Analysis of Preliminary Cal-IPC Noxious Weed Ratings



The California Invasive Plant Council (Cal-IPC) is a wonderful organization that is devoted to Protecting California's Wildlands from Invasive Plants Through Research, Restoration, and Education. This is an extremely-important mission, since otherwise we will lose much of our native species due to the introduction of non-native species. (If anyone naively thinks this is "survival of the fittest" at work, nothing could be farther from the truth.)

Cal-IPC broke ground in 1999 when they published their first Exotic Pest Plants of Greatest Ecological Concern in California. In the past year or so, they have been working on a revision of that list, and in August 2005 completed work on some 248 non-native species. A detailed Preliminary Assessment Form was filled out for each species, resulting in a score that represents exactly how noxious each of these noxious weeds are. The scores were then translated into an assessment of High, Medium, Low, or "Considered But Not Listed". A ranking of High means:

These species have severe ecological impacts on ecosystems, plant and animal communities, and vegetational structure. Their reproductive biology and other attributes are conducive to moderate to high rates of dispersal and establishment. These species are usually widely distributed ecologically, both among and within ecosystems.

Lower assessments are given to species with lesser impacts.

This page gives the lists and links to the Preliminary Assessment Forms: Invasive Plant Inventory Revision Completed Preliminary Assessment Forms, and asks for comments on the rankings.

This page reports my analysis of the scores, using my own experience and results from the database I have compiled from producing plant trail guides for 90 trails that span a large area of Southern California, and from digitizing floras that cover most of Southern California.

I have widespread recent expert experience in the current state of noxious weeds in the CA-FP section of Southern California, since I have botanized a full day in the field roughly every fourth day since 2001, and spent the intervening three days working on my field samples and data full time. My plant trail guides sample Southern California from the Santa Monica Mountains south and east, including the San Gabriel Mountains, the San Bernardino Mountains, the Palomar Mountains, the Laguna mountains, and the cismontane areas on the coastal side of all those ranges.

I have four trails in the Anza-Borrego Desert, so have a small sampling in that area as well, but this is not a large enough sample to use in a statistical analysis.

The database I have produced from these trail guides, and from the digitized floras compiled by others, is ideally suited for investigating which are the most widespread non-native plants with greatest impact to our native ecosystems. The trail guides not only give a plant list for a trail, but also include an estimate of the abundance of each species on that trail.

Using my personal experience, I have given Cal-IPC comments on three individual species that are much worse problems than were known in the preliminary assessment:

I have also gone through their entire list, and given my own rankings to those noxious weeds which are present in Southern California.

Personal experience can sometimes be misleading, however, so I used my database to produce a list of the Most Common Non-Native Plants in Southern California as well as the number of trails and floras that contain every single Non-Native Taxa in Southern California Observed On A Trail Or In A Flora.

The rest of this page compares the preliminary scores from both CAL-IPC and my own estimates to this quantitative list.


The Cal-IPC list available in the Excel spreadsheet contains 255 species, or groups of species in a few cases. This is slightly larger than the 248 species since it includes some that are still being assessed.

I have compared those to my list of Non-Native Taxa in Southern California Observed On A Trail Or In A Flora; 191 match a taxon on my list and 64 are unmatched. Of the 64 unmatched species, two of them are species that are considered native species in the Jepson Manual which apparently may have non-native strains confused with them or are now considered non-native. Those two species, Phragmites australis and Ludwigia hexapetala, are not considered further here.

The other 62 unmatched species are not found in Southern California, according to the Jepson Manual regions listed in the Excel spreadsheet. (Two of those species, Senecio jacobaea and Spartina patens, are erroneously listed in the excel spreadsheet as being in the Jepson Manual SW geographic region.)

I separated the 191 matched taxa into three groups:

The following two plots show a comparison of the preliminary rankings from Cal-IPC, and from my own assessment, for the Established Invaders, with the number of trails plus floras in which each of those species appear:

In both plots, the scores were translated to numbers as follows:

ScoresNumeric Value In Plot Above
Not Ranked0

Individual species are plotted with blue diamonds. Averages of 11 species, grouped in non-overlapping bins of the # of trails and floras, are shown with pink rectangles. There are not as many species with my scores (TJC scores) since I didn't assess every species.

