The Most Common Non-Native Plants of Southern California
As of 27 August 2005, there are 90 trails and 47 floras (lists for short in the following) in my Master List. As part of giving feedback to Cal-IPC for their invasive weed list, I sorted the Master List to find the most common non-native taxa.
I selected the species in two ways:
- First, I sorted the Master List by the number of trails in which each taxon appears. The maximum number was 55. I then selected all species that appeared on at least half this number of trails. It turned out that there was a break in the number of occurrences between an observed number of 26 and an observed number of 30, so this was a quite clean breakpoint. There were 17 taxa which were selected in this step. All of these taxa appeared in at least 24 floras, so these were frequently-found taxa in all lists.
- Second, I sorted the Master List by the number of floras in which each taxon appears. The maximum number was 41. I then selected all species that appeared in at least 20 floras, which was roughly half of the maximum number of 41. I selected the number of 20 in part to make sure I listed some species that were on the Cal-IPC list. This added another 50 taxa to my selection.
The final list contains 67 taxa, which are clearly widespread in Southern California. The specific lists on which they occur can be found by using the search feature here. In addition, most of the trail guides retrieved from such a search contain an estimate of the abundance of each species on that trail.
The primary reason that more taxa were selected using the number of floras is that floras always cover a much larger area than that of a single trail list. For example, the flora of the San Gabriel Mountains includes all the taxa found only at the lowest elevations, whereas none of the plant trail guides at high elevation contain taxa found only at the lowest elevations. This is a large effect, since the highest elevations are nearly weed-free (see, for example, the first plot here. Hence the selection criterion of half the maximum number found reaches deeper into the counts from the floras than into counts from the plant trail guides.
The relationship between the counts from the floras and the counts from the plant trail guides is shown in the next plot:
There is a very high correlation between the counts at high values, although there is significant scatter due primarily to the plant trail guides located at high elevation with few non-natives. Hence the most common non-native taxa selected using just the floras will be very similar to the most common non-native taxa selected using just the trail lists.
In contrast, uncommon non-native species are picked up much better in the floras, as would be expected due to the larger area covered by the floras.
For the curious:
- The taxon present in 15 floras but not found in any plant trail guide is barnyard grass, Echinochloa crus-galli. Interestingly, I just discovered a population of this species in one of the trail guides recently, but the Master List has not yet been updated to include it.
- The taxon present in 13 plant trail guides but only 3 floras is cut-leaved geranium, Geranium dissectum. Its overpopulation in the plant trail guides is due entirely to the high number of trails covered at the Santa Rosa Plateau Ecological Reserve, where 13 of the 16 trails there contain this species.
Of course, widespread does not mean they are also abundant (see the discussion of the many ways to define the "most common taxa"). However, nearly all the species on this list are in fact also abundant taxa where they occur, as might be expected of widespread non-native weeds.
A few caveats on the table below:
- Desert trails and floras are vastly undersampled in the Master List, and hence non-native plants in the desert are vastly undercounted relative to their true abundance in Southern California. Only 4 of the 90 trails and 1 of the 47 floras, all from the Anza-Borrego Desert, are included in the Master List presently. Hence this list should properly be considered primarily a list of non-natives observed in the Southern California section of the CA-FP.
For example, Asian mustard, Brassica tournefortii, is a very widespread, very abundant species in the DSon, but doesn't make this list due to the undersampling of the DSon in my Master List. This species is found on all 4 of those trails in Anza-Borrego, and even in four floras in addition to the Anza-Borrego Flora.
- The number for the Avena species from the plant trail lists is under-reported, since A. barbata and A. fatua can only be reliably distinguished for a short period. Hence if a trail wasn't covered during that period, the trail guide will list only A. species, and not entered in my Master List.
- Species that have several common subtaxa in Southern California will be underreported, since many floras and some trail guides list only the species, and some floras list only the genus name. I have not attempted to combine the subspecies or varieties in general, but I have for the following two species, which are very common species:
- Vulpia myuros is much more widespread than would be indicated if one just considered the varieties alone. Doing that would list only V. myuros var. hirsuta, found in 18 trails and 20 floras.
- Hordeum murinum, which wouldn't even appear on the list below if one treated the subspecies separately.
If anyone at Cal-IPC wishes me to get the accurate total count for any other such species with subtaxa, I can easily do that manually.
