13.2 POPULATIONS 13.2.1 Population Attributes NEET

13.2 POPULATIONS
13.2.1 Population Attributes 
  • In nature, we rarely find isolated, single individuals of any species; majority of them live in groups in a well defined geographical area, share or compete for similar resources, potentially interbreed and thus constitute a population. 
  • Although the term interbreeding implies sexual reproduction, a group of individuals resulting from even asexual reproduction is also generally considered a population for the purpose of ecological studies. 
  • All the cormorants (Birds) in a wetland, rats in an abandoned dwelling, teakwood trees in a forest tract, bacteria in a culture plate and lotus plants in a pond, are some examples of a population. 
  • In earlier chapters you have learnt that although an individual organism is the one that has to cope with a changed environment, it is at the population level that natural selection operates to evolve the desired traits. 
  • Population ecology is, therefore, an important area of ecology because it links ecology to population genetics and evolution.
  • A population has certain attributes that an individual organism does not. 
1. Birth (b) rates and Death rates (d): An individual may have births and deaths, but a population has birth rates and death rates. 
  • In a population these rates refer to per capita births and deaths, respectively. 
  • The rates, hence, expressed is change in numbers (increase or decrease) with respect to members of the population. 
  • Example-1. If in a pond there are 20 lotus plants last year and through reproduction 8 new plants are added, taking the current population to 28, we calculate the birth rate as 8/20 = 0.4 offspring per lotus per year. 
  • Example-2. If 4 individuals in a laboratory population of 40 fruitflies died during a specified time interval, say a week, the death rate in the population during that period is 4/40 = 0.1 individuals per fruitfly per week.
  • r value: intrinsic rate of natural increase (b - d = r)
2. Sex ratio: Another attribute characteristic of a population is sex ratio. 
  • An individual is either a male or a female but a population has a sex ratio (e.g., 60 per cent of the population are females and 40 per cent males). 
3. Age pyramid: A population at any given time is composed of individuals of different ages. 
  • If the age distribution (per cent individuals of a given age or age group) is plotted for the population, the resulting structure is called an age pyramid.
  • For 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.
PDF] Organisms and Populations.pmd | Semantic Scholar

4. Population density (N): The size of the population tells us a lot about its status in the habitat. 
  • Whatever ecological processes we wish to investigate in a population, be it the outcome of competition with another species, the impact of a predator or the effect of a pesticide application, we always evaluate them in terms of any change in the population size. 
  • The size, in nature, could be as low as <10 (Siberian cranes at Bharatpur wetlands in any year) or go into millions (Chlamydomonas in a pond). 
  • Population size, more technically called population density (designated as N), need not necessarily be measured in numbers only. 
  • Although total number is generally the most appropriate measure of population density, it is in some cases either meaningless or difficult to determine. 
  • Example: In an area, if there are 200 Parthenium plants but only a single huge banyan tree with a large canopy, stating that the population density of banyan is low relative to that of Parthenium amounts to underestimating the enormous role of the Banyan in that community. 
  • In such cases, the per cent cover or biomass is a more meaningful measure of the population size. 
  • Total number is again not an easily adoptable measure if the population is huge and counting is impossible or very time-consuming. 
  • If you have a dense laboratory culture of bacteria in a petri-dish what is the best measure to report its density? (number of bacteria/colony). ans- colony forming unit (CFU)
  • Sometimes, for certain ecological investigations, there is no need to know the absolute population densities; relative densities serve the purpose equally well. 
  • For instance, the number of fish caught per trap is good enough measure of its total population density in the lake. 
  • We are mostly obliged to estimate population sizes indirectly, without actually counting them or seeing them. 
  • The tiger census in our national parks and tiger reserves is often based on pug marks and fecal pellets.

Think/Answer

  • If a population of 50 Paramoecium present in a pool increases to 150 after an hour, what would be the growth rate of population?
  • What would be the per cent growth or birth rate per individual per hour for the same population mentioned in the previous question?

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