Managing Invasive Plants: Concepts, Principles, and Practices link

U.S. Fish & Wildlife Service
America's National Wildlife Refuge System

MANAGING INVASIVE PLANTS: Concepts, Principles, and Practices

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Natural resource monitoring is another form of assessment that provides land managers with information essential to making well-informed management decisions. Monitoring can play a vital role in invasive plant management and prevention-it provides the justification and knowledge needed for evaluating management actions, and adjusting them if necessary, to reach invasive plant management objectives and sustainable land management goals more effectively and efficiently.

What Is Monitoring?

Photo of USFWS staff monitoring purple loosestrife populations for biocontrol effectiveness.
Monitoring purple loosestrife (Lythrum salicaria) populations for biocontrol effectiveness. Photo credit: D Tibbetts/ USFWS

The USFWS defines monitoring as a survey repeated through time to determine changes in the status and demographics of abiotic resources, species, habitats, or ecological communities (701 FW 2 Policy on Inventory and Monitoring).

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The terms monitoring and inventory/survey are sometimes used interchangeably but they have specific purposes. Elzinga et al. (1998) make the following useful distinction:

"Inventory can be described as a point-in-time measurement of the resource . . . Information collected during an inventory may provide a baseline, or the first measurement, for a monitoring study. Often, however, the necessary type and intensity of monitoring will not be known until the inventory is completed. The information collected during inventory, while useful for the development of a monitoring study, may not be useful for monitoring itself."

Diagram illustrating populations or patches of invasive plants and monitoring plots.
Monitoring involves sampling repeatedly from plots within populations to detect changes in a resource (e.g., vegetation).


  • is conducted on a regular or systematic basis
  • follows the trend over time of an indicator or variable of the resource compared to predetermined management objectives
  • involves collecting data by sampling or on the entire resource if financially and logistically feasible

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Why Conduct Monitoring?

A flowchart illustrating feedback loop between monitoring and adjusting management to achieve invasive plant management objectives.

Diagram demonstrating how monitoring is used to detect when management actions are achieving management objectives and when ineffective actions should be modified.

Monitoring can play an essential role in managing invasive plants-it provides nonbiased information to make well-informed management decisions. As discussed in the Planning module, monitoring is a key component when employing an Adaptive Management framework. Monitoring results can be used to demonstrate where management actions (e.g., control treatments) are effectively and successfully meeting invasive plant management objectives, and to more quickly detect and modify actions that are ineffective.

For example, if an invasive plant management objective is to determine the effectiveness of a chemical herbicide and prescribed burning treatment in reducing stem density by 35%, a monitoring protocol or plan could be developed to measure the treatment's effectiveness. If monitoring results indicate that the objective (reduction in stem density) is not being achieved, the management or treatment would be adjusted appropriately and monitoring would again be implemented.

In some management situations, formal monitoring may not be necessary when the outcome of an action is well known (Williams et al. 2007).

Monitoring can also be used to

  • detect new populations
  • determine the status and temporal trends in population sizes and distributions over time (e.g., evaluate invasiveness)
  • determine effects of invasive plant species on biota and processes of the ecosystem
  • measure success of restoration and revegetation projects
  • measure success of best management practices (e.g., during road and building construction, fire-fighting, etc.) that are meant to prevent the introduction and spread of invasive plants into and throughout a management area

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Types of Invasive Plant Monitoring

The type of monitoring to implement in a management program will depend upon the management strategy (i.e., prevention, early detection, control, restoration). Below, four types of invasive plant monitoring are described and examples of agency and university projects are provided.

1. Monitoring for Early Detection


Early detection monitoring is implemented before unwanted species have arrived in an area. It is the most cost-effective monitoring because when rapid eradication takes place, control efforts are minimal.


  • finding species when they first appear in a management area
  • performed on a systematic schedule; either a predetermined one (e.g., every two years) or one that is based on known events of vector transport of new species through pathways into new areas. For example, an influx of firefighting vehicles may serve as a vector for seed dispersal along newly created firelines that serve as pathways.


  • sample target areas using inventory/survey methods or using information from predictive models based on ecosystem attributes, species establishment characteristics, and vectors and pathways
  • record non-infested sites during each monitoring event; this is a key part of early detection monitoring that is sometimes overlooked
  • may be able to use volunteers for detection of well-known, easily identified plants, and skilled field botanists or plant enthusiasts for new species and those that are difficult to identify


Early Detection of Target Populations
Probability map showing where cheatgrass may occur on refuge.
Probability map showing where cheatgrass (Bromus tectorum) may occur on refuge. See report (left) to view full size map. Map credit: USGS
  • Cooperative project with National Elk Refuge, US Geological Survey National Institute of Invasive Species Science, and Natural Resource Ecology Lab - Colorado State University
  • The refuge will be developing a monitoring plan for early detection and eradication of target species using probability of occurrence maps.
  • Probability models of nonnative, single-species occurrences were developed with vegetation and soil data collected on plots and mapped vegetation data collected using a handheld mapping system.
  • Sampling and modeling approaches are described in Barnett et al. 2007 and An Inventory of Nonnative Plant Species on the National Elk Refuge (1.5 MB PDF).

