. P OPULATIONS . . 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 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 because it links ecology to population genetics and evolution.
A population has certain attributes whereas, 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 capita births and deaths. The rates, hence, expressed are change in numbers (increase or decrease) with respect to members of the population.
Here is an example. If in a pond there were lotus plants last year and through reproduction new plants are added, taking the current population to , we calculate the birth rate as / = . offspring per lotus per year. If individuals in a laboratory population of fruitflies died during a specified time interval, say a week, the death rate in the population during that period is / = .
individuals per fruitfly 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., per cent of the population are females and per cent males). 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 (Figure . ). For human population, the age pyramids generally show age distribution of males and females in a diagram. The shape of the pyramids reflects the growth status of the population - (a) whether it is growing, (b) stable or (c) declining.