This MCQ module is based on: Population Attributes
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Population Attributes
🎓 Class 12
Biology
CBSE
Theory
Ch 11 – Organisms and Populations
⏱ ~14 min
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Organisms and Populations - Population Attributes
Our living world is fascinatingly diverse and amazingly complex. We try to understand its complexity by investigating processes at various levels of biological organisation — macromolecules, cells, tissues, organs, individual organisms, populations, communities, ecosystems and biomes. This chapter works at one of those levels: the population.
Two kinds of question
At any level of biological organisation we can ask two types of question. When we hear the bulbul singing early in the morning in the garden, we may ask ‘How does the bird sing?’ or ‘Why does the bird sing?’
The how-type questions seek the mechanism behind a process. The why-type questions seek the significance of the process. For the bulbul, the answer to the first might be in terms of the operation of the voice box and the vibrating bone in the bird; the answer to the second may lie in the bird's need to communicate with its mate during the breeding season.
Observe nature with a scientific frame of mind and both kinds of question will come. Why are night-blooming flowers generally white? How does the bee know which flower has nectar? Why does a cactus have so many thorns? How does the chick recognise her own mother? Neither kind of question is more scientific than the other — and a complete explanation in biology usually needs both.
What ecology is
Ecology is a subject which studies the interactions among organisms, and between an organism and its physical — that is, abiotic — environment.
It is basically concerned with four levels of biological organisation: organisms, populations, communities and biomes. In this chapter we explore ecology at the population level.
What a population is
In nature, we rarely find isolated, single individuals of any species. The 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 examples of a population.
Why the population, and not the individual, is the unit that matters here. 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.
Population attributes
A population has certain attributes whereas an individual organism does not. This is the central idea of this section, and it is worth holding onto: an individual may have births and deaths, but a population has birth rates and death rates.
Birth rate and death rate
In a population these rates refer to per capita births and deaths. The rates express change in numbers — increase or decrease — with respect to the members of the population.
📐 Activity: calculate the two rates
Two situations.
(a) In a pond there were 20 lotus plants last year, and through reproduction 8 new plants are added, taking the current population to 28.
(b) In a laboratory population of 40 fruitflies, 4 individuals died during one week.
Predict: what is the birth rate in (a) and the death rate in (b)? Take care with the units.
(a) Birth rate = 8/20 = 0.4 offspring per lotus per year. Note that the divisor is the initial population of 20, not the current 28. The rate measures what the existing individuals produced.
(b) Death rate = 4/40 = 0.1 individuals per fruitfly per week.
Two things to notice about the units. First, both are per capita — per lotus, per fruitfly — which is what makes them rates rather than counts, and what lets you compare a pond of 20 plants with a lake of 20,000. Second, both carry a time period — per year, per week. A rate quoted without a period means nothing at all.
Sex ratio
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 — for example, 60 per cent of the population are females and 40 per cent males.
Age distribution and the age pyramid
A population at any given time is composed of individuals of different ages. If the age distribution — the per cent of individuals of a given age or age group — is plotted for the population, the resulting structure is called an age pyramid. For human populations, age pyramids generally show the age distribution of males and females in a single diagram.
The shape of the pyramid reflects the growth status of the population: whether it is (a) growing, (b) stable, or (c) declining.
Population size, and how to measure it
The size of a population tells us a lot about its status in the habitat. Whatever ecological process we wish to investigate — the outcome of competition with another species, the impact of a predator, or the effect of a pesticide application — we always evaluate it in terms of any change in the population size.
Size in nature could be as low as fewer than 10 — Siberian cranes at Bharatpur wetlands in any given year — or go into millions, as with Chlamydomonas in a pond.
Population size, technically called population density and designated N, need not necessarily be measured in numbers only. Although total number is generally the most appropriate measure, in some cases it is either meaningless or difficult to determine.
