Prioritizing Weed Species and Sites in Deserts

Prioritizing Weed Species and Sites

Along with budgeting and seeking public support, determining priorities is an essential element of weed-control strategy. This article considers natural vegetation and does not deal with weeds in irrigated crops.

Prioritizing Species

Cost effective weed management focuses resources on the most dangerous species and the most valuable sites. Less than 10% of the hundreds of native and introduced alien weed species present in deserts will invade native vegetation. Many invasive species have little impact when they do. Because of financial limitations, this has prompted greater emphasis on passively monitoring weed infestations to learn whether they will cause problems. Prioritization equips weed managers for immediate response when aggressive species appear.

Locating information on the ecology, distribution, and control of weeds requires literature searches for each species. Though the information produced by the searches is variable and may not cover local conditions, it provides a first approximation of the relevant characteristics and control treatments for species that are present.

Weed scientists have developed many prioritization systems. For example, Byers et al. (2002) provide a risk-assessment guide that ranks species according to potential for:

  • Arrival (risk associated with entry pathways)
  • Establishment (risk of forming viable, reproductive populations)
  • Spread (risk of expanding into native vegetation)
  • Impact (risk of having a measurable effect on existing species or plant communities)

Species distribution information by country is available online from the Global Register of Introduced and Invasive Species (

The risks are uncertain for many species, populations, and places. And even after weed managers learn a species’ characteristics and the conditions of a site, a significant level of uncertainty remains. Environmental fluctuations, land-use changes, and species adaptations require frequent adjustments of prioritization systems.

According to Rew and Pokorny (2006), the ideal prioritization system combines weed and site ranking integrating ecological knowledge of species with site conditions, and adjusts with observed results of treatments.

Prioritizing Sites

Species information is often be useful across regions, but every site requires direct observation. Here is a sample recipe for the process.

First, divide large or complex sites into homogenous Weed Control Areas (WCAs) aligned with administrative or environmental boundaries. Next, use information from an initial inventory to subdivide the WCAs into Weed Control Zones (WCZs) based on weed distribution and planned treatments. Use the site categories outlined below to rank the WCZs.

Priority Site Categories

Hobbs and Humphries (1995) propose four categories of management action based on resource value and the level of site disturbance. Thus, parks, wilderness areas, critical habitats, etc. are in the highest category.

Desert streams and riparian areas play such an important role in arid ecosystems they have high value even though they are at high risk of invasion. Besides providing critical habitat, riparian vegetation occupying floodplains is an important natural shock absorber that reduces flooding, erosion, and sediment transport.

Highest Priority Sites

  • Upland sites that are relatively undisturbed, where entry pathways are controllable, and few weeds are present.
  • Moderately disturbed perennial stream segments with riparian vegetation on either bank that extends over 20-m laterally from the center of the stream channel. Weeds are a minor part of the vegetation. Entry pathways are limited, and managers can control human-caused disturbances from recreation and livestock grazing. Applies to hydrophytic shrub, woodland, and gallery forests in wildernesses and monuments. Such streams are rare and may no longer exist.

Objectives for Highest Priority Sites:

  1. Maintain weed-free by eradicating existing and new weed populations.
  2. Prevent disturbances and weed establishment by altering land use, managing access, and conducting frequent monitoring and rapid response to eliminate new populations.
  3. Restore native vegetation to increase invasion resistance and stabilize floodplains.

Highest priority areas include national parks, species and habitat conservation areas, research reserves, other important natural areas.

High Priority Sites

  • Upland sites moderately disturbed and susceptible to invasion.
  • Moderate to highly disturbed ephemeral stream segments with patches of native riparian vegetation extending more than 20m from either side of the stream. Weeds may be abundant. Apply this category only if access and human disturbances such as hiking and livestock grazing are controllable. Examples: willow thickets and hydrophytic woodland and gallery forests in parks and conservation areas.

Image:  Highly disturbed site with heavy weed infestation. Wide (>20m) riparian vegetation also highly disturbed and infested. Human disturbances are controllable.This is a perennial stream of critical importance for wildlife, but weed control is unlikely (photo by GR).

