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LESSON 3 Population Growth Guiding Question: What factors determine whether, and how, a population’s size changes? • Describe the factors that influence a population’s growth rate. • Explain exponential growth and logistic growth. • Explain how limiting factors and biotic potential affect population growth. Reading Strategy Create a KWL chart for each of the headings in this lesson. Before you read, fill in what you know and what you want to know. After reading, fill in what you learned. Vocabulary survivorship curve, immigration, emigration, migration, exponential growth, limiting factor, carrying capacity, logistic growth, density-dependent factor, density-independent factor, biotic potential Why are there so many bacteria and so few 4.3 LESSON PLAN PREVIEW Inquiry Model the effects of different factors that determine population growth. Real World Apply the concept of limiting factors to a classroom environment. Differentiated Instruction Support less proficient readers by guiding them to use headings to find information. whales? Place a few bacteria on a nutrient-rich petri dish, and a few hours later, each will have generated a colony of millions. But place a few whales in the middle of the ocean, and it takes a year or more for any new whales to be born. Why the difference? 4.3 RESOURCES In Your Neighborhood Activity, Migrating Populations • Scientific Method Lab, Yeast Population Growth • Real Data Online • Real Data Math Worksheet • Lesson 4.3 Worksheets • Lesson 4.3 Assessment • Chapter 4 Overview Presentation GUIDING QUESTION FOCUS Name several factors that can affect a population’s growth, such as disease, unusually large numbers of births, and limited food resources. Have students predict how each of these factors might affect a population’s growth. 110 Lesson 3 Proteus bacteria colonies and two humpback whales Factors That Determine Population Growth A population’s growth rate is determined by births, deaths, immigration, and emigration. In the simplest terms, populations increase in size when more individuals enter the population than leave it. Likewise, populations decrease in size when more individuals leave it than enter it. Two sets of factors influence the ratio of individuals entering and leaving a population: births and deaths, and immigration and emigration. Birth- and Death Rates Population size, density, distribution, sex ratio, and age structure can all influence the rates at which individuals within a population are born and die. The rate at which individuals are born is called natality. The rate at which individuals die is called mortality. Natality and mortality are usually expressed as the number of births or deaths per 1000 individuals over a given time. All else being equal, when a population’s birthrate is greater than its death rate, the population size increases. When its death rate is greater than its birthrate, the population size decreases. Individuals of different ages have different probabilites of dying. To show how the likelihood of death varies with age, population ecologists use graphs called survivorship curves. Figure 8 shows the three basic types. Organisms with a type I curve, such as humans, have higher mortality at older ages. Most individuals survive at young ages, and the likelihood of dying increases with age. If you were to follow a thousand 10-year-old and 80-year-old humans for a year, you would find that at year’s end more 80-year-olds had died than 10-year-olds. Amphibians such as the golden toad show a different survivorship pattern. Golden toads produced large numbers of young that suffered high death rates. This pattern, in which death is less likely (and survival more likely) at an older age than at a very young age, defines a type III survivorship curve. A type II survivorship curve indicates a population with equal mortality at all ages. Many bird species have type II curves. ▶ Survivorship Curves Recall that a population’s age structure describes the relative number of individuals within various age groups. The age structure of a population influences how relative birthrates and death rates affect its size. Consider a population following a type I survivorship curve. If the population is made up of mostly young, reproductive or pre-reproductive individuals, there will likely be more births than deaths, and the population size will increase. In a population of mostly older individuals, there are likely to be more deaths than births, and the population size will decrease. A population with an even age distribution will likely remain stable. ▶ Age Structure and Population Growth Reading Checkpoint hich type of survivorship curve describes W populations whose mortality is highest at young ages? Survivorship Curves Number of Survivors Type I Type II Type III Young Old Age Figure 8 Survivorship Curves In a type I survivorship curve, individuals are most likely to die when they are old. In a type III survivorship curve, mortality is highest for young members of the population. In a type II survivorship curve, mortality remains constant throughout an individual’s lifetime. ANSWERS Reading Checkpoint Type III Population Ecology 111 Figure 9 Seasonal Migration and Population Size The size of the turkey vulture population in Pennsylvania changes over the course of the year. Much of the change has to do with seasonal migration into and out of the area. Immigration and Emigration In addition to births and deaths, population size can also change because of individuals moving into or out of a population. Immigration is the arrival of individuals from outside a given area. Emigration is the departure of individuals from a given area. Immigration and emigration can have dramatic effects on a population’s size, especially when it comes to humans. Millions of people move around the world each year, driven by necessity, opportunity, or a desire to make new connections and see new places. Sometimes, organisms make brief movements into and out of an area as part of a seasonal routine. Migration is a seasonal movement into and out of an area. Many animals, including fishes, mammals, and birds, migrate. Every year, for example, the turkey vulture population near Hawk Mountain Sanctuary in Kempton, Pennsylvania, increases in late summer and early autumn. Part of the population increase is due to local births. Most of the increase, however, is due to birds arriving from the north as part of their annual migration cycle. In the early winter, the entire local population leaves the area as the migration continues south. Then, in early spring, they come back. Some stay, and some continue to breeding grounds farther north. The cycle starts again when temperatures begin to drop. Real Data Turkey Vultures 1. Interpret Graphs Describe the annual trend in turkey vulture sightings along the survey route. 