Plants of Southern California: Introduction and Explanation of Trail Guides

Introduction to Our Trail Guides
Explanation of Trail Guides
Trail Guide Caveats and Further Information
Search the Trail Guides
Tips For Printing the Trail Guides
Glossary and Footnotes (words and concepts explained in the glossary are linked at their first occurrence in the text below)


The ability to identify and recognize different species of plants is something that adds immeasurably to one's pleasure in hiking a given trail. Recognizing old friends, and discovering new ones, opens one's eyes to the incredible richness, beauty and diversity of our native and introduced plant species. Having a name for a plant is the only way to find out more information about that species. An ancient Chinese proverb says:

The beginning of wisdom is calling things by their right names.

In addition to learning a whole new way to appreciate nature, there are sometimes large aesthetic payoffs of acquiring this knowledge. For example, being able to identify plant species when they are not in bloom allows one to recognize areas that produce magnificent floral displays at other times of year, and to return to enjoy that peak display.

Many plant species or genera are easy to recognize at any time of year by anyone familiar with them. Thus many hikers have learned to recognize such plants as coast live oaks, lupines, and California poppies without much effort. Unfortunately, many plant species are difficult to identify. For example, there are ~48 different species of lupines in Southern California, with over 20 different subspecies within those species, for a total of nearly 70 different taxa in the lupine genera. Each of those different taxa has their own beauties, and being able to identify each different taxon allows one to appreciate those beauties at a whole new level.

Further complicating matters, plant identification often relies on features that are hard to observe, or features that may not be available for use at the time one sees a plant. For example, some keys require both flower and fruit, which sometimes are never available at the same time.

Finally, there are roughly 4,000 taxa of vascular plants in Southern California. Very few people can learn each of these species and remember how to identify them.

Fortunately, botanists long ago found a way to simplify the process of plant identification. A few of them study in detail a given area, and compile a list of all the species they find in that area. They then publish that as a plant list for that area, making the task of plant identification for that area much easier for everyone else.

There is a fundamental observational fact behind the power of a plant list. Botanists have found that the number of taxa in a given area scales with the area to roughly the 1/3 power. As an easy to remember rule of thumb, if one reduces an area by a factor of ten, that will halve the number of taxa found in that area compared to the ten times larger area. Thus if one is working with a smaller area than all of Southern California, one does not have to discriminate all the 4,000 taxa found in Southern California or all the 70 different taxa of lupines. For example, if one is hiking in the San Gabriel Mountains, there are only about 1,200 taxa and about 22 taxa of lupines found in that area. If one is hiking only in the Mt. Wilson area, there may be only ~500 taxa and ~8 taxa of lupines. Finally, if one is hiking only on the Echo Mountain Trail, there may be only ~100 taxa and a single taxon of lupines. If indeed there is only a single taxon of lupines on a trail, and you have a complete plant list for that trail, you will know the name of all the lupines you see on that trail, whether they are in flower or not.

In other words, most plant species have a restricted distribution, and thus there is not an equal probability of encountering every one of the 4,000 taxa at a given location. For reasons we don't fully understand, one area of a given habitat is often not exactly the same as another to many plant species. (This is why it is important to preserve as much natural habitat as possible.)

It may come as a surprise to the reader that plant lists do not exist for many large areas of Southern California, let alone for nearly any trail. This is mostly a reflection of the low funding that botany receives from the federal government relative to other disciplines of science. Thus even some large areas of Southern California, such as the San Gabriel Mountains, the San Bernardino Mountains, and the San Jacinto Mountains, have no published plant lists at all. (Ironically, plant lists are most often created for areas where all the plants will be killed, as part of the environmental impact report for property that is about to be "developed".)

Our Trail Guides are plant lists for a given trail with extra information: the location of the first occurrence of every taxon we find along a given trail in hundredths of a mile from the trailhead. We started making such lists for ourselves, to help learn the species of a given trail. By having the location identified, along with a written list of its common and scientific name, we could study each species throughout the year and get to know each one better.

We then realized that our collection of Trail Guides could be used to help others learn the species of a given trail. Since the species are in order of their first appearance on the trail, one can simply hike the trail with our Guide and identify the species encountered one by one. (See Trail Guide Caveats for some limitations in this procedure.)

