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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Map of sampling locations and photo of a researcher collecting data at Kenai NWR in Alaska.
 Map and photo credits: Kenai NWR/USFWS


Assessment is the process of gathering and evaluating information in a way that facilitates decision making. Through methods such as ranking systems, geospatial analysis, inventory/survey, and monitoring, assessments can provide a more accurate picture of problems and solutions associated with plant invasions so that land managers are better equipped to identify feasible management strategies, develop measurable objectives, select safe and effective methods, and evaluate program outcomes. Ranging from simple to complex, assessment activities should be designed to accommodate a project's specific needs and resources available for management.

In this module, you will become familiar with

  • the purpose and need for assessment in invasive plant management
  • a framework for assessing risk associated with plant invasions
  • applying invasive plant assessments to management
  • fundamentals of invasive plant inventory/survey
  • approaches to invasive plant monitoring

Role of Assessment in Invasive Plant Management

What is Assessment?

Assessment can refer to a number of activities used to achieve different goals. In the context of managing invasive plants, assessment is the process of gathering and evaluating information about the nature, quality, ability, extent, or significance of the many factors that influence invasive plant management decisions. These factors may include distribution, abundance, and impacts of invasive plants; ecological characteristics and conditions of a management area; and costs and effects of management activities, all within the context of operational constraints and socioeconomic factors. Assessment is ongoing and integrated throughout the invasive plant management process, evaluating new information and knowledge as it is acquired.

Gathering Information
Evaluating Information

The information used for assessments may come from

  • databases, reports, published literature, current research
  • knowledge and experiences of staff or other experts
  • inventory/survey and monitoring data collection

Evaluation and management decisions may be based on

  • judgment of knowledgeable personnel
  • precedent of what has been done before
  • formal analysis (e.g., ranking system, geospatial database analysis, statistical analysis)

Why Conduct Assessments?

Assessment is an important element of invasive plant management, providing a basis and rationale for management decisions that address newly invading, as well as established invasive plant populations. Assessments support developing and maintaining plans and/or programs for invasive plant management that are consistent with the standards of knowledge-intensive management frameworks, such as Integrated Pest Management (IPM) or Adaptive Management.

Knowledge gained from assessments can provide a deeper understanding of the problems and solutions associated with plant invasions so that land managers are better equipped to

  • identify management strategies and options
  • establish measurable management objectives and action thresholds
  • select safe and effective management methods
  • evaluate program outcomes

Risk and Invasive Plant Management

Risk is defined as the product of the likelihood and consequences of an event or process, such as a plant invasion or perhaps the management actions taken to control an invasive plant population (National Research Council 2002). Managing invasive plant populations within an IPM framework is essentially a form of risk management, where invasive plants or the methods used to control them may present a threat (risk) to land management goals for a valued resource such as an ecological community.

Risks associated with plant invasions and their management are highly variable and not fully understood.

  • Invasive plant species differ in the likelihood and consequences of their establishment and spread, and in their susceptibility to management actions.
  • Ecological communities differ in the values society, policy, and directives attach to them, in their vulnerability to invasion, and in their susceptibility to unintended effects of management actions.

Plant species or populations that are the intended object of a management activity or action are referred to as targets, and those plants or resources that are not the intended object are referred to as nontargets. Land managers are often tasked with managing large tracts of land where complete prevention or control of all target invasive plant populations would be economically or ecologically impossible. In addition to other demands required to adequately manage National Wildlife Refuge (NWR) System lands, operational resources must be allocated among current and potential invasions through a process of balancing risks, costs, and benefits. Managing invasive plants in terms of minimizing risk-whether it is economic, environmental, or societal risk-requires a thorough understanding of the problem (invasive plants) and the alternative solutions (management actions).

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Framework for Assessment

The National Research Council (2002) suggests that a risk assessment framework has the most practical value for prioritizing management actions because risk assessments are designed to evaluate both the likelihood and consequences of events such as the establishment and spread of invasive plants and/or the effects of methods used to control them.

