Sunshine StateSTANDARDSSC.G.1.3.2: The stu-dent knows thatbiological adaptationsinclude changes instructures, behaviors,or physiology thatenhance reproductivesuccess in a particularenvironment.SC.G.2.3.3: The stu-dent knows that abrief change in thelimited resources of anecosystem may alterthe size of a popula-tion or the averagesize of individualorganisms and thatlong-term change mayresult in the elimina-tion of animal andplant populationsinhabiting the Earth.

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VOCABULARYimmigration p. 537emigration p. 537limiting factor p. 538opportunist p. 541competitor p. 542

BEFORE, you learned

• Four characteristics are used todescribe a population

• Scientists study these four characteristics to predict popu-lation change

NOW, you will learn

• About limits to populationgrowth

• How population density affectslimiting factors

• About two reproductive strategies found within populations

KEY CONCEPT

Populations respondto pressures..

Population growth is limited.No population can grow forever. Every population has a limit to itsgrowth. For example, the cockroach has been around for more than300 million years. This insect has outlived the dinosaurs and may persist long after humans have become extinct. Yet even if cockroachesbecame the only species on the planet, several factors would limittheir population size.

CHOOSE YOUR OWNSTRATEGYUse a strategy from an earlier chapter or one ofyour own to take notes onthe main idea: Populationgrowth is limited.

EXPLORE Population Density

How does population density vary?

PROCEDURE

Choose three different locations in yourschool where you can observe how manypeople enter and leave an area during a specific time period.

Position three people at each location acounter, a timekeeper, and a recorder.

Count the number of people who passthrough the area for at least 2 minutes.Record the number.

Compare your data with the data collectedby other groups.

WHAT DO YOU THINK?• Where was the number of people the highest?

the lowest?• Explain what may have affected population

density at each location.

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MATERIALS• stopwatch• notebook

Birth, Death, Immigration, and EmigrationWhen scientists study how a population changes, they must considerfour things: birth, death, immigration, and emigration. There is even asimple formula to help scientists track population change.

Population change � (birth � immigration) � (death � emigration)

It is too simple to say that a high birth rate means populationgrowth, or that many deaths mean population decline. isthe movement of individuals into a population. For example, if astrong wind blows the seeds of a plant from one area into another, thenew plant would be said to immigrate into the new area. Immigrationcan increase a population or help stabilize a declining population.Birth and immigration introduce individuals into a population.

is the movement of individuals out of a population.If resources become scarce within a habitat, some of the individualsmight move to areas with greater supplies. Others may even die.Death and emigration remove individuals from a population.

Check Your Reading List two factors that lead to population growth and two thatlead to population decline.

Consider, for example, a flock of seagulls that flies inland during a storm. They stop at a city dump, where food is plentiful. Theseincoming seagulls become part of the seagull population that isalready living at the dump. A raccoon population living in the samearea has been eating the seagulls’ eggs, causing the number of seagullbirths to decrease. If enough seagulls immigrate to the dump, theseagull population would increase, making up for the decrease inbirths. Immigration would help keep the population stable. The seagull population would also increase if part of the raccoon popula-tion moved away.

Emigration

Immigration

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Limiting FactorsWhen a population is growing at a rapid rate, the birth rate is muchhigher than the death rate. That means that more individuals arebeing born than are dying during a particular time period. There areplenty of resources available, and the population size is increasingrapidly. Eventually, however, the population will stop growing, becausea habitat can support only a limited number of organisms.

A is a factor that prevents the continuing growth of a population in an ecosystem. Abiotic, or nonliving, limiting factorsinclude air, light, and water. Other limiting factors can be livingthings, such as other organisms in the same population or individualsbelonging to different species within the same area.

Check Your Reading What are two limiting factors?

Competition can occur between different populations sharing thesame habitat. Competition can also occur among individuals of thesame population. Suppose, for example, that a population of deer in aforest preserve were to increase, either through births or immigration.Population density at the forest preserve would go up. More and moredeer in that area would be competing for the same amount of food.

limiting factor

538 Unit 4: Life Over Time

Density-Dependent Factors

Factors may include

• Competition• Disease• Parasitism• Predation

Density-dependent factors have a greater effect onpopulations with many individuals in a small area.

Chapter 15: Population Dynamics 539

Effects of Population DensityIn the situation described above, the seagull population could decreaseas a result of competition for food. Competition is an example of adensity-dependent factor—that is, a limiting factor that affects a popu-lation when density is high. Disease is another density-dependentfactor. The more crowded an area becomes, the easier it is for disease tospread, so more individuals are affected. If population density is low,there is less contact between individuals, which means that disease willspread more slowly. Density-dependent factors have a greater effect onthe population as it grows. They can bring a population under control,because they apply more pressure to a growing population.

