【OG24-525题】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?_mofa留学圈mofaGRE在线题库系统_GRE在线搜题_GRE题库解析_GRE真题机经_GRE课程_GRE培训机构

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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 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 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?

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 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 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 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 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 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 balanced there:compared with the Sonoran Deses 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.

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?

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

正确答案： B

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文章大意逻辑
在墨西哥西北部和亚利桑那州南部的索诺兰沙漠，柱状仙人掌花曾经只在夜间由吸食花蜜的蝙蝠授粉，而其在墨西哥南部干旱热带地区的近亲仍保持这样。热带地区白天的访客授粉无效，因日出时花的柱头失去接受能力或花关闭，但索诺兰沙漠的仙人掌花日出后仍开放，能被蜜蜂和鸟类等白天访客授粉。这种变化可能是因为吸食花蜜的迁徙蝙蝠数量每年有变化。授粉者不可靠有多种原因：可能是饮食通才，对特定物种的忠诚度取决于替代食物来源；也可能是饮食专才，但数量每年变化大或长期相对花朵数量低。最近数据显示索诺兰沙漠春季时，吸食花蜜的蝙蝠是专才，但仙人掌花数量春季高时，蝙蝠种群密度除在繁殖栖息地附近外较低，且该地区白天授粉鸟类比蝙蝠多。 文章类型及逻辑简图
文章类型：现象解释型逻辑简图：现象：Sonoran Desert柱状仙人掌花授粉者范围扩大（In the Sonoran Desert...allowing pollination by such diurnal visitors as bees and birds.）解释：可能因migratory nect feeding bats数量年变化（This development...migratory nect feeding bats.）不可靠授粉者原因：饮食通才，忠诚度取决于替代食物源（They can be dietary generalists...alternative food sources.）饮食专才，数量年变化大（Or, they can be dietary specialists...variable pollination of their preferred food species.）饮食专才，数量长期相对花朵低（Finally, they may be dietary specialists...relative to the availability of flowers.）数据支持：春季蝙蝠是专才，但数量低，白天授粉鸟类多（Recent data reveals...are the nocturnal bats.） 问题类型及做题方法
问题类型：细节题。做题方法：根据题干中“unreliable pollinator”定位到文章中关于授粉者不可靠原因的描述部分。 选项分析
A选项：数量通常相对于花较高的饮食专才。文章说不可靠原因不是数量通常高，而是数量年变化大或长期低，所以该选项不符合，排除。 B选项：数量有时显著低于花的数量的饮食专才。符合文中提到的饮食专才数量长期相对花朵数量低导致不可靠这一原因，当选。 C选项：该花的花蜜不是首选食物但却是最稳定可得食物的饮食通才。文章未提及这种情况是不可靠原因，排除。 D选项：该花的花蜜比其他可得食物稍受青睐的饮食通才。文章没有相关表述，排除。 E选项：从饮食专才物种进化而来的饮食通才。文章未提及与不可靠授粉者的关系，排除。

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