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California Invasive Plant Inventory, 2006

Download 2006 Inventory (pdf)

The California Invasive Plant Inventory categorizes non-native invasive plants that threaten the state’s wildlands. Categorization is based on an assessment of the ecological impacts of each plant. The Inventory represents the best available knowledge of invasive plant experts in the state. However, it has no regulatory authority, and should be used with full understanding of the limitations described below.

California is home to 4,200 native plant species, and is recognized internationally as a “biodiversity hotspot.” Approximately 1,800 non-native plants also grow in the wild in the state. A small number of these, approximately 200, are the ones that this Inventory considers invasive. Improved understanding of their impacts will help those working to protect California’s treasured
biodiversity.

2017 Update

Cal-IPC’s Inventory review committee oversaw a 2017 update of the Inventory and released proposed additions to the inventory on Feb. 1, 2017.

The Inventory

Yellow starthistle (Centaurea solstitialis) flowerheads. Photo courtesy Bob Case

The Inventory categorizes plants as High, Moderate, or Limited, reflecting the level of each species’ negative ecological impact in California. Other factors, such as economic impact or difficulty of management, are not included in this assessment. It is important to note that even Limited species are invasive and should be of concern to land managers. Although the impact of each plant varies regionally, its rating represents cumulative impacts statewide. Therefore, a plant whose statewide impacts are categorized as Limited may have more severe impacts in a particular region. Conversely, a plant categorized as having a High cumulative impact across California may have very little impact in some regions.

Two additional categories are “Alert” and “Watch.” An Alert is listed on species with High or Moderate impacts that have limited distribution in California, but may have the potential to spread much further. Species on the “watch” list have been assessed as posing a high risk of becoming invasive in the future in California.

The Inventory Review Committee, Cal-IPC staff, and volunteers drafted assessments for each plant based on the formal criteria system described below. The committee solicited information from land managers across the state to complement the available literature. Assessments were released for public review before the committee finalized them.

The 2006 Cal-IPC Inventory list includes 39 High species, 65 Moderate species, and 89 Limited species. Additional information, including updated observations, will be added to this website periodically, with revisions tracked and dated.

Definition of Invasive Plants

The Inventory categorizes “invasive non-native plants that threaten wildlands” according to the definitions below. Plants were evaluated only if they invade California wildlands with native habitat values. The Inventory does not include plants found solely in areas of human-caused disturbance such as roadsides and cultivated agricultural fields.

Wildlands are public and private lands that support native ecosystems, including some working landscapes such as grazed rangeland and active timberland.
Non-native plants are species introduced to California after European contact and as a direct or indirect result of human activity.
Invasive non-native plants that threaten wildlands are plants that 1) are not native to, yet can spread into, wildland ecosystems, and that also 2) displace native species, hybridize with native species, alter biological communities, or alter ecosystem processes.

Criteria for Listing

The California Invasive Plant Inventory updates the 1999 “Exotic Pest Plants of Greatest Ecological Concern in California.” Cal-IPC’s Inventory Review Committee met regularly between 2002 and 2005 to review 238 non-native species with known or suspected impacts in California wildlands. These assessments are based on the “Criteria for Categorizing Invasive Non-Native Plants that Threaten Wildlands”, developed in collaboration with the Southwestern Vegetation Management Association in Arizona and the University of Nevada Cooperative Extension so that ratings
could be applied across political boundaries and adjusted for regional variation.

The goals of the criteria system and the Inventory are to:
Provide a uniform methodology for categorizing non-native invasive plants that threaten wildlands;
Provide a clear explanation of the process used to evaluate and categorize plants;
Provide flexibility so the criteria can be adapted to the particular needs of different regions and states;
Encourage contributions of data and documentation on evaluated species;
Educate policy makers, land managers, and the public about the biology, ecological impacts, and distribution of invasive non-native plants.

The criteria system generates a plant’s overall rating based on an evaluation of 13 criteria, which are divided into three sections assessing Ecological Impacts, Invasive Potential, and Ecological Distribution. Evaluators assign a score of A (severe) to D (no impact) for each criterion, with U indicating unknown. The scoring scheme is arranged in a tiered format, with individual criteria contributing to section scores that in turn generate an overall rating for the plant. Detailed plant assessment forms list the rationale and applicable references used to arrive at each criterion’s score. The level of documentation for each question is also rated, and translated into a numerical score for averaging. The documentation score presented in the tables is a numeric average of the documentation levels for all 13 criteria.

Inventory Categories

Each plant on the list received an overall rating of High, Moderate or Limited based on evaluation using the criteria system. The meaning of these overall ratings is described below. In addition to the overall ratings, specific combinations of section scores that indicate significant potential for invading new ecosystems triggers an Alert designation so that land managers may watch for range expansions. Some plants were categorized as Evaluated But Not Listed because either we lack sufficient information to assign a rating or the available information indicates that the species does not have significant impacts at the present time.

High – These species have severe ecological impacts on physical
processes, plant and animal communities, and vegetation structure. Their reproductive
biology and other attributes are conducive to moderate to high rates of dispersal
and establishment. Most are widely distributed ecologically.
Moderate – These species have substantial and apparent—but
generally not severe—ecological impacts on physical processes, plant
and animal communities, and vegetation structure. Their reproductive biology
and other attributes are conducive to moderate to high rates of dispersal,
though establishment is generally dependent upon ecological disturbance. Ecological
amplitude and distribution may range from limited to widespread.
Limited – These species are invasive but their ecological impacts
are minor on a statewide level or there was not enough information to justify
a higher score. Their reproductive biology and other attributes result in low
to moderate rates of invasiveness. Ecological amplitude and distribution are
generally limited, but these species may be locally persistent and problematic.