If you are interested in only one of the species on a trail, you can go directly to its first appearance on the trail. For example, if one of our Guides says the first occurrence of toyon (Heteromeles arbutifolia) is at mile 0.50 of the trail, then you can go to that location on that trail and find a real live toyon plant to examine. There are a number of ways to do this:

  1. Our Guides all list landmarks of each trail at regular intervals. Thus by simply paying attention to the landmarks, you can zero in on a given location. If you are able to recognize most of the species listed, this is even more helpful in using our Guides to find a given species, since the first occurrence of each species is then a landmark for you.

  2. If you have a pedometer, a GPS receiver or other means to mark your mileage from the trailhead, you can use the given mileage directly, especially if you use our landmarks to correct any deviations between your mileages and ours.

  3. You can use Topo! or one of the similar map programs to translate the trail mileage to a GPS location, and then use a GPS receiver to navigate to the species. We in fact have translated many of our mileage markers into GPS locations, and will make the GPS coordinates of each species available as time permits.

Our Guides also provide a detailed record of the species present along a trail that establishes a baseline with which to examine changes in the future. Our native plant species are undergoing high extinction rates due to habitat destruction and "unnatural" competition from non-native plants. Worse, these two causes often work together to create even worse loss of species then each cause would separately. Hence in a few decades some of our trails may have entirely different populations of species.

We are also compiling our Guides into a database that creates several more uses. The most helpful for plant identification is to find all occurrences of a given species on all the trails we have surveyed, so that one can go to the closest trail and actually find that species. We've found that among the most powerful methods of plant identification is simply to see all the closely related species that might be confused with each other. Often, they exhibit dramatic differences that are not captured or conveyed well in a key or in the terse written description found in most floral guides.

In this respect, all of the occurrences in our database are living vouchers.

We have only one request for the user of our Trail Guides. Please do not disturb the plants on a trail in any way. Do not pick the flowers, harvest the seeds, or eliminate plants you think are weeds unless you are an expert in discriminating weed species from native species. There have been many examples of well-meaning people destroying rare endemic thistles because they could not discriminate them from the common non-natives. Also, the horribly-invasive Arundo is sometimes hard to discriminate from two natives (Leymus sp. and Phragmites sp.). Also, note that in most localities it is actually illegal for you to disturb any of the plants without a permit to do so, even the weeds:

But officer, it was a WEED!
That's what they all say, sonny, I'm running you in....

One reason why we list only species very close to a trail is that the primary harmful effect of humans in a native habitat is habitat destruction by trampling. Please stay on the trail; by doing so, the habitat will not be harmed by your presence. If enough people go off-trail to see "something interesting", it will create a new trail, destroying all the habitat that formerly had no trail.

Explanation of Trail Guides

Our Trail Guides give the location of the first occurrence of every taxon we find along a given trail, as well as the number of plants or locations along that trail for each species. We assign numbers only to plants that can be touched while standing on the edges of the trail. Such plants can be examined closely, without trampling other plants, since one of the primary purposes of the list is for people to find a given species and study it. Species that are off-trail are either not noted at all, or mentioned in parentheses at a given location.

The definition of the trail becomes a bit looser in the desert and higher elevations, where there are not a lot of species right next to the trail, and one can go several steps off the trail without trampling other plants. I.e., some taxa are given numbers for trails in those areas that would not be numbered along a trail with dense vegetation alongside the trail.

The plant family for each species was added beginning on 7 October 2007, so most guides will not have it for some time.

The Trail Guides are best understood by going through an example. Consider these selected records from our Plant Guide to Vernal Pool Trail at the Santa Rosa Plateau (the families were added in this example version; they aren't yet present in the real guide):

Mile#Sid?FAMCommon NameScientific Name (Latin Name)#here#all
0.00   Trailhead on Via Volcano
0.001r POA*foxtail barleyHordeum murinum ssp. leporinum 20 / 17
0.002  POA*ripgut bromeBromus diandrus 99 / 913
0.008l AST*sow thistleSonchus oleraceus 20 / 57
0.009  LILblue dicksDichelostemma capitatum ssp. capitatum 99 / 97
0.0021  ONApurple clarkiaClarkia purpurea ssp. quadrivulnera 99 / 92
0.01 l Sign: Reserve Fees $2 adults, $1 children
0.03   (FAG coast live oak, Quercus agrifolia var. agrifolia)
 45  CAR*sand spurreySpergularia bocconei+ 5 / 21
0.0952 ~POAnodding needlegrassNassella cernua+ 99 / 54
0.1354  CACVasey's prickly pearOpuntia Xvaseyi+ 10 / 23
0.8395  CRAlanceleaf dudleyaDudleya lanceolata+ 5 / 26

