Everything you need to know about Population Growth

Population is a group of organisms of the same species occupying a particular space at a particular time. It has a definite organization and structure and various characteristics.

Demography

Demography is the study of various aspects of population. (generally human population).

Population Density

Population Density refers to the number of individuals present in unit area at a given time. It is usually determined by counting all individuals and dividing it by the number of units of space. If the total number of individuals is taken as N and the number of units of space or area as S. the population density can be expressed as:

$$ D=\frac {N}{S}$$

Using the above equation/formula measuring of the population density can be done. The unit of space or area can be square miles or square kilometers. So the unit of population density will be per square miles (mi⁻²) or per square kilometer, (km⁻² ).

Although total number is generally the most appropriate measure of population density, it is in some cases either meaningless or difficult to determine. For instance, in an area if there are 100 small herbs but only a single huge mango tree with a large canopy, it shows the population density of mango tree is low compared to the the small herb plants underestimating the enormous role of mango tree in that community.

In such cases, the per cent cover or biomass is a measure of the population size. Total number is again not an easily adoptable measure if a population is huge if counting is impossible or very time consuming. Sometimes for certain ecological investigations, there is no need to know the absolute population sizes densities; relative densities serves the purpose well enough.

We can also estimate population sizes indirectly, without actually counting them or seeing them. For instance, the tiger census in Indian National Parks and tiger reserves is often based on pug marks and fecal pellets.

What is the definition of Biotic Potential?

Biotic potential( reproductive potential)  refers to the inherent power of a population to increase in numbers when all environmental conditions are favorable. Mathematically, it may be defined as the slope of the population growth curve during the logarithmic phase of growth. It is an innate characteristic of every species.

Environmental Resistance

In nature, full biotic potential of population is not realized since conditions are rarely favorable. Unfavorable climatic changes, shortage of food, predator etc., curtail the growth of population and their combined effect is called environmental resistance. These factors fall into the following two categories:

1. Density dependent population growth
2. Density independent population growth

 

1. What is Density dependent population growth

It describes the growth of a simple population in a confined space, where resources are not unlimited. In the early stages resources are abundant, the death rate is minimal and reproduction can take place as fast as possible allowing the individuals to attain their intrinsic rate of increase.

An example for the density dependent population growth will be when a species discovers and inhabits a new habitat, there is no competition thus they can reproduce and grow exponentially. The population increases geometrically until the maximum number of individuals is approached that environment can sustainably support is approached. This  number is called the Carrying capacity (K).

The population growth rate then declined to zero and the population size gets stabilized. This can be
described as the logistic equation:

Rate of change of population
            size at a time 't'             $$=Intrinsic \ rate \ of \ increase  \times Population \ size \times Density \ dependent \ factor$$
                                                                                     

                                                      $$\frac{dn}{dt}= rN (-1-\frac {N}{K})$$

Where the density dependent factor \((-1-\frac {N}{K})\) approaches zero as the population approaches the
                                                                     
 carrying capacity and the intraspecific competition becomes more intense. This equation predicts growth of a population over time to be sigmoidal, as is commonly observed in real populations.

2. What is Density independent population growth

For example, Imagine the introduction into a pond of chemical that is toxic to water fleas. The size of the population will not influence its effect - if it is concentrated enough to kill the fleas it will do so whether there are ten or ten million.

Typical density-independent factors include temperature and natural catastrophes like floods, storms and fires. This is how density independent factors affect population growth.

Unlimited growth of this kind is  described by a continuous population model and expressed in terms of the rate of change in population numbers at a time :

Rate of change of population size at a time t=Intrinsic rate ✖ Population 

                                                       $$\frac{dn}{dt}= rN$$
   
These are the differences between Density dependent and density independent population growth factors.                                                        

Growth Models

The growth models of a population with time shows a specific and predictable pattern.
  

1. Exponential Growth of global Population

Resource( food and space) availability is essential for the unimpeded growth of a population. Ideally, when resources in the habitat are unlimited, each species has the ability to realise fully its innate potential to grow in number. In other words, the population grows in exponential or geometric fashion. Any species growing exponentially under unlimited resource conditions can reach enormous population densities in a short time.

Exponential Growth of global Population, J Shaped Curve or Exponential growth curve
J Shaped Curve or Exponential growth curve

2. Logistic Growth of  Population

No population of any species in nature has at its disposal unlimited resources to permit exponential growth. This leads to competition between individuals for limited resources. In nature, a given habitat has enough resources to support a maximum possible number, for that species in the habitat.

This limit is called the nature's Carrying Capacity (K) for that species in that habitat. A population growing in a habitat with limited resources show initially a lag phase, followed by acceleration and deceleration and finally the population density reaches the carrying capacity. This type of population growth is called Verhulst-Pearl Logistic Growth.

Logistic ( S Shaped or Sigmoid) Curve, Logistic Growth of  Population
Logistic ( S Shaped or Sigmoid) Curve

Population Growth

Population growth refers to the total number of increase in the organisms occupying a certain area. It keeps changing in the time, depending on various factors including food availability, predation pressure and adverse weather.

If favorable environmental conditions exist in a specific region and there is a mean but which a species may be introduced in the desired area, a group of the organisms exposed to such an environment will increase in number with the passage of time.

Population explosion alludes to the fast and sensational ascent in total Population that has happened in the course of the last hundred years. Somewhere in the range of 1959 and 2000, the total Population expanded from 2.5 billion to 6.1 billion individuals. As per United Nations projections, the total population will be between 7.9 billion and 10.9 billion by 2050.

The most common causes of Population explosion is

  1. Poverty
  2. Child Labor
  3. Poor contraceptive use
  4. Immigration
  5. Reduced Mortality Rates

Population growth comprises of two aspects namely natality (birth rate) and mortality (death rate). Natality refers to the inherent ability of a population to increase. The maximum possible rate of reproduction for the given species under optimal conditions is termed potential natality.

In natural conditions potential natality is rarely attained because birth rate may be adversely affected by many factors. Thus actual birth rate under existing conditions is called realized natality. On the contrary, mortality is defined as the number of death of organisms in a population of specific area with the passage of time.

It is also influenced by environmental conditions as natality. Similarly, lowest death rate for a given species is the best of circumstances constitutes potential mortality and actual death rate is the realized mortality.

Survivorship is the converse of mortality. The number of survivors is usually of more interest than the number dying, therefore mortality is often expressed as life expectancy-- the average number of years to be lived in the future by population members of a given age.

Survivorship data are often shown as a survivorship curve for a particular population, a graph showing the proportion of survivors on a log scale through each phase of life.

In general, population growth is determined by the number of organisms added to the population(through birth or immigration) minus the number of organisms lost (through death or emigration).

When the number of individuals added to the population equals the number of individuals lost, the population remains constant or shows zero population growth. Population increases when the number of births exceeds the number of deaths.

The factors of environmental resistance when do not produce zero growth, the population experiences exponential growth, which can be shown by the following equation

                                                                 ∆N   = rN
                                                                 ∆t
where N is the number of individuals, t is the time and    ∆N
                                                                                            ∆t 
is the change in population size with a change in time.

The maximum growth of rate is called intrinsic rate of growth.
As per recent studies the average population growth rate is 83 million people per year. So that is about 1.1% annually.

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