populations · ators. for example, populations of european dandelions and star-lings imported into...

POPULATIONS

194194Copyright© by Holt, Rinehart and Winston. All rights reserved.

3U N I T

CHAPTER 10

Biodiversity

This school of young striped cat-fish near the coast of Australiagathers into a huge, writhing ballto defend against predators.Forming a ball makes the fish looklike one large organism, and thefish's stripes may make it hard fora predator to see individual fish.

195

CHAPTER 8

UnderstandingPopulations

CHAPTER 9

The HumanPopulation

Copyright© by Holt, Rinehart and Winston. All rights reserved.

Understanding Populations

196 Chapter 8 Understanding Populations

C H A P T E R 81 How Populations Change

in Size

2 How Species Interact withEach Other

Before you read this chapter,take a few minutes toanswer the following questions in your EcoLog.

1. What might cause thenumber of organisms in a group to increase ordecrease?

2. What are some differentways that two speciescould affect each other?

READING WARM-UP

Orcas (also called killer whales) huntsea lions. The interaction betweenthese groups affects the number ofindividuals and the behavior of indi-viduals in each group.


Section 1 How Populations Change in Size 197

Biologist Charles Darwin once calculated that a single pair of ele-phants could theoretically produce 19 million descendants within750 years. Darwin made the point that the actual number of ele-phants is limited by their environment.

One way to study the relationship of elephants with theirenvironment is at the level of populations. Such a study wouldinclude tracking the number of elephants in an area and observ-ing the animals’ interactions with their environment.

What Is a Population?A is all the members of a species living in the sameplace at the same time. The bass in an Iowa lake make up onepopulation. Figure 1 shows other examples of populations. Apopulation is a reproductive group because organisms usuallybreed with members of their own population. For example,daisies in an Ohio field will breed with each other and not withdaisies in a Maryland population. The word population refers tothe group in general and also to the size of the population—thenumber of individuals it contains.

population

Objectives� Describe the three main properties

of a population.� Describe exponential population

growth.� Describe how the reproductive

behavior of individuals can affectthe growth rate of their population.

� Explain how population sizes innature are regulated.

Key Termspopulationdensitydispersiongrowth ratereproductive potentialexponential growth carrying capacity

S E C T I O N 1

How Populations Change in Size

Figure 1 � The palm trees on anisland (left) and a school of fish(below) are examples of populations.


Properties of PopulationsPopulations may be described in terms of size, density, or disper-sion, as shown in Figure 2. A population’s is the numberof individuals per unit area or volume, such as the number ofbass per cubic meter of water in a lake. A population’sis the relative distribution or arrangement of its individualswithin a given amount of space. A population’s dispersion maybe even, clumped, or random. Size, density, dispersion, and otherproperties can be used to describe populations and to predictchanges within them.

How Does a Population Grow?A population gains individuals with each new offspring or birth andloses them with each death. The resulting population change overtime can be represented by the equation below. A change in the sizeof a population over a given period of time is that population’s

The growth rate is the birth rate minus the death rate.

Over time, the growth rates of populations change becausebirth rates and death rates increase or decrease. Growth rates canbe positive, negative, or zero. For a population’s growth rate to bezero, the average number of births must equal the average numberof deaths. A population would remain the same size if each pair ofadults produced exactly two offspring, and each of those offspringsurvived to reproduce. If the adults in a population are notreplaced by new births, the growth rate will be negative and thepopulation will shrink.

growth rate.

dispersion

density


Figure 2 � Populations may havevery different sizes, densities, and dis-persions. Flamingos (right) are usu-ally found in huge, dense flocks,while most snakes (above) are soli-tary and dispersed randomly.

QuickLABPopulation GrowthProcedure1. Model the change in size of a

population by applying theequation at right.

2. Start with 100 g (3.5 oz) ofdry beans. Count out fivebeans to represent the startingpopulation of a species.

3. Assume that each year 20 per-cent of the beans each havetwo offspring. Also assume that20 percent of the beans dieeach year.

4. Calculate the number of beansto add or subtract for 1 y.Round your calculations towhole numbers. Add to orremove beans from your popu-lation as appropriate.

5. Continue modeling your popula-tion changes over the course of10 y. Record each change.

Analysis1. Make a graph of your data.

Describe the changes in yourpopulation.


