GRE阅读高频机经原文：Snowshoe hare population
<p> 29. <span word="Snowshoe">Snowshoe</span> <span word="hare">hare</span> <span word="population">population</span></p><p> <span word="The">The</span> <span word="snowshoe">snowshoe</span> <span word="hare">hare</span> <span word="is">is</span> <span word="one">one</span> <span word="of">of</span> <span word="the">the</span> <span word="northern">northern</span> <span word="mammals">mammals</span> <span word="that">that</span> <span word="go">go</span> <span word="through">through</span> <span word="periodic">periodic</span> <span word="cycles">cycles</span> <span word="of">of</span> <span word="changes">changes</span> <span word="in">in</span> <span word="their">their</span> <span word="populations">populations</span>. <span word="Over">Over</span> <span word="a">a</span> <span word="period">period</span> <span word="of">of</span> <span word="years">years</span>, <span word="their">their</span> <span word="population">population</span> <span word="will">will</span> <span word="increase">increase</span> <span word="in">in</span> <span word="numbers">numbers</span> <span word="and">and</span> <span word="then">then</span> <span word="suddenly">suddenly</span> <span word="crash">crash</span> <span word="to">to</span> <span word="very">very</span> <span word="low">low</span> <span word="numbers">numbers</span>, <span word="only">only</span> <span word="to">to</span> <span word="rise">rise</span> <span word="again">again</span>. <span word="Snowshoe">Snowshoe</span> <span word="hare">hare</span> <span word="tend">tend</span> <span word="to">to</span> <span word="cycle">cycle</span> <span word="about">about</span> <span word="once">once</span> <span word="every">every</span> <span word="ten">ten</span> <span word="years">years</span>. <span word="The">The</span> <span word="snowshoe">snowshoe</span> <span word="hare">hare</span> <span word="cycle">cycle</span> <span word="is">is</span> <span word="one">one</span> <span word="of">of</span> <span word="the">the</span> <span word="best">best</span> <span word="known">known</span> <span word="and">and</span> <span word="fur">fur</span> <span word="trappers">trappers</span> <span word="and">and</span> <span word="traders">traders</span> <span word="working">working</span> <span word="in">in</span> <span word="northern">northern</span> <span word="Canada">Canada</span> <span word="first">first</span> <span word="described">described</span> <span word="it">it</span>. <span word="In">In</span> 1865, <span word="traders">traders</span> <span word="with">with</span> <span word="the">the</span> <span word="Hudsons">Hudsons</span> <span word="Bay">Bay</span> <span word="Company">Company</span> <span word="were">were</span> <span word="flooded">flooded</span> <span word="by">by</span> <span word="hare">hare</span> <span word="pelts">pelts</span> <span word="brought">brought</span> <span word="in">in</span> <span word="by">by</span> <span word="trappers">trappers</span>. <span word="But">But</span>, <span word="by">by</span> 1870, <span word="only">only</span> <span word="a">a</span> <span word="few">few</span> <span word="hare">hare</span> <span word="pelts">pelts</span> <span word="trickled">trickled</span> <span word="in">in</span>. <span word="This">This</span> <span word="pattern">pattern</span> <span word="of">of</span> <span word="high">high</span> <span word="and">and</span> <span word="low">low</span> <span word="abundance">abundance</span> <span word="of">of</span> <span word="hare">hare</span> <span word="pelts">pelts</span> <span word="repeated">repeated</span> <span word="itself">itself</span> <span word="over">over</span> <span word="and">and</span> <span word="over">over</span>, <span word="roughly">roughly</span> <span word="every">every</span> <span word="ten">ten</span> <span word="years">years</span>.</p><p> <span word="Snowshoe">Snowshoe</span> <span word="hares">hares</span> <span word="are">are</span> <span word="a">a</span> <span word="dominant">dominant</span> <span word="herbivore">herbivore</span> <span word="in">in</span> <span word="northern">northern</span> <span word="forest">forest</span> <span word="ecosystems">ecosystems</span>. <span word="In">In</span> <span word="Alaska">Alaska</span> <span word="and">and</span> <span word="western">western</span> <span word="Canada">Canada</span>, <span word="their">their</span> <span word="populations">populations</span> <span word="cycle">cycle</span> <span word="over">over</span> 8 <span word="to">to</span> 11 <span word="years">years</span>. <span word="The">The</span> <span word="difference">difference</span> <span word="in">in</span> <span