Objectives for High Priority Sites:

  1. Manage weeds by eradicating high-priority species and controlling others.
  2. Limit weed introductions and establishment by managing access and altering land use to eliminate disturbances.
  3. Restore native vegetation in treated areas.

High priority areas may include boundary areas next to highest priority areas.

Medium Priority Sites

  • Upland sites of subject to frequent disturbance including livestock grazing, wood gathering, and recreation.
  • Highly disturbed ephemeral stream segments with narrow (<20m) bands of riparian vegetation at least 20m long measured along the mean course of the stream. Human access and disturbances such as recreation and livestock grazing are controllable.

Image:  Highly disturbed site with heavy weed infestation. Wide (>20m) riparian vegetation also highly disturbed and infested. This is a perennial stream of critical importance for wildlife, but weed control is unlikely.

Objectives for Medium Priority Sites:

  1. Monitor likely areas and control high priority weeds.
  2. Poorly accessible upland areas lightly grazed by livestock and ephemeral stream segments outside high-value management units fit this category.

Low Priority Sites

Repeatedly burned, heavily grazed, almost total weed cover. Former diverse woodland lost. Restoration would be difficult and very expensive (GR).

  • Upland sites subject to high levels of disturbance. These sites have so many weeds that restoration of native communities is unlikely. Roadways and transmission rights of way fit this category well. Hobbs and Humphries (1995) recommend that these sites receive no action.
  • Ephemeral stream channels with discontinuous riparian vegetation occurring in segments <20m. These sites are of higher concern when they contain high-priority species.

Objectives for Low Priority Sites:

  1. Monitor likely areas for new weed invasions and control high-priority weeds.

After initial inventories, analyze each WCZ using criteria in the prioritization table below and place it in a Management Priorities List. The Management Priorities List denotes the order and principal categories of management action and forms a core element of the regional weed management plan.

Many countries have developed prioritization systems and have weed management plans in place. I prepared the table below to illustrate the issues involved, not serve as a working system.

  • Table Explanation:  Following initial inventories, this table uses species and site prioritization to guide monitoring, and treatment.
  • Numbers in the left column show the minimum search frequency in years to detect new infestations. Highest-priority sites receive two or more visits each year because some species germinate throughout the growing season and others germinate during spring or summer.
  • Numbers in cells (not including the left column) indicate the recommended frequency in years for monitoring of existing infestations. The “0” in the cells for highest and high priority sites and the weed designations “Highest” and “High” show rapid response instead of monitoring. The seasonal timing of observations should follow maturation of the weeds. For weeds that germinate throughout the growing season, this would be autumn. The term “withdraw” indicates intervention to limit human disturbances. Monitor sites in the cells at lower right to detect appearances by high-priority species. Depending on resources and proximity to high-value sites, managers might eradicate such species.

Relevant Websites

Sources Cited

Byers, J. E., et al. 2002. Directing research to reduce the impacts of nonindigenous species. Conservation Biology 16: 630-640.

Hobbs, R.J. and Humphries, S.E. 1995. An integrated approach to the ecology and management of plant invasions. Conservation Biology 9: 761-770.

Rew, L. J., and M. L. Pokorny, eds. 2006. Inventory and survey methods for nonindigenous plant species. Montana State University Extension Service, Bozeman, MT. 75 p.


Controlling Invasive Weeds in Deserts: Strategic Concepts

Strategic Concepts for Invasive Weed Control in Deserts

This is the first in a series of articles on methods for preventing the devastation caused by weeds invading desert ecosystems.


Weed control efforts are more successful when strategy and tactics are carefully planned in advance. Strategy includes these concepts:

Adaptive Management

Weed managers can build periodic reviews into their plans so they can benefit from results and insights generated by the plan’s implementation. Adaptive management, also known as ecological management, uses analytical techniques to assess results of actions and provide feedback for improving methods. Adaptive management is useful when outcomes are uncertain but weed treatments are clearly defined. Analysis of outcomes may contribute new general knowledge, and it will help adjust policies and tactics.

Priority Weeds

Building a list of problem weeds for management units is an essential part of weed-control planning. Private and government organizations provide lists of known invasive species for countries and regions worldwide. These sources will provide a baseline list that will expand as observations and experience accumulate. The place to begin is the Invasive Species Compendium hosted by the Center for Agriculture and Biosciences International.

