Invasive exotic plants pose a problem for a natural reserve such as Edgewood. If invasive plants are allowed to take hold, the natural can quickly become unnatural. As plant habitats change, so do the populations of insects and animals that have adapted to the natural environment. In large part, Edgewood has remained natural because of its unique soil composition. The serpentine-rich soil enables serpentine-adapted native plants to thrive. Along with the native plants, insects and the birds and animals that feed on them have also been able to thrive. However, nitrogen from the traffic along I-280 has made Edgewood increasingly hospitable to aggressive exotic plants. Similarly, aggressive insects and animals are able to displace similar, but less aggressive, native species.
In this exhibit, the visitor is challenged to link an intriguing piece of trivia to the correct image of an invasive non-native, including Argentine ant, bigleaf periwinkle, Eastern fox squirrel, eucalyptus, Italian ryegrass, Italian thistle, and yellow star thistle.
In this Exhibit
Argentine ants (Linepithema humile) are very small and aggressive, but their greatest strength lies in their numbers. They do not have separate competing colonies like most ant species. In fact, the Argentine ants in California are so closely related to each other, they can move from one nest to any other and be considered a member of the group. Multiple fertile females (queens) can be found in most nests. With a ratio of one queen per 1,000 workers, each nest has close to 40,000 ants, and with one nest site every 10 to 20 feet, the number of ants available for any task is huge. These characteristics, among others, allow Argentine ants to overwhelm and displace other ant species.
Bigleaf periwinkle (Vinca major) is a perennial vine or groundcover with a blue flower. Periwinkle is native to the western Mediterranean, but has naturalized in California. It grows rampant in moist woodlands, and on stream banks. At each point where a stem touches the ground, it takes root and continues to spread. Periwinkle forms dense mats that smother native plants. This plant is commonly used in garden plantings, but frequently escapes into wild areas.
Eastern fox squirrel (Sciurus niger) was introduced into California in the early 1990s. This aggressive squirrel eats the same foods, such as acorns, as native squirrels and birds. Eastern fox squirrel can displace the western gray squirrel within a particular habitat in a short period of time. It also eats the eggs and nestlings of native songbirds.
Eucalyptus (Eucalyptus globulus) is a fast-growing, invasive Australian native. These trees not only invade native plant communities, they also chemically prevent other plants from growing near them, a condition called allelopathy. In addition, the oils they secrete are highly flammable, making eucalyptus an unwise choice for fire-prone areas of California.
Italian ryegrass (Lolium multiflorum) is an upright annual grass that grows vigorously in winter and early spring. It can grow to about 3 ft. tall; if left uncut, it will flower and set seeds which are spread by the wind and by animals. Its rapid spread both shades and crowds out native plant species. Well-timed mowing can contain this grass–one of 35 non-native grass species found at Edgewood.
Italian thistle (Carduus pycnocephalus) is a prickly weed with a pink/purple flower. Its height can range from around 8 in. to over 6 ft. It spreads via seeds, many of which drop close to existing plants, thus creating a dense mat of seedlings in winter. This dense growth crowds out native species that are also trying to get established at the same time. This highly invasive plant was first recorded in Pennsylvania in 1814, and has become a major threat to natural areas in Canada and the United States.
Yellow star thistle (Centaurea solstitialis) is a highly invasive plant that begins flowering in late June and continues flowering until October. It can be recognized by the 3/4 to 1 inch spines that surround the flower head. This plant grows in such dense mats that it takes up most of the available moisture in the soil. This prevents other species from getting established. Yellow star thistle infests over 8,000,000 acres in California. Many acres have been removed from Edgewood over the years, using a combination of mowing and hand pulling. It has now mostly been eradicated from Edgewood.