Organisms and Populations

Introduction 

All the organisms on this planet possess the capability of adapting to dissimilar and various environmental conditions. This happens because of the variation in the genes of the organisms. Due to this, their chances of survival increase. For instance, a polar bear has a body type and character traits that help in surviving in the extremely cold Antarctica. The dense fur coats on the body of a bear protect it from predators as well as the cold at the same time. Their body repels liquid and that keeps them warm in the cold.

Organisms

An organism functions and survives as an individual entity and is made up of cells. The organisms can be classified through taxonomy into multiple groups like plants, fungi, etc.

Populations

Individuals of any species survive and live their life in a group in a geographical area that is well defined. Moreover, they share or fight for some resources that are similar for them and then they create a population.

Population Attributes

A population has certain attributes that are absent in individual organisms. Such as an individual may die or take birth. Whereas, a population can have birth or death rates. The birth and death rates are said to be the per capita births or deaths. These rates might get reduced or increased depending upon the members of a population. There is one more attribute of the population that we call the sex ratio. Moreover, an individual can be counted as a male or female depending upon the gender, but a population can have a sex ratio at the same time. A population is composed of different ages of multiple individuals. In case the age distribution is planned for the population, it will result in a structure known as age pyramids. The structure of the pyramid defines the shape of the status of growth of the population, this can be the following:

  •       Expending
  •       Stable
  •       Declining

Population size § or the population density (N) can be measured in numerical values but it may not be appropriate all the time.

Population Growth

The size of the population is not fixed. It keeps on showing variations with time, depending upon the availability of food, predation pressure, and reduces weather. The major and primary aspects that define the growth of the population are as follows:

  •       Natality
  •       Mortality
  •       Immigration
  •       Emigration

If ‘N’ is the population density at a given time ‘T’, then its density at time T+1 will be = NT + 1 NT + [(B + I) – (D + E)]

Therefore, the density of the population will rise if the number of births + the number of immigrants i.e. (B + I) is greater than the number of deaths + the number of emigrants i.e. (D + E), else it will get reduced.

Growth Model

There are 2 main types of growth models and these are given below:

  • Exponential Growth – in this model, growth takes place when food and space are available in an adequate amount. When resources in the habitat are limitless, each species has the chance and ability to grow in number.The population rises in an exponential or geometric manner. If, in a population with size ‘N’, the birth rates are denoted as ‘b’ and death rates as ‘d’, then rise and decline in ‘N’ throughout the unit period time ‘t’ will be:

dN / dt = (b – d) × N

Let (b – d) = r, then

dN / dt = rN

Therefore, the ‘r’ in this equation will be called the “intrinsic rate of natural increase”.

  • Logistic Growth – in this model, there is a rivalry between the individuals in a population for food and shelter. Furthermore, the healthiest and the strongest one survives and reproduces.

For instance;

Here, ‘N’ = Population Density at a given time ‘t’

‘R’ = Intrinsic rate of natural increase

‘K’ = Carrying capacity

Since resources for the growth of the populations are limited. However, this model is considered a more genuine growth model.

Population Interaction

All the plants, animals, as well as microbes under a biological community, communicate and cooperate with each other. These interactions are sometimes beneficial and sometimes neutral or detrimental to the species, individually or to both the species. The following types of interaction are possible:

  •       Predation
  •       Competition
  •       Parasitism
  •       Commensalism
  •       Mutualism

Conclusion

The study of organisms and populations makes us understand that ecology is all about the relations among the living organisms and biotic as well as abiotic components of their environment. You can consider it with 4 levels of biological organisation, and these are: organisms, populations, biomes, and communities.

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