Since plants are a sedentary life form (not mobile like animals) they have evolved a number of means to disperse throughout the environment and to persist under conditions not always favorable to native species.  In addition, human activities often facilitate the dispersal of invasive plants in a number of ways.  In this unit the students will learn some of the mechanisms by which invasive plants spread throughout the ecosystem.  From windblown seed, to a hike across muddy fields, to the innocent purchase of wildflower seed in an effort to establish native plants, invasive weed species are spreading through our environment at alarming rates.

As seen in the Ecosystem Unit , native plants occupy particular ecological niches, within communities that occupy specific habits.  In a balanced ecosystem, natural systems of competition, predation, herbivory, disease and natural forces (such as fire) have evolved together (the Impacts Unit describes how these forces disrupt biodiversity).

Most alien weed species, like our native species, are also a part of a balanced ecosystem within their natural range.  The majority of alien weed species are native to other countries, predominantly Europe, the Mediterranean, and Asia.  Within those areas, plants function in the same manner as our own native species: in balance with nature.  Alien species become invasive if they are able to become established where natural forces that keep them in balance no longer exist.  If the animals or insects that feed on a plant, if the diseases that normally affect the plant, and if the competition that normally restricts their spread are removed, there is every likelihood that an alien plant will be successful in it's new habitat.  These events, combined with some general traits shared by most invasive plant species (high seed production, extensive root systems, and an ability to survive a wide range of environmental conditions), further enhance the ability for these plants to become invasive. 

In general terms, of every 100 species introduced to our country, 10 will likely become established, and one will become invasive.  Although 1% doesn't seem significant, when students calculate the exponential growth of alien plant species in a landscape devoid of natural limiting factors, the potential invasion of that 1% becomes a wildfire of biological pollution.  In 1998 the estimated acreage in California invaded by yellow starthistle was 41 million acres.  Eastern Oregon, Washington and parts of Idaho are inundated with starthistle from a single seeding project intended to restore burned over land during wildfires of the 1980s.  Starthistle seed was unknowingly a major contaminant of the grass seed used in the restoration project, and now affects ranch animals, agriculture, recreation, and the general economy of the area to this day.

In this unit students will discover how weeds can spread so quickly, and in the process completely disrupt the balance of natural ecosystems.  They will explore the sources of weed species and how human activities have encouraged both the establishment and eventual invasion of Aliens In Your Neighborhood.

Links to Activities

Your Curriculum - Many life science curricula follow a pattern of exploration in the study of plants.  Plant reproduction and seed development are essential topics of study and are the foundation to an understanding of how invasive plant spread. The following lessons can be incorporated with your regular curriculum in Life Science, ecology, the biosphere, plant reproduction, and the mechanism of plant dispersal in the environment. When you combine these lessons with your own curriculum you take the first step to integrating invasive plants species as a unifying theme.

Alien Species Research Discussion
Alien Seed Bank
Alien Shopping Spree
Aliens In Your Socks
Alien Invasion - Math

The glossary at the end of each lesson provides additional keywords that can assist you with background information for the integration of concepts similar to your own curriculum.

Last updated 06/26/03