word="densities">densities</span> <span word="and">and</span> <span word="abundance">abundance</span> <span word="during">during</span> <span word="the">the</span> <span word="cycle">cycle</span> <span word="are">are</span> <span word="extreme">extreme</span>. <span word="Densities">Densities</span> <span word="as">as</span> <span word="high">high</span> <span word="as">as</span> 4,000 <span word="hares">hares</span> <span word="per">per</span> <span word="square">square</span> <span word="kilometer">kilometer</span> <span word="have">have</span> <span word="been">been</span> <span word="estimated">estimated</span> <span word="at">at</span> <span word="the">the</span> <span word="peak">peak</span> <span word="of">of</span> <span word="the">the</span> <span word="cycle">cycle</span>. <span word="These">These</span> <span word="populations">populations</span> <span word="can">can</span> <span word="crash">crash</span> <span word="to">to</span> <span word="densities">densities</span> <span word="of">of</span> <span word="less">less</span> <span word="than">than</span> <span word="one">one</span> <span word="hare">hare</span> <span word="per">per</span> <span word="square">square</span> <span word="kilometer">kilometer</span> <span word="within">within</span> <span word="a">a</span> <span word="period">period</span> <span word="of">of</span> <span word="one">one</span> <span word="year">year</span>. <span word="The">The</span> <span word="populations">populations</span> <span word="slowly">slowly</span> <span word="increase">increase</span> <span word="to">to</span> <span word="peak">peak</span> <span word="densities">densities</span> <span word="over">over</span> <span word="the">the</span> <span word="next">next</span> 6 <span word="to">to</span> 13 <span word="years">years</span>. <span word="The">The</span> <span word="densities">densities</span> <span word="during">during</span> <span word="peak">peak</span> <span word="years">years</span>, <span word="however">however</span>, <span word="are">are</span> <span word="not">not</span> <span word="always">always</span> <span word="the">the</span> <span word="same">same</span>.</p><p> <span word="Just">Just</span> <span word="what">what</span> <span word="is">is</span> <span word="it">it</span> <span word="that">that</span> <span word="drives">drives</span> <span word="the">the</span> <span word="snowshoe">snowshoe</span> <span word="hare">hare</span> <span word="cycle">cycle</span>? <span word="This">This</span> <span word="question">question</span> <span word="has">has</span> <span word="puzzled">puzzled</span> <span word="people">people</span> <span word="living">living</span> <span word="in">in</span> <span word="northern">northern</span> <span word="areas">areas</span> <span word="and">and</span> <span word="scientists">scientists</span> <span word="for">for</span> <span word="many">many</span> <span word="years">years</span>. <span word="On">On</span> <span word="a">a</span> <span word="broad">broad</span> <span word="scale">scale</span>, <span word="it">it</span> <span word="may">may</span> <span word="be">be</span> <span word="the">the</span> 22- <span word="year">year</span> <span word="sunspot">sunspot</span> <span word="cycle">cycle</span> <span word="and">and</span> <span word="its">its</span> <span word="effects">effects</span> <span word="on">on</span> <span word="boreal">boreal</span> <span word="forest">forest</span> <span word="weather">weather</span> <span word="patterns">patterns</span> <span word="or">or</span> <span word="forest">forest</span> <span word="fires">fires</span>. <span word="On">On</span> <span word="a">a</span> <span word="smaller">smaller</span> <span word="scale">scale</span>, <span word="the">the</span> <span word="cycles">cycles</span> <span word="may">may</span> <span word="be">be</span> <span word="affected">affected</span> <span word="by">by</span> <span word="over">over</span> <span word="browsing">browsing</span> <span word="the">the</span> <span word="food">food</span> <span word="supply">supply</span>, <span word="predators">predators</span>, <span word="shock">shock</span> <span word="disease">disease</span> <span word="due">due</span> <span word="to">to</span> <span word="stress">stress</span>, <span word="parasites">parasites</span>, <span word="or">or</span> <span word="a">a</span> <span word="combination">combination</span> <span word="of">of</span> <span word="these">these</span>. <span word="The">The</span> <span word="primary">primary</span> <span word="theory">theory</span> <span word="pertaining">pertaining</span> <span word="to">to</span> <span word="the">the</span> <span word="snowshoe">snowshoe</span> <span