Population

A population is a group of individuals of one species that live in a defined area, share or compete for similar resources and potentially interbreed. This page covers the attributes of a population (birth rate, death rate, sex ratio and age pyramids), the ways of measuring population density, the four processes that change it, and the exponential and logistic growth models with their equations. It follows the Class 12 NCERT chapter Organisms and Populations, and NEET often tests growth curves, carrying capacity and the intrinsic rate of natural increase of a population.

1. Ecology and Levels of Organisation

1.1 Ecology as a unifying thread

• Biology is often split into botany, zoology and microbiology, or into classical and modern (molecular) biology.
• Ecology is one thread that joins these areas into a unifying principle. It gives a holistic view of biology.
• Each organism remains an individual, yet it interacts with other organisms and with its physical habitat as part of a group.
• So organisms behave like organised wholes: a population, a community, an ecosystem or even the whole biosphere.
• Ecology also studies anthropogenic (human-caused) environmental degradation and the socio-political issues it has raised.

1.2 Ramdeo Misra: Father of Ecology in India

• ★ Exam imp Ramdeo Misra (1908-1998) is revered as the Father of Ecology in India. He was born on 26 August 1908.
• He obtained a Ph.D. in Ecology (1937) under Prof. W. H. Pearsall, FRS, from Leeds University, UK.
• He established ecology teaching and research in the Department of Botany, Banaras Hindu University, Varanasi.
• His research laid foundations for understanding tropical communities and their succession, and the environmental responses of plant populations.
• He also worked on productivity and nutrient cycling in tropical forest and grassland ecosystems.
• He formulated the first postgraduate course in ecology in India. Over 50 scholars obtained a Ph.D. under him and spread ecology teaching across the country.
• Honours: Fellowships of the Indian National Science Academy and the World Academy of Arts and Science, and the Sanjay Gandhi Award in Environment and Ecology.
• His efforts led the Government of India to set up the National Committee for Environmental Planning and Coordination (1972).
• This committee later paved the way for the Ministry of Environment and Forests (1984).

1.3 Levels of biological organisation

• The living world is diverse and complex. We study it at different levels of biological organisation.
• The levels, from small to large: macromolecules, cells, tissues, organs, individual organisms, populations, communities, ecosystems and biomes.
• At any level we can ask two types of questions.
• ★ Exam imp How-type questions seek the mechanism behind a process. Why-type questions seek its significance.

• Other questions of both types: Why are night-blooming flowers generally white? How does a bee know which flower has nectar?
• Why does a cactus have so many thorns? How does a chick recognise its own mother?

1.4 What ecology studies

• ★ Exam imp Ecology is basically concerned with four levels of biological organisation: organisms, populations, communities and biomes.
• This page studies ecology at the population level.

2. Population and Its Attributes

2.1 What is a population?

• In nature, isolated single individuals of a species are rare. Most live in groups.
• Interbreeding implies sexual reproduction. Yet a group produced by asexual reproduction is also treated as a population in ecological studies.
• An individual organism has to cope with a changed environment. But natural selection operates at the population level to evolve the desired traits.
• ★ Exam imp So population ecology is important: it links ecology to population genetics and evolution.

Examples to Remember

2.2 Attributes of a population

• A population has certain attributes that an individual organism does not have.
• ★ Exam imp In a population, birth rate and death rate are per capita rates: the change in numbers (increase or decrease) with respect to the members of the population.

• Birth rate example: a pond had 20 lotus plants last year. Reproduction added 8 new plants, so the population is now 28.
• Birth rate $=8/20=0.4$ offspring per lotus per year.
• Death rate example: in a laboratory population of 40 fruitflies, 4 died during a week.
• Death rate $=4/40=0.1$ individuals per fruitfly per week.
• Sex ratio: an individual is either male or female, but a population has a sex ratio, for example 60 per cent females and 40 per cent males.