Public Perception and Support for Weed Management

The field of Weed Science developed in response to the measurable economic costs of weed invasions of farms. Measuring economic costs of weed invasions on non-agricultural wildlands is more difficult. Aldo Leopold argued that applying economics to ecosystems was not appropriate.

“One basic weakness in a conservation system based wholly on economic motives is that most members of the land community have no economic value. Wildflowers and songbirds are examples. Of the 22,000 higher plants and animals native to Wisconsin, it is doubtful whether more than five percent can be sold, fed, eaten, or otherwise put to economic use. Yet these creatures are members of the biotic community, and if (as I believe) its stability depends on its integrity, they are entitled to continuance” –Leopold, 1949, 210.

Example weed-prevention poster by U. S. State of Minnesota

During the years since Leopold wrote the above comment, environmental economics has made substantial progress. For instance, weed reductions in species richness and diversity and harvests of wildland plants have been measured (e.g., Costanza, R., d’Arge, R., de Groot, R. et al., 1997), and benefit-cost analysis supports stringent weed control strategies (Naylor, R. L., 2000).

As weed management has progressed toward integrating complex tactics and ecosystem concepts, the issues may have become opaque to nonscientists. Conducting weed-management discussions within a framework of human economic and environmental security can awaken public interest.

An effective strategy for encouraging public support is to provide examples that involve local communities. Field trips to view fire scars or transmission corridors dominated by weeds will illustrate how weeds are transforming desert vegetation. Community leaders can use the illustrations to create weed prevention and eradication programs within their community.

No one wants to lose the value and character of the land, but many users of the land are unaware of the full significance of the weed invasion. Weed managers can find partners willing to help publicize weed problems among organizations, such as native plant societies and wildlife conservation organizations. Weed problems will also interest recreation users, ranchers, hunters, and others spending time outdoors. School visits, field demonstrations, films, and tours for individuals and groups of users should be an initial element in weed control plans. Education materials are available online from the North American Invasive Species Management Association.


Prevention planning and execution precedes or is concurrent with initial weed inventories. This is necessary, because continual appearance of weeds erases the effectiveness of eradication, control, or restoration efforts. Weed prevention is much less expensive than any form of treatment. Regional and national plans listed on the U. S. National Invasive Species Information Center website present techniques for preventing weed invasions and spread. The focus of the plans is on movement of people, livestock, and equipment, and on restoration of native vegetation. Major prevention strategies include:


National and regional restrictions can limit introduction of invasive plants. Regulation is most effective if there is public awareness and participation.

Ecosystem Health and Invasive Weeds

Biological soil crusts block invasive weeds in deserts. Livestock grazing damages crusts and heavy trampling can eliminate them. (Collema spp. In Colorado Plateau division of the Great Basin Desert. © GR.

Supporting and restoring native plant communities and ecosystems increases resistance to invasion while reestablishing lost biodiversity and ecosystem value (Blumenthal, D. M., Jordan, N. R. & Svenson, E. L., 2003). Habitats and ecological processes can suffer cumulative impacts from the direct and indirect effects of invasive species. Ecological relationships that have evolved over evolutionary timescales are at risk. Invasive species cause disturbances that have multiple effects throughout an ecosystem, and human alterations of the environment can exacerbate them. Healing these disturbances is difficult. Thus, restoration treatments are an integral part of control and management efforts. They help guard against future re-infestations and the potential for further harm.

Minimizing Disturbance

Strict fire-control programs and grazing management to allow recovery of soil microorganisms and native vegetation removed by fires or excessive grazing are essential.

Ecological Restoration

Healthy native vegetation resists weed invasions. The Society for Ecological Restoration supplies information with examples and techniques on the organization’s website.

Early Detection, Rapid Monitoring and Response

Effective weed control requires rapid responses to invasive weeds. Early detection is essential. It requires preliminary inventories and regular monitoring to discover new populations. The later section on inventory describes monitoring tactics. Management responses including mechanical, chemical, biological (including grazing), and fire treatments are described in the section on treatment tactics.

