Population interaction


A population is defined as a group of individuals of the same species living and interbreeding within a given area. Members of a population often rely on the same resources, are subject to similar environmental constraints, and depend on the availability of other members to persist over time.

A population at any given time is composed of individuals of different ages. If the age distribution (percent individuals of a given age or age group) is plotted for the population, the resulting structure is called an age pyramid. For the human population, the age pyramids generally show age distribution of males and females in a combined diagram. The shape of the pyramids reflects the growth status of the population – (a) whether it is growing, (b) stable or (c) declining.

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Age Pyramid for Human Population

Population Growth

The size of a population for any species is not a static parameter. it keeps changing in time, depending on various factors including food availability, predation pressure and adverse weather. The density of a population in a given habitat during a given period, fluctuates due to changes in four basic processes, two of which (natality and immigration) contribute to an increase in population density and two (mortality and emigration) to a decrease.

  1. Natality refers to the number of births during a given period in the population that are added to the initial density. 
  2. Mortality is the number of deaths in the population during a given period. 
  3. Immigration is the number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration. 
  4. Emigration is the number of individuals of the population who left the habitat and gone elsewhere during the time period under consideration.
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Population Interaction 

In nature , animals , plants and microbes do not and can not live in isolation , they interact in various ways to form a biological community. 

Interaction of populations of two different species gives rise to interspecific interactions. Such interactions could be beneficial, detrimental or neutral (neither harm nor benefit) to any of the participating species which are part of such interactions.

Here in the following table ‘+’ sign has been assigned for beneficial interaction , ‘- ’ sign has been assigned for rimental and ‘0’ for neutral interactions.

InteractionsSpecies ASpeciesBExamples
Mutualism/ Symbiotic++
Leguminous plants and nitrogen-fixing bacteria.Process of pollination in plants. 
Commensalism+0Remoras eat leftover food from sharks without depleting sharks’ resources.
Amensalism0Shading out of one plant by a taller and wider one.Allelopathy – inhibition of one plant by the secretions of another. 
Parasitism +Ticks, and the protozoan that causes malaria.
Competition Lions and tigers are in the same niche. 
Predation+Lion and zebra, bear and fish, fox and rabbit.
  • Mutualism – Such interactions confer benefits on both the interacting species. For example, Lichens exhibit such a relationship between fungus and photosynthesising algae or cyanobacteria.  Similarly , the mycorrhizae are associations between fungi and the roots of higher plants. The fungi helps the plant in absorption of nutrients from the soil while the plant in turn provides the fungi with carbohydrates. 
  • Commensalism – In this type of interactions one species benefits and the other is neither harmed nor benefited. For example epiphyte growing on a tree or Barnacles growing on the back of a whale , neither the tree nor the whale derives any apparent benefit.
  • Amensalism – In such interactions one species is harmed whereas the other remains unaffected. For example, Herd of animals trampling over a grass field.
  • Parasitism and Predation – In both of these interactions only one species benefits i.e. Parasite and Predator respectively and such interaction proves detrimental to the other species i.e. host and prey respectively. For example, Leeches on host bodies (Parasitism) or Tiger Hunting a deer (Predation).  
  • Competition –  Organisms of two species depend and compete for the same limited resource and have a negative effect on each other. For ex- Hippopotamus and crocodile competing for the same limited amount of resources in a dried up water body.
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