The National Park Service manages over 330 different parks, monuments, memorials, battlefields, historic sites, pathways, trails, recreation areas, seashores, and lake shores and have many different objectives. They are responsible for preserving natural landscapes and ecosystems, preserving and interpreting the country's historic and cultural heritage, and providing certain types of recreation. While most of the National Parks are cultural areas (such as battlefields and historic sites), out west large spectacular parks preserve wildlife, protect wilderness areas, and provide spectacular scenery on a scale not found elsewhere in state or local parks. These parks originated from federal lands that were set aside by Congress. Generally, national parks are more prestigious that national monuments(which may be designated by a president).

Saguaro National Park, like other Parks, has the primary purpose of preserving nationally significant natural resources (plants and animals). They are to provide "present and future visitors with the opportunity to see and appreciate the natural scenery and the native plant and animal life". The park must maintain all ecosystems in a "dynamic equilibrium that does not upset the interdependent biomasses of plant and animal life". Therefore, alien (invasive or non-native) plant species are not welcome. Alien plant species can and do compete with native plants for sun, water, soil nutrients, and space. They can often displace the native plant species by reproducing faster than the natives. Many alien plants do not have any natural pests or predators in the habitat that they are introduced to; consequently, they have an advantage over the natives. Animals that depend on these native plants for food or shelter are also disrupted because the variety of the native habitat is reduced. The less variety of plants in an area, the less variety of animals. This can completely change the "natural scenery" the park is supposed to protect.

The students are introduced to National Parks and their purpose through discussion, readings, and videos. Lead into a discussion about alien species, their impact on the environment, and control strategies. Review basic ecological concepts including: niches, competition and endangered species. Discuss the three ways alien (non-native) species are contained or removed from a habitat: mechanical (physically get rid of the plants; ex. pull weeds, burn them, trap and remove animals), chemical (use pesticides to kill them) or biological (use natural pests/disease agents and predators). Make certain that they understand that alien plants compete with native plants, often at an advantage because it is not their natural habitat, and that alien plant species can be controlled mechanically, chemically or biologically. The following student activities are recommended: