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 an a- sexual reproduction is also generally considered a population for the purpose of ecological studies. All the cormorants 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. An individual may have births and deaths, but a population has birth rates and death rates. In a population these rates refer to per capital births and deaths, respectively. The rates, hence, are expressed is change in numbers (increase or decrease) with respect to members of the population. Here is an example. 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. If 4 individuals in a laboratory population of 40 fruit flies 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 fruit fly per week.

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).

Ecological significance of flora & fauna in knowing its population attributes for building up population growth models:

Population Growth Models: We have been concerned about unbridled human population growth and problems created by it in our country in particular and throughout the world in general. It is therefore natural for us to be curious if different animal populations in nature behave the same way or show some restraints on growth. Perhaps we can learn a less or two from nature on how to control population growth.

(i)   Exponential growth Model: Resource (food and space) availability is obviously essential for the unimpeded growth of a population. Ideally, when resources in the habitat are unlimited, each species has the ability to realize fully its innate potential to grow in number, as Darwin observed while developing his theory of natural selection. Then the population grows in an exponential or geometric fashion. This innate potential of a population to grow in size or number is referred to as the ‘intrinsic rate of natural increase’ usually represented by ‘r’. This is indeed a very important parameter chosen for assessing impacts of any biotic or abiotic factor on population growth.

      It must be noted that any specie’s growth exponentially under unlimited resource conditions can reach enormous population densities in a short time. Darwin showed how even a slow growing animal like elephant could reach enormous numbers in the absence of any checks and balances.

Connecting concepts: How big a population can grow, if growing exponentially?

Here is an anecdote popularly narrated to demonstrate dramatically how fast a huge population could build up when growing exponentially.

The king and the minister sat for a cheese game. The king, confident of winning the game, was ready to accept any bet proposed by the minister. The ministry humbly said that if he won, he wanted only some wheat grains, the quantity of which is to be calculated by placing on the chess board on grain in Square 1, then two in Square 2. Then four in Square 3, and eight in Square 4, and so on, doubling each time the previous quantity of wheat on the next square until all the 64 squares were filled. The king accepted the seemingly silly bet and started the game, but unluckily for him, the minister won. The king felt that fulfilling the minister’s bet was so easy. He started with a single grain on the first square and proceeded to fill the other squares following minister’s suggested procedure, but by the time he covered half the chess board, the king realized to his dismay that all the wheat produced in his entire kingdom pooled together would still be inadequate to cover all the 64 squares. Now think of a tiny Paramecium staring with just one individual and through binary fission, doubling in numbers every day, and imagine what a mind-boggling population size it would reach in 64 days. (provided food and space remain unlimited). Does this really happen in nature?

We may find its answer in another population growth model called as:

(ii)  Logistic growth Model: This is nature’s universal fact that 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. Eventually, the ‘fittest’ individual will survive and reproduce. The governments of many countries have also realized this fact and introduced various restrains with a view to limit human population growth. In nature, a given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible. We may call this limit imposed by nature as 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 phases of acceleration and deceleration and finally an asymptote, when the population density reaches the carrying capacity.

      Since resources for growth for most animal populations are finite and become limiting sooner or later, the logistic growth model is considered a more realistic one.

Life History Variation

Populations evolve to maximize their reproductive fitness, also called Darwinian fitness (higher r value), in the habitat in which they live. Under a particular set of selection pressures, organisms evolve towards the most efficient reproductive strategy. Some organisms breed only once in their lifetime (Pacific salmon fish, bamboo) while others breed many times during their lifetime (most birds and mammals). Some produce a large number of small-sized offspring (Oysters, pelagic fishes) while others produce a small number of large-sized offspring (birds, mammals). So which is desirable for maximizing fitness? Ecologists suggest that life history traits of organisms have evolved in relation to the constraints imposed by the biotic components of the habitat in which they live. Evolution of life history traits in different species is currently an important area of research being conducted by ecologists.