The Problem with Invasive Plants
Adapted from an Article by Naturalist Rosemary Flynn that appeared in the March 2005 edition of Tracks
Diversity is a term used a lot today. We refer to it when we discuss an investment portfolio or when we select the fruits and vegetables we eat. We may even consider it where we work and where we go to school. Diversity in the natural world, or biodiversity, is also a good thing. It is one of the reasons that Upper Newport Bay (UNB) is so special. Unfortunately, at UNB as at other locations, biodiversity is threatened by adjacent uses, pollution and other factors. In some cases the threat is, in fact, other plants!
By definition, biodiversity represents the sum total of species that occur naturally (native) in an area. It also encompasses the relationships and interactions that occur between these organisms. In terms of ecosystems, coastal wetlands are particularly rich in biodiversity; it is here that the exchange of nutrients and energy between land, fresh water and salt water can take place. As demonstrated at UNB, these interactions support not only open water, mudflat and saltmarsh habitats (and organisms) but also the outlying freshwater marsh, riparian and multiple upland communities. Coastal wetlands also contribute, via tides and currents, to biodiversity on a regional and even global scale.
In recent years, there has been a growing emphasis on the role of native species in biodiversity. Why is it important that species be native? The reason is that plants that are not native tend to upset the interactions and relationships that support biodiversity. Specifically, when non-natives are introduced, they have usually been selected (or self-select) to do well. Invariably, they also lack natural predators. Unfortunately, this advantage is often to the detriment of native species as well as the multitude of organisms that have evolved to rely on these species for nesting, shelter and/or food. When an aggressive non-native comes in, the natives — along with all their supporting interactions and relationships — loose out.
In considering this, it is important to keep in mind that it takes time for an organism to evolve. Indeed, the genetic make-up of any one species has been influenced by geologic, climatic and biotic events that have occurred over millions of years. This is well demonstrated in southern California where much of our flora migrated millions of years ago from surrounding areas of northern Mexico, the Sierra’s and Great Basin Desert. Eventually, as time passed and the relationship of these physical features was changed, plants were no longer able to migrate in and out of the region as they had. Instead, these plants came to be a keenly adapted to the setting: the ancestral base for today’s native plant species.
Obviously, since these early days, the region of what is now UNB has undergone of a great number of changes and withstood many impacts. Uses and activities in the Bay have ranged from Indian hunting and gathering, commercial salt harvesting, fishing and ranching to boating, camping and general recreation. Most recently, there has been on-site construction and a great deal of adjacent land development. Such events have involved clearing and disturbing the land, which provide ideal opportunities for non-native plants to become established.
Today, perhaps reflecting the attention the subject now receives, there is a whole vocabulary associated with the topic of native/non-native species. A plant is said to be endemic when it is native and restricted to a given area, region, state, country, or continent. A plant is rare when its populations are small and it is considered at risk of disappearing. An endangered species is a species that is in immediate danger of becoming extinct. Technically, a rare or endangered plant is not necessarily a native. Rare or endangered status is, however, closely tied to habitat loss and it is native species that are most affected by that loss.
At this time, UNB is home to several rare species. Southern spikeweed, Hemizonia parryi ssp austrailus,is a native that is considered rare but, remarkably, does quite well in the UNB riparian and upland habitats. Saltmarsh bird’s beak, Cordylanthus maritimus, is a native recognized by both the state and the federal government as a rare and endangered species.
The term alien describes plants that are not native and came to the area by chance, perhaps by attaching to a traveler’s clothing or escaping household propagation. Examples include the familiar and prolific yellow-flowered black mustard, Brassica nigra, and white blooming wild radish, Raphanus raphanistrum. Both plants cover the hillsides each spring. In some cases, non-native plants were deliberately introduced. Pampas grass, Cortaderia selloana, is such a species. A native of South America, pampas grass was bought in during the 1800’s to use as an ornamental and was actively used for erosion control by public agencies. Other introduced species include California or Peruvian pepper tree, Schinus molle, introduced in the 1830’s for some plantings in San Diego as well as Myoporum laetum, a tough native of New Zealand. It was originally used for screening along the highway and seashore in the San Francisco area.
In their native settings, many of these non-native plants would not be considered pests. Unfortunately, with so much critical habitat already lost in the area, the additional threat represented by non-natives is a very real concern. Diversity is one thing. But when it comes to biodiversity, the natives make it work. Protecting our native species is an ongoing challenge in the Bay.
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