The numbers presented in the table below are:
Title Meaning #all number of all trails and floras in the Master List that contain this taxon (137 maximum) #trails number of our trails that contain this taxon (90 maximum) #floras number of all floras in the Master List that contain this taxon (47 maximum)
The Most Common Non-Native Plants of Southern California
The table is sorted by declining number of #all.
#all #trails #floras Latin Name Common Name 96 55 41 Erodium cicutarium redstem filaree 90 51 39 Bromus diandrus ripgut brome 89 52 37 Bromus madritensis ssp. rubens red brome 88 51 37 Hirschfeldia incana shortpod mustard 81 45 36 Centaurea melitensis tocalote 77 39 38 Avena barbata slender wild oats 75 42 33 Lactuca serriola prickly lettuce 75 41 34 Sonchus oleraceus sow thistle 75 40 35 Medicago polymorpha California burclover 71 45 26 Filago gallica narrowleaf filago 70 36 34 Hordeum murinum foxtail barley 70 32 38 Marrubium vulgare horehound 69 43 26 Vulpia myuros rattail fescue 69 40 29 Bromus hordeaceus soft chess 69 35 34 Hypochaeris glabra smooth cat's ear 69 33 36 Silene gallica windmill pink 67 32 35 Anagallis arvensis scarlet pimpernel 60 26 34 Avena fatua wild oats 60 25 35 Salsola tragus Russian thistle 59 24 35 Melilotus indicus sourclover 58 30 28 Cynodon dactylon Bermuda grass 58 25 33 Stellaria media common chickweed 56 19 37 Nicotiana glauca tree tobacco 55 23 32 Rumex crispus curly dock 54 30 24 Sonchus asper ssp. asper prickly sow thistle 54 21 33 Lamarckia aurea goldentop 52 20 32 Erodium moschatum white-stemmed filaree 52 19 33 Malva parviflora cheeseweed 50 22 28 Polypogon monspeliensis rabbits-foot grass 50 18 32 Polygonum arenastrum knotweed 49 18 31 Senecio vulgaris common groundsel 48 22 26 Schismus barbatus Mediterranean schismus 47 19 28 Cerastium glomeratum mouse-ear chickweed 47 19 28 Piptatherum miliaceum smilo grass 46 20 26 Sisymbrium officinale hedge mustard 46 19 27 Plantago lanceolata English plantain 45 17 28 Cotula australis Australian brass-buttons 44 19 25 Chamomilla suaveolens pineapple weed 44 11 33 Raphanus sativus purple wild radish 43 15 28 Ricinus communis castor bean 43 12 31 Sisymbrium orientale Oriental mustard 42 21 21 Carduus pycnocephalus Italian thistle 42 16 26 Conyza bonariensis little horseweed 42 11 31 Chenopodium album white goosefoot 41 21 20 Bromus tectorum downy brome 41 17 24 Cirsium vulgare bull thistle 40 13 27 Foeniculum vulgare fennel 40 11 29 Erodium botrys long-beaked filaree 39 12 27 Amaranthus albus tumble pigweed 36 16 20 Gnaphalium luteo-album common cudweed 35 11 24 Poa annua annual blue grass 35 9 26 Silybum marianum milk thistle 35 9 26 Capsella bursa-pastoris shepherd's purse 34 11 23 Pennisetum setaceum fountain grass 33 7 26 Plantago major common plantain 32 10 22 Atriplex semibaccata creeping Australian saltbush 31 9 22 Sisymbrium irio London rocket 31 8 23 Melilotus albus white sweetclover 31 8 23 Arundo donax giant reed 30 10 20 Euphorbia peplus petty spurge 30 8 22 Spartium junceum Spanish broom 30 5 25 Chenopodium ambrosioides Mexican tea 29 6 23 Lobularia maritima sweet alyssum 28 5 23 Brassica nigra black mustard 28 5 23 Conium maculatum poison hemlock 27 3 24 Chenopodium murale nettle-leaved goosefoot 24 2 22 Brassica rapa field mustard 23 3 20 Apium graveolens celery
The complete list of all non-native species present in my Master List in either a plant trail guide or a digitized flora is also available, but keep in mind that this list simply counts the number of lists in which they occur. The abundance of these species may be quite different from that count.
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: 28 August 2005