Comments On Specific Species

Spergularia bocconei. This weedy non-native species is on the Red List as a Critically endangered plant in its native land, the United Kingdom!

Opuntia Xvaseyi. The identification for these plants came from plants with flowers near the Main Vernal Pool.

Nassella cernua. The identification for these plants is tentative since the plants did not bloom in 2002.

Dudleya lanceolata. A few plants are visible some distance from the trail prior to this point. These occurrences are just north of the Coast Live Oak here, with a few more along the next part of the trail.

Species in parentheses. These are species that are not within touching distance from a trail, but are usually at least easily visible in the distance. Some of these species can easily be reached off-trail; some cannot.

Mile, #

The plant guide begins at the trailhead, in this case, the entrance to the trail on Via Volcano Road, and this is mile 0.00. The mile counter is exactly equivalent to the odometer in your car, or a pedometer on your belt, and thus everything seen in the first 0.01 mile will be recorded as being at mile 0.00. In the first 0.01 mile of the trail, we found 21 different species (since all of them are recorded at mile 0.00, before the first landmark at mile 0.01, the Sign: Reserve Fees $2 adults, $1 children). Signs along a trail are almost always recorded as landmarks, as are all trail junctions, many switchbacks, and other such easy-to-recognize features.

Every plant guide that we have done has many more species found in the first half of the trail than in the last half. The same result would be found if we reversed the direction of all our trails. That is, there is nothing special about the beginning of a trail that causes so many new species at the beginning. It simply results from the fact that no matter where you start on a trail, you will find a lot of species within a tenth mile or so from that spot. Then once you've picked up all the species in that tenth mile, new ones appear more and more slowly as you move away from that spot. (Actually, if a trailhead is at a roadside, as is frequently the case, there is often a handful of weed species found only along the roadside. This increase in the number of species at the trailhead is usually negligible compared to the effect discussed here.)

The plants are in order of appearance when we did our surveys, cumulatively from all of our surveys. Thus immediately at the trailhead, the very first plant we ever saw on any of our surveys was foxtail barley, Hordeum murinum ssp. leporinum, and thus this species is #1 on the list. If this year, ripgut brome, the #2 species we found, happens to drop one of its seeds closer to the trailhead than does foxtail barley, it may be the first species you encounter next year. Hence the order of annual species at the very beginning of the trail may be somewhat different from year to year. Perennial species are likely to remain in the same order for years.

Also, you could very well be at the trail on a day when the first example of a species hasn't yet bloomed, or has finished blooming, and you may therefore encounter the first bloom from that species later on the trail. Purple clarkia, #21, is a good example. The first bloom to occur on the trail might be at mile 0.72, and this might be the first one you notice. But if you know that there is a purple clarkia just before the sign at mile 0.01, you could look for it and find out what it looks like before it begins to bloom.

Although it is our intention to give mileages for every plant, in some cases we may not yet have the exact location. For #45 in the example, we obtained its rough location from a list of plants blooming made in 2001. Because the plant never germinated in 2002, we were not able to pin down its location exactly.


This column gives the side of the trail on which the first occurrence of the taxon is found. We began adding this column more recently, at the request of botanists, since sometimes the first occurrence of a taxon is hard to see. Hence lists begun prior to this addition may not be complete for this column. The abbreviations are:

bBoth sides
lLeft side
rRight side
cIn the center of the trail


The id? column is extremely important, and is summarized in the table below followed by further information for each identifier.

 We have determined the identification with very high confidence, well over 99% confidence, usually by keying out the species or by recognizing the species from previous identifications elsewhere.
~We have not yet been able to make a very high confidence identification, but we feel that we have identified the species with at least 95% confidence.
sspWe have identified the species with very high confidence, but have not yet been able to identify the subspecies.
spWe have identified the genera with high confidence, but have not yet been able to identify the specific species.
?We either have no idea what this species is (if the name is unk shrub, for example), or only a guess if a species name is given.

