Newly-Introduced Non-Native Species at the Santa Rosa Plateau Ecological Reserve, and the Rate of Introduction of New Species With Time

UNFINISHED DRAFT; ADD WATERLINE ROAD effect; mention of other species such as african daisy, yellow star thistle, artichoke thistle; species extirpated at the srp (aloe, nicotiana glauca)

Introduction and Summary
The Newly-Introduced Non-Native Species
Rate of Introduction of New Species With Time

Introduction and Summary

Non-native species are a huge problem nearly everywhere in the world, and the Santa Rosa Plateau Ecological Reserve is no exception. This page lists the 34 non-native species found from my 2005 survey of the trails of the Santa Rosa Plateau Ecological Reserve that were not found here in previous surveys by Lathrop and Thorne in 1985 and earlier. (There are additional newly-introduced species at the Reserve not found on trails that are not included in this list.)

The total of 34 species in 20 years gives a raw average introduction rate of 1.7 species per year. Analysis of this list shows that it is unlikely that most of these species were missed in the Lathrop and Thorne surveys, and hence there has indeed been an explosion in the rate of introduction of new species since approximately 1995.

This huge increase in the rate of introduction of new species is probably due to the following factors:

Note that the introduction rate for new non-native species for Riverside County as a whole will be larger than the number quoted above, which only includes new non-native species found at the Reserve.

The Newly-Introduced Non-Native Species

The following non-native species were not found in the area of the Santa Rosa Plateau Ecological Reserve in the Lathrop and Thorne 1985 Flora of the Santa Rosa Plateau, Southern California. A few of these species are listed in that flora, but were only found in restricted localities in a small portion of the 82% of the area in that Flora outside the Reserve itself. See Flora of the Greater Santa Rosa Plateau Region for more information about the difference in areas.

For each species, information about their spread in Southern California was deduced from the following sources:

1959Munz, A California FloraMunz 1959
1968Lathrop and Thorne, Aliso 6:17-40LT 1968
1974Munz, A Flora of Southern CaliforniaMunz 1974
1978Lathrop and Thorne, Flora of the Santa Ana Mountains, California Aliso 9:197-278LT 1978
1985Lathrop and Thorne, Flora of the Santa Rosa Plateau, Southern California, Southern California Botanists Special Publication No. 1LT 1985
1986Beauchamp, Flora of the San Diego County, Southern CaliforniaBeauchamp 1986
2004Fred M. Roberts, Jr., Scott D. White, Andrew C. Sanders, David E. Bramlet, and Steve Boyd, The Vascular Plants of Western Riverside County, California: An Annotated Checklist, F.M. Roberts PublicationsWRC 2004
2005Rebman et al, San Diego County Plant AtlasSDCPA 2005

San Diego County is only 2.2 miles south-southwest of the Mesa de Colorado, and hence is as important a source of information about nearby taxa as Riverside County. Orange County is 13 miles west, and largely on the other side of the Santa Ana Mountains from the Santa Rosa Plateau, but still is an important source of information about nearby taxa. Unfortunately, there is no flora of Orange County with locations and abundance that can easily be checked for this study.

Information about the spread at the Santa Rosa Plateau Ecological Reserve is from my observations from 2001-2005, augmented by those of Zach Principe, the Reserve Biologist from 1990 to 2005.

The species are given in order of their current abundance on the trails of the Santa Rosa Plateau Ecological Reserve. For each taxon, I give my estimate of the introduction date to the Santa Rosa Plateau Ecological Reserve and the earliest voucher in the SMASCH database for Riverside County. The SMASCH database includes the vouchers from UC Riverside, but does not yet include vouchers from Rancho Santa Ana Herbarium. I will attempt to get any earlier dates from RSA in the future. Vouchers I collected in 2005 have not yet been given to UCR, but I treat those vouchers as if they were in SMASCH and report that date if it is the only voucher.

Discussion of each species follows the list, and is linked to the name for each species in the table.