When total number is meaningless
In an area, suppose there are 200 carrot grass (Parthenium hysterophorus) plants but only a single huge banyan tree with a large canopy. Stating that the population density of banyan is low relative to that of carrot grass amounts to underestimating the enormous role of the banyan in that community. In such cases, per cent cover or biomass is a more meaningful measure of population size.
When counting is impractical
Total number is also not an easily adoptable measure if the population is huge and counting is impossible or very time-consuming. If you have a dense laboratory culture of bacteria in a petri dish, the sensible measure is not a head count but something like biomass, optical density, or colony-forming units per unit volume.
Sometimes, for certain ecological investigations, there is no need to know the absolute population densities — relative densities serve the purpose equally well. For instance, the number of fish caught per trap is a good enough measure of the total population density in a lake.
In fact we are mostly obliged to estimate population sizes indirectly, without actually counting or seeing the individuals. The tiger census in our national parks and tiger reserves is often based on pug marks and fecal pellets.
| Situation | Best measure | Why |
|---|---|---|
| A countable population of similar-sized individuals | Total number | Generally the most appropriate measure |
| One banyan among 200 carrot grass plants | Per cent cover or biomass | Numbers would underestimate the enormous role of the banyan in the community |
| A dense bacterial culture in a petri dish | Biomass or an indirect index | Counting is impossible or very time-consuming |
| Fish in a lake | Number caught per trap — a relative density | Absolute density is not needed for many investigations |
| Tigers in a reserve | Indirect estimate from pug marks and fecal pellets | The animals are rarely seen; density must be inferred |
🎯 Interactive: Which Attribute, Which Measure?
Six ecological situations. Choose one and see which population attribute or measure applies.
🎯 Competency-Based Questions
Q1. List the attributes that populations possess but individuals do not, and explain in each case why the attribute is meaningless for a single organism.
(i) Birth rate and death rate. An individual may have births and deaths, but a population has birth rates and death rates. These are per capita figures — the change in numbers with respect to the members of the population — so they require a number of individuals to divide by. A single lotus plant either produces offspring or does not; the figure 0.4 offspring per lotus per year describes a group.
(ii) Sex ratio. An individual is either a male or a female. A ratio such as 60 per cent females and 40 per cent males can exist only across a collection of individuals.
(iii) Age distribution. An individual has one age at a time. A distribution is the per cent of individuals in each age or age group, plotted as an age pyramid, whose shape shows whether the population is growing, stable or declining.
(iv) Population density (N). Density is a property of a group in an area — measured as total number, per cent cover, biomass or a relative index. A single organism has no density.
The general principle. Each of these is a statistical property: it emerges only from many individuals considered together. This is why natural selection, although it acts on individuals, is studied at the population level — and why population ecology links ecology to population genetics and evolution.
Q2. Distinguish 'how-type' from 'why-type' questions in biology, using the cactus's thorns as your example, and explain why a complete account needs both.
The distinction. The how-type questions seek the mechanism behind a process; the why-type questions seek the significance of the process.
Applied to the cactus. How does a cactus have thorns? The answer is developmental and anatomical: the thorns are modified leaves, formed from leaf primordia whose growth is arrested and whose cells become sclerified rather than expanding into a blade. Why does a cactus have so many thorns? The answer is ecological and evolutionary: thorns are the most common morphological means of defence against herbivores, and a plant in a dry habitat that cannot afford to lose tissue or water benefits doubly, since reduced leaf area also reduces transpiration.
Why both are needed. The mechanism without the significance leaves the fact arbitrary — you would know how a thorn is built but not why any plant should build one. The significance without the mechanism leaves the fact unexplained — you would know what thorns are for but not how a leaf becomes one, nor how the trait could change. Biology is unusual among sciences in having both kinds of answer available for the same observation, because living things have a history as well as a machinery.
The chapter's other examples work the same way: why are night-blooming flowers generally white (visibility to nocturnal pollinators) versus how does a bee know which flower has nectar (scent and visual cues and learning).
Q3. Ecologists studying the effect of a pesticide on a pest population report their result as a change in population density rather than as the number of insects killed. Explain why, and explain how they might measure density in this case.
