Ecological impact of invasive plants

Nonindigenous species spread at the rate of ~700,000 hectares per year in the US (Pimentel et al 1999). Nonindigenous plants are spreading rapidly. Cytisus scoparius (scotch broom) has invaded over 1 million acres in Oregon and Washington; Centaurea maculosa (spotted knapweed) now covers 7.2 million acres in North America; Ligustrum japonica (Privet) has invaded 2.4 million acres in the Southern United States (Meier 1999). Horticultural invasive plants are changing the nation's landscape. They have changed the way our natural areas look, feel, and function. Most importantly, invasive plants create permanent impacts on ecosystems and ultimately contribute to the loss of biodiversity.

Changes to Ecosystems

Invasive plants compete with native plants for resources, thereby becoming dominant. More importantly they can outcompete plants that are food supplies for animals in the ecosystem. This may result in animals depending on nonnative plants for food or, if they are specialists, losing their food source entirely. Invasive plants normally lack predators and may more easily outcompete natives with their natural predators. Invasive plants are a problem because they alter the invaded ecosystem and species composition (Woods 1997) to such an extent that they threaten native flora and fauna. Invasive species capitalize on many techniques in order to invade ecosystems. There are three ways that biological invasions alter ecosystems according to D’Antonio and Vitousek (1992). Invasive exotic plants alter rates of resource supply, trophic level relationships, and the disturbance regime. Walker and Smith (1997) add changes in stability to this list. While these effects have been identified, it is often difficult to separate out the effects of invasive plants from the effects of anthropogenic disturbance – in fact, they are intricately linked (Woods 1997). A highly disturbed ecosystem is susceptible to invasion (Hobbs and Huenneke 1992).

Resource Supply

Plants require a few key resources to survive. These are light; water; carbon dioxide; the following macronutrients (required in large amounts): nitrogen (N),  potassium (K), calcium (C), Phospohorous (P), Magnesium (Mg), and Sulfur (S); and  the following micronutrients (required in trace amounts): iron (Fe), Chlorine (Cl), Copper (Cu), Manganese (Mn), Zinc (Zn), Molygedum (Mo), and Boron (B). Some plants also require sodium (Na) or Cobalt (Co). Plants are adapted to the nutrients available in their environments. In nitrogen-poor environments, some plants are able to convert nitrogen in the air to nitrogen in the soil, where they can use it. For instance, plants in the bean family (Fabaceae), along with Myrica (sweet gales), Alnus (Alder trees),  and Ceanothus (mountain lilacs), and Comptonia (sweet ferns), have a symbiotic relationship with bacteria which form nodules on their roots (Raven et al 1992). These bacteria convert nitrogen in the air (N2) to ammonium (NH4+) which is the form useful to plants. Myrica faya (fayatree) is a member of Myricaceae (the bayberry family) that invades nutrient-poor lava flows in Hawaii. By adding nitrogen to the soil, Myrica faya can survive, but it severely alters the nutrient balance in the area and creates a habitat uninhabitable by the native species that are adapted to limited nutrient conditions (Vitousek et al 1987).

Soil water and salt content

Some invasives have been implicated in changing the hydrology and salinity of an area. Tamarix (salt-cedar) species alter the desert riparian areas that they invade in the American southwest through increasing evapotranspiration, perhaps adding to the desiccation of flood plains (Walker and Smith 1997). Mesembryanthemum crystallinum (an iceplant) in Coastal California takes up salt from soils and deposits it on the surface, making it uninhabitable by indigenous plants. Iris pseudocarus (Yellow iris) was instrumental in changing a Potomac River march to mesic forest by creating a raised seed bed with its rhizomes that favored Fraxinus (ash) trees over Salix (willows) (Woods 1997).

Trophic Levels

The second alteration by invasive species is of trophic levels (D’Antonio and Vitousek 1992). An invading species can act as a new predator or a new food source, thereby throwing off the normal relationships within an ecosystem. With invasive plants, they usually have no predators in their new environments and they may outcompete an important food source for native animals.