List of New Non-Native Species

Family NameScientific NameCommon Name
Widespread and abundant Taxa
20031967AsteraceaeCarduus pycnocephalusItalian thistle
19851961AsteraceaeHedypnois creticaCrete weed
19851980AsteraceaeCirsium vulgarebull thistle
19901971FabaceaeVicia villosa ssp. variawinter vetch
19852005PoaceaeAegilops cylindricajointed goatgrass
Common Taxa or Taxa Rapidly Expanding Their Range
20031999ApiaceaeTorilis arvensisfield hedge-parsley
20001966AsteraceaeTragopogon dubiusyellow salsify
20031986FabaceaeTrifolium hirtumrose clover
20001992FabaceaeVicia sativa ssp. sativaspring vetch
19851908PlantaginaceaePlantago lanceolataEnglish plantain
Locally-Abundant Taxa
20001954AsteraceaePicris echioidesbristly ox-tongue
19951994GeraniaceaeErodium botryslong-beaked filaree
19851941MalvaceaeMalva parvifloracheeseweed
20002001RubiaceaeGalium muraletiny bedstraw
19951956PoaceaePhalaris aquaticaHarding grass
Infrequent Taxa
20001904AsteraceaeSilybum marianummilk thistle
20001999CrassulaceaeCrassula tillaeaMediterranean pygmy-weed
20002001RubiaceaeGalium parisiensewall bedstraw
20001915PoaceaeDactylis glomerataorchard-grass
20002006PoaceaeBrachypodium distachyonpurple false-brome
19951995PoaceaeHainardia cylindricabarbgrass
Scarce Taxa
20002005AsteraceaeAcroptilon repensRussian knapweed
20001934AsteraceaeAnthemis cotuladog-fennel
20051919AsteraceaeCotula australisAustralian brass-buttons
20051965BrassicaceaeLobularia maritimasweet alyssum
20002004BrassicaceaeSinapis arvensischarlock
20001895ConvolvulaceaeConvolvulus arvensisfield bindweed
20001962LamiaceaeLamium amplexicaulehenbit
20001996LiliaceaeAloe saponariasoap aloe
20001897PoaceaeDigitaria sanguinaliscrabgrass
20051952PoaceaeEleusine indicagoose grass
20002005PoaceaeElytrigia repensquackgrass
20001950PoaceaeFestuca arundinaceatall fescue
20051965PoaceaePiptatherum miliaceumsmilo grass

Discussion of Each New Non-Native Species

Widespread and Abundant Taxa

Rate of Introduction of New Species With Time

A histogram of the estimated introduction dates to the Santa Rosa Plateau Ecological Reserve for all observed non-native species (not just the new ones listed above) shows that new introductions exploded after 1995:

The above plot includes additional non-native species over the list of 34 presented above, with their introduction dates estimated primarily from Munz 1959, LT 1968, LT 1978, and LT1985. No dates were obtained prior to 1960 since I had no earlier sources to pin down earlier dates.

The number of introduced species per decade was 26 new species in the decade from 1995 to 2005, after being between 0 and 5 new species per decade in the previous four decades.

This dramatic increase is not due to omissions in the Lathrop and Thorne surveys. The detailed discussion for each of the species above shows that many of the species have clear support from many sources that they are recently introduced in this area. Analysis given below will also show that many of these species are recent introductions.

Instead, the increase is most likely the result of three factors, which will be explored in turn below.

Rate of Introduction of New Non-Native Species to Riverside County As a Whole

Part of the increase is due to introductions in Riverside County (and often Southern California) as a whole. To estimate how much of the increase is part of this area-wide trend, I histogrammed the dates for the first voucher for these species in Riverside County. The following histogram shows that there has been a significant increase in the number of introduced non-native species to Riverside County as a whole.

The number of introductions to Riverside County increased from 1-2 species per decade in 1895 to 1945 to an average of 3 per decade in 1945 to 1985 to 4 in 1985 to 1995 and 9 in 1995 to 2005. Purely statistically, the average of 9 in 1995 to 2005 is a two-sigma increase over the average of 3 per decade and thus is formally significant at the 95% confidence level.

However, voucher collections vary tremendously with time, geographic location, and with the abundance of a given taxon. It is possible that the dramatic increase in the number of first collections in the last decade is due to one or more of the following effects:

To understand these factors, I created a control population within the Santa Rosa Plateau Trail Flora that has similar characteristics to the new non-native species. Analysis of the control population, given below, verifies that the number of introductions to Riverside County as a whole has indeed increased significantly, and that it is not due to any effect of the Riverside County vouchering.

To obtain the control population, I sorted the Trail Flora first by the number of trails on which I observed each taxon in my 2005 survey and then by family. I then selected the taxon immediately following each one of the new non-native taxa at the Santa Rosa Plateau, irrespective of whether it was native or non-native, and histogrammed their first Riverside County voucher.

This procedure ensured the selection of plants with very similar characteristics to the new non-native species in terms of the above factors which might compromise deductions from the date of the first voucher. The species in the control population are from the same geographic area, and have the same abundance distribution. If the new non-native species I have found at the Santa Rosa Plateau Ecological Reserve had actually been present earlier, but missed by previous surveys of the Santa Rosa Plateau, one would expect them to have similar dates for their first voucher collection in Riverside County.

