Reading Essentials for Biology Glencoe, страница 11

If a population growslarger than the carrying capacity of the environment, there will bemore deaths than births. The number of organisms in a population is sometimes more than the environment can support andsometimes less than the environment can support.1. What limits a population’sgrowth?________________________________________________________________________Reproduction PatternsAn organism’s population growth is shaped by its reproductivepattern, or life-history pattern. If an organism has a rapid lifehistory pattern, it will reproduce early in life. It also will producemany offspring in a short period of time.

A mosquito is an example of a rapid life-history organism. Rapid life-history organismshave a small body size and a short life span. They live in environments that change extensively. Their populations grow quicklybut decline when the environment becomes unfavorable. As soonas conditions are favorable, the small population will grow rapidly.READING ESSENTIALS2. Analyze What type ofbody size do rapid lifehistory organisms have?(Circle your choice.)a. smallb. largeChapter 435NameDateClassSection4.1Population Dynamics, continuedSlow life-history patterns belong to large species that have longlives.

They usually grow into adults slowly and reproduce slowly.They tend to live in more stable environments. Often, slow lifehistory organisms will keep their populations at carrying capacity.Humans, elephants, and trees are examples of slow life-historyorganisms. Slow life-history organisms have far fewer young during their lifetime than do rapid life-history organisms.________________________________________________________________________________________________________________________4. Analyze An example of adensity-independent factor is (Circle your choice.)a.

a frost that destroystomato plants.b. a fungus that spreadsfrom plant to plant.36Chapter 4Density, or the number of organisms in a given area, affectspopulation growth. Factors that are related to the density of thepopulation are called density-dependent factors. They includedisease, competition, predators, parasites, and food. These factorsbecome more important as the population increases.

For example,when members of a population live far apart, disease spreadsslowly. When the members live close together, disease spreadsquickly. This is true for both plants and animals. It is true forpopulations of people. Some scientists think that the presence ofHIV/AIDS in populations is a limiting factor in the growth ofthose populations.Density-independent factors affect populations, no matterhow large or small. Density-independent factors include volcaniceruptions, temperature, storms, floods, drought, chemical pesticides, and major habitat disruption. Imagine a pond containing apopulation of fish.

If a drought caused the pond to dry up, thenthat population of fish would die. It would not matter if therewere 10 or 100 fish in the pond. Drought is a density-independent factor. Human populations also can be affected by densityindependent factors. Rivers sometimes overflow their banks after aheavy rain. If a town is flooded, it does not matter how many people live in the town; everyone feels the effects of the flood.READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.3.

Name five densitydependent factors.How does the number of organisms in an areaaffect population growth?NameDateClassSection4.1Population Dynamics, continuedOrganism Interactions Limit Population SizeCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Populations also are limited by contact with other organisms ina community. The relationship between predator and prey is agood example. A cat controls the population of mice around ahouse.

A swarm of locusts eats and destroys acres of lettuce on afarm. Bears eat the salmon swimming up a river. All of these areexamples of interactions between organisms in a community.Sometimes predators can have a large impact on a population.For example, when brown tree snakes were brought to the islandof Guam, they had no natural predators on the island. The snakepopulation increased unchecked. As a result, bird populationshave been almost completely destroyed by the snakes.

Whenpredators consume prey on a large enough scale, the size of theprey population can be greatly reduced.Sometimes populations of predators and their prey experiencechanges in their numbers over time. Scientists observe cycles ofpopulation increases and decreases. Some are quite regular andpredictable. An example is the interaction of the snowshoe hareand the Canadian lynx.

The lynx, a member of the cat family, eatsthe snowshoe hare. As you can see, predator populations affectthe size of prey populations.5. Why did the browntree snake populationon Guam increaseunchecked?________________________________________________________________________Hare populationincreases.With fewer hareto eat, the lynxpopulation declines.More food isavailable for thelynx population.The hare populationdeclines.Lynx populationincreases.The larger lynxpopulation eatsmore hare.READING ESSENTIALSChapter 437NameDateClassSection4.1Population Dynamics, continuedUsually the lynx population catches the young, old, injured,or sick members of the hare population. This makes moreresources available for the remaining healthy members of thehare population.Is there competition among members of thesame population?◗ After You ReadMini Glossarycarrying capacity: the number of organisms ofone species that an environment can support indefinitely; populations below carrying capacity tend to increase; those abovecarrying capacity tend to decreasedensity-dependent factors: limiting factors, suchas disease, parasites, or food availability,that affect growth of a populationexponential growth: growth pattern where apopulation grows faster as it increases insize; a graph of an exponentially growingpopulation resembles a J-shaped curvelife-history pattern: an organism’s pattern ofreproduction; may be rapid or slowdensity-independent factors: factors, such astemperature, storms, floods, drought, orhabitat disruption, that affect all populations, regardless of their density1.

Review the terms and their definitions in the Mini Glossary above. Then choose a term andwrite a sentence describing how the term relates to population dynamics.________________________________________________________________________________________________________________________________________________________________________38Chapter 4READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.The lynx and hare are members of different populations, butmembers of the same population also interact with each other.Populations can increase so that members are competing for food,water, and territory.

Competition is a density-dependent factor.When only a few individuals need the available resources, there isno problem. When the population becomes so large that demandfor resources is greater than the supply of resources, the population size will decrease.Sometimes populations become crowded and organisms beginto show signs of stress. Individual animals may become aggressive.They may stop caring for their young or even lose their ability tobear young.

Stress also makes animals more at risk for disease.All of these stress symptoms are limiting factors for growth. Theykeep populations below carrying capacity.NameDateClassSection4.1Population Dynamics, continued2. The web diagram below identifies three factors that affect the growth of populations. Writetwo examples of each factor in the diagram.Life-HistoryPatterns1.2.DensityIndependent FactorsDensityDependent Factors1.1.2.2.PopulationGrowthCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Visit the Glencoe Science Web site at science.glencoe.com to findyour biology book and learn more about population dynamics.READING ESSENTIALSChapter 439NameDateClassSection4.2 Human PopulationSC.G.2.4.5 The student understands that the amount of life any environment can support is limited and that humanactivities can change the flow of energy and reduce the fertility of Earth.◗ Before You ReadThis section discusses human population.