The appearance of a non-native species does not always signal the onset of an invasion. Unfavorable environmental conditions, inherent demographic and biological limits, and resistance of native vegetation can prevent spread or cause demise of a weed. Because of the uncertain survival of new weeds, the first response to most species should be monitoring. This does not apply to weed species known to be invasive in similar sites nearby. Since it is possible for a static weed patch to serve as a source population, monitoring must include a thorough survey of the surrounding area. Repeated observations of existing populations is ‘type-two’ monitoring in the discussion below.


Monitoring comprises repeated observations to detect new weed infestations, to keep track of changes in an existing weed patch, and to test the results of management activities. When effort is evenly divided between monitoring and treatment, weed control becomes more efficient. I recognize three types of monitoring. Type-one comprises repeated searches of sites for new infestations, type-two comprises repeated observations of existing infestations, and type-three comprises repeated observations to learn the results of management actions.

Type-one monitoring

Periodic visits to likely sites for new infestations are the primary defense against new weed invasions. Variations in weed visibility and elevated threat levels associated with highest priority sites and weed species might require multiple visits each year, but most areas can be visited yearly or every two or three years. Because desert precipitation is often irregular, monitoring schedules must be flexible. Some weeds germinate in spring when there is sufficient moisture; and others germinate after summer rain. If the weed list for a particular management unit includes species from both groups, two visits each year are required. Single yearly visits should follow summer rains when winter annuals are still standing and summer annuals are likely to have germinated. Recommended frequencies are included in Table 1.

Table 1. Decisions Based on Combined Weed Species and Site Priorities.
Weeds Across

Sites Down









Highest (<1) (0) withdraw, eradicate, restore (0) withdraw, eradicate, restore (2) monitor, treat if expanding, restore (3) monitor, treat if expanding, restore
High (1) (0) withdraw, evaluate for eradication or control, treat, restore (1) withdraw, evaluate for eradication or control, treat, restore (2) monitor, treat if expanding, restore (3) monitor, treat if expanding, restore
Medium (2) (0) withdraw, evaluate for eradication or control, treat, restore (2) monitor, treat if expanding, restore (3) monitor (4) monitor
Low (3) (1) withdraw, evaluate for eradication or control, treat, restore (3) monitor, treat if expanding, restore (4) monitor (5) monitor


This table shows management actions for sites with varying weed threats and resource values. Managers use it to set inventory and monitoring schedules after the initial weed inventory is complete.

Numbers in the left column of Table 1 are the minimum frequency in years for type-one monitoring to detect new infestations. Schedule highest-priority sites for two or more visits each year to spot species that develop differentially during spring and summer.

Numbers in cells (not including the left column) are the recommended frequency in years for type-two monitoring of infestations. Zero (0) in the cells for highest and high priority sites and the weed designations “Highest” and “High” indicate that rapid response instead of monitoring is required. Highly invasive weeds are eradicated as soon as possible after discovery. Seasonal timing of type-two monitoring should match the maturation of individual weed species. Withdraw indicates intervention to limit human disturbances. The term “monitor” refers to type-two monitoring. Type-three monitoring will follow all treatments and restorations. Eradicate refers to the immediate management response for new or small areas of weeds. Eradication of widespread weeds requires a preliminary effort to secure financial and public support.

Type-two monitoring

Type-two monitoring applies to new and existing infestations. It is used to determine stability and potential threat. Since weeds often establish in plant communities and on soils that are not optimal for their survival and growth, many patches found along roads or around disturbed areas may never spread into adjacent vegetation. It is cost effective to monitor them while devoting resources to treating infestations of species of highest concern in high-value areas and infestations known to be spreading. Recommended frequencies are included in Table 1.

Type-three monitoring

Type-three monitoring gauges ecosystem responses to natural and human-caused disturbances, weed treatments, and restorations. Landuse such as livestock grazing, weed treatments, and restoration projects incorporate measurements made at regular intervals. Type-three monitoring will often occur more than once during growing seasons.