The US Department of Agriculture and other government agencies have been using a risk assessment framework to address invasive species for several years, primarily to evaluate the potential invasiveness of proposed imported plant species and to identify susceptible resources (e.g., agricultural commodities, ecological communities, or property) and pathways of established species (APHIS 2004, EPA 1998). The ecological risk assessment framework published by the Environmental Protection Agency (1998) has been adapted to evaluate risk for invasive species (see Suedel et al. 2007) and invaded habitats (see Stohlgren and Schnase 2006).

The goals of large-scale assessment frameworks that address risks associated with plant invasions on a national level are essentially the same as those that are applied to small-scale assessments for local management areas:

  • evaluate the risk of introduction and establishment of potential invasive plant populations
  • allocate resources to control existing invasive plant populations

Regardless of scale and scope, assessments that follow a risk assessment framework consist of three major phases.

Risk Assessment Framework Phases

1.  Problem

  • a planning and scoping process that establishes the goals, breadth, and focus of the assessment
  • requires identification of known or potential stressors (i.e., invasive plants) and susceptible ecological resources

2.  Analysis

  • a process of information gathering and evaluation used to develop “exposure and effect” profiles of the invasive plants within the context of affected ecological communities
  • exposure = likelihood of introduction, establishment, and spread; effect = probability and severity of economic and ecological consequences of invasion

3.  Risk

  • compares exposure and effect profiles using a variety of techniques to draw conclusions and make decisions

Within this basic framework, assessments can be conducted in a variety of ways to achieve different outcomes. They can be simple, rapid, and inexpensive; or complex, time-intensive, and costly. While assessments should be conducted in a way that provides sufficient information and understanding to make sound decisions that support management goals, it is important to conduct the best assessment possible within a reasonable time frame and budget.

Prior to initiating an assessment, it will be important to consider the overall goals and objectives of the assessment, the scale and scope, the type and detail of data to be used, and methods for collecting and analyzing data and making decisions.

Considerations for Conducting Assessments

Goals and objectives

An important first step for conducting an assessment is establishing the purpose and desired outcomes, and how the outcomes will contribute to management objectives and overall land management goals.

Scale and scope

Resources are rarely available to thoroughly assess risks associated with all invasive plant species at all sites within a managed area. For detailed assessments, it is often necessary to narrow the scope and scale of an assessment to include only a few target species, or a small area or ecological site. More general assessments may include a number of species across larger areas.

Data type and

An important step for preparing to conduct an assessment is deciding what type of information (e.g., qualitative or quantitative) is needed to meet the requirements of established goals and objectives. It is important to consider how the data will be used, the level of detail required, what information is already available, and whether the required standards for data are feasible considering the operational resources required to collect them.

Analysis and
decision making

Decision making processes are used to analyze and evaluate assessment information. In general, management decisions are made in one of three ways: judgment and expertise of land managers, precedent of what has been done before, or through a formal analysis. The judgment of experienced professionals is irreplaceable in decision making. However, a formal analysis provides a consistent, objective approach for making decisions, as well as well-documented justification for those decisions.

Several assessment systems have been devised for characterizing plant species in terms of their invasiveness, impacts, and feasibility of control; and for characterizing areas in terms of their ecological value or vulnerability to invasion. Together, these systems can help identify which invasive plant species are likely to invade and be disruptive in a particular area, and which areas are most susceptible to invasion by which species. This knowledge can be applied to prioritizing prevention and control efforts to specific areas and invasive plant populations.

Characterizing Species

Although an inventory or survey of an area may indicate there are a number of undesirable plant species present, only a small percentage may become invasive and cause harm. Those species that are considered to be invasive will vary in likelihood and consequences (risk) of their establishment in ecological communities, and differ in their susceptibility to management methods. It may be prudent to prioritize work efforts towards those species or populations that are most likely to establish and spread, most likely to cause harm, and most likely to be successfully controlled.

