Organisms do not live in isolation. Besides the internal hierarchy of the individual organism, there is also an external hierarchy of the biological world (Figure 1.10B). The populations of all the species that live and interact in a defined area are called a community. Communities together with their abiotic, or physical, environment constitute an ecosystem.

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Individuals in a population interact in many different ways. Animals eat plants and other animals and compete with other species for food and other resources. Some animals prevent other individuals of their own species from exploiting a resource, be it food, nesting sites, or mates. Animals may also cooperate with members of their own species, forming social units such as a termite colony or a flock of birds. Such interactions have resulted in the evolution of social behaviors such as communication and courtship displays.

Plants also interact with their external biotic and abiotic environment. Plants living on land depend on partnerships with fungi, bacteria, and animals. Some of these partnerships are necessary to obtain nutrients, some to produce fertile seeds, and still others to disperse seeds. Plants compete with each other for light and water and have ongoing evolutionary interactions with the animals that eat them. Through time, many adaptations such as thorns and toxins have evolved that protect plants from predation. Other adaptations such as flowers and fruits help attract the animals that assist in plant reproduction. The interactions of populations of plant and animal species in a community are major evolutionary forces that produce specialized adaptations.

The major ecosystems of Earth that cover broad geographic areas with distinguishing physical features and communities of organisms are known as biomes. Examples of biomes include Arctic tundra, coral reefs, and tropical rainforests. All of the biomes on our planet make up the biosphere. The ways in which species interact with one another and with their environment in populations, communities, and ecosystems is the subject of ecology, covered in Part Ten of this book.