Why density is the right measure. Whatever ecological process we wish to investigate in a population — the outcome of competition with another species, the impact of a predator, or the effect of a pesticide application — we always evaluate it in terms of any change in population size. The number killed is not the outcome that matters: a pesticide that kills a million insects has failed if the survivors reproduce back to the same density within a fortnight, and a small kill may matter greatly if it holds the population below the level at which it damages the crop. Density also allows before-and-after comparison, and comparison between fields of different size.
How they would measure it. Rarely by counting every individual. Population size need not necessarily be measured in numbers only, and we are mostly obliged to estimate population sizes indirectly, without actually counting or seeing them. Reasonable choices here would be a relative density — the number of insects caught per trap per night, on the same principle as the number of fish caught per trap in a lake — or counts on a fixed number of sampled plants, or the proportion of plants showing damage.
What must be controlled. Density changes anyway, because of natality, mortality, immigration and emigration, so an untreated control area is essential. Otherwise a decline caused by weather or by a natural predator would be credited to the pesticide.
Q4. Two age pyramids are drawn for the same country, fifty years apart. The first has a very broad base and narrow top; the second has nearly vertical sides. What has changed, and what would you predict for the next fifty years?
What the shapes mean. The shape of an age pyramid reflects the growth status of the population. A very broad base with a narrow top means a large proportion of young individuals and relatively few old ones — a growing population. Nearly vertical sides mean each age group is of similar size — a stable population.
What has changed. Two things must have happened together. The birth rate has fallen, which narrows the base; and survival has improved, which fills out the middle and upper age groups so that individuals now live to occupy them. A stable pyramid is not a population that has stopped changing — it is one in which births and deaths have come into balance.
The prediction for the next fifty years. The population will roughly hold its size, but its composition will keep shifting: the large cohorts that were young in the first pyramid move upwards, so the proportion of older individuals rises. If the birth rate continues to fall, the base narrows further and the pyramid begins to take the declining form, with a narrow base and a wide middle.
The caution to add. An age pyramid describes only natality, mortality and age structure. Immigration and emigration also change density, and in a human population they can change it faster than births do — so a pyramid forecasts, it does not determine.
Q5. In each of the following, say which measure of population density you would use and defend the choice: (a) banyan trees and carrot grass in the same plot; (b) bacteria in a petri dish; (c) fish in a lake; (d) tigers in a reserve.
(a) Banyan and carrot grass: per cent cover or biomass. If 200 carrot grass plants stand against a single huge banyan with a large canopy, saying the banyan's density is low relative to the carrot grass amounts to underestimating the enormous role of the banyan in that community. Number treats a seedling and a century-old tree as equal; cover and biomass do not.
(b) Bacteria in a petri dish: biomass or an indirect index. Total number is not an easily adoptable measure if the population is huge and counting is impossible or very time-consuming. Biomass, turbidity or colony-forming units per unit volume all work, and all scale with the quantity that matters.
(c) Fish in a lake: a relative density — the number caught per trap. For certain ecological investigations there is no need to know the absolute densities; relative densities serve the purpose equally well. If the trapping effort is kept constant, catch per trap tracks the real density faithfully, and that is enough to detect change.
(d) Tigers in a reserve: an indirect estimate from pug marks and fecal pellets. The tiger census in our national parks and tiger reserves is often based on exactly this. The animals are few, secretive and dangerous to approach, so direct counting is not an option; the signs they leave are.
The principle behind all four. Population density is a question about the ecological weight of a species in its habitat, and the appropriate unit is whatever best reflects that weight — number, biomass, cover, or an index — not whatever is easiest to state.
🧠 Assertion–Reason Questions
For each pair, decide whether both statements are true and whether the reason correctly explains the assertion.
Assertion (A): Population ecology is an important area of ecology.
Reason (R): Natural selection operates at the population level to evolve the desired traits, even though it is the individual that must cope with a changed environment.
