What is Alpha, Beta and Gamma Diversity ?
(a) Richness of species
(b) Simpson's Diversity Index
(c) For measuring biodiversity over geographical scale
(d) Whittaker (1972) suggested these terms
Which

Question

What is Alpha, Beta and Gamma Diversity ?
(a) Richness of species
(b) Simpson's Diversity Index
(c) For measuring biodiversity over geographical scale
(d) Whittaker (1972) suggested these terms
Which of the statements given above is/are correct ?

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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.  (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.  (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.

Concept: 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.

State PCS 2022 Q18 — Environment & Ecology PYQ with Answer

State PCS 2022 Q18 — Environment & Ecology PYQ with Answer