One way to prioritize management actions is use a ranking system that helps separate innocuous species from disruptive species. This separation allows land managers to then concentrate efforts on species in the disruptive category. Ranking systems are also designed to identify those species that have the potential to become a threat.

Alien Plant Ranking System computer output graph showing invasive species rank.
After a user inputs data about species characteristics, site characteristics, and control difficulty, the Alien Plants Ranking System computer program produces a report that ranks species by plotting relative impacts against feasibility of control.

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Several systems have been designed to assist in ranking or prioritizing species at various spatial scales (see Hiebert and Stubbendieck 1993, APRS Implementation Team 2001, Morse et al. 2004, Cal-IPC 2006, Pheloung et al. 1999). Species ranking systems typically use a numeric scoring system to assess species based on their abundance and distribution, innate ability to be invasive, current or potential impacts, and management difficulty. Although most systems account for some level of uncertainty, the user should be able to accurately answer questions about the ecology, biology, and control methods for each species.

The Alien Plants Ranking System (APRS) is a computer application that analyzes user-provided data about a species' impacts, invasiveness, and ease of control within the context of a specific ecological site. Based on successful application to National Park System lands and other areas such as Silvio O. Conte NWR, the US Fish and Wildlife Service (USFWS) is exploring ways that APRS can be adapted as a standardized ranking and prioritizing system for refuges.

Applying a standardized systematic method for assessing and prioritizing invasive plant species on a small scale can provide valuable site-specific information relevant for management in a particular area. When applied as part of a coordinated effort, a standardized ranking system can contribute to developing priorities on a regional or national scale.

An Informal Ranking System: Invasive Free Zone Management Plan
Image of invasive plants management plan cover.
  • The Whittlesey Creek NWR in Wisconsin, along with numerous agencies and individuals, prioritized invasive species using an informal ranking system.
  • Resource professionals involved in the project ranked invasive plant species for the "Invasive Free Zone" management plan based on their experience and knowledge of species invasiveness.
  • As more data are available from inventory/survey and monitoring, invasiveness rankings will be reassessed and adjusted as needed.
  • Invasive Free Zone: Invasive Plants Management Plan
    (2 MB PDF)

Characterizing Sites

Ecological communities differ in their susceptibility to invasion. Characterizing sites that have been invaded or are susceptible to invasion can provide valuable insight for prioritizing areas for prevention or early detection strategies, or for prioritizing populations for control based on their location in high priority areas (e.g., areas with high ecological value or high susceptibility).

Researchers have attempted to identify general site attributes and conditions that make some ecological communities more susceptible to invasion than others (see Stohlgren et al. 2002, Endress et al. 2006). Others have described ecological communities in terms of their susceptibility to invasion by one or more particular invasive plant species (see Mantas 2003, Shafii et al. 2003, Rice 2007, Rew et al. 2005, Martin et al. 2007). Both approaches consider site characteristics and conditions that may promote establishment and spread of invasive plants. The latter approach also requires knowledge about the invasive plant species such as habitat requirements, life history characteristics, and other factors that may influence a plant's establishment and survival.

Geospatial database analysis is often used to characterize and prioritize sites in terms of vulnerability to invasion. Geographic Information System (GIS) computer software can be used to analyze complex combinations of spatial data and develop models and maps that identify areas where plant invasions have a high probability of occurrence and where they may have the greatest impacts. Extensive GIS databases are available for most federally managed lands and include attributes such as elevation, geological character, soil associations, vegetation types, habitats, land use, etc. Invasive plant inventory/survey and monitoring data can be incorporated into a GIS database and analyzed with other ecological data to identify susceptibility or resistance of ecological communities to plant invasions.