2. Apply Concepts What factors might be increasing the vulture population’s size? What factors decrease population size? 3. Infer Turkey vultures arrive from the north onto sanctuary lands and reside there for a while before migrating south. When do you think the vultures from the north arrive? When do you think they all leave? 112 Lesson 3 Turkey Vultures Per Survey, 1992–2008 Average number of vultures per survey Hawk Mountain Sanctuary in Kempton, Pennsylvania, is a protected area for birds of prey. Scientists at the sanctuary monitor bird populations by conducting roadside surveys. Scientists drive slowly along a set route and count the birds they spot. The graph at right shows the average number of turkey vultures surveyed along a 48-kilometer route near the sanctuary early and late month throughout the year. 40 35 30 25 20 15 10 5 0 Early month Late month J F M A M J J A Month S O N D Data from Hawk Mountain Data Archives 4. Perform Error Analysis What is one potential source of error when conducting a roadside survey? Births Deaths Population size increases Immigration Population size decreases Golden toad population Emigration How Populations Grow BIG QUESTION How do changes in population size relate to environmental conditions? Perspective Ask students to consider the environmental conditions that support the exponential growth of a single population. Then, ask them to discuss how exponential growth relates to resource availability. Suggest that students consider not only the resources used by the growing population, but also how the resources used by other populations in the ecosystem may change. Figure 11 Exponential Growth Populations growing by a fixed percent experience exponential growth. Every incremental increase in number is larger than the one before it. The Scots pine population grew exponentially in Great Britain following the last ice age. Population Size Exponential Growth Time 114 Lesson 3 Populations can grow exponentially or logistically. Growth rates tend to change depending on the resources available to the organisms in the population. Population ecologists recognize two basic patterns of population growth: exponential and logistic. Exponential Growth Based on a population of 1000 individuals, if the population grows by 10 percent per year, there will be 1100 individuals in the population next year (1000 + 100). The year after that, there will be 1210 (1100 + 110), and then 1331 (1210 + 121) the next year. Notice that even though the growth rate (10%) remains the same, each increase in population size is larger than the one before it. Only 100 individuals were added in the first year. But 110 were added in Year 2, and 121 were added in Year 3. Each year, a 10% increase means more and more individuals are added to the population. When a population increases by a fixed percentage each year, it is said to undergo exponential growth. Changes in population size are shown with population growth curves. The J-shaped curve in Figure 11 shows exponential growth. Normally, exponential growth occurs in nature only when the starting population is small and environmental conditions are ideal. Most often, these conditions occur when organisms are introduced to a new environment. Mold growing on a piece of bread and bacteria colonizing a recently dead animal are examples. But species of any size may show exponential growth under the right conditions. A population of the Scots pine (Pinus sylvestris) grew exponentially when it began colonizing the British Isles after the end of the last ice age. Receding glaciers had left conditions ideal for their expansion. Logistic Growth When it happens, exponential Logistic Growth growth rarely lasts long. After all, if a single species increased exponentially for many generations, it would take over the planet. Instead, most populations are eventually constrained by limiting factors. Limiting factors are characteristics of the environment that limit population growth. Limiting factors determine a population’s carrying capacity. Carrying capacity is the largest population size a given environment can sustainably support. Logistic growth describes how a population’s initial exponential increase is slowed and finally stopped by limiting factors. Figure 12 shows the S shape of logistic growth. Notice that the population size increases sharply at first, but then begins to level off as the effects of limiting factors become stronger. Eventually, population size stabilizes around its carrying capacity. lg row th ne nti a Time Figure 12 Logistic Growth The logistic growth curve shows how population size may increase rapidly at first, then grow more slowly, and finally stabilize at the carrying capacity. Population Growth in Nature The logistic curve is a simplified model, and real populations, like those shown in Figure 13, can behave differently. Some may fluctuate, or cycle, indefinitely above and below the carrying capacity. Others may rise quickly, overshoot the carrying capacity, and then crash. Carrying capacities are not fixed. As limiting factors in an environment change, so does its carrying capacity. Plants in the understory of a dense forest, for example, may be limited by the amount of sunlight available. If a large tree dies, however, and sunlight pours in through a gap overhead, then the carrying capacity for understory plants in the area may increase. Fluctuating Rise and