Exotic Species on the Central Coast

This month’s President’s notes will be divided into two parts, with the second part included in the November issue.

As someone who frequently hikes in our California landscape, I often ponder over the abundance and diversity of exotic species that have naturalized here, and how that came to pass, specifically during the past two centuries. This is not to say humans did not move plants around California prior to the arrival of Europeans, but historical records point to the introduction and, in some cases, rapid dispersal of non-native species in our state, as a post-European-arrival event. So, I set out to satisfy my curiosity, and in the process found some interesting facts.

How did so many non-native species find their way to California and how could they spread so quickly and in such large populations – was it intentional or by accident – by air, by water, by animal, by human, etc.? I reckon humans have had preferences for certain plant species since the beginning of time, thus altering the landscape to create a more desirable food supply. As North Americans, most of the plant species we consume are actually native to somewhere else, where simple domestication events started eons ago. However, as non-native plant species were carried to other continents during the last 500 years, adequate safeguards for their containment were apparently not well understood, and the botanical invasion we see today in California correctly had its origins in the not so distant past.

Here’s one example. I am astonished to see European fennel (Foeniculum vulgare) growing in such numbers in abandoned fields in my neighborhood. It is particularly thick along the railway right-of-way that bisects this vicinity. How did those plants get so well established here? One possible scenario could be – it’s originally from Greece, then traveled to Italy, then to Spain, to Mexico, and finally to California, where it was planted in the early gardens in this area, and then transported via the hooves of horses and on the wagons they pulled, later on the trains, trucks and tractors that worked and subdivided the land, and finally via the shoes, socks, and pockets of humans. I wonder if that is how it really occurred.

This hypothesis comes from observations I have made while hiking in some of the more remote areas of our county, where it can be hard to find exotic species that have become established. In most cases, the further one travels away from human civilization, the fewer number and variety of exotic plants are found. There are exceptions to this rule, especially where soil type and/or moisture do not afford these plants adequate resources for growth. Here are a couple of examples of what I have seen in recent years, illustrating to me how new species have potentially moved into our area.

On some of the hiking trails, one finds an introduced species growing right along the path, but rarely more than a few feet off the path. Torilis arvensis, known as hedge parsley, or “sock destroyer”, is a European native having seeds covered with tiny hooks, similar to Velcro. The plant grows within the range of most people’s socks, i.e., 1-2 feet tall. Socks pick up the seeds when the plant is shaken and then transport them to new locations along the side of the trial, thus engineering this unique dispersal pattern.

The other example concerns two species; Canadian horseweed, Erigeron canadensis, and Italian thistle, Carduus pycnocephalus. Both of these plants were uncommon in our county 15-20 years ago. Later, they began showing up along roadsides and even later spreading along the trail systems, to the point where they now dominate sizeable portions of our open spaces. Canadian horseweed, although native to North America, has become an “unwanted plant” in many places, common in disturbed areas in full sun, while Italian thistle does best in partial shade, especially under coast live oaks. Both are in the Asteraceae family and produce copious quantities of seeds. Using its pappus, a small parachute connected to one end of the seed, these species achieve phenomenal aerial dispersal potential, even from a single plant. Seeds move with the wind, easily get lodged in vehicles, horses’ hooves, people’s shoes, etc. Thus, these seeds first arrived along roadsides, then moved along the trails into our wildlands.

So, I guess the finger of blame for these introductions, and rapid disseminations, points to us, because the patterns of dispersal invariably involve human activity. Next month, I will cite some historical records to further illustrate my point.

Torilis. Photos by David Chipping

Carduus and Torilis, right) revealing two different seed dispersion strategies.