Satellite Populations

Weed populations grow by expanding their boundary and by long-distance dispersal to establish outlying or satellite populations. Where habitat conditions and dispersal vectors are spatially uniform, propagule dispersal from each plant follows a simple curvilinear process to create a circular pattern with decreasing propagule density in all directions from the center. As the area of an infestation grows, the proportion of each plant’s propagules falling among the plants within the occupied area will increase and the proportion falling outside the area will decline. Thus, the rate of expansion will decrease as the infested area grows. Since the rate of expansion is greater for small populations, satellite populations should be treated first.

Horehound (Marrubium vulgare) beside a fresh road cut through weed-free oak-dominated Interior Chaparral. Each plant produces thousands of hooked seed pods.

Since a single plant can produce thousands of seeds, very small satellite populations of one or a few plants can serve as significant sources for further invasion. Searches in concentric circles around the source population should find fewer satellites as the circles are enlarged. Of course, neither available germination sites nor dispersal vectors such as wind vary with topography, vegetation, and transient air pressure. However, without preliminary data on vectors and preferred habitats, the more refined and accurate analytical techniques geographers use (e.g., R. Abler, J. S. Adams, and P. Gould, 1971) to calculate rates of spread and predict the impact of barriers (absorbing, filtering, reflecting), shapes of source areas, hospitable sites, and so forth cannot be used. Thus, the inventory system presented below assumes symmetrical dispersal and is appropriate for general use to locate satellite populations. Information about a particular species’ dispersal mechanisms and response to vectors and barriers in particular locations can be developed and used to modify the search pattern (see M. B. Soons and J. M. Bullock 2008).

Coordination with Neighbors

Property boundaries do little to influence weed dispersal and establishment. Fences may stop the first wave of tumbleweeds, and they may create rows of bird-dispersed weeds, but they are not an effective barrier. Coordinating prevention, inventory, and control with neighboring agencies and private landowners is essential.

Control vs. Eradication

Immediate eradication is sometimes more cost effective than the various other treatment options. Annual costs to control weeds increases as new species arrive and existing species establish satellite populations. Eradication of a widespread species is difficult and costly, but might be essential to protect native ecosystems and reduce long-term costs. Mack, R. N. & Foster, S. K. (2009) found that eradication of a widespread species requires most of the following conditions.

  1. An effective prevention plan is in place.
  2. An exhaustive inventory is available.
  3. The target species are readily detectable. Failure to find all individuals creates opportunities for new seed banks to form.
  4. Terrain is accessible and easily inventoried and monitored.
  5. Target species’ seed bank is short lived (no more than 3 years).
  6. Target species’ current range is small enough that available resources are adequate for inventory, eradication, and monitoring.
  7. Concurrent treatment of satellite and source populations is possible.
  8. Monitoring will continue well past the expected duration of the seed bank. Otherwise, seedlings and other individuals missed in inventories, treatments, and subsequent monitoring will produce new seed banks.
  9. Public and financial support is constant.


  • Abler, R., Adams, J. S. & Gould, P. (1971). Spatial organization: The geographer’s view of the world. Englewood Cliffs, NJ, Prentice-Hall.
  • Blumenthal, D. M., Jordan, N. R. & Svenson, E. L. (2003). Weed control as a rationale for restoration: example of tallgrass prairie. Conservation Ecology 7, 1.
  • Costanza, R., d’Arge, R., de Groot, R., et al (1997). The value of the world’s ecosystem services and natural capital. Nature 387, 253-260.
  • Mack, R. N., & Foster, S. K. (2009). Eradicating plant invaders: Combining ecologically-based tactics and broad-sense strategy. In Inderjit, (ed.) Management of invasive weeds. Pp 35-60. Dordrecht: Springer.
  • Leopold, A. 1949. A Sand County almanac and sketches here and there. New York: Oxford University Press.
  • Naylor, R. L. (2000). The economics of alien species invasions. In Mooney, H. A. & Hobbs, R. J. (eds.) Invasive species in a changing world. Pp 241-259. Washington, D. C.: Island Press.
  • Soons, M. B. & Bullock, J. M. ( 2008). Non-random seed abscission, long-distance wind dispersal and plant migration rates. J. Ecology 96, 581-590.