Crash 2500 Population size Population size 6000 4000 2000 0 20 60 Time (day) 2000 1500 1000 500 0 1910 100 Data from Huffaker, C.B. 1958. Experimental studies on predation: Dispersion factors and predator-prey oscillations, Hilgardia 27: 343–383. (a) Limiting factors: • Water • Space • Food • Predators • Disease Ex po Population Size Carrying capacity Stabilized population size 1920 1930 1940 Time (year) 1950 Data from Scheffer, V.C. 1951. Rise and fall of a reindeer herd, Scientific Monthly 73: 356–362. (b) Figure 13 Population Growth in Nature Population growth in nature does not often follow an idealized logistic curve. (a) The population size of some organisms, such as mites, fluctuates around its carrying capacity. (b) Some populations grow rapidly and use resources too quickly, causing their numbers to crash suddenly. Reindeer introduced to the Bering Sea island of St. Paul showed this pattern. Population Ecology 115 Limiting Factors and Biotic Potential Limiting factors and biotic potential regulate a population’s growth. ANSWERS Reading Checkpoint Severe weather can affect a population regardless of the population’s density. Lesson 3 Assessment 1. –3/1000; smaller 2. During exponential growth, the population continues to grow by a fixed percent. Logistic growth starts out as exponential growth, but then limiting factors cause the growth rate to slow down and even out. Logistic growth is more common in nature. 3. A limiting factor is any environmental characteristic that limits population growth. Biotic potential is the maximum offspring an organism can produce under ideal conditions. 4. Answers will vary. Figure 14 Density Dependence Competition is a density-dependent limiting factor. Here, great blackbacked gulls are fighting over a fish. The more gulls, the more intense the competition. Limiting factors slow population growth either by decreasing birthrates or immigration, increasing death rates or emigration, or some combination of these events. Some limiting factors have more of an effect in dense populations. Other limiting factors affect all populations in the same way. Density-Dependent Factors Recall that high population density increases competition for resources such as food and water. Competition, as shown in Figure 14, is a density-dependent factor because its influence changes with population density. The higher the population density, the less food and water will be available per individual. In turn, this causes competition for those resources to intensify. Predation and disease are two other examples of density-dependent factors. Density-Independent Factors Density-independent factors are limiting factors whose influence is not affected by population density. Catastrophic events such as floods, fires, and landslides are considered density-independent factors. It does not matter if the original population was dense or not. The result is always the same: a dramatic and sudden reduction in population size. A change in the region’s climate brought devastation to the golden toads of Monteverde. In the spring of 1987, unusually warm and dry conditions caused the breeding pools used by the toads to dry up almost completely. In the process, nearly all of the eggs and tadpoles within the pools were killed. This climate change was a density-independent factor that caused the golden toad population to crash. As you’ll recall, by 1990, the toads were extinct. Reading Checkpoint hy is severe weather considered a density-independent W factor? (a) (b) Biotic Potential Limiting factors from an organism’s environment provide only half the story of population regulation. The other half comes from the characteristics of the organism itself. For example, organisms differ in their biotic potential, or maximum ability to produce offspring in ideal conditions. Many factors influence biotic potential, including gestation and generation time. Gestation time is how long it takes for an embryo or fetus to develop and be “born.” The span from an organism’s birth to the time it has its own offspring is called generation time. Generation time is largely controlled by how long it takes an animal to reach sexual maturity. The number of offspring born at a time also affects biotic potential. Cabezon, or “scorpion fish,” and orangutans (Figure 15) vary greatly in their biotic potential. Once female cabezon are mature, at about 3 to 5 years old, they can release 50,000 to 100,000 eggs every year. Once fertilized, the eggs take just 12 to 16 days to hatch. Clearly, scorpion fish have a very high biotic potential. Orangutans, however, have a very low biotic potential. Females are not sexually mature until they are about 10 years old, and they give birth to a single baby only about once every 8 years. Populations of individuals with high biotic potential recover more quickly from declines than those of individuals with low biotic potential. Figure 15 Biotic Potential Organisms differ in their biotic potential. (a) Cabezon, like many fish species, have a very high biotic potential. In just one year, a female can release tens of thousands of eggs. (b) Female orangutans, however, usually only have three or four offspring in their lifetime. 3 1. Calculate A population has a birthrate of 10/1000, a death rate of 9/1000, an immigration rate of 3/1000, and an emigration rate of 7/1000. What is the population’s growth rate? Is the population getting larger or smaller? 2. Compare and Contrast What is the difference between exponential growth and logistic growth? Which is more common over long terms in nature? 3. Apply Concepts In your own words, define limiting factor and biotic potential. 4. You are a population ecologist studying white-tailed deer populations in your state. Populations have been growing exponentially for some time, and food is becoming a limiting factor. Many deer are dying of starvation, and others are in bad health. What do you recommend to state officials? Should people intervene and try to limit deer populations through relocation or hunting? Or should they do nothing and wait for the population to regulate itself? Explain your reasoning. Population Ecology 117