There are many reasons we cannot always make a very high confidence identification for a given species:

In these cases, our probable identifications are often based on other factors than the characteristics that enter into the key. Those factors come from the descriptions of the possible species in the floras, from geographic ranges of the different possible species, as well as any other information we can find.

In the example above, Nassella cernua has a ~ in the id? column, and the reason is given in the Comments On Specific Species section that is linked to its Latin name. In this case, the awn and lemma lengths are critical to the identification in the key, and neither was available in 2002 since none of the specimens here flowered due to the drought.


Only a few plant trail guides have this column, since we found it clutters up the guide too much.

The family for each taxon is given as the first three letters of the full scientific family name, with all letters capitalized. See the key to the three letter abbreviations.

The plant family for each species was added beginning on 7 October 2007, so most guides will not have it for some time.

Common Name, Scientific Name (Latin Name)

Each plant is identified by a common name, which should always be taken with caution, since there is no standard convention as to which species has which common name. In this case, in our database, foxtail barley refers to Hordeum murinum ssp. leporinum as well as the parent species itself, Hordeum murinum.

The Scientific name for a species is given in italics, and sometimes even that name is not unique to a given species without qualification. Only the full Scientific name that includes a reference to the author who specified that name is unique. In this case, the full Scientific name is "Hordeum murinum L. ssp. leporinum (Link) Arcang". To avoid having to give this ungainly mouthful for each species, we use the names as published in the latest manual of California vascular plants. For plant guides prior to 2014, we used the Jepson Manual, 1993, third printing with corrections, 1996. For plant guides in 2014 and later, we used the 2012 Second Edition names, with a few changes detailed here.

In earlier pages, we've referred to the Scientific Name as the Latin Name.

An asterisk before the common name, as in this example, indicates a plant that is not native to California, having been introduced intentionally or accidentally beginning with the first European contact here. Sometimes plants native to California are not native to a given area, such as specimens of Sequoiadendron giganteum, giant sequoia, planted in various places in southern California.

The Latin name is sometimes linked to any comments about that species in the section Comments On Specific Species. Comments could be further information in some cases, such as the location of the plant, if it is hard to see, as for #95, interesting tidbits about a species, as for #45, or the reason why a definite identification was not made for a species, as for #52.

In some cases, comments are given in that section without being linked to the scientific name.


This column reports a very rough estimate of the number of specimens along the entire trail (the number before the "/") and the number of locations along the entire trail (the number after the "/") where a given taxon is found. The intent of this column is to alert the reader to species that are found only at a few locations, as well as to species that are ubiquitous along the trail. If there is only a single pitiful specimen of a given taxon along a trail, you might choose to take another trail containing this taxon if the main purpose of taking the trail is to see that taxon. Similarly, if you know that a taxon is ubiquitous along a trail, you might make a special effort to hike that trail to see many examples of that taxon.

The reported number of specimens ranges from 1 to 99. All species with more than 99 specimens on the trail get the maximum value of 99.

In the examples above, there are zillions of ripgut brome, blue dicks, purple clarkia, and nodding needlegrass specimens on the trail, and hence all get the number 99. We recall only 5 plants of sand spurrey on the entire trail in 2001, so that species gets the number 5.

The number of locations along the entire trail is a less precise number, since we use no rigorous definition of what constitutes a separate location. The most important number here is "1", meaning all the specimens along the trail are found next to each other. If you hiked this trail to see that taxon, you would want to spend all your time at this location. This number ranges from 1 to 9.

Sometimes the separate locations are clear. For example, sand spurrey is found in just two locations. Its first occurrence, #49, is between mile 0.05 and 0.08. The second location is just beyond the Main Vernal Pool at about mile 0.73. So everyone would agree that "2" is the right number of locations.

Sometimes the separate locations are less clear. For example, we report that nodding needlegrass has 5 locations. But it often occurs in large patches going along the trail for a tenth of a mile or so. And in this case, it was hard to tell this species from purple needlegrass in 2002 without the flowers. So the number "5" is a bit imprecise, which is intentional. An intermediate number like "5" primarily tells you that there is more than one location, but the species is not ubiquitous along the trail.


