MPSC 2022 Prelims Paper-I · Question 18 of 100

Correct — option (3). The stem asks what alpha, beta and gamma diversity are, and statement (c) gives the answer: they are the means by which biodiversity is measured across geographical scale. That is the whole point of the three terms. Alpha diversity is the diversity found within a single community or habitat — the number of species present at one site and how evenly the individuals are distributed among them. Beta diversity is not a count at all but a comparison: it measures how much the species composition changes as one moves from one habitat to the next along an environmental gradient, so a hillside on which the vegetation turns over completely between the foot and the ridge has high beta diversity while a uniform stretch of plain has low beta diversity. Gamma diversity is the total diversity of the whole landscape or region taken together, the several communities and the turnover between them combined. Held that way, the three form a single nested scheme: alpha is diversity within a place, beta is diversity between places, and gamma is diversity across the region that contains them, and the reason ecologists need all three is that a region can be rich in species while each of its habitats is poor, or the reverse. So the three terms are a framework for measuring diversity at different spatial scales, which is exactly what statement (c) says, and the key takes that statement alone. Statement (d), which records who proposed the terms, is a note on their authorship rather than an account of what they are — Whittaker's name is genuinely attached to these measures, and his paper on the evolution and measurement of species diversity is the standard citation for them — but it does not answer the question the stem asks. Option (3), which names statement (c) and nothing else, is the answer.

• (1)Only (a) and (b) — This row accepts statements (a) and (b), and neither describes what alpha, beta and gamma diversity are. Statement (a) offers richness of species, which is a narrower idea altogether: species richness is simply the number of species present, one of the two components of diversity, the other being evenness, and it is a quantity that alpha diversity may be measured by rather than a definition of the three-part scheme. Answering that alpha, beta and gamma diversity are richness of species leaves out the entire spatial dimension that the terms exist to capture, and in particular leaves beta diversity, which is a measure of turnover between habitats rather than a count within one, without any meaning at all. Statement (b) fails for a separate reason set out below.
• (2)Only (b) — This row accepts only statement (b), which names Simpson's Diversity Index. That index is a real and important measure but it is a different thing from the scheme the question asks about. Simpson's index expresses the probability that two individuals drawn at random from a community belong to the same species, so it is a measure of dominance, or in its complementary form of diversity, within a single community; the Shannon index is the other measure commonly used for the same purpose. Both are ways of putting a number on alpha diversity at one site. Neither is a name for the alpha, beta and gamma framework, which is a matter of the scale at which diversity is assessed rather than of the formula used to assess it. The row therefore confuses an index with the scheme within which such an index is applied.
• (4)Only (c) and (d) — This row adds statement (d) to statement (c), and the key takes statement (c) alone. The stem asks what alpha, beta and gamma diversity are, and statement (d) does not answer that question: it records who proposed the terms, which is a fact about the history of the concept rather than about its content. The distinction is worth taking seriously as a matter of examination technique, because a set of statements attached to a what-is stem will often include one that is a true remark about the topic without being an answer to the question asked, and the row that gathers up every true-sounding statement is exactly the row such an item is built to catch. Read the stem first, decide what kind of answer it calls for, and admit only the statements that supply that kind of answer.

Biodiversity is measured at three scales, and the three measures answer different questions. Alpha diversity describes a single community: how many species live there and how evenly the individuals are shared among them, which is what indices such as Simpson's and Shannon's are constructed to summarise. Beta diversity describes the difference between communities: it is the rate at which species composition changes along a gradient of altitude, moisture, soil or disturbance, and it is high where habitats are sharply distinct and low where the same species run on from one to the next. Gamma diversity describes the whole region: the total number of species in a landscape containing many communities, which depends on both the richness of the individual communities and the turnover between them. The practical value of the scheme is that it separates two ways in which a region can be diverse. A tract of the Western Ghats where each valley carries its own endemic species has high beta diversity, so its gamma diversity is far greater than any single site would suggest; a uniform plantation may have moderate alpha diversity at every site and almost no beta diversity, so the whole region holds little more than one site does. The distinction matters for conservation, because protecting one large area serves a region whose diversity is mostly alpha, while a region whose diversity is mostly beta needs a network of sites spread across its gradients.