Uses and Limitations

The California Invasive Plant Inventory serves as a scientific and educational report. It is designed to prioritize plants for control, to provide information to those working on habitat restoration, to show areas where research is needed, to aid those who prepare or comment on environmental planning documents, and to educate public policy makers. Plants that lack published information may be good starting points for student research projects. The Inventory cannot address, and is not intended to address, the range of geographic variation in California, nor the inherently regional nature of invasive species impacts. While we have noted where each plant is invasive, only the cumulative statewide impacts of the species have been considered in the evaluation. The impact of
these plants in specific geographic regions or habitats within California may be greater or lesser than their statewide rating indicates. Management actions for a species should be considered on a local and site-specific basis, as the inventory does not attempt to suggest management needs for specific sites or regions. The criteria system was designed to be adapted at multiple scales, and local groups are encouraged to use the criteria for rating plants in their particular area.

Providing Input for Future Revisions

If you have additional information to add to a plant assessment, please complete this form and email to info@cal-ipc.org.

Summary of the Criteria

Section 1. Ecological Impact
1.1   Impact on abiotic ecosystem processes (e.g. hydrology, fire, nutrient cycling)
1.2   Impact on native plant community composition, structure, and interactions
1.3   Impact on higher trophic levels, including vertebrates and invertebrates
1.4   Impact on genetic integrity of native species (i.e. potential for hybridization)

Section 2. Invasive Potential
2.1   Ability to establish without anthropogenic or natural disturbance
2.2   Local rate of spread with no management
2.3   Recent trend in total area infested within state
2.4   Innate reproductive potential (based on multiple characteristics)
2.5   Potential for human-caused dispersal
2.6   Potential for natural long-distance (>1 km) dispersal
2.7   Other regions invaded worldwide that are similar to California

Section 3. Distribution
3.1 Ecological amplitude (ecological types invaded in California)
3.2 Ecological intensity (highest extent of infestation in any one ecological type)

Documentation Levels
Assessed as highest level of documentation for each criterion.
4 = Reviewed scientific publications
3 = Other published material (reports or other non-peer-reviewed documents)
2 = Observational (unpublished information confirmed by a professional in the field)
1 = Anecdotal (unconfirmed information)
0 = No information

References

Bossard, C. C., J. M. Randall, and M. C. Hoshovsky.2000. Invasive Plants of California’s Wildlands. University of California Press: Berkeley, CA.
Cal-EPPC. 1999. The Cal-EPPC List: Exotic Pest Plants of Greatest Ecological Concern in California. California Exotic Pest Plant Council: San Juan Capistrano, CA.
Warner, P.J., C. C. Bossard, M.L. Brooks, J. M. DiTomaso, J. A. Hall, A. M. Howald, D. W. Johnson, J. M. Randall, C. L. Roye, and A. E. Stanton.2003. Criteria for Categorizing Invasive Non-native Plants that Threaten Wildlands. California Exotic Pest Plant Council and Southwest Vegetation Management Association.
Hickman, J. C. (ed.) 1993. The Jepson Manual: Higher Plants of California. University of California Press: Berkeley, CA.
WSSA. 2005. Composite List of Weeds. Weed Science Society of America.
Holland, R. F. 1986. Preliminary Descriptions of the Terrestrial Natural Communities of California. Unpublished report. California Department of Fish and Game: Sacramento, CA.

2006 Inventory Review Committee

Dr. Carla Bossard, Professor
St. Mary’s College of California
Moraga, CA

Dr. Matt Brooks, Research Botanist
US Geological Survey
Henderson, NV

Dr. Joseph DiTomaso, Extension Non-Crop Weed Ecologist
University of California-Davis
Davis, CA

Dr. John Randall, Director, Invasive Species Initiative
The Nature Conservancy
Davis, CA

Cynthia Roye, Environmental Scientist
California State Parks
Sacramento, CA

Jake Sigg
California Native Plant Society
San Francisco, CA

Alison Stanton, Research Botanist
BMP Ecosciences
South Lake Tahoe, CA

Peter Warner, Environmental Scientist
California State Parks
Mendocino, CA

Acknowledgments

We gratefully acknowledge the effort of all those who volunteered their time to write plant assessment forms, provide comments on assessments, or add observations to fill gaps in information. Too many people contributed information for us to list them individually, but each assessment contains the name of its author and those who provided information on that species. In particular, we thank those who helped develop the criteria, including John Hall of The Nature Conservancy in Arizona, Ann Howald of Garcia and Associates, and Maria Ryan of University
of Nevada Cooperative Extension. We also wish to thank Kristin Dzurella of UC Davis and John Knapp of the Catalina Island Conservancy for their contributions of time and data.

Financial Support

Funding for this project was provided by the Center for Invasive Plant Management and the Exotic/Invasive Pests and Diseases Research Program of the UC Statewide IPM Program and UC Riverside Center for Invasive Species Research, funded by USDA/CREES Special Research Grant Exotic Pests and Diseases (CA). General operating support provided by the San Francisco Foundation, the Switzer Foundation, and the True North Foundation.

California Invasive Plant Council

Protecting California’s environment and economy from invasive plants