How Fast Can a Population Grow?A female sea turtle may lay 2,000 eggs in her lifetime innests she digs in the sand. Figure 3 shows newly hatchedsea turtles leaving their nest for the ocean. If all of themsurvived, the turtle population would grow rapidly. Butthey do not all survive. Populations usually stay aboutthe same size from year to year because various factorskill many individuals before they can reproduce. Thesefactors control the sizes of populations. In the long run,the factors also determine how the population evolves.

Reproductive Potential A species’ biotic potential isthe fastest rate at which its populations can grow. Thisrate is limited by the maximum number of offspringthat each member of the population can produce,which is called its Some specieshave much higher reproductive potentials than others.Darwin calculated that it could take 750 years for apair of elephants to produce 19 million descendants. Incontrast, a bacterium can produce 19 million descendants in a few days or weeks.

Reproductive potential increases when individuals producemore offspring at a time, reproduce more often, and reproduceearlier in life. Reproducing earlier in life has the greatest effect onreproductive potential. Reproducing early shortens the generationtime, the average time it takes a member of the population toreach the age when it reproduces.

Small organisms, such as bacteria and insects, have short gen-eration times. These organisms can reproduce when they are onlya few hours or a few days old. As a result, their populations cangrow quickly. In contrast, large organisms, such as elephants andhumans, become sexually mature after a number of years. Thehuman generation time is about 20 years, so humans have amuch lower reproductive potential than insects.

Exponential Growth Populations sometimes undergowhich means they grow faster and faster. For example, if a

pair of dogs gives birth to 6 puppies, there will be 6 dogs in onegeneration. If each pair of dogs in that generation has 6 puppies,there will be 18 dogs in the next generation. The following genera-tion will contain 54 dogs, and so on. If the number of dogs is plot-ted versus time on a graph, the graph will have the shape shown inFigure 4. In exponential growth, a larger number of individuals isadded to the population in each succeeding time period.

Exponential growth occurs in nature only when populationshave plenty of food and space, and have no competition or pred-ators. For example, populations of European dandelions and star-lings imported into the United States underwent exponentialgrowth. Similar population explosions occur when bacteria ormolds grow on a new source of food.

growth,exponential

reproductive potential.


Figure 3 � Most organisms have areproductive potential that farexceeds the number of their off-spring that will survive. Very few ofthese baby sea turtles will survivelong enough to breed.

0 4 8 12 16 20

Number of months

Exponential Growth

0

100,000

200,000

300,000

400,000

500,000

600,000

700,000

800,000

Num

ber

of

ind

ivid

uals

Figure 4 � Population growth isgraphed by plotting population sizeover a period of time. Exponentialpopulation growth will look like thecurve shown here.


What Limits Population Growth?Because natural conditions are neither ideal nor constant, popula-tions cannot grow forever and rarely grow at their reproductivepotential. Eventually, resources are used up or the environmentchanges, and deaths increase or births decrease. Under the forcesof natural selection in a given environment, only some membersof any population will survive and reproduce. Thus, the prop-erties of a population may change over time.

Carrying Capacity The blue line in Figure 5 represents a popula-tion that seems to be limited to a particular size. This theoreticallimit, shown by the dashed yellow line, is called carrying capacity.The of an ecosystem for a particular species isthe maximum population that the ecosystem can support in-definitely. A population may increase beyond this number, but itcannot stay at this increased size. Because ecosystems change, car-rying capacity is difficult to predict or calculate exactly. However,carrying capacity may be estimated by looking at average popula-tion sizes or by observing a population crash after a certain sizehas been exceeded.

The history of rabbits in Australia demonstrates both expo-nential growth and carrying capacity. Originally, there were norabbits in the native ecosystems of Australia. When rabbitswere introduced there in 1859, their numbers increased rapidlybecause they had plenty of vegetation to eat, no competition,and no predators. But eventually, disease and starvation caused the rabbit population to crash. Over time, the veg-etation recovered, and the rabbit population increased again.The population continues to increase and decrease, but lessdramatically.

carrying capacity


Figure 5 � An example of carryingcapacity is shown by the dashed yel-low line in the graph (right). This lineseems to be a limit on the size of theexample population (blue line).When rabbits were introduced intoAustralia (below), their populationquickly exceeded the carrying capac-ity of the area.

