首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
考研
As Gilbert White, Darwin, and others observed long ago, all species appear to have the innate capacity to increase their numbers
As Gilbert White, Darwin, and others observed long ago, all species appear to have the innate capacity to increase their numbers
admin
2017-06-25
66
问题
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.【F1】
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 population 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.【F2】
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).【F3】
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.【F4】
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.
【F5】
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.
【F4】
选项
答案
对于那些数量相对稳定或者以某种循环周期摆动的种群而言,这种“信号”的特征比较容易被察觉.而且也很容易描述它的影响,即使引起这种情况的生物机制仍是未知的。
解析
转载请注明原文地址:https://www.kaotiyun.com/show/MXbZ777K
0
考研英语一
相关试题推荐
Aneweconomicspaperhassomeold-fashionedadviceforpeoplenavigatingthestressesoflife:Findaspousewhoisalsoyourb
Nationally,anageingpopulationisaproblem.Butlocallyitcanbeaboon.Theover-50scontrol80%ofBritain’swealth,and
Between1998and2013,theEarth’ssurfacetemperatureroseatarateof0.04°Cadecade,farslowerthanthe0.18°Cincrease
Picture-takingisatechniquebothforreflectingtheobjectiveworldandforexpressingthesingularself.Photographsdepicto
Researchershaveestablishedthatwhenpeoplearementallyengaged,biochemicalchangesoccurinthebrainthatallowittoact
Severalweeksago.threeofthecountry’smostrespectedinstitutionsofhigherlearning,Harvard,Princeton,andtheUniversity
Whenoneusesthescientificmethodtostudyorinvestigatenatureortheuniverse,oneispracticingscientificthinking.Alls
TheFederationofAmericanSocietiesforExperimentalBiology(FASEB)hasjustreleasedareportonthecareertrajectoriesofyou
Everyhumanbeinghasuniquearrangementofskinonhisfingersandthisarrangementisunchangeable.Scientistsandexpertshav
随机试题
BrotherlyLoveAdidasandPumahavebeentwoofthebiggestnamesinsportsshoemanufacturingforoverhalfacentury.
二尖瓣狭窄伴有何种心律失常最易并发栓塞( )
某咨询机构接受A市的委托,从就业、失业和价格三个方面进行了调查和研究。甲为刚刚毕业的学生,目前正在寻找工作;某公司进行战略整合将原来的技术研发中心撤销,企业外包给一家专业公司,企业本身则专一做市场销售,乙为公司技术研发中心人员,因公司业务转换,不
计算机打印输出的凭证,账簿,报表,其保存期限与手工方式不完全一致。
中国农业发展银行的主要业务包括()。
企业采用成本分析模式管理现金,在最佳现金持有量下,下列各项中正确的有()。2016(卷Ⅰ)(卷Ⅱ)
完颜希尹家族墓地在今()。
我国独立自主、自力更生方针的理论依据是()。
【偷袭珍珠港】
在嵌入式系统硬件设计中,可以采用______方法减少信号的辐射。
最新回复
(
0
)