Chapter 18

Ecosystems and Human Populations

18.1 Ecosystems

Lesson Objectives

• Discuss the importance of chemical and physical factors to living organisms.


• Describe the role of different species in an ecosystem.


• Describe the function of an ecosystem, and how different species fill different roles in
  different ecosystems.


• Describe energy transfer from the lowest to the highest trophic level in a chain, includ-
  ing energy loss at every trophic level.


• Discuss how materials are cycled between trophic levels and how they can enter or
  leave a food web at any time.

Introduction

An ecosystem is made up of the living creatures and the nonliving things that those creatures
need within an area. Energy moves through an ecosystem in one direction. Nutrients cycle
through different parts of the ecosystem and can enter or leave the ecosystem at many points.

Biological Communities

Apopulationconsists of all individuals of a single species that occur together at a given
place and time. Aspeciesis a single type of organisms that can interbreed and produce
fertile offspring. All of the populations living together in the same area make up acom-
munity. Anecosystemis all of the living things in a community and the physical and
chemical factors that they interact with. The living organisms within an ecosystem are its