The header is an abbreviation for Number Of All Trails With Plant Guides In The Database That Contain This Species. The abbreviation was used to minimize the column width displayed by your browser.

This column reports how many of our trails contain each species. Thus foxtail barley (Hordeum murinum ssp. leporinum) has been found on seven trails, this one and six others. The number of trails in the database is given on each Trail Guide in the section Number of Unique Taxa On This Trail. In the example given above, there were 24 trails in our database. So this taxa is fairly common, appearing on 29% of all our trails.

The next species, ripgut brome (Bromus diandrus) is very common, appearing on 13 out of 24 trails, 54% of them.

Sand spurrey (Spergularia bocconei) has a "1" in this column, indicating that this is the only trail of the 24 trails that contain this species.

The section Number of Unique Taxa On This Trail gives a histogram of all the numbers in this column. This histogram tells the reader how many species are found only on this trail, and thus gives an idea of how different this trail is from the others in our database. In this case, the histogram for the trail as if 9/5/02 showed that 48 out of the 139 taxa, 35%, had been found only on this trail.

Trail Guides Caveats and Further Information

Search the Trail Guides

A detailed description of how to search just our field guides to find all occurrences of a given taxon is given in Search

Tips For Printing the Trail Guides

If you simply print a typical plant trail guide out from the web page, it is about ten pages in length, plus a couple more pages for the rest of the information on the webpage. In order to provide just the trail guide in a more compact format, on every guide updated after early 2003, we provide a link to a page that contains just the guide itself. That link is found just above the trail guide portion of the webpage, and is always called Version for printing, without lines and other text on this page (n pages), where n is the number of pages that Firefox 1.5 would print.

On 4/3/06, we have begun to add an Adobe Acrobat version of just The Plant Guide itself, without the Introduction et al. This has the advantage that it can be printed to make a double-sided booklet, with the pages reduced so that two pages fit on a normal single sheet of paper. This version is linked immediately under the header for The Plant Guide itself. In this way, four pages fit on both sides of a normal single sheet of paper, and the printout can be folded to produce a compact booklet.

For more information about either of the two options for printing, see Printing Instructions.

If you have the expertise and software, you can create a customized printed version by opening the webpage in an html editor (recent versions of Microsoft Word will do it) and edit it before printing. Here's what we do:

A typical plant guide then becomes a handy 4 to 8 page booklet, of size equal to a sheet of paper folded lengthwise.

Glossary and Footnotes

A genus, plural genera, is a collection of closely-related plants with the same first scientific name. Grape-soda lupine has the species name Lupinus excubitus, and it belongs to the Lupinus genera along with all the other lupines. Some species of plants also have subspecies or varieties (these words are mostly interchangeable in actuality, although some botanists attempt to make a distinction between them). Grape-soda lupine has five varieties, including one, var. hallii, which does not have the strong grape-soda smell that other of its varieties do!

A taxon is a unique subspecies or a unique species without subspecies. Each taxon is recognizably different in some way from all the other taxa. Taxa is the plural of taxon.

A key is used to identify plant species. Commonly, a key consists of a tree-like structure made of many pairs of statements. One begins with the first pair of statements, and chooses the statement that describes the taxon under consideration better than the other one. By repeating this procedure, one will eventually arrive at a unique taxon, which will be the identification if the key has no errors and the person keying out the taxon has made no errors. Our estimate is that this situation of no errors in either place occurs in less than half of all keying attempts for experts, and significantly less than half of all attempts for students!

It Is Important To Preserve As Much Natural Habitat As Possible.

Most non-botanists do not know this basic finding of botany, that the number of taxa in a given area scales with the area to roughly the 1/3 power. This is the fundamental reason why botanists have so much difficulty convincing the general public of the seriousness of the on-going habitat loss.

Most species are restricted to a given habitat. Coastal dunes, coastal valleys, deserts and mountains are examples of different habitats. Many people feel that preserving 10% of a given habitat surely is sufficient to preserve all the plant species. However, the basic finding of botany means that we will lose half of all the plant species in that habitat if we destroy 90% of a given habitat.

Additional Problems Caused by Habitat Loss.

