This chapter places your study of human biology in the

broader context of the whole living world (1.3).

You will gain a global perspective on the energy and raw

materials that sustain living organisms (2.1, 3.13). You will

also learn how a global water cycle makes water available

for life processes in organisms (2.5).

Key ConCepts

LinKs to earLier ConCepts

Principles of Ecology

Ecology is the study of interac-

tions of organisms with one another

and the environment. Energy flows

through ecosystems, passing from

organism to organism by way of

food webs. Sections 24.1–24.3

Chemical Cycles

Nutrients cycle in ecosystems.

In biogeochemical cycles, water

and nutrients such as carbon and

nitrogen move from the physical

environment to organisms, then

back to the environment. Sections

24.4 –24.6

Top: George H.H. Huey/Encyclopedia/Corbis; Bottom: © Cengage Learning

Firefighters in dry areas of the western United States

are bracing for an increasingly busy future. Our planet is warming, a

trend that experts expect will change climates and weather patterns

for the long term. In the mountains of western North America, there will

be less snow, drier forests and grasslands, and more intense wildfires.

Elsewhere, climate change is linked to melting glaciers, rising sea

levels, and other impacts that will ripple through Earth’s ecosystems

and affect a variety human activities, from where people choose to

live to the kinds of crops farmers can grow.

In this chapter we start thinking about the science of ecology:

how we and other organisms interact with our environment.

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prinCipLes of eCoLogy

24

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