This is the third PowerPoint population presentation in a series of six. Viewing a passenger bus, students wonder how many passengers might be able to climb aboard without breaking or damaging critical machinery. (They are surprised to imagine that of all the systems stressed by increased crowding, the restroom at the rear of the vehicle might be the first to fail.) The lesson then raises similar questions concerning earth's "carrying capacity" for an industrialized humanity and the "limiting factors" that tend to govern ultimate population sizes.

POPULATION GROWTH Populations can reach limits to growth. Carrying capacity is the maximum number of individuals that can be supported in a given environment. The limitation is not the availability of space, but the number of people in relation to resources and the capacity of earth systems to support human beings. Changes in technology can cause significant changes, either positive or negative, in carrying capacity. Populations grow or decline through the combined effects of births and deaths, and through emigration and immigration. Populations can increase through linear or exponential growth, with effects on resource use and environmental pollution. Various factors influence birth rates and fertility rates, such as average levels of affluence and education, importance of children in the labor force, education and employment of women, infant mortality rates, costs of raising children, availability and reliability of birth control methods, and religious beliefs and cultural norms that influence personal decisions about family size. NATURAL RESOURCES Human populations use resources in the environment in order to maintain and improve their existence. Natural resources have been and will continue to be used to maintain human populations. The earth does not have infinite resources; increasing human consumption places severe stress on the natural processes that renew some resources, and it depletes those resources that cannot be renewed. Humans use many natural systems as resources. Natural systems have the capacity to reuse waste, but that capacity is limited. Natural systems can change to an extent that exceeds the limits of organisms to adapt naturally or humans to adapt technologically. ENVIRONMENTAL QUALITY Natural ecosystems provide an array of basic processes that affect humans. Those processes include maintenance of the quality of the atmosphere, generation of soils, control of the hydrologic cycle, disposal of wastes, and recycling of nutrients. Humans are changing many of these basic processes, and the changes may be detrimental to humans. Materials from human societies affect both physical and chemical cycles of the earth. Many factors influence environmental quality. Factors that students might investigate include population growth, resource use, population distribution, overconsumption, the capacity of technology to solve problems, poverty, the role of economic, political, and religious views, and different ways humans view the earth. Living organisms have the capacity to produce populations of infinite size, but environments and resources are finite. This fundamental tension has profound effects on the interactions between organisms. Human beings live within the world's ecosystems. Increasingly, humans modify ecosystems as a result of population growth, technology, and consumption. Human destruction of habitats through direct harvesting, pollution, atmospheric changes, and other factors is threatening current global stability, and if not addressed, ecosystems will be irreversibly affected.

This is a self-contained unit and students can complete the PowerPoint presentation itself by simply reading each slide and using the mouse to click through the slides of the program. The enrichment activities suggested below, if assigned, offer opportunities to enhance student skills using search engines, entering URLs to access internet information, developing graphs, tables and charts, preparing PowerPoint or multimedia presentations, etc.

Would you like your students to apply the understandings gained in this unit to the real world? Consider a follow-up project involving the following student research and activities: Assign student teams to research a nation and all its characteristics (economic, political, resources, wildlife, climate, language, employment, etc.) Then the students are to take on the following role: They have been hired by the government of that nation to develop policy recommendations to optimize, insofar as possible, employment, freedom, protection of the wildlife, genetic resources, and the environment, cultural diversity, standard of living, levels of education, and sustainability. (1) Student teams choose a nation and use internet and digital resources to identify its: current population, system of government, per capita income, number of births, annual rate of growth. (2) Have the teams prepare a map, graphs, and a computer or multimedia presentation on the nation selected, identifying its major mountains, rivers, natural resources, cities, natural areas, native wildlife species, national parks, languages, agricultural areas, historical developments to date, etc. (3) Students use their data to: (a) estimate and project future conditions in the nation over the next 30-50 years; (b) make policy recommendations that will feed, clothe, and educate its population, optimize standard of living, health, and employment, minimize waste and depletion of resources, minimize levels of pollution, and maintain the climatic, biological, and genetic integrity of its natural systems. (4) Each team presents its results to the class and identifies their top five policy recommendations, the rationale supporting those recommendations, and the costs and sources of funding associated with implementing the recommendations. (5) Finally, each team should identify two "alternative futures" for the nation: One future should presume that the recommended policies are adopted and implemented -- the second future should presume that the policies are not implemented and existing policies, conditions, and trends continue. You may find the following web-sites useful: www.census.gov and www.cia.gov

(1) Students click through PowerPoint lesson (27 slides). (2) Students answer worksheet items based on lesson information. (3) Students take "self-quiz" (see closing screens of the presentation). (4) Students take turns reading worksheet items aloud, and specify the correct answer to each item. (5) Class discussion and further research may follow at the option of the instructor