome, these interactions between two species are classified as competition (both species suffer), predation and parasitism (one benefits and the other suffers), commensalism (one benefits and the other is unaffected), amensalism (one is harmed, other unaffected) and mutualism (both species benefit). Predation is a very important process through which trophic energy transfer is facilitated and some predators help in controlling their prey populations. Plants have evolved diverse morphological and chemical defenses against herbivory. In competition, it is presumed that the superior competitor eliminates the inferior one (the Competitive Exclusion Principle), but many closely related species have evolved various mechanisms which facilitate their co-existence. Some of the most fascinating cases of mutualism in nature are seen in plant-pollinator interactions.


Ecology at the organismic level is essentially physiological ecology which tries to understand how different organisms are adapted to their environments in terms of not only survival but also reproduction. Regional and local variations within each biome lead to the formation of a wide variety of habitats. Major biomes of India are shown in Figure.

Major Abiotic Factors:


Ø  Temperature decreases progressively from equator towards the pole and high altitudes to > 50°C in tropical deserts in summer.

Ø  Thermal springs and deep-sea hydrothermal vents are unique with >100°C.

Ø  Temperature affects the kinetics of enzymes, BMR and other physiological actions.

Ø  Eurythermals: organism which can tolerate wide range of temperatures.

Ø  Stenothermal: organism which can tolerate narrow range of temperatures.


Ø  Water is also important factor that influence the life of organism.

Ø  The productivity and distribution of plants is also depends on water.

Ø  The salinity varies in aquatic environment:

  • 5% in inland waters (fresh water)
  • 30-35 in sea water
  • More than 100percent in hyper saline lagoons.

Ø  Euryhaline: organism which can tolerate wide range of salinity

Ø  Stenohaline: organism which can tolerate narrow range of salinity.


Ø  Plant produce food by photosynthesis, which only possible in presence of light. Hence it very important for autotrophs.

Ø  Plant species (herbs and shrubs) adapted for photosynthesize under canopy

Ø  Sunlight is required for photoperiodic response like flowering.

Ø  Animals use diurnal and seasonal variations in light intensity and photoperiod as cues for timing their foraging, reproductive and migration.


Ø  Properties of soil vary according to the climate, the weathering process.

Ø  Soil composition, grain size and aggregation determine the percolation and water holding capacity of the soil.

Ø  These characteristic along with pH, mineral composition and topography determine to a large extent the vegetation in any area.

Ø  The sediment-characteristic often determine the type of benthic animal in aquatic environment.

Response to Abiotic Factors:

Homeostasis; the process by which the organism maintain a constant internal environment in respect to changing external environment.

How does organism cope with the changing environment?


Ø  Some organisms are able to maintain homeostasis physiological (sometimes behavioral also) means which ensures constant body temperature, constant osmotic concentration.

Ø  All birds and mammals and few lower invertebrates are capable of such regulation i.e. thermoregulation and osmoregulation.

Ø  Success of mammals is due to thermoregulation.

Ø  We maintain a constant body temperature of 37°C.

Ø  When outside temperature is high we sweat profusely and evaporative cooling take place to bring body temperature down.

Ø  In winter due to low temperature outside our body temperature falls below 37°C, we start to shiver, to generate heat to raise body temperature.


Ø  Majority (99%) of animals and plants cannot maintain a constant internal environment; their body temperature varies according to ambient temperature.

Ø  In aquatic animals the osmotic concentration of body fluid varies with ambient water osmotic concentration.

Ø  All the above animals and plants are simply called as conformer.

Why the conformer not evolved to became regulators?

Ø  Thermoregulation is energetically expensive for many animals.

Ø  Small animal like shrews and humming birds cannot afford so much energy for thermoregulation.

Ø  Heat loss or heat gain is a function of surface area.

Ø  Small animals have larger surface area relative to their volume, they tend to lose body heat very fast when it is cold outside; then has to expend much energy to generate body heat through metabolism.

Ø  This is why very small animals are rarely found in Polar Regions.

Alternative response for stressful conditions is localized or remains for short duration


Ø  The organism moved away temporarily from the stressful habitat to a more hospitable area and return when stressful condition is over.

Ø  Bird migrate form the colder region to warmer region.


Ø  Thick walled spores are formed in microbes to overcome unfavourable stressful external environment. Spores germinate in favourable condition.

Ø  In higher plants seeds and other vegetative reproductive structures are means to tide over the stress. They reduce their metabolic activity and going into a state of ‘dormancy’.

Ø  Hibernation: during winter animals like bears escape in time

Ø  Aestivation: animals like snail and fish avoid summer related problem like heat and desiccation.

