UPSC Prelims 2011 · CSAT (Paper II): Consider the following assumptions :1. The food chains/food web in an ecosystem are influenced…
Read the passage
A species that exerts an influence out of proportion to its abundance in an ecosystem is called a keystone species. The keystone species may influence both the species richness of communities and the flow of energy and materials through ecosystems. The sea star Pisaster ochraceus, which lives in rocky intertidal ecosystems on the Pacific coast of North America, is also an example of a keystone species. Its preferred prey is the mussel Mytilus californianus. In the absence of sea stars, these mussels crowd out other competitors in a broad belt of the intertidal zone. By consuming mussels, sea star creates bare spaces that are taken over by a variety of other species.
A study at the University of Washington demonstrated the influence of Pisaster on species richness by removing sea stars from selected parts of the intertidal zone repeatedly over a period of five years. Two major changes occurred in the areas from which sea stars were removed. First, the lower edge of the mussel bed extended farther down into the intertidal zone, showing that sea stars are able to eliminate mussels completely where they are covered with water most of the time. Second, and more dramatically, 28 species of animals and algae disappeared from the sea star removal zone. Eventually only Mytilus, the dominant competitor, occupied the entire substratum. Through its effect on competitive relationships, predation by Pisaster largely determines which species live in these rocky intertidal ecosystems.
Consider the following assumptions :
- 1. The food chains/food web in an ecosystem are influenced by keystone species.
- 2. The presence of keystone species is a specific characteristic of aquatic ecosystems.
- 3. If the keystone species is completely removed from an ecosystem, it will lead to the collapse of the ecosystem.
With reference to the passage, which of the above assumptions is/are valid ?
Answer (c) & explanation
Assumptions 1 and 3 are valid per UPSC's key (c). Step 1: the passage says predation by the sea star 'largely determines which species live' there, so keystone species shape the food web: assumption 1. Step 2: when sea stars were removed, 28 species vanished and only mussels remained, a collapse of the community: UPSC accepts assumption 3, though 'collapse of the ecosystem' slightly overstates a single study. Step 3: assumption 2 is wrong: one marine example cannot show that keystone species are found only in aquatic ecosystems. So (a), (b) and (d) fail.
Tip: one example never proves 'specific to' or 'only'. Generalising from a single case is the trap.
Answer as per the official UPSC answer key.
Related questions from other years
Question : X receives three coins of different denominations : 1, 2, 5, 10 and 20. If the total amount received by X is m, does X receive a coin of denomination 5?
Statement I : m is not a prime number.
Statement II : The sum of the digits of m is greater than 5.
Answer (a) & explanation
Statement II alone is enough; Statement I alone is not (a). Step 1: three different coins from 1, 2, 5, 10, 20 give ten totals. With a 5: 8, 16, 17, 26, 27, 35. Without a 5: 13, 23, 31, 32. Step 2: Statement I (m not prime): 8, 16, 26, 27, 35 have a 5, but 32 (2 + 10 + 20) does not. Mixed answers, so I fails. Step 3: Statement II (digit sum above 5): 8, 16, 17, 26, 27, 35 qualify, and every one has a 5. (13, 23, 31, 32 have digit sums 4, 5, 4, 5.) So II answers "yes". Options (b), (c) and (d) fail.
Tip: when cases are few, list them all. Then check each statement against the full list.
don't assume both statements are needed. Listing all ten cases takes thirty seconds and settles it.
Answer as per the official UPSC answer key.
Read the passage
How is deflation done? Most countries use a method called ‘double deflation’, where input and output prices are deflated separately. Consider a manufacturer importing oil for use in production. If oil prices fall, output prices do not and quantities remain the same, real value added should not change. But if the same deflator is used for inputs and outputs, as in India, it would look as if the manufacturer had become more productive.
Which of the following assumptions is/are valid?
- 1. Deflation strategies can be used to make manufacturers appear to be doing better than they actually are.
- 2. When input and output prices are both deflated against a single input price, it is referred to as ‘double deflation’.
Select the answer using the code given below.
Answer (a) & explanation
Only assumption 1 is valid (a). Step 1: the passage says that using one deflator for inputs and outputs, as in India, makes it "look as if the manufacturer had become more productive". So deflation methods can make firms appear better than they are (1). Step 2: assumption 2 says double deflation means deflating both against a single price. But the passage defines double deflation as deflating inputs and outputs separately. So 2 reverses the definition. Step 3: options (b), (c) and (d) fail.
Tip: check definitions word for word against the passage. Deflating means removing the effect of price changes to find real growth, as in real GDP.
Answer as per the official UPSC answer key.
P, Q, R, S and T are ranked 1 to 5 (not necessarily in that order). The rank of P is 4, the rank of Q is not 5, the rank of R is 1, the rank of S is not 2, the rank of T is not 3. Then which of the following is/are correct?
- I. If the rank of S is 3, then that of T is 2.
- II. If the rank of Q is 3, then that of T is 5.
Select the answer using the code given below.
Answer (d) & explanation
Neither statement is correct (d). Step 1: P = 4 and R = 1 are fixed. Ranks 2, 3 and 5 remain for Q, S and T, with rules: Q is not 5, S is not 2, T is not 3. Step 2: statement I: if S = 3, then Q and T share 2 and 5. Q cannot be 5, so Q = 2 and T = 5, not 2. I is wrong. Step 3: statement II: if Q = 3, then S and T share 2 and 5. S cannot be 2, so S = 5 and T = 2, not 5. II is wrong. Options (a), (b) and (c) fail.
Tip: fix the known ranks first, then apply each "if" condition and let the "not" rules decide the rest. Write the small table on paper.
Answer as per the official UPSC answer key.
How many times does 5 appear in all two-digit positive integers?
Answer (b) & explanation
The digit 5 appears 19 times (b). Step 1: 5 in the tens place: 50, 51, …, 59 = 10 times. Step 2: 5 in the units place: 15, 25, 35, 45, 55, 65, 75, 85, 95 = 9 times (05 is not a two-digit number). Step 3: total = 10 + 9 = 19. The number 55 has two 5s, and both are counted. Option (a), 18, counts 55 once. Options (c) and (d) wrongly add 05 or 5.
Tip: count place by place. The question asks how many times the digit appears, not how many numbers contain it (that would be 18).
Answer as per the official UPSC answer key.