Habitat destruction has additional effects beyond the loss of species that are only found in the destroyed habitat. The remaining native habitats become "islands" no longer connected to other native habitats, which can result in the loss of species within the "islands" in a number of ways:

Scientific Name In Biology Is Not Unique To A Given Species Without Qualification

Unlike other disciplines such as astronomy, biologists developed a convention that the name of a species would serve two roles: as the identifier for a given species and as an indicator of the relationship of that species to all other species. This would not be a problem if the species relationships were clear-cut. Unfortunately, species relationships in plants are often very hard to discern. This means that everytime someone thinks the relationship is different than before, a species has to be given a new name to indicate the new relationship.

An example will make this clear. In astronomy, one source of names for stars is to give them in brightness order in a given constellation. Thus the brightest star in the constellation Lyra, Vega, is also known as alpha Lyrae. The second brightest star in that constellation is known as beta Lyrae. These names are fixed forever, since the International Astronomical Union has a rule that names once given cannot be changed.

If the star known as beta Lyrae suddenly becomes brighter than Vega, astronomers would still call it beta Lyrae, but everyone would just note that beta Lyrae is now brighter than alpha Lyrae. Biologists, however, would insist that the names now be swapped, so that anyone reading literature prior to the brightness change would have to remember that alpha Lyrae then is equal to beta Lyrae now.

The way they would keep track is that whoever published the original star names, say T. Chester, for example, would have had their name attached to that name, such as "alpha Lyrae Chester". If J. Strong had then been the first to notice that beta Lyrae was now brighter than alpha Lyrae and publish the new name and "relationship", the new name for the same star would be "beta Lyrae (Chester) Strong". This name would be synonymous with "alpha Lyrae Chester".

The actual situation in biology is worse than indicated by this example. In this example, nearly everyone would agree that beta Lyrae was now brighter than alpha Lyrae and accept the new name. In botany, it seems that there is never anything close to unanimous agreement with a just-proposed revision. So different countries, different sections of the United States, and even different nearby universities may not agree on the proper name for a given species.

Confusing? You bet.

One way to help sort out this mess is what are called "synonomy lists", which keep track of all the names a given species has been given. It would be amusing to know what the record is for the species with the most names. An example we picked almost at random, green-leaf manzanita (Arctostaphylos patula), has had 12 other Latin names given to it, including subspecies names no longer recognized. We're sure the record number is much higher...

However, in defense of botanists, it is very useful for all related species have the same genus name, since the relationship is obvious just from the name. Thus if you encounter slender tarweed (Hemizonia fasciculata) on a trail, and you are familiar with San Diego tarweed (Hemizonia paniculata), you will be instantly alerted to look for the similarities and differences between the species. In contrast, if botanists used the astronomical convention, by now many genera would have a mish-mash of genus names that no longer match the group genus name. One would have to carry around a book that keeps track of relationships.

In addition, new names are always needed when it is recognized that one species is actually made of two or more separate species. In this case, botanists follow the same convention as astronomers - the first assigned name is retained for one of the newly-separated species, and a new name is given to the newly-recognized species. Botanists always preserve an example of a new species that is given a name for the first time. Hence that preserved plant is tracked down to find out which of the newly-separated species will retain the old name.

This is why most scrub oaks had their name changed from Quercus dumosa to Quercus berberidifolia. It was recognized that the scrub oaks were actually two separate species, a relatively-uncommon coastal species and a very common inland species. The original specimen, collected centuries ago, was tracked down, and it was the coastal one. Hence the inland scrub oaks had to take on a new name. Astronomers would have done the same thing.

"Unnatural" Competition From Non-Native Plants

We use the word "unnatural" simply to convey in a quick and clear fashion the effect discussed in this section. See the text below to clarify what is actually occurring.

Before we knew much about botany, we used to think that all the brouhaha about introduced plants wiping out native species was misplaced criticism. Surely this was just evolution at work, with the fittest species surviving.

We were surprised to learn that this was a na´ve view, since we didn't understand the whole picture.

In a nutshell, the whole picture is that plants and animals evolved together. Plants are the primary producers of food, which they make from sunlight, carbon dioxide and water. Animals depend on eating plants for their survival, or eating other animals that eat plants. Thus plants and animals have an ongoing war that has created intimate associations, since plants don't want to be eaten. As a result, plants have evolved physical and chemical defenses, which require resources from the plant.