Ø  Diapauses: many zooplanktons undergo a stage of suspended development in unfavourable conditions.


Adaptation: is any attribute of the organism (morphological, physiological, and behavioral) that enables the organism to survive and reproduce in its habitat.

Adaptation of animal in desert:

Ø  Kangaroo rat meets their water requirement from oxidation of fat.

Ø  Excrete very concentrate urine to conserve water.

Adaptation of plant in desert (xerophytes)

Ø  Thick cuticle on their leaf surfaces.

Ø  Sunken stomata, both to reduce transpiration.

Ø  Have special photosynthetic pathway (CAM), stomata closed during day time and remained open during night.

Ø  Opuntia has no leaf- they are reduced to spines.

Ø  Photosynthesis takes place in flat green stems.

Adaptation of animal in cold climate:

Ø  Allen’s Rule: mammals from colder climates generally have shorter ears and limbs to minimize heat loss.

Ø  Seals of polar aquatic seas have a thick layer of fat called blubber below their skin that acts as insulator and reduces loss of body heat.

Adaptation in high altitude:

Ø  A person move to high altitude (>3,500 meter), develop altitude sickness.

Ø  Symptoms developed are nausea, fatigue and heart palpitations.

Ø  This is due to low atmospheric pressure of high altitudes; the body does not get enough oxygen.

How the bodies solve the problem?

Ø  The body compensates low oxygen availability by increasing red blood cell production.

Ø  The body compensates decreasing binding capacity of hemoglobin with oxygen by increasing rate of breathing.

Behavioral adaptation:

Ø  Desert lizards are conformer hence they cope with the stressful environment by behavioral adaptations:

  • They bask in the sun and absorb heat when their body temperature drops below the comfort zone in winter.
  • Move to shade when the ambient temperature starts increasing.
  • Some species burrowing into the soil to hide and escape from the above-ground heat.


Population attributes:

Ø  Population: a group of individual living in a well defined geographical area, share or compete for similar resources, potentially interbreed.

Ø  Birth rate and death rate refers to per capita births and deaths respectively.

Ø  Another attribute is sex ratio. The ratio between male female in a population.

Ø  If the age distribution is plotted for a population the resulting structure is called age pyramid.


Ø  The shape of the pyramids reflects the growth status of the population like growing, stable or declining.

Ø  The population size is more technically called as population density.

Methods for measurement of population density:

Ø  Counting the number

Ø  Percent cover

Ø  Biomass.

Ø  Pug marks and fecal pellets for tiger census

Population growth:

Ø  The size of the population changes depending on food availability, predation pressure and reduce weather.

Ø  Population size fluctuated due to changes in four basic processes, two of which (Natality and immigration) contribute an increase in population density and two (mortality and emigration) to a decrease.

Ø  Natality: number of birth in given period in the population.

Ø  Mortality: number of deaths in the population in a given period of time.

Ø  Immigration: is the number of individuals of same species that have come into the habitat from elsewhere during a given period of time.

Ø  Emigration: number of individuals of the population who left the habitat and gone elsewhere during a given time period.

Ø  If ‘N’ is the population density at time ‘t’, then its density  at time t + 1 is :


B = the   number of          births I =the number of immigrants

D = the number of deaths

E = the number of Emigrants.

N = Population Density

r = Intrinsic rate of natural increase t = Time period

K = Carrying capacity (The maximum population size that an environment can sustain).

Exponential growth:

Ø  The Exponential growth equation is Nt = N0ert

Ø  Nt = Population density after time t

Ø  N0 = Population density at time zero

Ø  r = intrinsic rate of natural increase

Ø  e = the base of  natural logarithms (2.71828)

Exponential growth (‘J’ shape curve is obtained).

Ø When resources are not limiting the growth.

Ø Any species growth exponentially under unlimited resources conditions can reach enormous population densities in a short  time.

Ø Growth is not so realistic.

Logistic growth model

Ø  Verhulst-Pearl Logistic Growth is described by the following equations

Ø  dN/dt = rN  (K–N / N)

Ø  Where N = Population density at time t

Ø  r = Intrinsic rate of natural increase

Ø  K = Carrying capacity

Logistic Growth (Sigmoid curve is obtained)

Ø  When responses are limiting the Growth.

Ø  Resources for growth for most animal populations are finite and become limiting.

Ø  The logistic growth model is a more realistic one.

Population Interactions:


Ø  Organism of higher trophic level (predator) feeds on organism of lower trophic level (prey) is called the predation.

Ø  Even the herbivores are not very different from predator.

Ø  Predator acts as a passage for transfer of energy across trophic level.

Ø  Predators keep prey populations under control.

