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KEY CONCEPT Populations respond to pressures. . Sunshine State STANDARDS SC.G.1.3.2: The student knows that biological adaptations include changes in structures, behaviors, or physiology that enhance reproductive success in a particular environment. SC.G.2.3.3: The student knows that a brief change in the limited resources of an ecosystem may alter the size of a population or the average size of individual organisms and that long-term change may result in the elimination of animal and plant populations inhabiting the Earth. BEFORE, you learned NOW, you will learn • Four characteristics are used to describe a population • Scientists study these four characteristics to predict population change • About limits to population growth • How population density affects limiting factors • About two reproductive strategies found within populations EXPLORE Population Density How does population density vary? PROCEDURE 1 Choose three different locations in your school where you can observe how many people enter and leave an area during a specific time period. MATERIALS • stopwatch • notebook 2 Position three people at each location a counter, a timekeeper, and a recorder. 3 Count the number of people who pass through the area for at least 2 minutes. Record the number. VOCABULARY immigration p. 537 emigration p. 537 limiting factor p. 538 opportunist p. 541 competitor p. 542 CHOOSE YOUR OWN STRATEGY Use a strategy from an earlier chapter or one of your own to take notes on the main idea: Population growth is limited. 536 Unit 4: Life Over Time 4 Compare your data with the data collected by 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. Population growth is limited. No population can grow forever. Every population has a limit to its growth. For example, the cockroach has been around for more than 300 million years. This insect has outlived the dinosaurs and may persist long after humans have become extinct. Yet even if cockroaches became the only species on the planet, several factors would limit their population size. Birth, Death, Immigration, and Emigration When scientists study how a population changes, they must consider four things: birth, death, immigration, and emigration. There is even a simple 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 population growth, or that many deaths mean population decline. Immigration is the movement of individuals into a population. For example, if a strong wind blows the seeds of a plant from one area into another, the new plant would be said to immigrate into the new area. Immigration can increase a population or help stabilize a declining population. Birth and immigration introduce individuals into a population. Emigration is the movement of individuals out of a population. If resources become scarce within a habitat, some of the individuals might 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 that lead 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. These incoming seagulls become part of the seagull population that is already living at the dump. A raccoon population living in the same area has been eating the seagulls’ eggs, causing the number of seagull births to decrease. If enough seagulls immigrate to the dump, the seagull population would increase, making up for the decrease in births. Immigration would help keep the population stable. The seagull population would also increase if part of the raccoon population moved away. 537 Limiting Factors When a population is growing at a rapid rate, the birth rate is much higher than the death rate. That means that more individuals are being born than are dying during a particular time period. There are plenty of resources available, and the population size is increasing rapidly. Eventually, however, the population will stop growing, because a habitat can support only a limited number of organisms. A limiting factor is a factor that prevents the continuing growth of a population in an ecosystem. Abiotic, or nonliving, limiting factors include air, light, and water. Other limiting factors can be living things, such as other organisms in the same population or individuals belonging to different species within the same area. Check Your Reading What are two limiting factors? Competition can occur between different populations sharing the same habitat. Competition can also occur among individuals of the same population. Suppose, for example, that a population of deer in a forest preserve were to increase, either through births or immigration. Population density at the forest preserve would go up. More and more deer in that area would be competing for the same amount of food. Density-Dependent Factors Density-dependent factors have a greater effect on populations with many individuals in a small area. Factors may include • • • • 538 Unit 4: Life Over Time Competition Disease Parasitism Predation Effects of Population Density In the situation described above, the seagull population could decrease as a result of competition for food. Competition is an example of a density-dependent factor—that is, a limiting factor that affects a population when density is high. Disease is another density-dependent factor. The more crowded an area becomes, the easier it is for disease to spread, so more individuals are affected. If population density is low, there is less contact between individuals, which means that disease will spread more slowly. Density-dependent factors have a greater effect on the 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 factors have the same effect on a population, whether it has a high density or a low density. Freezing temperatures could be considered a density-independent factor. A freeze might kill all of the flowering plants in an area, whether or not the population density is high. A natural event such as a wildfire is another example of a densityindependent factor. When a wildfire occurs in a forest, it can wipe out an entire ecosystem. Check Your Reading How are limiting factors that are density-dependent different from limiting factors that are density-independent? Density-Independent Factors Density-independent factors are typically changes in weather. These factors affect low-density and high-density populations equally. Factors may include • • • • • Drought Hurricanes Tornados Fires Floods Chapter 15: Population Dynamics 539 Limiting factors include nonliving factors in the environment and natural events such as earthquakes, fires, and storms. During times of drought, there may not be enough food to meet the needs of all the organisms in an area. The quality of the food declines as well. For example, a lack of water may cause a population of trees to produce fewer pieces of fruit, and the fruit itself may be smaller. If there is little food available, a condition called famine arises. If the famine is severe, and if death rates exceed birth rates, then the population size will fall dramatically. 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 these limits. In Section 15.3 you will read about how the human response to limits differs from that of other biological populations. Limiting Factors What limiting factors determine plant growth? SKILL FOCUS Designing experiments Using the materials below, design an experiment to test how limiting factors such as sunlight or water can determine how well a plant population will grow. PROCEDURE 1 Decide how to use the beans, soil, and water. 2 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. 540 Unit 4: Life Over Time MATERIALS • • • • 6 paper cups potting soil beans water TIME 20 minutes Populations have distinct reproductive survival strategies. Although reproduction of offspring is not necessary for the survival of an individual organism, it is necessary for the survival of a species. Scientists studying populations observe patterns in the reproductive strategies used among species. There are two main strategies that many species use. There are also many species whose strategies fit somewhere in between. Strategies of Opportunists Opportunists are species that reproduce rapidly if their population falls below carrying capacity. They share many characteristics, including a short life span and the ability to reproduce large quantities of offspring. Their population size tends to change often, and opportunists live across many areas. Opportunists include algae, dandelions, bacteria, and insects. These species can reproduce and move across an area quickly. In addition, they can adapt quickly to environmental changes. Populations of opportunists often grow rapidly. FLORIDA Content Preview reminder Species reproduce either asexually or sexually. Read more about this on page 817. VOCABULARY Remember to make a word magnet for the term opportunist. Include examples in your diagram. Opportunists Pine trees are opportunists that can spread across an area quickly. Pine cones release huge amounts of pollen into the air. Chapter 15: Population Dynamics 541 Competitors Wolves are examples of competitor species. These cubs will be cared for by adults until they are able to hunt. Strategies of Competitors You might be familiar with the term competitor as meaning an organism that struggles with another to get resources. Scientists who study population growth use the term competitor in another way. Competitors are species with adaptations that allow them to remain at or near their carrying capacity for long periods of time. Competitors have many characteristics that differ from those of opportunists. Species that have a competitive reproductive strategy often live longer and have fewer offspring. Elephants and saguaro cacti are two examples of competitors. The offspring of competitors take longer to develop than those of opportunists. Also, animals with this strategy tend 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. KEY CONCEPTS CRITICAL THINKING 1. What four factors do scientists consider when they measure population change? 4. Analyze Why would it be a mistake to predict population growth based on birth rate alone? 2. Give two examples of densitydependent factors and two examples of densityindependent factors. 3. Other than life span, how do opportunists and competitors differ? 542 Unit 4: Life Over Time 5. Apply Give an example of a factor that limits a population near you. CHALLENGE 6. Synthesize There has been an oil spill along a waterway famous for its populations of seals, dolphins, and sea birds. Six months later, all populations show a decline. Explain what factors might have caused such a change and whether the oil spill was a density-dependent or densityindependent factor or both.