Predictive Modeling: National Elk Refuge
Map of Canada thistle probability from National Elk Refuge report
Map showing the probability of Canada thistle (Cirsium arvense) occurrence based on mapping and plot data. See report (left) to view full size map. Map credit: USGS
  • Cooperative project with the National Elk Refuge (Wyoming), US Geological Survey National Institute of Invasive Species Science, and Natural Resources Ecology Lab - Colorado State University.
  • Researchers performed intensive inventories/surveys to gather detailed ecological data from areas where invasive plant species occurred as well as from areas where invasive plants did not occur.
  • From these data, associations of invasive species presence or absence with certain ecological parameters were identified and used to develop predictive models.
  • Managers can use these models to anticipate where future invasions may occur and then conduct monitoring to support prevention and early detection strategies.
  • 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)


NASA/USGS Invasive Species Forecasting System
Screenshot of the Invasive Species Forecasting System homepage.
The Invasive Species Forecasting System is an information management and modeling environment.
  • The NASA Office of Earth Science and the US Geological Survey are working together to develop a National Invasive Species Forecasting System for the early detection and management of invasive species on Department of Interior and adjacent lands.
  • The forecasting system provides a framework for using predictive geospatial models to process NASA and commercial data to create on-demand, local- and regional-scale maps of invasive species patterns and vulnerable habitats.
  • When fully implemented, the web-based forecasting system will provide a dynamic and flexible mechanism for generating maps of areas with high potential for nonnative species invasions.
  • See National Invasive Species Forecasting System and Schnase et al. 2002

Assessing Management Alternatives

Managing large acreages with limited means requires careful allocation of resources to areas with the highest return. The economic and ecological costs and benefits of various management activities are often used to prioritize invasive plant management actions and guide efforts towards those areas that are considered to be the most valuable and/or have a high probability of success.

Management methods applied to control invasive plants can have environmental and economic consequences. For example, the use of chemicals to manage invasive plants has risks, many of which are assessed during the Environmental Protection Agency's process for registering an herbicide. Similarly, classical biological control agents undergo rigorous testing that evaluates their potential impacts and risks prior to their release in the United States. Other methods such as prescribed grazing, prescribed burning, or physical methods are not federally regulated, but may present some risks as well.

Effective management of invasive plants will require comparisons of invasive plant management methods that consider

  • effectiveness of a method in controlling a particular species
  • potential nontarget or side effects a method may have in a particular ecological setting

The potential impacts associated with various management alternatives proposed for federal lands, such as NWR System lands, are typically evaluated in Environmental Assessments or Environmental Impact Statements required by the National Environmental Policy Act. On refuges, impacts of proposed management activities are further evaluated during development of Habitat Management Plans or Comprehensive Conservation Plans, and sometimes examined in more detail in Integrated Pest Management Plans or Invasive Species Management Plans.

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Applying Assessments to Management

Knowledge gained from assessments-risk assessments, inventory/survey, and monitoring- provides a basis for making management decisions fundamental to the invasive plant management process. Through a deeper understanding of relative risks associated with invasive plants and accurate assessment of population status and trends, land managers can determine when control action is necessary, establish measurable achievable objectives, and select feasible management strategies and options.

Action Thresholds and Management Objectives

Action thresholds are helpful in prioritizing and selecting management strategies and options. An action threshold is the invasive plant population level at which a decision is made that some intervention is needed. Originally developed for agricultural systems where economic damage is measurable, action thresholds are more difficult to determine for invasive plants occurring in wildland ecosystems (see Briske et al 2006, Brown et al. 1999, Coble and Mortensen 1991).

By conducting invasive plant assessments, levels of acceptable and unacceptable risk or damage are more clearly defined. For example, action thresholds may reflect a very low tolerance for high risk invasive plants in high value areas, whereas an abundance of low risk species occurring in low value areas may be tolerated before action is required.

Management objectives that describe desired invasive plant population levels in specific and measurable terms may help define action thresholds in some situations. For example, in an area where low densities of an invasive plant occur, management objectives may be to maintain population levels at or below the current density. Until densities exceed that level no management action is required, but the site may be monitored instead. When monitoring detects densities at or above acceptable levels, the site may be made a higher priority for management action. When invasive plant populations are successfully reduced to the desired (acceptable) level within a plant community, management actions may be stopped.

