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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Invasive plant inventories/surveys can provide fundamental information used for assessing and prioritizing invasive plant management efforts. Inventories/surveys are conducted for different purposes, use a wide range of methods, and vary in scales and level of detail (resolution).

In light of the growing threat of invasive species, the Refuge System's National Invasive Species Program launched a Collaborative Volunteer Invasive Monitoring Project in 2003. This project was designed both to provide a tool for generating quantitative data that will help refuges to prioritize areas for control of invasive plants and to engage volunteers in mapping new and established infestations on refuge lands.

What Are Inventory, Survey, and Mapping?

The USFWS defines inventory as a survey to determine the presence, relative abundance, status, and distribution of abiotic resources, species, habitat, or ecological communities at a particular point in time (701 FW 2 Policy on Inventory and Monitoring). In the case of invasive plant inventories, it is the presence, relative abundance, status, and distribution of invasive plant populations that are determined.

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Although the USFWS definition suggests that inventory and survey are synonymous, other authors (e.g., Pokorny et al. 2006, Elzinga et al. 1998) distinguish inventory and survey as different types of activities; yet both are conducted to acquire information about the location and or abundance of a resource at one point in time. Furthermore, some resource managers use the term monitoring to include inventories, surveys, and mapping, and others use the term mapping to refer to inventories. The difference among these activities is important to recognize as they all have specific purposes.

Inventory (a census)
Survey (a sample)
Diagrams illustrating distinctions between inventory, survey, mapping, and monitoring.

catalogues all species or a subset of species, such as invasive plants, within an entire area

Diagrams illustrating distinctions between inventory, survey, mapping, and monitoring.

samples a representative portion of an area; usually at points, along transects, or within swaths

Diagrams illustrating distinctions between inventory, survey, mapping, and monitoring.

a way to gather, record, depict, or communicate inventory and survey information

Diagrams illustrating distinctions between inventory, survey, mapping, and monitoring.

samples repeatedly from plots within populations to detect changes in a resource (e.g., vegetation)

Throughout this module, the term inventory/survey is used to refer to techniques that may be applied to either an inventory or a survey. The term mapping is used to refer to inventory or survey techniques that use maps as a method of gathering, recording, or depicting data. Monitoring is discussed on the following page of this module.

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Why Conduct Inventories/Surveys?

Invasive plant inventories/surveys play an important role in assessing and managing invasive plants and can be used to achieve different outcomes. An inventory/survey provides qualitative or quantitative information about the identity, location, and abundance of invasive plants within the management area, which is critical for making well-informed management decisions. Knowing which invasive plant species are present in or near the management area, and where and to what extent their populations occur increases the ability to assess and prioritize invasive plant management, direct work efforts, and improve cost effectiveness. More detailed inventories/surveys that identify ecological or anthropogenic associations with invasive plant occurrence can also enhance the ability to develop predictive models that facilitate more proactive approaches to invasive plant management such as prevention and EDRR strategies.

Conducting inventories/surveys can have additional benefits:

  • Maps created from inventory/survey data are valuable planning references that document invasive plant population status and management events, improving continuity of staff knowledge over the long-term.
  • Detailed inventories/surveys can provide baseline data for developing monitoring programs.
  • Maps created from inventories/surveys can be used in education and outreach efforts for the public, policy makers, or other personnel.
  • Quantifying invasive plant status in a management area can help justify funding or support other requests.

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Types of Inventories/Surveys

Inventories/surveys can be classified based on what is already known about invasive plant populations in a particular area, the desired outcomes and how they will be used to achieve land management goals and invasive plant management objectives, and the operational resources required to carry out an inventory/survey. Kuchler (1988) describes different types of inventories/surveys as exploratory, reconnaissance, extensive, and intensive.

In general, inventories/surveys generally fall into two different categories based on their overall purpose:

  • those that document the distribution and abundance of invasive plant populations for making management decisions (i.e., exploratory and reconnaissance)
  • those that attempt to further understanding of invasive plant populations or plant communities (i.e., extensive and intensive)

Different types of inventories/surveys can be combined to accomplish one or both of these outcomes, and one type of inventory/survey can be used to support another. For example, an inventory/survey that collects detailed data to increase knowledge about ecological relationships between invasive plant species and a particular plant community can support management decisions.

The following descriptions of exploratory, reconnaissance, extensive, and intensive inventories/surveys are summarized from Kuchler (1988) and Pokorny et al. (2006). There is a gradual change from exploratory to intensive inventories/surveys and it may not be necessary to define an effort as any one type.