word="hare">hare</span> <span word="population">population</span> <span word="cycle">cycle</span> <span word="describes">describes</span> <span word="the">the</span> <span word="mechanics">mechanics</span> <span word="as">as</span> <span word="a">a</span> <span word="combination">combination</span> <span word="of">of</span> <span word="predators">predators</span> <span word="and">and</span> <span word="browse">browse</span> <span word="availability">availability</span>. <span word="During">During</span> <span word="the">the</span> <span word="peak">peak</span> <span word="of">of</span> <span word="their">their</span> <span word="cycle">cycle</span>, <span word="hares">hares</span> <span word="eat">eat</span> <span word="a">a</span> <span word="large">large</span> <span word="proportion">proportion</span> <span word="of">of</span> <span word="the">the</span> <span word="shrubs">shrubs</span> <span word="available">available</span> <span word="in">in</span> <span word="their">their</span> <span word="area">area</span>. <span word="They">They</span> <span word="can">can</span> <span word="also">also</span> <span word="kill">kill</span> <span word="many">many</span> <span word="of">of</span> <span word="the">the</span> <span word="shrubs">shrubs</span> <span word="in">in</span> <span word="their">their</span> <span word="area">area</span> <span word="by">by</span> <span word="chewing">chewing</span> <span word="through">through</span> <span word="the">the</span> <span word="outer">outer</span> <span word="layer">layer</span> <span word="of">of</span> <span word="bark">bark</span>. <span word="Over">Over</span>-<span word="browsing">browsing</span> <span word="of">of</span> <span word="shrubs">shrubs</span> <span word="results">results</span> <span word="in">in</span> <span word="a">a</span> <span word="food">food</span>-<span word="shortage">shortage</span> <span word="that">that</span> <span word="causes">causes</span> <span word="malnourishment">malnourishment</span>, <span word="starvation">starvation</span>, <span word="and">and</span> <span word="reduced">reduced</span> <span word="reproduction">reproduction</span>. <span word="This">This</span> <span word="leads">leads</span> <span word="to">to</span> <span word="the">the</span> <span word="start">start</span> <span word="of">of</span> <span word="the">the</span> <span word="population">population</span> <span word="crash">crash</span> <span word="or">or</span> <span word="decline">decline</span>. <span word="Also">Also</span>, <span word="as">as</span> <span word="food">food</span> <span word="supplies">supplies</span> <span word="decrease">decrease</span>, <span word="the">the</span> <span word="health">health</span> <span word="of">of</span> <span word="hares">hares</span> <span word="declines">declines</span> <span word="and">and</span> <span word="makes">makes</span> <span word="many">many</span> <span word="individuals">individuals</span> <span word="more">more</span> <span word="vulnerable">vulnerable</span> <span word="to">to</span> <span word="predators">predators</span> <span word="and">and</span> <span word="disease">disease</span>. <span word="In">In</span> <span word="the">the</span> <span word="western">western</span> <span word="Yukon">Yukon</span>, <span word="scientists">scientists</span> <span word="believe">believe</span> <span word="that">that</span> <span word="the">the</span> <span word="hare">hare</span> <span word="cycle">cycle</span> <span word="is">is</span> <span word="caused">caused</span> <span word="by">by</span> <span word="changes">changes</span> <span word="in">in</span> <span word="food">food</span> <span word="supply">supply</span> <span word="and">and</span> <span word="predation">predation</span>. <span word="The">The</span> <span word="declines">declines</span> <span word="and">and</span> <span word="increases">increases</span> <span word="in">in</span> <span word="the">the</span> <span word="population">population</span> <span word="of">of</span> <span word="hares">hares</span> <span word="were">were</span> <span word="influenced">influenced</span> <span word="by">by</span> <span word="the">the</span> <span word="reproductive">reproductive</span> <span word="rates">rates</span> <span word="of">of</span> <span word="adult">adult</span> <span word="hares">hares</span> <span word="and">and</span> <span word="the">the</span> <span word="survival">survival</span> <span word="rates">rates</span> <span word="of">of</span> <span word="all">all</span> <span word="ages">ages</span> <span word="of">of</span> <span word="hares">hares</span>.</p>
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