2.3 Age distribution and age pyramids

• At any given time, a population has individuals of different ages.
• Age distribution: the per cent of individuals of a given age or age group.
• ★ Exam imp When the age distribution is plotted, the resulting structure is called an age pyramid.
• Human age pyramids generally show the age distribution of males and females in one diagram.
• The age groups shown are pre-reproductive, reproductive and post-reproductive.
• The shape of the pyramid reflects the growth status of the population: growing, stable or declining.

3. Population Size and Population Density

• The size of a population tells us a lot about its status in the habitat.
• Any ecological process is evaluated as a change in population size: the outcome of competition with another species, the impact of a predator, or the effect of a pesticide.
• ★ Exam imp Population size in nature may be as low as under 10 (Siberian cranes at Bharatpur wetlands in any year) or run into millions (Chlamydomonas in a pond).

• In the carrot grass example, calling the banyan's density 'low' would underestimate its enormous role in that community. Per cent cover or biomass shows that role better.
• We are mostly obliged to estimate population sizes indirectly, without actually counting or seeing the individuals.

4. Population Growth

4.1 The four processes that change density

• Population size is not static. It changes with time due to food availability, predation pressure and adverse weather.
• These changes show whether a population is flourishing or declining.
• ★ Exam imp Density fluctuates through four basic processes: two increase it (natality and immigration) and two decrease it (mortality and emigration).

• Natality ($B$): the number of births during a given period in the population that are added to the initial density.
• Mortality ($D$): the number of deaths in the population during a given period.
• Immigration ($I$): the number of individuals of the same species that have come into the habitat from elsewhere during the period.
• Emigration ($E$): the number of individuals of the population who left the habitat and went elsewhere during the period.

• If $N$ is the population density at time $t$, its density at time $t+1$ is:

• ★ Exam imp Density increases if births plus immigrants $(B+I)$ are more than deaths plus emigrants $(D+E)$.
• Under normal conditions, births and deaths are the most important factors influencing density.
• Immigration and emigration become important only under special conditions.
• For example, when a new habitat is just being colonised, immigration may contribute more to growth than birth rates.

5. Growth Models

• Does population growth follow a specific, predictable pattern? Unbridled human population growth in our country makes this question important.
• Studying whether animal populations show restraints on growth may teach us how to control population growth.

5.1 Exponential growth

• Availability of resources (food and space) is essential for the unimpeded growth of a population.
• When resources are unlimited, each species can fully realise its innate potential to grow in number. Darwin observed this while developing his theory of natural selection.
• ★ Exam imp The population then grows in an exponential or geometric fashion.

1. Let $b$ = per capita birth rate and $d$ = per capita death rate (rates, not total numbers).
2. The change in $N$ per unit time is $\frac{dN}{dt}=(b-d)\times N$.
3. Let $(b-d)=r$. Then $\frac{dN}{dt}=rN$.

Numbers to Remember

• $r$ for the Norway rat = $0.015$
• $r$ for the flour beetle = $0.12$
• $r$ for the human population in India in 1981 = $0.0205$
• $e$ (base of natural logarithms) = $2.71828$

• ★ Exam imp Plotting $N$ against time for exponential growth gives a J-shaped curve (Figure 3, curve a).
• Its integral form (derived by calculus) is:

• $N_t$ = population density after time $t$; $N_0$ = population density at time zero.
• $r$ = intrinsic rate of natural increase; $e$ = the base of natural logarithms ($2.71828$).
• A species growing exponentially with unlimited resources can reach enormous densities in a short time.
• Darwin showed that even a slow-growing animal like the elephant could reach enormous numbers in the absence of checks.

1. The chessboard story: a king, sure of winning a chess game, accepts any bet the minister proposes.
2. The minister asks for wheat grains: one grain on square 1, two on square 2, four on square 3, eight on square 4, doubling on each next square up to square 64.
3. The minister wins. By the time the king covers half the board, he realises that all the wheat of his kingdom cannot fill all 64 squares.
4. In the same way, one Paramecium doubling by binary fission every day would reach a mind-boggling number in 64 days, if food and space stayed unlimited.