When used

  • little is known about the location and species of invasive plants in large areas
  • existing knowledge is based mostly on casual observations


When used

  • general abundance and/or distribution of common invasive plant species are already known, and maps or data including basic information exist
  • conducted periodically to detect new populations/patches; may be limited to areas considered to be most susceptible to new introductions


  • search as many acres as possible in the least amount of time and at the lowest possible cost, while providing basic information needed to guide initial management efforts
  • create a basic invasive plant map indicating the species present, their general distribution, and relative abundance


  • locate and record as many small populations/patches of early-stage invaders as possible to support early detection and rapid response elements of existing invasive plant management program
  • more accurately define the perimeter of large infestations and locate all isolated populations/patches discovered beyond the main infestation


Creating Basic Invasive Plant Maps
Photo of volunteers mapping invasive plants.
Volunteers mapping invasive plants at Great Meadows NWR, Massachusetts. Photo credit: USFWS
  • The USFWS Northeast Region initiated a region-wide, systematic effort to identify, locate, and map invasive plant species on refuge lands.
  • Following a standardized protocol, refuge staff, volunteers, and contractors walk line transects to cover as much of the refuge as possible.
  • Mapping information is interpreted to determine the size of the invasive populations, the direction and rate of spread, and other relevant information.
  • Using maps and inventory information, managers can develop strategies focused on removing new and isolated infestations while containing or suppressing the core infestation.
  • Surveying and Mapping Invasive Plants on Northeast Refuges (216 KB PDF) (Taylor 2004)


When used

  • exploratory and/or reconnaissance inventory/survey has been completed
  • data are required that are more detailed, more accurately delineated, and/or at a finer resolution than those collected by exploratory and/or reconnaissance inventories/surveys


When used

  • as much information as possible is needed about invasive plant species, other plant species, and environmental factors at a level of accuracy and detail sufficient to allow meaningful scientific interpretation of ecological relationships
  • data may be useful as baseline for future monitoring projects


  • build upon and refine data gathered previously through exploratory and/or reconnaissance inventories/surveys
  • identify general correlations or associations between invasive plant species, other dominant or abundant vegetation, and/or certain environmental factors or conditions that appear to promote or suppress plant invasions


  • locate invasive plants and record detailed information about all plant species and the role they play in the plant community; may include data on phenology, floristic composition, dynamic features, land use, elevation, relief, slope, exposure, soil, water, or geologic character
  • data from a relatively small area can be extrapolated to a much larger area that was not sampled to develop predictive models


Identifying Ecological Relationships: Hart Mountain National Antelope Refuge
Map showing plot sampling locations from Hart Mountain National Antelope Refuge report.
Map showing plot sampling locations in different types of plant communities. See report (left) to view full size map. Map credit: USFWS
  • Cooperative project with Hart Mountain National Antelope Refuge (Oregon), US Geological Survey National Institute of Invasive Species Science, and Natural Resources Ecology Lab - Colorado State University.
  • Researchers conducted successive inventories/surveys in three steps to develop ecological models (see Stohlgren and Schnase 2006).
  • Models provide an indication of how environmental variables contribute to the distribution of invasive plant species, and can be useful for directing control and assessing impact to natural resource assets and management objectives.
  • Sampling and modeling approaches are described in Iterative Sampling of Nonnative Plant Species at Hart Mountain National Antelope Refuge (1.7 MB PDF) (Barnett et al. 2005)

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Considerations for Conducting an Inventory/Survey

General considerations for conducting inventories/surveys range from establishing goals and objectives, to defining the target area and species to include in the effort, determining what data is required and at what level of detail, and establishing sampling protocols.

Goals and Objectives

It is important to establish a clear purpose for conducting an inventory/survey and determine how the information will contribute to invasive plant management objectives and overall land management goals. Established goals and objectives will guide the selection of data collection methods used in inventories/surveys to ensure that relevant information is provided at the scale and detail/resolution required. For example, an inventory/survey designed to locate, document, and map the occurrences of invasive plants so that they may be prioritized for management will require a different set of data standards and field protocols than an inventory/survey that is intended to further understanding about invasive plant population ecology through detailed ecological sampling.

Scale and Scope

Essential to any invasive plant inventory/survey project is identifying the search area and target species that are to be included. Resources are rarely (if ever) available to inventory/survey all plant species within an entire management unit. Therefore, a subset of priority target species is often selected.

