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OG In the Sonoran Desert of northwestern Mexico

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AbeNeedsAnswers Master | Next Rank: 500 Posts Default Avatar
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OG In the Sonoran Desert of northwestern Mexico

Post Sat Aug 19, 2017 2:00 pm
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    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
    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 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.

    439) The primary purpose of the passage is to
    A. compare the adaptive responses of several species of columnar cacti in the Sonoran Desert with those in the arid tropical regions of southern Mexico
    B. discuss some of the possible causes of the relatively low abundance of migratory nectar-feeding bats in the Sonoran Desert
    C. provide a possible explanation for a particular evolutionary change in certain species of columnar cacti in the Sonoran Desert
    D. 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
    E. compare the effectiveness of nocturnal and diurnal pollination for several different species of columnar cacti in the Sonoran Desert

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

    441) 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
    A. have flowers that remain open after sunset
    B. are pollinated primarily by dietary specialists
    C. can be pollinated by nectar-feeding bats
    D. have stigmas that are unreceptive to pollination at night
    E. are sometimes pollinated by diurnal pollinators

    Q439: C
    Q440: B
    Q441: E

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    NandishSS Master | Next Rank: 500 Posts Default Avatar
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    Post Tue Sep 19, 2017 7:55 pm
    Hi Ceilidh/Dave,

    How to deal with this long passage?

    439) The primary purpose of the passage is to
    A. compare the adaptive responses of several species of columnar cacti in the Sonoran Desert with those in the arid tropical regions of southern Mexico
    B. discuss some of the possible causes of the relatively low abundance of migratory nectar-feeding bats in the Sonoran Desert
    C. provide a possible explanation for a particular evolutionary change in certain species of columnar cacti in the Sonoran Desert
    D. 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
    E. compare the effectiveness of nocturnal and diurnal pollination for several different species of columnar cacti in the Sonoran Desert

    OA is C but are we discussing evolutionary changes? Isn't it about the pollinated. How does it happen in lack of availability?

    Can you pls explain Q2 & Q3?

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