There are also density-independent factors. These limiting fac-tors have the same effect on a population, whether it has a highdensity or a low density. Freezing temperatures could be considereda density-independent factor. A freeze might kill all of the floweringplants in an area, whether or not the population density is high.A natural event such as a wildfire is another example of a density-independent factor. When a wildfire occurs in a forest, it can wipeout an entire ecosystem.

Check Your Reading How are limiting factors that are density-dependent differentfrom limiting factors that are density-independent?

Density-Independent Factors

Factors may include

• Drought• Hurricanes• Tornados• Fires• Floods

Density-independent factors are typically changes in weather. These factors affect low-density and high-density populations equally.

540 Unit 4: Life Over Time

Limiting factors include nonliving factors in the environment andnatural events such as earthquakes, fires, and storms. During times ofdrought, there may not be enough food to meet the needs of all theorganisms in an area. The quality of the food declines as well. Forexample, a lack of water may cause a population of trees to producefewer pieces of fruit, and the fruit itself may be smaller. If there is littlefood available, a condition called famine arises. If the famine is severe,and if death rates exceed birth rates, then the population size will falldramatically.

Check Your Reading How do limiting factors affect populations? Remember: a summary includes only the most important information.

Limiting factors affect human populations as well. However,humans have found different ways to help overcome many of theselimits. In Section 15.3 you will read about how the human response tolimits differs from that of other biological populations.

What limiting factors determine plant growth?Using the materials below, design an experiment totest how limiting factors such as sunlight or watercan determine how well a plant population will grow.

PROCEDURE

Decide how to use the beans, soil, and water.

Write up your experimental procedure. Include any safety tips.

WHAT DO YOU THINK? • What variables did you use in your experiment?

• What evidence do you expect to see to support the goal of your experiment?

CHALLENGE Conduct your experiment. Be sure to observe your beans daily and note which ones are most healthy. Make a chart and record your observations. The beans should grow for at least two weeks before you make your conclusion.

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Limiting FactorsLimiting Factors

SKILL FOCUS Designing experiments

MATERIALS• 6 paper cups• potting soil• beans• water

TIME20 minutes

Pine trees are opportunists thatcan spread across an area quickly.

Opportunists

Chapter 15: Population Dynamics 541

Populations have distinct reproductivesurvival strategies.

Although reproduction of offspring is not necessary for the survival ofan individual organism, it is necessary for the survival of a species.Scientists studying populations observe patterns in the reproductivestrategies used among species. There are two main strategies thatmany species use. There are also many species whose strategies fit somewhere in between.

Strategies of Opportunistsare species that reproduce rapidly if their population

falls below carrying capacity. They share many characteristics, includ-ing a short life span and the ability to reproduce large quantities ofoffspring. Their population size tends to change often, and oppor-tunists live across many areas. Opportunists include algae, dandelions,bacteria, and insects. These species can reproduce and move across anarea quickly. In addition, they can adapt quickly to environmentalchanges. Populations of opportunists often grow rapidly.

Opportunists

Pine cones releasehuge amounts ofpollen into the air.

VOCABULARYRemember to make aword magnet for theterm opportunist. Include examples in your diagram.

reminder

Species reproduce eitherasexually or sexually. Readmore about this on page817.

Content PreviewFLORIDA

Strategies of CompetitorsYou might be familiar with the term competitor as meaning an organ-ism that struggles with another to get resources. Scientists who studypopulation growth use the term competitor in another way.

are species with adaptations that allow them to remain ator near their carrying capacity for long periods of time. Competitorshave many characteristics that differ from those of opportunists.

Species that have a competitive reproductive strategy often livelonger and have fewer offspring. Elephants and saguaro cacti are twoexamples of competitors. The offspring of competitors take longer todevelop than those of opportunists. Also, animals with this strategytend to take care of their young for a longer period of time.Competitors are not distributed across areas as widely as opportunists,but greater numbers of their offspring survive to reproductive age.

Competitors

KEY CONCEPTS 1. What four factors do scientists

consider when they measurepopulation change?

2. Give two examples of density-dependent factors and twoexamples of density-independent factors.

3. Other than life span, how doopportunists and competitorsdiffer?

CRITICAL THINKING4. Analyze Why would it be a

mistake to predict populationgrowth based on birth ratealone?

5. Apply Give an example of afactor that limits a populationnear you.

CHALLENGE6. Synthesize There has been

an oil spill along a waterwayfamous for its populations ofseals, dolphins, and sea birds.Six months later, all popula-tions show a decline. Explainwhat factors might havecaused such a change andwhether the oil spill was adensity-dependent or density-independent factor or both.

542 Unit 4: Life Over Time

Competitors

Wolves are examples of competitor species. These cubswill be cared for by adults until they are able to hunt.