The scale of an inventory/survey will depend on what type of inventory/survey is to be performed. A large scale inventory/survey may collect invasive plant data from an entire refuge, whereas a smaller scale inventory/survey may focus only on select sites or habitats. Topography, accessibility, size, and susceptibility of an area to invasion can determine the search or sample area.

When habitat requirements of a target invasive plant species are well understood, focusing efforts on areas with similar ecological conditions-such as soil type, aspect, elevation, precipitation, disturbance regime, etc.-can improve efficiency and reduce costs. Similarly, high-risk areas or pathways for invasive plant transport and establishment, such as roads, trails, waterways, or frequently disturbed sites, may also be high priorities.

Data Type and Resolution

Inventories/surveys can be used to collect very general information, such as presence or absence of a particular species; or very detailed information, such as the size and distribution of population, phenology, associated floristic composition, soil properties, etc. It will be important to determine what information is essential or optional, and what level of detail or resolution is required to meet the goals and objectives of the inventory/survey.

For example, in an area where little is known about the occurrence of invasive plant species, an exploratory inventory or survey may collect very general information about invasive plant populations that are encountered, such as presence or absence, with the goal of compiling an invasive plant list. However, a follow-up reconnaissance inventory/survey would require more detail about the precise size and shape of the infestation, and abundance of plants.


Once the goals and objectives, scale and scope, and data type and resolution have been clearly defined, the next step is to select methods that best meet these established parameters of the inventory/survey. There are numerous ways to conduct effective invasive plant inventories/surveys, and it is unlikely that there will ever be agreement on any one "best" way given vast variation in goals, landscapes, target species, available operational resources, and other factors.

Methods should be tailored to meet the needs of a specific inventory/survey project. However, using established standardized field procedures can facilitate collaboration among refuges or other entities and improve data quality and consistency. If an organization does not use a standardized inventory procedure, then the information collected may only be useful to that project (NAWMA 2002). Procedures describe observation approaches, search strategies and sampling design, and protocols for collecting and recording data.

A variety of approaches exist for mapping vegetation that range from field visits to remote sensing.

Inventory/Survey Approaches


Observations are made directly by traveling through an area on foot, horseback, ATV, or other vehicle and recording findings on a standardized form, a topographic map, aerial photo, or a GPS (Global Positioning System) unit.


Observations are made directly by traveling over an area in an aircraft and recording findings on a standardized form, a topographic map, aerial photo, or an aircraft mounted GPS unit.


Observations are made by interpreting aerial photography, satellite imagery, multispectral-sensors, or any method where the infestation was not directly observed.

The approach used to collect data is determined by the desired scale and resolution of the inventory. In rough terrain or very large management areas, ground-based surveys can be too time consuming or costly. Inventory/survey techniques such as Digital Aerial Sketch Mapping can be used to cover larger areas in less time by detecting and recording invasive plant infestations from an airplane. Despite their utility as rapid and cost-effective methods, aerial-based and remote inventories/surveys generally do not provide the level of detail and accuracy that ground-based inventories/surveys do. Often, the areas observed remotely are checked on the ground to verify and collect supplemental data.

Map showing results of saltcedar inventory/survey activities at Sevilleta NWR in New Mexico.
Map displaying saltcedar location data collected using ground-based and remote inventory/survey approaches in the Palo Duro Canyon at Sevilleta NWR. Map credit: USFWS

The refuge system's volunteer invasive plant mapping program generally employs field visits to map invasive plant occurrences. However, some refuges have access to remote sensing technology that can significantly improve inventory/ survey efficiency in large management areas.

For example, the 230,000-acre Sevilleta NWR in New Mexico used natural color ortho-rectified digital imagery acquired in 2005 by the National Science Foundation's Long-term Ecological Research Network station located on the refuge. Stands of saltcedar (Tamarix spp.) were delineated from the images and helped focus ground-based field inventory/surveys conducted by refuge Invasive Species Strike Teams. Using handheld GPS units with the Refuge Lands Geographic Information System (RLGIS) data model, strike teams collected detailed saltcedar occurrence data along 14 miles of riparian area in just two days. Within one week, the maps created through this effort were used to acquire funding to treat the infested area.