List of Relevant Websites

Invasive species shift Great Lakes ecosystems – Summit County Citizens Voice

GR:  Invasive species are weakening and eliminating ecosystems around the world. In many instances, people introduced the invasives to increase food production for domestic livestock. In other instances, people introduce the invasives to control other invasives. The search for magic bullets such as pesticides, diseases from the original homeland of an invasive, and introduction of more competitive aliens from the homeland continues, but without much success. Many times, the only effective means to eliminate an invasive species is to apply manual labor: catching, pulling, or mowing.

“The Great Lakes have seen successive invasions by non-native species that alter the ecosystem, including quagga mussels that filter the water and remove nutrients. At least partly as a result of the invasive mussels, Lake Michigan is becoming less hospitable to Chinook salmon, according to a new study led by scientists with the U.S. Geological Survey and Michigan State University.

“The scientists concluded that stocking could help sustain a population of Chinook salmon, but that the lake’s ecosystem is now more conducive to stocking lake trout and steelhead salmon. These two species can switch from eating alewife, which are in decline, to bottom-dwelling round goby, another newly established invasive prey fish that feeds on quagga mussels.

“Findings from our study can help managers determine the most viable ways to enhance valuable recreational fisheries in Lake Michigan, especially when the open waters of the lake are declining in productivity,” said Yu-Chun Kao, an MSU post-doctoral scientist and the lead author of the report.” –Summit County Citizens Voice (Continue: Invasive species shift Great Lakes ecosystems – Summit County Citizens Voice).

Behind New Zealand’s wild plan to purge all pests

Invasive Species

GR:  After 1500 AD, sailing ships and then later on, motor-powered ships began transporting and introducing plant and animal species all over the globe. Freed from the predators and diseases of their homes, some of the introduced species became invasive–that is, they began spreading, replacing native species, and decreasing ecosystem stability and productive. This is not news, of course, biologists have long been aware of the devastation caused by invasive species.

Eradicating invasive species is very expensive and very difficult. National resolve and full public support are required. Eradication is something that we humans, who are responsible for spreading the invasive species, should be about everywhere.

However, it is essential to place greater focus preventing the initial introduction of non-native species. Prevention is cheaper and kinder than eradication. And again, prevention is not a new idea. Natural resource managers have known how to prevent invasions for the past century. In many instances, they just don’t take the necessary steps. Here are some articles on invasive plants.

New Zealand has one of the worst invasive plant and animal problems in the world. The article below describes an ambitious and necessary plan to do something about it.

New Zealand Eradication Plan

New Zealand has three invasive species of rat. The Pacific rat, or kiore (Rattus exulans), was introduced from Polynesia in about the twelfth century; the ship rat (Rattus rattus) arrived in the late 1700s; and the Norway rat (Rattus norvegicus) became established in the 1860s. All three prey on native birds, insects and lizards, and have been blamed for the decline or extinction of a variety of species.

“Razza the rat nearly ended James Russell’s scientific career. Twelve years ago, as an ecology graduate student, Russell was releasing radio-collared rats on to small islands off the coast of New Zealand to study how the creatures take hold and become invasive. Despite his sworn assurances that released animals would be well monitored and quickly removed, one rat, Razza, evaded capture and swam to a nearby island.

“For 18 weeks, Russell hunted the animal. Frustrated and embarrassed, he fretted about how the disaster would affect his PhD. “I felt rather morose about the prospects for my dissertation,” he says.

“Although there was a lot of literature on controlling large rat populations, little had been written about tracking and killing a single rodent, which turns out to be rather important in efforts to completely eradicate a species. “It demonstrated how hard it is to catch that very first rat as it arrives on an island — or, conversely, the very last rat that you’re trying to get off,” says Russell, now at the University of Auckland.

Brushtail possums are among the numerous invasive pests regularly culled in New Zealand.

“Razza’s escape became the subject of a paper in Nature1 as well as a popular children’s book. And now, with more than a decade of successful pest-eradication projects behind him, Russell is taking on a much bigger challenge. He is coordinating research and development for a programme that the government announced last July to eliminate all invasive vertebrate predators — rats, brushtail possums, stoats and more — from New Zealand by 2050 to protect the country’s rare endemic species.” –Brian Owens (Continue reading:  Behind New Zealand’s wild plan to purge all pests : Nature News & Comment)