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Population Interactions Notes | Study Biology Class 12 - NEET

Document Description: Population Interactions for NEET 2022 is part of Chapter 13 - Organisms and Populations for Biology Class 12 preparation. The notes and questions for Population Interactions have been prepared according to the NEET exam syllabus. Information about Population Interactions covers topics like and Population Interactions Example, for NEET 2022 Exam. Find important definitions, questions, notes, meanings, examples, exercises and tests below for Population Interactions.

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Due to increase in different species in community, interaction (for food, habitat) also starts between them.

Interspecific InteractionsInterspecific Interactions

(a) Positive or beneficial interaction – Member of one or both the interacting species are benefitted but neither is harmed.

(b) Negative interaction – One or both interacting species is harmed.  


It is a wide spread phenomenon, it includes → mutualism, commensalism, protocooperation.

1. Mutualism (+/+) or Symbiosis :– (co-evolution) Positive inter specific interaction in which members of two different species completely depend on each other for growth and survival, physical contact is present in between both the interacting species. It is obligatory relationship.

  • Mutualism between animal and animal– e.g.Termites and Flagellates (Trichonympha).

  • Mutualism between plant & animals – e.g. Yucca plant flowers and Pronuba insects – Pollination of yucca plant by pronuba (Female yucca moth).

  • Mutualism between plant and bacteria – e.g. legume plant and Rhizobium.

  • Mutualism between algae and higher plant – e.g. Nostoc, Anabaena and Anthoceros plant.

  • Mutualism between algae and fungi– e.g. Lichens.

  • Mutualism between fungi and higher plants– e.g. Mycorrhizal association.

Note : Extra eg.

  • Fig tree and wasp species.

In many species of fig trees, there is a tight one to one relationship with the pollinator species of wasp. It means that a given fig species can be pollinated only by its partner wasp species and no other species.
The female wasp uses the fruit not only as an oviposition (egg-laying) site but uses the developing seeds with in the fruit for nourishing its larval. The wasp pollinates the fig inflorescence while searching for suitable egg-laying sites. In return for the favour of pollination the fig offers the wasp some of its developing seeds, as food for the developing wasp larvae.

  • Bees and orchid flower.

Orchids show diversity of floral patterns, which have evolved to attract the right pollinator insect (bees and bumblebees) and ensure guaranteed pollination by it.
The mediterranean orchid Ophrys employs "sexual deceit" to get pollination done by a species of bee. One petal of its flower beans resemble to the female of the bee in size, colour and markings. 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, it transfers pollen to it and thus, pollinates the flower.

2. Commensalism (+ / 0) – Association between members of two species in which one is benefitted while other is almost unaffected.

  • Lianas – are woody plants. Their roots are present in soil but their stem use other plant or object for support to get better light. They are found in dense forest. No nutritional relationship.  Lianas are the speciality of tropical rain forest. e.g. Bauhinia, Tinospora.

  • Epiphytes – Small plants grow on other plants in tropical rain forest. They utilise only the space of host plant for light & humidity e.g. Orchids, Hanging mosses.

  • Epizones – Those animals which depends on plants or other animals.
    Sucker fish (Echeneis) – Shark
    Pilot fish – Shark
    E.coli bacteria – Intestine of man
    Clown fish – Sea anemone
    Barnacles – Whale
    Cattle egret birds – Cattle

3. Proto-cooperation (+/+) – Association in which both organisms are benefited but can live separately, it is a facultative or optional or occasional association also called as non-obligatory relationship. e.g.

  • Hermit crab – Sea anemone

  • Tick bird (Red-billed or yellow billed) – Rhinoceros

  • Crocodile – Bird

Scavenging – Association in which one partner called scavenger or saprobiont, eats the dead bodies of other animals, which have died naturally or killed by another animal. e.g. Jackal, Vulture, Ant, Crow

Helotism – Association in between two organisms, when one behaves as a master and another as slave. e.g. Lichen.



Two types of negative interaction: 

(1) Exploitation 

(2) Amensalism 

(1) Exploitation – One species harms the other by making its direct or indirect use of support, shelter or food.