Ø  Exotic species have no natural predator hence they grow very rapidly. (prickly pear cactus introduced in Australia created problem)

Ø  Predators also help in maintaining species diversity in a community, by reducing the intensity of competition among competing prey species. (Pisaster starfish field experiment).

Defense developed by prey against predators:


Ø  Insects and frogs are cryptically coloured (camouflaged) to avoid being detected by the predator.

Ø  Some are poisonous and therefore avoided by the predators.

Ø  Monarch butterfly is highly distasteful to its predator (bird) due to presence of special chemical it its body. The chemical acquired by feeding a poisonous weed during caterpillar stage.


Ø  Thorns in Acacia, Cactus are morphological means of defense.

Ø  Many plants produce and store some chemical which make the herbivore sick if eaten, inhibit feeding, digestion disrupt reproduction, even kill the predators.

Ø  Calotropis produces poisonous cardiac glycosides against herbivores.

Ø  Nicotine, caffeine, quinine, strychnine, opium etc. are produced by plant actually as defenses against the grazers and browsers.


Ø  Interspecific competition is a potent force in organic evolution.

Ø  Competition generally occurs when closely related species compete for the same resources that are limiting, but this not entirely true:

Ø  Firstly: totally unrelated species could also compete for the same resources.

  • American lakes visiting flamingoes and resident fishes have their common food, zooplanktons.

Ø  Secondly: resources need not be limiting for competition to occur.

  • Abingdon tortoise in Galapagos Islands became extinct within a decade after goats were introduced on the island, due to greater browsing ability.

Ø  Competitive release: A species, whose distribution is restricted to a small geographical area because of the presence of a competitively superior species, is found to expand its distributional range dramatically when the competing species is experimentally removed.

Ø  Connell’s elegant field experiment showed that superior barnacle Balanus dominates the intertidal area and excludes the smaller barnacle Chathamalus from that zone.

Ø  Gause’s ‘competitive Exclusion Principle’: two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior will be eliminated eventually.

Ø  Resource partitioning: If two species compete for the same resource, they could avoid competition by choosing, for instance, different times for feeding or different foraging pattern.

Ø  MacArthur showed five closely related species of warblers living on the same tree were able to avoid competition and co-exist due to behavioral differences in their foraging activities.


Ø  Parasitic mode of life ensures free lodging and meals.

Ø  Some parasites are host-specific (one parasite has a single host) in such a way that both host and parasite tend to co-evolve.

Parasitic adaptation:

Ø  Loss of unnecessary sense organs.

Ø  Presence of adhesive organs or suckers to cling on to the host.

Ø  Loss of digestive system.

Ø  High reproductive capacity

Ø  Parasites having one or more intermediate host or vectors to facilitate parasitisation of its primary host.

Ø  Liver fluke has two intermediate hosts (snail and a fish) to complete its live cycle.

Effects on the host:

Ø  Parasite always harms the host.

Ø  They reduce the survival, growth and reproduction of the host.

Ø  Reduce its population density.

Ø  They make the host more vulnerable to the predators, by making it physically weak.

Ø  Ectoparasite: feeds on the external surface of the host.

  • Lice on human
  • Ticks on dog
  • Marine fish infested with copepods
  • Cuscutaa parasitic plant grow on hedge plants.

Ø  Endoparasites: are those that live inside the host body at different sites.

  • Life cycle is more complex.
  • Morphological and anatomical features are greatly simplified.
  • Highly developed reproductive system.

Brood parasitism:

Ø  Special type of parasitism found in birds.

Ø  The parasitic birds lay its eggs in the nest of its host and let the host incubate them.

Ø  The egg of the host is very similar with the egg of the host.

Ø  Cuckoo lays eggs in the nest of the crow.


This is the interaction in which one species benefits and the other is neither benefited nor harmed.

  • Orchids growing as an epiphyte on a mango branch.
  • Clown fish living among tentacles of sea anemone.
  • Barnacles on back of whales.
  • Cattle Egret and grazing cattle.


Interaction between two living organism, both are equally benefited, no one is harmed.

  • Lichen: a mycobiont and a Phycobiont.
  • Mycorrhiza: relationship between fungi and root of higher plant.
  • Pollinating insects and flowering plants.
  • Fig trees and its pollinating agent wasp.

Sexual deceit:

  • Mediterranean orchid Ophrys employs ‘sexual deceit’.
  • Petal of the flower resembles the female bee.
  • The male bee attracted to what it perceives as a female, ‘pseudocopulates’ with the flower but does not get any benefits.

Disclaimer: All contents are originally prepared by Shri K C Meena Ji, Principal, KVS. 

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