Carryingcapacity

Population overshootscarrying capacity

Population runs out ofresources and declines

Population recoversand stabilizes

Exponentialgrowth

Time

Pop

ulat

ion

siz

e

Population crashes

EcofactCarrying Capacity of IslandsIslands are good places to studycarrying capacity because islandshave clear boundaries. The PribilofIslands near Alaska were the site ofa well-studied population explosionand crash. In 1911, 25 reindeerwere introduced on one of theislands. By 1938, the herd hadgrown to 2,000 animals. The rein-deer ate mostly lichens, whichgrow back very slowly. By 1950,there were only 8 reindeer alive onthe island.

www.scilinks.orgTopic: Populations andCommunitiesSciLinks code: HE4085


Resource Limits A species reaches its carrying capacity when itconsumes a particular natural resource at the same rate at whichthe ecosystem produces the resource. That natural resource isthen called a limiting resource for the species in that area. Forexample, plant growth is limited by supplies of water, sunlight,and mineral nutrients. The supply of the most severely limitedresources determines the carrying capacity of an environment fora particular species at a particular time.

Competition Within a Population The members of a populationuse the same resources in the same ways, so they will eventuallycompete with one another as the population approaches its carry-ing capacity. An example is the fate of mealworm larvae in a sackof flour. Adults of this type of beetle will find a sack of flour, laytheir eggs in the sack, and leave. Most of the first larvae to hatchwill have plenty of flour to eat and will grow to adulthood. How-ever, the sack has a limited amount of food, and mealworms fromeggs that were laid later may not have enough food to survive toadulthood.

Instead of competing directly for a limiting resource, mem-bers of a species may compete indirectly for social dominanceor for a territory. A territory is an area defended by one ormore individuals against other individuals. The territory is ofvalue not only for the space but also for the shelter, food, orbreeding sites it contains. Competition within a population ispart of the pressure of natural selection. Many organismsexpend a large amount of time and energy competing withmembers of the same species for mates, food, or homes fortheir families. Some examples of competition within species areshown in Figure 6.


Figure 6 � Members of a populationoften compete with each other. Theseplants (above) are growing over eachother as they compete for light.These wolves (left) are competing forfood and for social dominance.

MATHPRACTICEGrowth Rate A growthrate is a change in a popu-lation’s size over a specific period of time.

Imagine a starting population of100 individuals. If there were 10births and 5 deaths in a given year,what was the population’s growthrate for the year? In the next year,if there were 20 births and 10deaths, what would the newgrowth rate be? If births increasedby 10 and deaths increased by 5for each of the next 5 years, howwould you describe the growth ofthis population?

change in population

timegrowth

rate =


Two Types of Population RegulationPopulation size can be limited in ways that may or may notdepend on the density of the population. Causes of death in apopulation may be density dependent or density independent.

When a cause of death in a population is density dependent,deaths occur more quickly in a crowded population than in a sparsepopulation. This type of regulation happens when individuals of apopulation are densely packed together, such as when a populationis growing rapidly. Limited resources, predation, and disease resultin higher rates of death in dense populations than in sparse popula-tions. The pine trees in Figure 7 are infected with a disease that isspreading in a density-dependent pattern. Many of the same kind ofpine tree are growing close to each other, so a disease-carrying bee-tle easily spreads the disease from one tree to another.

When a cause of death is density independent, a certain pro-portion of a population may die regardless of the population’sdensity. This type of regulation affects all populations in a generalor uniform way. Severe weather and natural disasters are oftendensity-independent causes of death. The winter storm shown inFigure 8 froze crops and fruiting trees regardless of the density ofplants in the area.


Figure 7 � The way a diseasespreads through a population isaffected by the population’s density.These pine trees have been infectedby a disease carried by the southernpine beetle. This disease has spreadrapidly through U.S. timber forests.

Figure 8 � Weather events usuallyaffect every individual in a similarway, so such events are considereddensity-independent regulation.

1. Compare two populations in terms of size, density,and dispersion. Choose any populations you know of.

2. Describe exponential population growth.

3. Describe three methods by which the reproductivebehavior of individuals can affect the growth rate of apopulation.

4. Explain how population sizes in nature are regulated.

CRITICAL THINKING5. Making Predictions How accurately do you think

the size of a population can be predicted? What infor-mation might be needed to make this prediction?

6. Compare and Contrast Read the description ofthe populations of rabbits in Australia and reindeer inthe Pribilof Islands. List the similarities and differencesbetween these two histories. READING SKILLS

S E C T I O N 1 Review

HistoryConnection to

Density and Disease The blackplague of 14th-century Europewas spread in a density-dependent pattern. About one-third of Europe’s population diedfrom the highly contagious dis-ease. Most of the deaths occurredin the crowded towns of thetime, and fewer deaths occurredin the countryside.


populations · ators. for example, populations of european dandelions and star-lings imported into...

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