5.2 Logistic growth

• No population in nature has unlimited resources for exponential growth.
• This leads to competition between individuals for limited resources. Eventually the 'fittest' individuals survive and reproduce.
• Many governments have realised this and introduced restraints to limit human population growth.

• A population growing in a habitat with limited resources passes through these phases:

1. Lag phase: growth is slow at first.
2. Acceleration: growth speeds up.
3. Deceleration: growth slows down as resources become limiting.
4. Asymptote: the population density reaches the carrying capacity, $K$.

• ★ Exam imp Plotting $N$ against time $t$ gives a sigmoid curve. This growth is called Verhulst-Pearl Logistic Growth (Figure 3, curve b).
• It is described by the equation:

• $N$ = population density at time $t$; $r$ = intrinsic rate of natural increase; $K$ = carrying capacity.
• Resources for most animal populations are finite and become limiting sooner or later. So the logistic model is considered more realistic.
• Activity: collect India's census figures for the last 100 years, plot them and check which growth pattern they show.

6. Life History Variation

• ★ Exam imp Populations evolve to maximise their reproductive fitness, also called Darwinian fitness (a high $r$ value), in their habitat.
• Under a particular set of selection pressures, organisms evolve towards the most efficient reproductive strategy.

Examples to Remember

• Which strategy maximises fitness? Ecologists suggest that life history traits evolved in relation to the constraints imposed by the abiotic and biotic components of the habitat.
• The evolution of life history traits in different species is currently an important area of research in ecology.

7. Pairs to Match and Numbers to Remember

Pairs to Match

Numbers to Remember

• Lotus pond: 20 plants, 8 new, now 28; birth rate $0.4$ offspring per lotus per year.
• Fruitflies: 4 of 40 died in a week; death rate $0.1$ individuals per fruitfly per week.
• Sex ratio example: 60 per cent females, 40 per cent males.
• Carrot grass example: 200 Parthenium plants against a single banyan tree.
• Chessboard: 64 squares; Paramecium: doubling every day for 64 days.
• Ramdeo Misra: born 1908, Ph.D. 1937, over 50 Ph.D. scholars; National Committee for Environmental Planning and Coordination 1972; Ministry of Environment and Forests 1984.

8. Quick Revision

• Ecology: interactions of organisms with each other and with the abiotic environment; levels: organisms, populations, communities, biomes.
• Ramdeo Misra (1908-1998): Father of Ecology in India; Ph.D. 1937 under Pearsall at Leeds; ecology at BHU; his efforts led to the 1972 national committee, which paved the way for the 1984 Ministry of Environment and Forests.
• How-type questions seek mechanism; why-type questions seek significance.
• Population: one species, a defined area, shared or competed-for resources, potential interbreeding; asexual groups count too.
• Natural selection operates at the population level; population ecology links ecology to population genetics and evolution.
• Population attributes: birth rate, death rate (per capita), sex ratio, age distribution.
• Age pyramids: expanding (broad base), stable, declining (narrow base).
• Density $N$: number, per cent cover, biomass, relative density (fish per trap) or indirect signs (pug marks, fecal pellets).
• Natality and immigration raise density; mortality and emigration lower it: $N_{t+1}=N_t+[(B+I)-(D+E)]$.
• Exponential: $dN/dt=rN$, $N_t=N_0{e}^{rt}$, J-shaped, unlimited resources.
• $r=b-d$: intrinsic rate of natural increase (Norway rat $0.015$, flour beetle $0.12$, India 1981 $0.0205$).
• Logistic (Verhulst-Pearl): $dN/dt=rN(K-N)/K$; sigmoid; lag, acceleration, deceleration, asymptote at $K$; more realistic.
• Life history: breed once (Pacific salmon, bamboo) or many times (most birds and mammals); many small (oysters, pelagic fishes) or few large offspring (birds, mammals).

9. Solved Examples

Common Mistakes to Avoid