Search Strategies and Sampling Design

Map displaying plot sampling scheme for inventory/survey activities at Kenai NWR  in Alaska.
Map displaying plot sampling locations at Kenai NWR in Alaska. Map credit: Kenai NWR/USFWS

Where to search for invasive plant populations and where sampling will occur is an important element of an inventory/ survey protocol. Search patterns and sampling locations should be unbiased and achieve good interspersion across the entire area of interest.

Procedures developed for the USFWS Northeast Region Survey and Mapping Initiative describe a systematic search pattern using a grid system or line transects to cover the entire refuge on foot at least once. Spacing between transect lines or the grid systems is variable, reflecting a distance that is the most suitable for the area or habitat based on visibility.

For larger refuges, such as the two million-acre Kenai NWR in Alaska, attempting to achieve full coverage of the refuge on foot is impractical. Instead, refuge biologists used a helicopter to access and systematically sample 255 points at 5 km intervals to establish a baseline landscape scale survey of native and nonnative flora throughout the refuge. This survey was done collaboratively with the USDA Forest Inventory & Analysis program.

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Data Collection

Once a population is found in a landscape, procedures for recording information about the size, shape, or other characteristics of the invasive plant occurrence will help ensure accuracy and consistency among different observers. Protocols for collecting supplemental or detailed ecological data during extensive or intensive surveys will also need to be developed and described in detail. Depending on the resolution required to meet inventory/survey objectives, specific criteria will be necessary to determine the following:

  • how to define one invasive plant occurrence from another (minimum patch separation distance)
  • when points, lines, or areas/polygons should be used to capture the approximate size and shape of the invasive plant occurrence (population or plant), and if the size and shape should be estimated or measured
  • how to describe the abundance and distribution of invasive plants within the infested area (i.e., cover, density, or frequency), and if the abundance should be estimated or measured

The USFWS Northeast Region's inventory/survey procedures include qualifying criteria for describing the distribution of invasive plants detected during an inventory/survey. For example, an infestation distribution is described as "evenly throughout" if individual plants occur at fairly regular intervals and may be separated by anywhere from 25 to 50 meters. Distribution for infestations that have many plants growing singly or in clumps close together or touching one another is described as "densely throughout."

Data Standards

Establishing or using preexisting standards for data collection can make inventory/survey data useful beyond the project area since it can be shared among refuges or other entities. The USFWS follows national data standards established by the North American Weed Management Association (NAWMA). The NAWMA system creates a standardized format for collecting and mapping nonnative plant species that allows for information to be shared and transferred between different areas. These standards represent a minimum of what should be collected during inventory/survey efforts, such as information about the identity, location, and abundance of invasive plants.

Researchers from the US Geological Survey and the Natural Resource Ecology Laboratory - Colorado State University suggest a new standard that incorporates plot-based protocols to supplement NAWMA standards and provide information to further understanding of ecological parameters associated with invasive plant populations. The protocol emphasizes collecting detailed plant community data from infested and uninfested sites. The purpose is to enhance understanding about areas that might be resistant or vulnerable to invasive plant establishment, and the rate of invasive plant population spread-all important components of predicting invasive plant invasions and increasing the ability to manage established invasive plant populations. The protocol also stresses the importance of improving quality control by conducting independent audits to verify taxonomy and accuracy of inventories/surveys.

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Data Models

Many refuges depend on computer programs such as Refuge Lands Geographic Information System (RLGIS) and the Weed Information Management System (WIMS) to collect and manage invasive plant inventory/survey data. Both of these programs function in mapping software called ArcPad and users take handheld GPS devices into the field to record the information about invasive plant infestations.

Photo of volunteer participating in a classroom training to use WIMS.
A refuge volunteer learns about using the WIMS data model for mapping invasive plants. Photo credit: USFWS

Conforming to NAWMA standards, RLGIS and WIMS track three kinds of data records:

  1. invasive plant occurrences (locations mapped as GPS point, line, or polygon features)
  2. infestation characteristics/attributes (size and status of the infestation)
  3. management treatments applied to infestations

Although a number of refuges use the WIMS data model, RLGIS is a standard GIS data model designed specifically for refuges by USFWS staff. RLGIS includes invasive plant data fields that integrate seamlessly into the larger geodatabase designed to manage spatial data on all aspects of refuge management including infrastructure, wildlife, and habitat. The RLGIS data model also integrates information about invasive plant management activities and tracks information about where and how surveys are conducted, regardless of whether invasive plant populations were found. It is important to know where invasive plant populations do NOT occur so these areas can be identified for prevention or early detection efforts.

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