2. Monitoring for the Effect of Management Actions on Target Invasive Plants


Monitoring the effects of management actions (i.e., a control treatment) on the target invasive plant populations is implemented unless the effects of that management action are already well understood and predictable. Such monitoring helps determine the most effective control method.


  • the effectiveness of control treatments (e.g., mowing, herbicide spraying, prescribed grazing and burning) in suppressing, containing or eradicating target invasive species is quantified


  • knowledge of target species characteristics and site conditions
  • monitoring before and after treatment events


Monitoring Effect of Herbicide Treatments on Perennial Pepperweed
Map showing monitoring plots  in different control treatments for perennial pepperweed.
Monitoring plots in different control treatments for perennial pepperweed. Map credit: Hogle et al. 2007
  • The San Pablo Bay NWR comprehensive control plan for perennial pepperweed (Lepidium latifolium) includes monitoring the effectiveness of herbicide treatments on perennial pepperweed patches that were prioritized by invasion risk.
  • Results from analysis of monitoring data are used to adjust management actions.
  • San Pablo Bay National Wildlife Refuge Lepidium latifolium Control Plan (3 MB PDF) (Hogle et al. 2007)
Monitoring Effect of Treatments on Prioritized Noxious Weeds
Photo of quadrat frame for estimating vegetation cover.
Quadrat frame for estimating vegetation cover. Photo credit: US Air Force

3. Monitoring for the Effect of Management Actions on Nontarget Species and the Environment


Monitoring for the effect of management actions on nontarget species and the environment is ideally employed when management actions are being implemented. Given time and money considerations, this type of monitoring can be used when it is suspected that native species or ecological processes may be negatively impacted. It can also help determine whether it is better to leave the invasive plant species untreated rather than risk damage to the ecosystem.


  • measuring the positive or negative effects of control treatments on other species (e.g., plants, animals, fungi, microbes) or ecological processes (e.g., soil stability, water quality). An example of a negative effect is contamination of ground or surface water by herbicides that are toxic to aquatic organisms. An example of a positive effect is an increase in abundance of desired plant species.


  • knowledge of target species characteristics and site conditions
  • knowledge of ecosystem components and processes in the area where treatments will occur
  • monitoring before and after treatment events


Effect of Management Treatments on Native Plant Species
Photo of Asian sand sedge population.
Asian sand sedge population. Photo credit: PCA- APWG
  • The effect of herbicide used to control Asian sand sedge (Carex kobomugi) on adjacent native plants was studied on coastal dunes in New Jersey.
  • Although this is not a true monitoring project because the study occurred only once, the methods could readily be extended into a long-term monitoring protocol.
  • Carex kobomugi - Louise Wootton, Georgian Court University (Wootton)

4. Monitoring for the Status and Trends of Target Species Populations


The current status and trends of target species populations can be monitored when management actions are not being implemented. Such monitoring determines when a threshold has been reached for a particular population, and at which point a management action may begin (e.g., if species is increasing) or end (e.g., if species is decreasing).


  • measuring the current status or characteristics of a population parameter such as abundance or distribution
  • measuring the trend or change in population abundance or vigor over a period of time


  • knowledge of target species characteristics and site conditions
  • measure once for status (i.e., inventory/survey) and more often for trend


Evaluating and Monitoring Invasive Plant Processes
Photo of yellow toadflax population.
Yellow toadflax population. Photo credit: ME Harte/
  • Several populations of yellow toadflax (Linaria vulgaris) in Montana were monitored to determine invasiveness (increase in density and/or spatial extent over time). The results showed that invasiveness varied across spatial scales and habitats.
  • An invasiveness index was developed to prioritize populations for management actions.
  • Evaluating and Monitoring Invasive Plant Processes (1.8 MB PDF) (Repath 2005)

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Invasive Plant Monitoring Plans

When a decision is made to implement monitoring in an invasive plant management program on a refuge, a monitoring plan (or protocol) is developed. This plan may be integrated into larger planning documents. Like any planning document, a monitoring plan provides the blueprint to reach invasive plant management objectives and facilitates continuity in the program from implementation, to evaluation, to adapting management if needed.

Developing a monitoring plan generally occurs after an inventory/survey of the management area is conducted, threats to high-value ecological sites/resources are identified, target plant species or populations are prioritized, and invasive plant management objectives are developed. Collectively, this information provides the foundation from which the plan is developed.

Elements of a monitoring plan include

  • statement of problem and invasive plant management objectives

  • monitoring objectives for target species (level of accuracy and precision)

  • sampling design (to achieve monitoring objectives)

  • field sampling methods

  • data management and analyses

  • evaluation of monitoring results in achieving invasive plant management objectives

  • adjustment of management actions or invasive plant management objectives if needed

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