In the Sonoran Desert of Northwestern Mexico and Southern Arizona GMAT Reading Comprehension

Reading Passage Question

In the Sonoran Desert of northwestern Mexico and southern Arizona, the flowers of several species of columnar cacti—cardon, saguaro, and organ pipe—were once exclusively pollinated at night by

(5) nectar-feeding bats, as their close relatives in arid tropical regions of southern Mexico still are. In these tropical regions, diurnal (daytime) visitors to columnar cactus flowers are ineffective pollinators because, by sunrise, the flowers' stigmas become unreceptive

(10) or the flowers close. Yet the flowers of the Sonoran Desert cacti have evolved to remain open after sunrise, allowing pollination by such diurnal visitors as bees and birds. Why have these cacti expanded their range of pollinators by remaining open and receptive in daylight?

(15)This development at the northernmost range of columnar cacti may be due to a yearly variation in the abundance—and hence the reliability—of migratory nectar-feeding bats. Pollinators can be unreliable for several reasons. They can be dietary generalists

(20) whose fidelity to a particular species depends on the availability of alternative food sources. Or, they can be dietary specialists, but their abundance may vary widely from year to year, resulting in variable pollination of their preferred food species. Finally, they

(25)may be dietary specialists, but their abundance may be chronically low relative to the availability of flowers. Recent data reveals that during spring in the Sonoran Desert, the nectar-feeding bats are specialists feeding on cardon, saguaro, and

(30)organpipe flowers. However, whereas cactus-flower abundance tends to be high during spring, bat population densities tend to be low except near maternity roosts. Moreover, in spring, diurnal cactus-pollinating birds are significantly more abundant in

(35)this region than are the nocturnal bats. Thus, with bats being unreliable cactus-flower pollinators, and daytime pollinators more abundant and therefore more reliable, selection favors the cactus flowers with traits that increase their range of pollinators. While data suggest

(40) that population densities of nectar-feeding bats are also low in tropical areas of southern Mexico, where bats are the exclusive pollinators of many species of columnar cacti, cactus-flower density and bat population density appear to be much more evenly

(45)balanced there: compared with the Sonoran Desert's cardon and saguaro, columnar cacti in southern Mexico produce far fewer flowers per night. Accordingly, despite their low population density, bats are able to
pollinate nearly 100 percent of the available flowers.

“In the Sonoran Desert of Northwestern Mexico and Southern Arizona GMAT Reading Comprehension” - is a GMAT reading comprehension passage with answers. Candidates need a strong knowledge of English GMAT reading comprehension.
This GMAT Reading Comprehension consists of 3 comprehension questions. The GMAT Reading Comprehension questions check the candidates’ abilities in understanding, analyzing, and applying information. Candidates can actively prepare with the help of GMAT Reading Comprehension Practice Questions.

Solution and Explanation

Question 1

The primary purpose of the passage is to

1. compare the adaptive responses of several species of columnar cacti in the Sonoran Desert with those in the arid tropical regions of southern Mexico
2. discuss some of the possible causes of the relatively low abundance of migratory nectar-feeding bats in the Sonoran Desert
3. provide a possible explanation for a particular evolutionary change in certain species of columnar cacti in the Sonoran Desert
4. present recent findings that challenge a particular theory as to why several species of columnar cacti in the Sonoran Desert have expanded their range of pollinators
5. compare the effectiveness of nocturnal and diurnal pollination for several different species of columnar cacti in the Sonoran Desert

Question 2

According to the passage, which of the following types of nectar-feeding pollinators is likely to be an unreliable pollinator of a particular cactus flower?

1. A dietary specialist whose abundance is typically high in relation to that of the flower
2. A dietary specialist whose abundance is at times significantly lower than that of the flower
3. A dietary generalist for whom that flower’s nectar is not a preferred food but is the most consistently available food
4. A dietary generalist for whom that flower’s nectar is slightly preferred to other available foods
5. A dietary generalist that evolved from a species of dietary specialists

Question 3

According to the passage, present-day columnar cacti in the Sonoran Desert differ from their close relatives in southern Mexico in that the Sonoran cacti

1. have flowers that remain open after sunset
2. are pollinated primarily by dietary specialists
3. can be pollinated by nectar-feeding bats
4. have stigmas that are unreceptive to pollination at night
5. are sometimes pollinated by diurnal pollinators

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