Populations and Communities

Transcription

1 Populations and Communities Populations What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Key Concepts What defines a population? What factors affect the size of a population? Before Statement After 1. Some life exists in the ice caps of the North Pole and the South Pole. 2. A community includes all organisms of one species that live in the same area. The Biosphere and Ecological Systems Earth s biosphere (BI uh sfihr) is the parts of Earth and the surrounding atmosphere where there is life. The biosphere includes all the land of the continents and islands. It also includes all of Earth s oceans, lakes, and streams. It includes the ice caps at the North Pole and the South Pole. Parts of the biosphere with large numbers of plants or algae often contain many other organisms. What is a population? The Kalahari Desert in Africa is a part of Earth s biosphere. Several groups of meerkats live there in a wildlife refuge. Meerkats are small mammals that live in family groups and help each other care for their young. Meerkats interact with each other for survival. They sleep underground in burrows. They hunt for food during the day. They stand upright to watch for danger and call out warnings to others. Meerkats are part of an ecosystem. An ecosystem is a group of organisms that live together in an area at one time. It also includes the climate, soil, water, and other nonliving parts of the environment. The Kalahari Desert is one of many ecosystems that makes up Earth s biosphere. The study of all ecosystems on Earth is ecology. Make Flash Cards Think of a quiz question for each paragraph. Write the question on one side of a flash card. Write the answer on the other side. Work with a partner to quiz each other using the flash cards. 1. Explain Why are meerkats considered to be part of an ecosystem? Reading Essentials Populations and Communities 349

2 Community The figure below shows a family group of 2. Describe What defines a population? meerkats on the bottom right. Many species besides meerkats also live in a wildlife refuge in the Kalahari Desert. They include scorpions, spiders, insects, snakes, birds, zebras, giraffes, lions, shrubs, grasses, small trees, and melon vines. All these plants and animals form a community. A community is all the populations of different species that live together in the same area at the same time. Population All the family groups of meerkats that live in this refuge form a population. A population is all the organisms of the same species that live in the same area at the same time. A species is a group of organisms that have similar traits and are able to produce fertile offspring. Ecosystem Biosphere Population Visual Check 3. Identify Name three populations shown in the figure. 350 Populations and Communities Competition At times, there is not enough food for every organism in a community. Members of a population must compete with other populations and each other for enough food to survive. Competition is the demand for resources, such as food, water, and shelter, in short supply in a community. When there are not enough resources available to survive, there is more competition in a community. In the Kalahari Desert, where water is scarce, the meerkats compete with other animals for resources such as food and water. Reading Essentials Community

3 Population Sizes When there is less food available, a population of meerkats gets smaller. Female meerkats cannot raise as many young. Some meerkats might leave the area to find food elsewhere. If there is plenty of food, the size of a population of meerkats grows larger. More meerkats survive to adulthood and live longer. Changes in environmental factors can result in changes to the size of a population. Limiting Factors Environmental factors, such as available food, water, shelter, sunlight, and temperature, are possible limiting factors for a population. A limiting factor is anything that restricts the size of a population. If there is not enough sunlight, green plants cannot make food by photosynthesis. A lack of green plants affects organisms that eat green plants. Temperature is a limiting factor for some organisms. When the temperature drops below freezing, many organisms die because it is too cold for them to survive. Disease and predators animals that eat other animals can be limiting factors for organisms. Natural disasters such as fires and floods also limit the size of populations. Measuring Population Size Measuring the size of a population can be difficult. Biologists often use the capture-mark-and-release method to count and observe animal populations. A population of lynx in Poland is counted and monitored using this method. To use the capture-mark-and-release method, biologists capture several animals of a species. They sedate the animals and put a radio collar on each one. Then they release the animals back into the wild. The radio collars help biologists estimate the size of the population and track the animals movements. Population density is the size of a population compared to the amount of space available. Biologists estimate population density by a sample count. Suppose you want to know how closely together Cumberland azaleas (uh ZAYL yuhz), a type of flower, grow in the Great Smoky Mountains National Park. Rather than counting every azalea shrub, you would count only the azalea shrubs in an area, such as 1 km 2. By multiplying the number of square kilometers in the park by the number of azaleas in 1 km 2, you would find the estimated population density of azalea shrubs in the entire park. 4. Specify What factors affect the size of a population? Reading Check 5. Describe two ways you can estimate population size. Reading Essentials Populations and Communities 351

4 Make a half-book. Use it to take notes on the relationship between population size and carrying capacity. Carrying Capacity Reading Check 6. Define What is carrying capacity? 7. Summarize How can overpopulation affect a community? Biotic Potential Imagine that a population of raccoons has plenty of food, water, and den space. The population has no disease and is not in danger from other animals. The only limit to the size of this population is the number of offspring the raccoons can produce. Biotic potential is the potential growth of a population if it could grow in perfect conditions with no limiting factors. No population on Earth ever reaches its biotic potential because no ecosystem has an unlimited supply of natural resources. Carrying Capacity What happens when a population reaches its biotic potential? It stops growing when the available resources in the ecosystem are used up. The largest number of individuals of one species that an environment can support is the carrying capacity. A population grows until it reaches the carrying capacity of an environment. Disease, space, food, and predators are some of the factors that limit the carrying capacity of an ecosystem. The carrying capacity of an environment does not stay the same. It increases and decreases as the amount of available resources increases and decreases. At times, a population can briefly grow beyond the carrying capacity of an environment. Overpopulation Populations can grow so large that they cause problems for other organisms in the community. Overpopulation occurs when a population becomes larger than the carrying capacity of its ecosystem. For example, meerkats eat spiders. An overpopulation of meerkats causes a decrease in the size of the spider population in that community. Populations of birds and other animals that eat spiders also decrease when the number of spiders decreases. Elephants in Africa s wild game parks present another example of overpopulation. Elephant herds searching for food can cause tree damage. They push over trees to feed on treetops. Other animals that use those trees for food and shelter must compete with the elephants. Also, the loss of trees can damage the soil. This might prevent other trees and plants from growing in that area. 352 Populations and Communities Reading Essentials

5 Mini Glossary biosphere (BI uh sfihr): the parts of Earth and the surrounding atmosphere where there is life biotic potential: the potential growth of a population if it could grow in perfect conditions with no limiting factors carrying capacity: the largest number of individuals of one species that an environment can support community: all the populations of different species that live together in the same area at the same time competition: the demand for resources, such as food, water, and shelter, in short supply in a community limiting factor: anything that restricts the size of a population population: all the organisms of the same species that live in the same area at the same time population density: the size of a population compared to the amount of space available 1. Review the terms and their definitions in the Mini Glossary. Write a sentence that explains how limiting factors and biotic potential are related. 2. Identify each example in the flowchart as a population, an ecosystem, or a community. Write the correct term in the box with its example. a. the Amazon Rain Forest of Brazil 3. List two facts about population that you learned from your partner s flash cards. What do you think Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? b. all species of monkeys, parrots, frogs, and plants that live in the Amazon Rain Forest ConnectED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. c. all poison dart frogs that live in the Amazon Rain Forest END OF LESSON Reading Essentials Populations and Communities 353