For small, localized populations, delaying actions can result in increased cost of control and decreased chance of success (Hobbs and Humphries 1995). Therefore, management objectives for newly invading populations often reflect very low action thresholds.

Management Strategies and Options

Invasive plant assessment activities that describe current status and trends in distribution and abundance of invasive plant populations (i.e., inventory/survey and monitoring) can help identify appropriate management strategies by characterizing the phase of the plant invasion process for populations.

Management and the Invasion Process
Screenshot of invasion process animation.

The process of plant invasions can occur in three phases (Cousens and Mortimer 1995):

  1. introduction phase
  2. colonization phase
  3. naturalization phase

Invasive plant management strategies can be described in four categories:

  1. prevention
  2. early detection
  3. control
  4. restoration

When and where these strategies are applied directly relates to the phase of the invasion process (Hobbs and Humphries 1995, Chippendale 1991, Radosevich in Chapter 3 of CIPM Online Textbook).

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Preventing invasive plants from becoming established is less expensive, less time-consuming, and more effective than trying to control them after they have become widespread or have had significant ecological impacts. Prevention can be applied only to invasive plant species that do not occur on the site or in areas where invasive plant species do not occur. It requires identification of potential invaders, awareness of vectors and pathways of introduction, mechanisms of reproduction and spread, habitat requirements, and conditions that favor the species. Assessments can help identify areas where invasive plant populations do not occur as well as habitats that may be particularly susceptible to introduction and spread of new infestations.

Early Detection

Once unwanted plants begin to invade an area, detecting and controlling invasions when they are small and localized provides the next best return. Early detection and rapid response (EDRR) is the key to eliminating the plants before their population expands.

Detecting and responding to plant invasions requires a complex series of actions that can be grouped into three main categories:

Diagram of early detection, rapid assessment, rapid response series of actions.

EDRR efforts are designed to DETECT new invasive plants in time to allow efficient and environmentally sound ASSESSMENTS to be made; and RESPOND to invasions in an effective, environmentally sound manner that will prevent the spread and permanent establishment of invasive species. Ranking systems and GIS analysis can help prioritize plant species or areas that will be targeted for EDRR strategies. Inventory/surveys and monitoring activities can be designed specifically to detect new invasive plant populations when they are small.


As populations begin to expand, control strategies may be required, demanding significantly more resources. Control options will depend on the compatibility of available management methods and technology with the characteristics of the infestation (size, abundance, and distribution), invasive plant characteristics, ecological and environmental conditions, and available resources (budgets, personnel, training, etc.). Control options can be grouped as follows:

Control Options


  • eliminate all invasive plants and their propagules
  • practical on small-scale infestations, generally in the introduction phase


  • reduce invasive plant populations in size or abundance (i.e., density and cover); promote desirable vegetation
  • effective suppression of large-scale, naturalized infestations is unlikely without massive resource inputs


  • prevent large infestations of invasive plants from spreading to uninfested areas
  • may involve methods that prevent reproduction and propagule dispersal, treating the perimeter of a large infestation, or eliminating small satellite infestations

Multiple strategies and options are often applied to manage a complex of invasive plant infestations throughout a management area.

A Typical Prioritization Scheme for Management Strategies and Control Options

Strategy: Prevention

prevent new infestations from becoming established

Strategy: Control

eradicate small or isolated “satellite” infestations

Strategy: Control

contain the perimeter of large “core” infestations

Strategy: Control

suppress the interior of large “core” infestations


Extensive, naturalized infestations may be beyond feasible control with conventional methods because the desirable plant community composition may not be sufficient to fill niches left open by controlled/suppressed invasive plants, invasive plant populations have altered the site, or other reasons. In these cases, further efforts may be necessary to restore the site. Restoration can be very labor intensive and costly. Restoration may be considered for high value areas that are relatively small. Assessment can help determine if expending resources into extensive restoration is worth the relative benefit.