The histogram for the control population, shown on the above plot, is dramatically different from that of the new non-native species, with a huge peak in the 1895 - 1905 decade that is completely absent from the histogram of the new non-native species. Except for that peak, the numbers are essentially constant with time, varying from 0-4 taxa per decade.

One variable that couldn't be controlled as well in the above selection was native vs. non-native, due to the smaller numbers of other non-native taxa. I explored whether there was any difference in collecting non-native taxa vs. native taxa in two ways.

First, albeit with less statistical significance, I can break down my control population into native and non-native species. There were 25 native species in the control population, and 8 non-native species. The separate histogram for the non-native species is similar to the separate histogram for the native species, with a huge peak in the 1895 - 1905 decade (three of the eight taxa were collected in 1901, 1902 and 1906). Clearly, non-natives were being well-collected as far as 100 years ago.

Second, I can histogram the first SMASCH voucher in Riverside County for every non-native species in the Santa Rosa Plateau Ecological Reserve Trail Flora. The following histogram shows that plenty of non-natives were vouchered one hundred years ago, with a peak in 1895-1905 that is very similar to the peak seen in the control population as a whole.

Furthermore, this plot shows that the number of species introduced to Riverside County, and found on trails at the Santa Rosa Plateau, has apparently increased from an average of 0.5 species per year in the 1905-1945 period to an average of 0.9 species per year in the 1945-2005 period. This is very different from the decline seen in recent decades in the control sample plotted above, especially when compared against just the native population in that control sample. There is a slight tendency toward more recent collections for the non-natives species in my control sample: the latest voucher for a native species is 1980, whereas two of the non-native control sample were collected first in 1985 and 1993.

I therefore conclude that the observed recent increase in the introduction rate of non-native species into Riverside County is not due to under-collection of non-native species in previous decades. This increase is a real effect that has been going on for perhaps a half century.

One should note that a newly-introduced non-native species will appear much earlier in a large area such as Riverside County than for any smaller specific area within that larger area. It takes time for a newly-introduced species to spread from the introduction point to another location. Hence it is not surprising at all that the rate of introductions to Riverside County is a smoother function than the rate of introductions to a specific area, especially in light of the factors considered next.

Introduction of Non-Native Species By Development Around the Santa Rosa Plateau

As late as 1995, the environs of the Santa Rosa Plateau Ecological Reserve remained largely undeveloped. However, beginning around 1995 the Temecula - Murrieta area experienced explosive growth. The north and east sides of the Santa Rosa Plateau Ecological Reserve became essentially fully-developed by 2005, and significant development occurred on the western side as well.

Developments on the west side, at the origins of several branches of Cole Creek, have likely been the source for four species: Lobularia maritima, sweet alyssum; Lamium amplexicaule, henbit; Aloe saponaria, soap aloe; and Elytrigia repens, quackgrass. Aloe saponaria appeared about a half mile inside the Reserve, directly downstream and along Stevenson Canyon Creek, and probably resulted from someone either dumping a plant outside the Reserve at the head of the Creek. Lamium amplexicaule appeared just across Clinton Keith Road along the stream from Sylvan Meadows homes. Lobularia maritima appeared much deeper in the Reserve along Cole Creek in the heavy flooding of 2005. This is a common garden plant, and there were probably numerous sources in the headwaters of Cole Creek. Elytrigia repens is a common lawn, garden or crop weed, which has never before been recorded in Riverside County. New developments, which often bring in nursery plants from outside the County, are by far the most likely suspect for this record.

All of these four species are garden plants or garden or lawn weeds, rarely found in natural settings.

Nearby developments may also be a factor in the introduction of two other taxa: Silybum marianum, milk thistle; and Dactylis glomerata, orchard-grass (at least in Sylvan Meadows). All of these species appeared in drainages below developments outside the Reserve.

These two species are not ones new to the Riverside County Flora. Their first vouchers are 1904 (Silybum marianum) and 1915 (Dactylis glomerata). Silybum marianum is sometimes planted as an ornamental, and Dactylis glomerata is usually a cultivated species, primarily for hay and / or forage for horses.

In summary, new developments near the Santa Rosa Plateau Ecological Reserve are likely the source for 4-6 species of the 34 listed here.

Introduction of Non-Native Species By Visitors to the Santa Rosa Plateau

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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
Updated 12 October 2005.