北京赛车PK10冠军大小计划当天计划表歡迎您的到來!

您所在的位置: 首頁 > 其它課程/考試 > GRE

GRE閱讀題型分類——復述題

作者:新通教育

2015-10-28 新通教育

GRE閱讀一直是中國考生的薄弱項,新通教育為幫助廣大GRE考生突破閱讀瓶頸,提高閱讀成績,特地精心整理了閱讀高分分類素材庫。那快來GRE閱讀高分素材庫汲取營養吧!

   As Gilbert White, Darwin, and others observed long ago, all species appear to have the innate capacity to increase their numbers from generation to generation. The task for ecologists is to untangle the environmental and biological factors that hold this intrinsic capacity for population growth in check over the long run. The great variety of dynamic behaviors exhibited by different populations makes this task more difficult: some populations remain roughly constant from year to year; others exhibit regular cycles of abundance and scarcity; still others vary wildly, with outbreaks and crashes that are in some cases plainly correlated with the weather, and in other cases not.
  To impose some order on this kaleidoscope of patterns, one school of thought proposes dividing populations into two groups. These ecologists posit that the relatively steady populations have “density-dependent” growth parameters; that is, rates of birth, death, and migration which depend strongly on population density. The highly varying populations have “density-independent” growth parameters, with vital rates buffeted by environmental events; these rates fluctuate in a way that is wholly independent of population density.
  This dichotomy has its uses, but it can cause problems if taken too literally. For one thing, no population can be driven entirely by density-independent factors all the time. No matter how severely or unpredictably birth, death and migration rates may be fluctuating around their long-term averages, if there were no density-dependent effects, the population would, in the long run, either increase or decrease without bound (barring a miracle by which gains and losses canceled exactly). Put another way, it may be that on average 99 percent of all deaths in a population arise from density-independent causes, and only one percent from factors varying with density. The factors making up the one percent may seem unimportant, and their cause may be correspondingly hard to determine. Yet, whether recognized or not, they will usually determine the long-term average population density.
  In order to understand the nature of the ecologist’s investigation, we may think of the density-dependent effects on growth parameters as the “signal” ecologists are trying to isolate and interpret, one that tends to make the population increase from relatively low values or decrease from relatively high ones, while the density-independent effects act to produce “noise” in the population dynamics. For populations that remain relatively constant, or that oscillate around repeated cycles, the signal can be fairly easily characterized and its effects described, even though the causative biological mechanism may remain unknown. For irregularly fluctuating populations, we are likely to have too few observations to have any hope of extracting the signal from the overwhelming noise. But it now seems clear that all populations are regulated by a mixture of density-dependent and density-independent effects in varying proportions.

21. According to the passage, all of the following behaviors have been exhibited by different populations EXCEPT:
  (A) roughly constant population levels from year to year
  (B) regular cycles of increases and decreases in numbers
  (C) erratic increases in numbers correlated with the weather
  (D) unchecked increases in numbers over many generations
  (E) sudden declines in numbers from time to time

查看正確答案和解析

GRE 保分課程

課程名稱 學習目標 班級特色
GRE精品VIP課程 GRE310分 私人訂制短期提升 報名
GRE鉆石VIP沖刺班 GRE315分 分析最新官方真題 報名
GRE 應考班 310-320分 強化訓練 短期提高 報名
GRE精品沖刺班 保315分沖320分 獨家授課快速提分 報名
GRE 1對1強化班 310-320分 閱卷老師授課 報名
GRE精講精練小班 GRE310分 專項突破全面提高 報名

更多分享

GRE復習資料免費打包下載
  • GRE真題集錦

    免費下載

    格式: doc 大。564KB

    免費下載
  • GRE全真試題匯總

    免費下載

    格式: doc 大。393KB

    免費下載
  • GRE寫作真題-高分范文

    免費下載

    格式: doc 大。392KB

    免費下載
語培留學服務
  • 考試提醒
  • 報名
  • 考試
  • 查分
2018年GRE考試報名時間

近期考試:7月5日 7月15日

報名 GRE備考
2018年GRE考試時間

近期考試:7月5日 7月15日

2018年GRE成績查詢入口

近期考試:7月5日 7月15日

查分
  • 考前預測
  • 名師押題 縮小范圍100%命中
北京赛车PK10冠军大小计划当天计划表 北京PK10全天两期计划 北京赛车PK10计划开奖号