Both A and R are true, and R is the correct explanation of A.
This is precisely why population ecology links ecology to population genetics and evolution. Selection acts on individuals but its result — a change in the frequency of traits — can exist only in a population.
Assertion (A): A group of organisms produced by asexual reproduction can be treated as a population.
Reason (R): For the purpose of ecological studies, such a group is generally considered a population even though interbreeding implies sexual reproduction.
Both A and R are true, and R is the correct explanation of A.
The definition of a population — individuals of a species in a well defined geographical area, sharing or competing for similar resources and potentially interbreeding — is applied flexibly in ecology. Bacteria in a culture plate are a population in every sense that matters for studying density, growth and interactions.
Assertion (A): Total number is always the best measure of population density.
Reason (R): Total number is generally the most appropriate measure of population density.
A is false but R is true.
Total number is generally the most appropriate measure — but in some cases it is either meaningless or difficult to determine. One banyan among 200 carrot grass plants is better described by per cent cover or biomass, a dense bacterial culture cannot practicably be counted, and tigers are estimated indirectly from pug marks and fecal pellets. 'Generally' is not 'always'.
Frequently Asked Questions - Population Attributes
What is ecology and what levels of organisation does it deal with?
Ecology is the subject which studies the interactions among organisms and between an organism and its physical, abiotic environment. It is basically concerned with four levels of biological organisation: organisms, populations, communities and biomes.
What is the difference between how-type and why-type questions in biology?
The how-type questions seek the mechanism behind a process, while the why-type questions seek the significance of the process. For a singing bulbul, the how-type answer lies in the operation of the voice box and the vibrating bone, while the why-type answer may lie in the bird's need to communicate with its mate during the breeding season.
What is a population?
A population is a group of individuals of a species living in a well defined geographical area, sharing or competing for similar resources and potentially interbreeding. Although interbreeding implies sexual reproduction, a group of individuals resulting from asexual reproduction is also generally considered a population for the purpose of ecological studies. 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 examples.
What attributes does a population have that an individual does not?
Birth rates and death rates, sex ratio, and age distribution. An individual may have births and deaths, but a population has per capita birth rates and death rates. An individual is either male or female, but a population has a sex ratio. And an individual has one age, while a population has an age distribution which can be plotted as an age pyramid.
How are birth rate and death rate calculated?
Both are per capita rates for a stated period. If a pond had 20 lotus plants and 8 new plants were added, the birth rate is 8 divided by 20, that is 0.4 offspring per lotus per year. If 4 individuals in a laboratory population of 40 fruitflies died during a week, the death rate is 4 divided by 40, that is 0.1 individuals per fruitfly per week.
What is an age pyramid and what does its shape indicate?
If the age distribution, that is the per cent of individuals of a given age or age group, is plotted for a population, the resulting structure is called an age pyramid. For human populations it shows the age distribution of males and females together. The shape of the pyramid reflects the growth status of the population, that is whether it is growing, stable or declining.
What is population density and is it always measured in numbers?
Population density, designated N, is the technical term for population size. It need not necessarily be measured in numbers only. Although total number is generally the most appropriate measure, in some cases it is either meaningless or difficult to determine, and then per cent cover or biomass is more meaningful.
Why is per cent cover sometimes better than total number?
Because individuals may differ enormously in ecological importance. If an area has 200 carrot grass plants but only a single huge banyan tree with a large canopy, stating that the population density of banyan is low relative to that of carrot grass amounts to underestimating the enormous role of the banyan in that community.
What is relative density and when is it used?
Relative density is a measure that reflects the real density without giving an absolute figure. For certain ecological investigations there is no need to know the absolute population densities, and relative densities serve the purpose equally well. For instance, the number of fish caught per trap is a good enough measure of the total population density in a lake.
How is the tiger population estimated in India?
Indirectly. We are mostly obliged to estimate population sizes without actually counting or seeing the individuals, and the tiger census in our national parks and tiger reserves is often based on pug marks and fecal pellets.
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