Many species, including nearly every butterfly species, have evolved to detoxify or otherwise nullify the chemical defenses of a very few selected plant species. An equilibrium develops between the plant and the animal, since if the animal wipes out the entire species, it is "eating itself to death".

This is true for nearly every species of plant.

What happens when a non-native plant is brought into this ecology? In general, it doesn't bring along its own predators and may not bring along its own diseases. So it thrives without those restraining forces, while its native competitors must still face those forces. Eventually, the non-native plant may through chance start devoting fewer resources to its no-longer-needed defense, and more resources toward reproduction. It grows more vigorously, and either produces many more viable seeds (like curly dock), or vigorously reproduces vegetatively (arundo). When that happens, the non-native species becomes an invasive weed, which is "unfairly" competing with the native species.

That this process has nothing to do with survival-of-the-fittest is dramatically illustrated by the fact that many of our native plants have also become invasive weeds in other countries, where they become the non-natives. Thus two species could outcompete each other, each being superior in its non-native land.

This process doesn't always happen for every species, and sometimes takes decades or centuries for a given species to become a problem. The human penchant for disturbing natural areas also gives many opportunities for non-native species to undergo this process.

However, many non-native species would become invasive problems even without any disturbance. In particular, arundo and tamarisk are wiping out areas left and right that aren't disturbed, as well as areas that are. Of course, we humans have changed nearly every regime in nature, but if we could do an experiment of introducing arundo into a California untouched by human influence, there is no doubt it would still take over most, if not all, of the streambeds. Disturbance is not required for arundo to win.

Star thistle and tocalote are other examples. Tocalote has taken over patches of the Santa Rosa Plateau that are virtually completely undisturbed, including areas within chaparral that were (probably) never touched by cattle.

The key point here is the "unnatural" competition part. It has got to be the case that arundo et al have a (temporary) competitive advantage if they are free of predators and the natives aren't. In the absence of any habitat destruction or ecosystem changes, some non-natives will still become invasive species.

What happens when one of a non-native plant's predators arrives on the scene? Anyone who has eucalyptus trees knows the answer first-hand. The previously-thriving trees are nearly wiped out by the predator, because there is nothing preying on the predator.

Living Vouchers

A voucher is a plant collected from a known location, with enough parts present to identify it, that is pressed, dried, and then stored in an herbarium. Vouchers are extremely important to botany:

In fact, vouchers are the foundation of botany. The voucher that was used to name a species defines that species forever. Nearly every botanical key is a key to classify dead pressed plants, not living plants.

Such standard vouchers, however, have a number of drawbacks:

We call every single one of the species on our lists a living voucher, since we in essence have a GPS location for every one. These living vouchers are an essential complement to dead vouchers stored in herbaria. With living vouchers, botanists can examine many samples of a given species in many locations, and overcome the drawbacks mentioned above. They can study a population at stages that may not have any representatives in herbaria, from seedlings to plants near death.

In addition, living vouchers have every positive attribute of a dead voucher except the ability to be mailed, the ability to be the defining voucher for a species name, and the ability to provide all characteristics of a species at any given time. Specifically, any botanist can go to the exact location of each one of our species and verify our taxon determination. It is probably more likely that an herbarium specimen will be destroyed by fire or insects than for the species to become extinct on the trails we survey.

Botany has not had living vouchers before, since previously no one has combined GPS locations with a plant list. Standard plant lists are not of much help in locating a given species. It does little good to know that a given species is found somewhere in 8,000 acres if you have to spend days looking for that species. But having a database with precise locations allows botanists to examine live plants almost as easily as traveling to an herbaria to examine dead ones.

By the way, in general, we do not produce dead vouchers for the species on our trails. If we did that, the number of trails for which we could make plant lists would be severely restricted due to the time needed to properly collect and dry the vouchers. However, whenever we find something surprising or out-of-the-ordinary, we will produce vouchers to provide an easily-accessible record of it. In addition, we would be happy to supply vouchers for researchers studying a given species, especially those doing DNA-based studies, as we have already begun doing.

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Last update: 15 September 2014