It is of two types: 

(a) Parasitism 

(b) Predation 

(a) Parasitism (+ / –) → This association involves individuals of two species of different size in which smaller (Parasite) is benefitted and larger (host) is harmed. The parasite gets nourishment and shelter from host but does not kill the host.

Type of Parasites : 

(i) Ectoparasite →  lives on the body of host.

  •  Ectozooparasite – Leech on cattle, ticks on dogs, copepods on marine fish and lice, mosquitoes, sandfly live on man

  •  Ectophytoparasite – Amphids, Lac insects, Red cotton bug.

(ii) Endoparasites → lives in the body of host.

  •  Tapeworm, Taenia, Ascaris, Entamoeba → live in intestine of man.

  •  Plasmodium → live in R.B.C. of human.

  • Hyper parasitism →  A parasite living on another parasite. e.g. plasmodium on female anopheles mosquito, Bacteriophages on bacteria.

  • Brood parasitism → Parasitism in which the parasitic bird (cuckoo) lays its eggs in the nest of its host (crow) and lets the host incubate them, this relation is known as brood parasitism.

  • Holo parasite → Parasites which are totally dependent upon the host for their requirement. e.g. Rafflesia, (Total root parasite) Cuscuta (Total stem parasite)

  • Hemiparasite → Parasite which partially  depend on the host.

Population Interactions Notes | Study Biology Class 12 - NEET

Note : Arceuthobium is the smallest parasite.


(b) Predation (+/–) :– A free living organisms which catches and kills another species for food.

Insectivores fungi – Dactylella, Dactylaria, Arthrobotrys.

Carnivores animals – Lion, snake.

Insectivores plants – Drosera, Utricularia, Nepenthes.

(2) Amensalism (– / 0) – amensal = (–)       inhibitor = (0)

In this interaction one species is inhibited by toxic secretion of another species. Inhibitor species is neither benefitted nor harmed.

Type of Amensalism

(i) Allelopathy (ii) Antibiosis

(i) Antibiosis – Secretion of antibiotics –

  • Penicillium fungi secretes penicillin which inhibits growth of Staphylococcus bacteria.

  • Chlorella algae secretes bacteriocytes which not only kills but also inhibits growth of the bacteria.

  • Microcystis (BGA) secretes hydroxyl  amine, this causes the death of fishes.

(ii) Allelopathy – Secretion of toxic chemical –

  • Parthenium – Trans Cinnamic acid is secreted by Parthenium which inhibits the growth of some plants like Cassia tora and Vincaregia. This phenomenon is known as allelopathy.

  • Sunflower, Barley, Sorghum, Occimum also show allelopathy.



  •  Silver oak shows autopathy – It destroys own seed.

Allochemics – Chemicals secreted by a species are known as allochemics.

Types of allochemics 

  •  Allamon – Chemicals which repel other enemies is called allamon. e.g. Cuttle fish (Sepia)

  •  Depressants – Chemicals which kill or inhibit the growth of other organism e.g. Parthenium

  •  Kairmons – The chemicals secreted by one organism which benefits the another organism e.g. Nematodes in soil →  stimulate growth of fungi.

  • Cannabalism →  organisms eaten by own species e.g. cockroach, termites

  •  Competition (–, –) : Process in which the fitness of one species is significantly lower in the presence of another species.



On the basis of position and the degree of protection to perennating organs (bud) during adverse season, Raunkiaer divided the plant in to five life forms (growth process)

1. Phanerophytes – The buds are located much above the surface of the earth.

Such type of plants are found more in tropics, epiphytes are included in phanerophytes.

2. Chamaeophytes – The buds are situated very close/near to the ground surface. Such types of plants are found in cold places or high altitudes.

3. Hemicryptophytes – The buds present just below or just above the surface of ground. Such plants are found in temperate climate.

4. Cryptophytes – The buds lie very deep in the ground.

5. Therophytes – Those plants which are passing the unfavourable climate through their seeds.


(1) Formation →  A well developed plant community in any climate is called formation.

Eg.→ Tropical forest.

