While studying vegetation of an area, terms like 'population' and 'community' are often used. Which one of the following statements best describes a population?
- (a)A group of organisms of one species, living in the same area at the same time.
- (b)A group of organisms of one species living in different areas during different seasons.
- (c)A unit consisting of biotic and abiotic components.
- (d)A group of organisms of more than one species, living in the same area at the same time.
Correct — A, A group of organisms of one species, living in the same area at the same time. A population is a group of individuals of a given species sharing or competing for similar resources in a defined geographical area — that is the standard textbook definition, and every element of it appears in option (a). One species is the crucial restriction, because the moment more than one species is involved the correct word is community. Sharing a defined area matters, since scattered individuals of the same species on different continents do not compete for the same resources. Being there at the same time matters too: teak trees standing in a forest tract today and teak trees that stood there two centuries ago are not one population. The textbook's own examples make the point concrete — all the cormorants in a wetland, the rats in an abandoned dwelling, the teakwood trees in a forest tract, bacteria in a culture plate, lotus plants in a pond. Each is one species, one place, one time.
- (b)A group of organisms of one species living in different areas during different seasons. — Breaks both the space and the time conditions at once. Individuals scattered across different areas at different seasons are not sharing or competing for the same resources, which is what makes a population a unit.
- (c)A unit consisting of biotic and abiotic components. — That is an ecosystem — a biological community together with the non-living surroundings it interacts with. A population is purely a set of organisms.
- (d)A group of organisms of more than one species, living in the same area at the same time. — The definition of a community. It differs from the right answer by exactly one clause, which is why it is the option most often chosen by mistake.
Ecology is studied at four levels of organisation — organisms, populations, communities and biomes. A population is one species in one place at one time; a community is all the populations of different species sharing that place; an ecosystem adds the abiotic environment; a biome is a large regional unit defined by climate and dominant vegetation. Populations have attributes that individuals do not, such as birth rates, death rates, sex ratio and age distribution, and it is at the population level that natural selection operates.
The four options form a deliberate ladder, and reading them as a ladder is the fastest route through. Option (a) restricts to one species, one area, one time. Option (d) relaxes the species restriction and becomes a community. Option (c) relaxes it further and adds the non-living surroundings, giving an ecosystem. Option (b) keeps one species but throws away the shared place and time, which destroys the very thing a population is for. Candidates who go wrong here almost always pick (d), because in ordinary speech 'population' suggests any large collection of living things. Holding on to one worked example is enough insurance: all the lotus plants in a single pond are a population, while the lotus plants, fish, insects and bacteria of that pond together are a community.
- A population is a group of individuals of a given species sharing or competing for similar resources in a defined geographical area.
- Textbook examples include 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.
- Populations have attributes individuals lack — birth rates, death rates, sex ratio and age distribution — and age structure is often drawn as an age pyramid.
- A community is made up of the populations of all the species living in an area; adding the abiotic surroundings gives an ecosystem.
- Natural selection operates at the population level, which is why population ecology links ecology to evolution.
- Sliding from population to community by dropping the one-species restriction.
- Confusing a population with an ecosystem, which includes the non-living surroundings.
- Ignoring the time clause — individuals of one species in one place but in different seasons or centuries are not a single population.
As a best-description item like this one, as a level-of-organisation match, or as an application question asking whether a stated group of organisms counts as a population or a community.
Which one of the following is the best description of the term ‘ecosystem’?
- (a) A community of organisms interacting with one another
- (b) That part of the Earth which is inhabited by living organisms
- (c) A community of organisms together with the environment in which they live
- (d) The flora and fauna of a geographical area
Answer(c) A community of organisms together with the environment in which they live
The next rung of the same ladder, tested the same way. Its correct option is almost word for word this question's option (c), which is offered here as a distractor — proof that the levels are being examined by their exact wording.
Which one of the following terms describes not only the physical space occupied by an organism, but also its functional role in the community of organisms?
- (a) Ecotone
- (b) Ecological niche
- (c) Habitat
- (d) Home range
Answer(b) Ecological niche
The same family of definitions, sorted on a different axis. Where population and community are counted by how many species are present, habitat and niche are separated by whether the address or the occupation is being described.
- practice — not a real PYQ
All the fish, insects, plants and bacteria living in a single pond together constitute
- (a)a population
- (b)a community
- (c)a biome
- (d)a niche
Answer(b) a community — more than one species sharing the same area at the same time; a single species there would be a population.
- practice — not a real PYQ
Which one of the following is an attribute of a population but not of an individual organism?
- (a)Birth
- (b)Death
- (c)Sex ratio
- (d)Growth in size
Answer(c) Sex ratio — an individual may be born or die, but only a population has birth rates, death rates and a sex ratio.