One important reason for this difference is the sedentary lifestyle of plants. Some animals, such as barnacles and sponges, are sedentary, but most animals are capable of moving. When conditions become uncomfortable, animals can move to another location. Plants are generally unable to move to a new location. They must cope with the circumstances that surround them. For example, if a porcupine began to nibble your leg, you would probably try to run away. However, if that porcupine began to nibble a plant, the plant could not move away. Plants must survive in their environment by producing noxious chemicals, growing spines, and generating sticky saps. Plants cannot move when they are too hot, cold, or thirsty, or when dramatic changes occur to their environment.

Invasive plant species are able to invade an area because these species have developed special adaptations that enable them to survive a wide range of environmental conditions. When an environment is altered and disturbed, usually at the hand of humankind, the delicate web of environmental connections is broken, thus changing the conditions to which plants and animals have adapted. Invasive plant species have developed mechanisms that enable them to cope with environmental changes, establish themselves in disturbed environments, establish themselves before native species can recover, and crowd out the more finely-adapted native species. Invasive plants have developed the following adaptations that enable them to out-compete native species:

	Seed heads with thousands of seeds in each pod
	Seeds that germinate within a wide range of temperature and moisture conditions
	Rapid growth patterns that use soil nutrients before native plants can use them
	Extensive root systems that rob native plants of precious water reserves

To understand the effect of invasive plant species, it is important to first understand what makes an ecosystem unique. This unit focuses on extending students’ understanding of the following concepts:

Lessons in the Weed Facts unit provide students with a comprehensive introduction to invasive plants by covering the following topics:

What is a Weed – Defines the term weed and introduces students to the problem of weeds.

An Ecosystem is Like a Bowl of Milk – Discusses ecosystems, cycles within ecosystems, and how invasive plant species impact a stable ecosystem.

Where in the World Do They Come From? – Compares and contrasts human dispersal mechanisms with those of weeds, discusses adaptations that enable weeds to spread, and explains negative consequences that result from the spread of weeds.

The Great Race for Survival – Explores how adaptations enable invasive plants to establish themselves and out-compete native plants and the consequences of an alien plant invasion.

When is a Rose Not a Rose? – Investigates the diversity of invasive plant characteristics and adaptations by having students conduct plant research.

Ecosystem Canaries: Assessing Rangeland Health – Introduces rangeland assessment, its purpose, and methods of conducting an assessment.

Alien Species Research Discussion – Compares and contrasts similarities in habitat and plant growth characteristics among invasive weed species.

Alien Invasion – Investigates the means by which invasive weeds spread and the rate at which they spread.

Make-Believe Aliens – Allows students to express their scientific knowledge of plants in an artistic manner by creating an imaginative, make-believe alien weed.

Facts and Fiction – Encourages students to conduct research on the origin of a noxious weed in their area and then express their knowledge in a creative manner.

Weeds of Mass Destruction – Explores how seeds are dispersed by weeds, humans, and animals and how weeds can disrupt life.

Gone With the Wind – Explores the role that wind plays in seed dispersal.

Curses! Foiled by a Seed – Introduces the field of forensic science, especially forensic botany.