(2) Association →   A sub-unit of formation. A smaller community of plant formation with its two or more dominant species is known as association. A formation has many association.

(3) Consociation →  Sub unit of association, means a community with a single dominant species.


1. Direct Method – This method is used to find out the species and members present in a community and study about them. This method can be used only for small community.

2. Sampling method – In this method we study the community in the form of sample. This method is also useful for large community. Samples are taken in the form of quadrats. The quadrats are formed with the help of rope to know the size of a quadrat, sequentially increase the size of quadrats according to increase in number of species, when the no. of the species becomes constant then estimate the number of plants present in the same size of quadrats. Then after that table work is done.

(i) Frequency →  No. of (Percentage) of samples on sampling unit in which species is present. It indicates species dispersal.

Population Interactions Notes | Study Biology Class 12 - NEET

e.g. The plants of prosopis species were present in 90 samples out of 100 sample,

Population Interactions Notes | Study Biology Class 12 - NEET

(ii) Density – The number of individuals per unit area, it indicates abundance of a species.

Population Interactions Notes | Study Biology Class 12 - NEET

e.g. 10,000 prosopis were present in 100 sample

Population Interactions Notes | Study Biology Class 12 - NEET


Analysis of community characters is generally done for :-

  •  Recording variations within and between communities.

  •  Naming and classifying communities.

Community analysis involves measurements of various characters in sample plots or quadrats located with in the community.

Frequency – Based on percentage of plots or quadrat in which a species (which is being studied) is present.

Diversity (Density) –  Number of individuals per unit area.

Cover –  Percentage land area occupied by a species-cover is expressed by basal cover (basal cover area occupied by stem bases) or crown cover (crown cover-area covered by canopy).

Biomass – Quantity of living materials per unit area.

Various community characters can be categorised as : 

(i) Analytic characters – Characters which are directly observed or measured in sample plots or quadrats.

(ii) Synthetic characters – Which are derived from the measurements of analytic chracters.

Analytic characters Synthetic characters 

  • Frequency → Dispersion of species

    Note :- Frequency is an analytic character which show or indicate dispersion of species which is a synthetic character.

  • Diversity → Relative abundance

  • Biomass → Growth of plant

  • Cover → Influence zone of species


Ecotone – The transition zone in between two communities is called ecotone or tension zone. It has greater number of species & density or it is a transition zone between two communities where one type of community is modified into another type of community is known as ecotone.

Edge effect – Species which occur most abundantly and spend their time in ecotone are called edge species. The tendency to increase variety and density of some organism at the community border is known as edge effect.

Biological (phyto-climatic) spectrum – The ratio or percentage distribution of different life forms in any plant community is called biological spectrum.

Biotic potential (Reproductive potential/or potential ability) → The term biotic potential was first used by Chapmann .

Under most favourable environmental conditions the maximum reproductive capacity of an species is known as biotic potential.

Vitality –Capacity of normal growth and reproduction for survival of a species. It depends upon weight of plant , stem height, root length, leaf number etc.

Ecological Niche – Word is given by Grinnel. It is the functional role of any species in a ecosystem or community. In other words it is a occupational address or profession of a species it means it is a functional position or status in an ecosystem.

Gause's competitive exclusion principle – This principle state that two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior are will be eliminated eventually. This may be true if resources are limiting but not otherwise.

Ecological Equivalents– Organism that occupy the same or similar ecological niche in different geographical regions are known as Ecological equivalents. e.g. Arctic fox and African Jackal, both are scavengers Grazers of North America and Kangaroo's of Australia

Habitat – Physical area covered by any organism

Micro climate and Micro habitat – Subdivision of habitat is called micro habitat. It is an immediate climate (real climate) of an organism which is different from the average climate of region. e.g. Forest floor, burrow and surface of desert

Competitive release –A species whose distribution is restricted to a small geographical area because of the presence of a comparatively superior species. If one of the superior competitor is removed then range of the distribution of inferior species get increased.

Resource partitioning – If two species compete for the same resource, they could avoid competition by choosing, for instance different time for feeding or different foraging patterns.


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