Before these plots can be analyzed, I need to assess what behavior should be seen in these plots.

First, for established Invaders, a species with a high ranking ought to be found in many trails and floras, as stated in the definition: These species are usually widely distributed ecologically, both among and within ecosystems. A species with a low ranking ought to be found in very few trails and floras, again from its definition: Ecological amplitude and distribution are generally limited.

This immediately implies, for established invaders, that high rankings should be found only for species also found in a large number of trails plus floras.

The plots show that my high scores satisfy this expectation, but the Cal-IPC scores do not. My high rankings are found predominantly on the right hand side of these plots, showing that these species are indeed widely distributed ecologically, both among and within ecosystems. (It is important to note that my trail guides and floras are widely distributed with respect to ecological habitats, essentially sampling every habitat in Southern California.)

In contrast, the Cal-IPC high scores, and the Cal-IPC medium scores, show, if anything, a concentration toward the left hand side of these plots.

Second, the definition of the high ranking includes: These species have severe ecological impacts on ecosystems, plant and animal communities, and vegetational structure. Hence a species can be widespread, present in many trails and floras, without having severe ecological impacts.

This implies that, without further information, low rankings could be found anywhere in the plots. Thus one cannot conclude anything about the low rankings simply by looking at the above plots without knowing how abundant each of those species are, and whether they displace native plants in ecosystems.

To go further requires looking at the individual species ranked low or lower (not ranked) yet are found on a high number of trails plus floras.

In order of the species found on the highest number of trails plus floras, the ones with a Cal-IPC ranking of low or lower are:

Name# Trails Plus FlorasCal-IPC RankingTJC Ranking
Erodium cicutarium9613
Lactuca serriola7503
Medicago polymorpha7513
Marrubium vulgare7012
Bromus hordeaceus6913
Hypochaeris glabra6913
Rumex crispus5513
Sonchus asper5401

Here is a brief assessment of some of those species, accompanied by estimates of their abundance from various trails. The abundance is given as the minimum number of plants / number of locations along a trail that can be touched without leaving the trail. The maximum numbers assigned are 99/9, which are reserved for the most abundant species along a trail. Taxa recorded as 50/9 are very abundant taxa, and usually have way more than 50 individuals along the trail.

This analysis reveals that the most widespread species should have a ranking of Medium or High.

Now I can return to the analysis of the plots, and state that there should also be an absence of low rankings for widespread species.

Together with the deduction that high rankings should be found only for species also found in a large number of trails plus floras, this implies that there should be a clear trend in the plot for average rankings for widespread species to be significantly higher than the average rankings for non-widespread species.

Examining the pink rectangles in the above plots shows that my gut rankings exhibit that behavior, with an average ranking near High for the most-widespread taxa, and an average ranking near Low for the least widespread taxa. However, the Cal-IPC rankings exhibit hardly any trend at all.

I thus recommend that the Cal-IPC rankings should be revisited in light of the data from my survey of Southern California trails and the data from the floras, with some tweaking needed to produce a list that correctly reflects how invasive species are in Southern California.

This conclusion is not terribly surprising, given the lack of quantitative data available previously to researchers trying to assess the state-wide or region-wide impacts from various species.

Finally, for completeness, here are plots showing the preliminary rankings from Cal-IPC for the New Invaders and the Obscure Invaders, with the number of trails plus floras in which each of those species appear:

The first plot above shows that some of the New Invaders are already fairly widespread; their high rankings are definitely deserved, especially for the least-widespread ones. It is far better to make these a high priority and prevent them from becoming widespread!

The second plot above shows why I classified the taxa as Obscure Invaders, since I have virtually no experience with them, nor are they in many Southern California floras. If I had more time, I'd look at the preliminary assessments for some of the high and low ranked ones, to see how the rankings were determined.

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Copyright © 2005 by Tom Chester
Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
Comments and feedback: Tom Chester
Last update: 30 August 2005