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wk 9 Lec 2sf - C hapte 18 20 rs Ecology and Hum Im an pacts...

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Chapters 18 & 20 Ecology and Human Impacts
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Ecology and Environmentalism Our current awareness of the biosphere’s limits stems mainly from the 1960s, a timeof growing disillusionment with environmental practices of thepast. The pesticide DDT , which improved agricultural production, Was developed in the1950s. Was determined to haveharmful sideeffects. Rachel Carson Was oneof thefirst ecologists to perceivetheglobal dangers of pesticideabuse. Helped initiateour current environmental awareness with her book Silent Spring. The modern environmental movement was catalyzed by Rachel Carson .
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The human population explosion Humans areby far themost abundant largeanimals. Humans thereforehave a disproportionately high impact on theenvironment. Thehuman species Requires vast amounts of materials and space . Has devastated theenvironment for many other species.
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Population growth models Two models, theexponential growth model and thelogistic growth model, will help us understand population growth. Thegrowth rateis thechangein population sizeper timeinterval.
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The Exponential Growth Model : TheIdeal of an Unlimited Environment The exponential growth model describes the rate of expansion of a population under ideal, unregulated conditions. A key feature of the exponential growth model is that the rateat which a population grows depends on the number of individuals already in the population .
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The Logistic Growth Model : TheReality of a Limited Environment In nature, a population may grow exponentially for a while, but eventually one or more environmental factors will limit its growth . Population-limiting factors restrict population growth. The logistic growth model describes growth of an idealized population that is slowed by limiting factors.
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Carrying capacity is thenumber of individuals in a population that theenvironment can just maintain with no net increaseor decrease. Comparing growth models
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Regulation of Population Growth Density-Dependent Factors The logistic model is actually a description of intraspecific competition. Describes population growth as density- dependent. Density-dependent factors Arepopulation-limiting factors whose effects intensify as the population increases in size . Increase a population’s death rate and decrease the birth rate.
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Density-Independent Factors Density-independent factors Arepopulation-limiting factors whose intensity is unrelated to population density . Include events such as seasonal freezing. In many natural populations, density- independent factors limit population size before density-dependent factors become important. Over thelong term, most populations are probably regulated by a mixtureof density-independent and density- dependent factors.
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Human Population Growth The History of Global Population Growth Thehuman population has been growing almost exponentially for centuries.
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Human population growth is based on the same parameters that affect other populations: birth rates and death rates.
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