In previous units students saw how alien weed species came to be in The Invasion, and the manner in which the exponential growth of weeds allow them to spread when natural limiting factors are not present.  Once weeds are established, students investigated The Impact upon biodiversity and the aesthetic and practical consequences to humans.  In this final unit before students embark on inventory and monitoring of weeds in their community, students will investigate the human response to this biological pollution through chemical, mechanical (including fire), and biological control.

It is important to remember that controlling alien plant species is a response to biological pollution, to a problem that will persist unless restoration of a healthy environment also becomes part of the solution.  Pulling weeds may slow the spread of weeds, but it does not alter the conditions that first favored the invasion.  Students who participate in NatureMapping, the capstone unit of Aliens In Your Neighborhood, contribute valuable information about the entire ecosystem affected by alien weeds.  By understanding the complex web of interactions, land managers may better prescribe a combination of control methods that eliminate the alien species, as well as, obtain from the students valuable information for the restoration of the land.

Alien weed species have the advantage of few natural enemies and an ability to persist in a wide range of habitat and environmental conditions.  For these reasons, it is rare that a single method of control is effective... as any child will tell you, after being told to weed the garden, it is a job that never seems to go away!  Besides hand pulling, land managers have a variety of other means for controlling invasive weeds.

Chemical control includes a wide range of methods to apply herbicides (or for non-plant alien species, insecticides, fungicides and pesticides).  Concerns about chemical control include pollution of the land and water, the killing of non-selected plants and animals, and the ability for chemicals to persist and accumulate in the environment.  The potential to persist and accumulate in both abiotic and biotic, non-target areas is particularly disturbing when it may take many years, if ever, to discover the effects.

Mechanical control may include that dreadful call to pull weeds by hand, or remove them with weedeaters, chain saws, and mowing (both rotary and flailing).  Like chemical controls, many of these methods are not specific to the alien weed, e.g. machinery typically cut, chip and grind everything in their path (not to mention the insects, small mammals and reptiles along the way).  For some alien species, and with the proper timing during the plant's growth cycle, mechanical control methods can be effective.  Fire is included in this category as one other method for controlling weeds.  Of important consideration in the use of fire is the timing and understanding of fire's effect on the plant.  Many plants are rated with regard to their resistance to fire; a plant with low resistance to fire (flammable) may respond to control by fire.  Fire is not necessarily the best control for all plants with low resistance though, for example, cheatgrass is highly flammable (low resistance) but it's prolific seeds are resistant to fast, low temperature fires, and may also benefit from the nutrients released to the soil by the fire.  Mechanical control and fire may have significant costs due to the labor intensiveness of the work, may accelerate the invasion rather than eliminate it, and requires specific knowledge of the target plants and the timing of the work.

A third approach used to control invasive plants is biological control.  Biological control involves some change in the environment that prevents the opportunity for alien species to dominate an ecosystem.  The most common form of biological control is the introduction of a host specific herbivore.  Host specificity is the ideal, for if an insect only feeds upon a particular plant, the plant is eradicated as the insect attacks, and the insect is eradicated as the food source disappears.  Unfortunately nature doesn't always work so smoothly.  Because alien weed species arrive without their natural herbivores, the insect must be imported as well, which introduces yet another alien to the ecosystem.  More than once an imported predator has found native species tastier than the target, and suddenly the new alien is eliminating the competition for the alien plant it was intended to destroy.  Biological control also includes changing the environment (ex. through flooding or drying) in an effort to create extreme conditions where the alien cannot survive (and unfortunately where some native species also suffer).  In the past few years gene manipulation has become another potential method for controlling invasive species.  One additional use of biological control involves the concept of facilitation.  Facilitation is a relationship between two or more species which benefits both species.  Recent research on Santa Cruz Island off the California coast demonstrated how honey bees (Apis sp.), itself an invasive species, facilitated the spread of yellow starthistle in their role as pollinators.  The introduction of host specific mites on the honey bees was used to eliminate the bees in the hopes that the rate of starthistle invasion would be reversed.  Final data is not yet in on this research (see Barthell, J. F., R. W. Thorp, A. M. Wenner & J. M. Randall. 2000. Yellow star-thistle, gumplant and feral honey bees on Santa Cruz Island: a case of invaders assisting invaders. Pages 269-273. In D. R. Browne, K. L. Mitchell and H. W. Chaney (eds.), Fifth California Islands Symposium. Santa Barbara Museum of Natural History, Santa Barbara, California. MBC Applied Environmental Sciences).  

Regardless of the methods used, whether introducing aliens to fight aliens, altering an ecosystem that once was in balance, or manipulating the genetic code, biological control has complex ethical issues, and short or long term consequences to the natural world.

More often we find land managers using a combination of several methods, especially when the combination can narrow the range of impact to other species, i.e., when the control is host-specific.  This unit allows students to investigate various methods of control and evaluate the effectiveness of that control on various species.  Because of the complexity involved with controlling alien species, the intent is not to teach students how to control invasive weeds, but rather, to evaluate methods that might be most effective, and more importantly, to understand  why some methods are inappropriate. The final NatureMapping Unit provides students with the opportunity to inventory and map infested areas in order to understand the complexity of the target area, secondly to apply their knowledge of various control methods, and ultimately to participate in the restoration of the land, to contribute to healing.