Ecology questions of this kind test whether a candidate has understood a framework or merely collected the words that belong to it, and MPSC builds the option set accordingly: it mixes a genuine but different concept, an index, a correct statement of the framework and a remark about its authorship, and offers rows that combine them in various ways. Two disciplines get a candidate through. The first is to read the stem for the kind of answer it demands — this one asks what the three terms are, so only a statement that says what they are can qualify, and a statement about who coined them, however accurate, is answering a different question. The second is to keep an index apart from the property it measures: Simpson's and Shannon's indices are ways of putting a number on diversity within a community, while alpha, beta and gamma name the scales at which diversity is considered, and confusing the two is the commonest error in this block. The topic itself is worth preparing carefully, because biodiversity measurement connects to conservation planning, to the Western Ghats and to the environment sections that now occupy a large share of the paper.

• Alpha, beta and gamma diversity are a framework for measuring biodiversity at different spatial scales, from a single habitat through the differences between habitats to a whole region.
• Alpha diversity is the diversity within one community or habitat, comprising the number of species present and the evenness with which individuals are distributed among them.
• Beta diversity measures the change in species composition between communities along an environmental gradient, so it is high where habitats are sharply distinct and low where they share their species.
• Gamma diversity is the total diversity of a landscape or region containing several communities, and it depends both on the richness of each community and on the turnover between them.
• Simpson's index and the Shannon index are measures of diversity within a single community; Simpson's index expresses the probability that two individuals drawn at random belong to the same species.

• Confusing an index of diversity, such as Simpson's, with the scheme of scales within which such an index is applied
• Reducing the three-part framework to species richness, which leaves beta diversity, a measure of turnover rather than a count, without any meaning
• Gathering every true-sounding statement into the answer, when a what-is stem admits only statements that say what the thing is
• Treating beta diversity as a count of species in a second habitat, when it measures the degree of change in composition between habitats

Biodiversity measurement appears in MPSC papers as statement questions of this kind, as definition questions asking what beta diversity means, and as application questions asking which region would show high beta diversity or why a network of small reserves may protect more species than one large one. The Commission tends to place the correct statement among others that are true of the topic but do not answer the stem, so the format punishes a candidate who reads for subject matter rather than for the question. The related terms — species richness, evenness, the Simpson and Shannon indices, and the three levels of biodiversity at genetic, species and ecosystem scale — are examined in the same block and are worth learning as one set, since the wrong options in any one of these questions are usually drawn from the others.

No directly related past PYQ was found.

• practice — not a real PYQ

  Beta diversity refers to

  • (a)the number of species within a single community
  • (b)the change in species composition between communities along an environmental gradient
  • (c)the total number of species in a large geographical region
  • (d)the number of individuals of the commonest species in a habitat

  Answer(b) The change in species composition between communities along an environmental gradient — beta diversity is a measure of turnover rather than a count, and it is high where neighbouring habitats share few species. Diversity within a single community is alpha diversity, the total for a region containing many communities is gamma diversity, and the abundance of the commonest species is a matter of dominance rather than of any of the three.

• practice — not a real PYQ

  Simpson's index is used in ecology to measure

  • (a)the rate of turnover of species between habitats
  • (b)the diversity of species within a community
  • (c)the area required for a viable protected area
  • (d)the rate of extinction in a region

  Answer(b) The diversity of species within a community — Simpson's index expresses the probability that two individuals drawn at random from the community belong to the same species, so it summarises richness and evenness at a single site, as the Shannon index also does. Turnover between habitats is beta diversity, which is a comparison rather than an index applied to one community.