In order to determine the effect of weeds or non-indigenous plant species (NIS) on an area and the appropriate weed management steps to reduce any negative effect, you first need to define the land use/management goals.  This step is important as it helps you untangle what you want from the land and which species you should be most concerned with: these things are often entwined. Begin with a description of the site, including specific biological communities, habitat type, rare or valued species, land use history, and the parties responsible for management.  Discuss adjacent lands and how their management may influence the management area.  

Once the site description is complete, state clearly and specifically the land management goals for the area.  The goals should be stated so that the outcome is specific and can be quantified.  For example, you may be managing for sustainable forage production for X number of cattle, conservation of a wilderness area to maintain high biodiversity, protection of one or several rare species, creation of a migratory corridor for wildlife, or maintenance of a healthy productive forest for logging.  The more specifically you can describe your goals, the more effective the weed management plan will be.  For example, if you are trying to preserve riparian plant communities it would be useful to clarify whether you are focused on preserving plant communities solely as wildlife habitat or as areas of native plant diversity as well.  If your goal is to preserve a whole community, mention if there are any specific species that you are most focused on conserving.  If there are several management goals, it is important to prioritize and list goals as primary or secondary.

In large areas it may be necessary to divide the land into management sub-units.  For example, if you were managing a forest service district you may have different land management goals for different areas, such as increasing forage for grazing in some regions and maintaining wilderness areas in others.  In this scenario the weed management objective may be to increase non-native grasses such as smooth brome (Bromus inermis) and timothy (Phleum pratensis) in some areas while preventing the introduction or spread of those same species in the wilderness areas.

Define NIS Management Objectives to Achieve Goals

In order to define NIS management objectives, it is first necessary to identify any NIS which may occur in the management area.  The definition of a NIS is a non-native plant species introduced to an area by people, either intentionally or accidentally.  A NIS population is considered invasive when it is increasing in density and/or spatial extent. 

First you should name all NIS for which there is evidence of occurrence in the management area.  This “evidence” generally falls into one of four categories, which can drawn upon and weighted according to one’s management directives.  The following forms of evidence are listed in order of increasing certainty of their accuracy:

  1. Local historic knowledge
  2. Popular literature, web sites, extension bulletins, etc.
  3. Peer reviewed scientific journal articles
  4. Local quantitative measure of the species

Beside each NIS you need to make an estimate (guesstimate) of the potential effect (impact) the species could have on your land management goals, again stating the evidence as described above. For example, saltcedar (Tamarix spp.) may change surface soil salinity and prevent growth of native riparian species, thus decreasing habitat quality and diversity.  The spread of saltcedar would therefore interfere with land management goals if the purpose of the area were to protect wildlife habitat or maintain high levels of biodiversity. However, if your management area was in northwestern USA you should note that this information is from the literature (3 above) but that the data were collected in the southwest USA and therefore the species may behave differently in your area.

Next, you effectively rank your NIS, and how many you can target for management will depend on your resources.  To do this you need to state concrete objectives for the management of each species. For instance, is the objective to increase the cover and biomass of certain non-native grasses for forage, or to reduce the number or cover of NIS populations that may be degrading habitat quality?  A critical point here is creating quantitative objectives so that progress can be measured.  For example, one objective may be to reduce species X by Y% throughout the entire area, or it may be to reduce cover of this species only along trails and roads.  Defining the objective in this way means that you can determine if you achieved it – if you just said "reduce NIS" that would be impossible for anyone to "prove".  Describe NIS management objectives for each region or habitat within the management area (i.e. riparian areas, trails and roads, grasslands etc.).  If you find that the presence or potential impacts of certain NIS may not actually affect overall land management goals, these are the species which do not need to be managed  - at least initially, monitoring these populations may alter this over time.  
Finally in your NIS management plan you need to discuss / define how the NIS management objectives will facilitate the achievement of overall land management goals.  It is best to adopt an adaptive management approach and understand that land management goals and NIS management objectives may change because prediction of management outcomes in ecosystems is inherently imprecise.  It is necessary to monitor populations and ecosystems.