Disturbance Regime

Ecosystems are dynamic. When a component of the system, such as the climate, the landscape, or the living organisms change, the rest of the system must adapt. Over evolutionary time, systems have produced organisms that are adaped to the regular changes that occur. When these changes are altered in time or area scale, some of the ecosystem may not survive- providing opportunities for nonindigenous organisms to move in.

An example of this is Bromus tectorum (cheat grass) in the American intermountain west. It has invaded land traditionally dominated by perennial grass which experienced fire infrequently - on a 60-110 year frequency. Rangeland degradation allowed the winter annual Bromus tectorum to invade, fueling fires every 3-5 years which kill of the native shrubs and perennials (Pimentel et al 1999, D'Antonio and Vitousek 1992). Bromus tectorum then easily invades burnt areas before the natives, creating an ever-enlarging monoculture of the exotic grass and frequent fires. Tamarix has also contributes to increasing fires (Walker and Smith 1997).

Hybridization

Invasives can effect closely related indigenous plants by hybridizing with them and creating even more aggressive invaders. Thompson (1991) showed that Spartina alterniflora, a cordgrass indigenous to the East Coast of North America, hybridized with Spartina maritima after being accidentally introduced to the south coast of England. The hybrid species resulted in an aggressive invader of salt marshes that often become dominant and result in a homogenous flora. While the hybrid species are very aggressive, S alterniflora is a fairly poor invader of the south coast of England. However, it has become a problem on the west coast of the United States (Gould 2000, lecture).

Biodiversity reduction and extinction rates

Maintaining biodiversity is the motivating factor behind most conservation issues. Invasive plants are no exception, as non-native species are the second highest threat to the threatened and endangered species in the United States behind habitat loss (Wilcove et al 1998). Of the 958 species listed, about 400 (42%) are threatened by non-native species (Pimentel 1999, TNC 1996, 49% according to Wilcove et al 1998).

Biodiversity

The importance preserving biodiversity may not be immediately apparent. A diverse ecosystem can provide many utilitarian and intrinsic services. Our ecosystems provide us with goods, services, and information. Goods includes fod, fuel, fiber, medicine, and even organic pesticides. Services are everything from pollination, recycling of organic wastes, fixation of atmospheric nitrogen, and regulation of the atmosphere. Information provided by diverse ecosystems can help us develop genetically modified goods, further our knowledge of evolution, and provide the tools for other science (Meffe and Carroll 1994). Beyond utilitarian goods, services, and information, a diverse world provides humans with aesthetic beauty to enjoy. Many ecologists and philosophers believe that we must look beyond the anthropocentric values of utility and beauty and realize that every organism has an intrinsic value beyond its relationship to the human world because it has its own values and goals (Meffe and Carroll 1994). Conserving biodiversity is advantageous for both the human and non-human world.

The number of plant species in the United States and the world is not precisely known, though it has been estimated that in addition to the 17,000 native plants in the US, there are about 25,000 non-native plants (including agricultural and horticultural), only 5,000 of which have escaped and established in natural ecosystems (Pimentel 1999). According to another source, the number of non-natives that occur outside of cultivation is far fewer; only 400 species (The North Carolina Botanical Garden’s Biota of North America Program in TNC 1996). As the number of non-native plants and animals increases (it cannot decrease, as complete eradication of a species is nearly impossible), more native plants will come into direct competition with and be threatened by non-native species, become endangered, and possibly extinct. The Invasive Plants Fact Book refers to invasive species as "biological pollution"; it is far more potent than chemical pollutants that degrade over time, because biological pollution reproduces and spreads on its own. Local diversity may increase as non-native species establish in new ecosystems, but world biodiversity will decrease as rare species are displaced by invaders. In Hawaii, where the pre-human rate of introduction was one species per 100,000 years, when Polynesians arrived the rate jumped to 1 species per 50 years, and after European colonization the rate exceeded one species per 0.25 years. In the past 200 years, more than 869 species have become established in Hawaii (Reichard and White 2000).

Through the increased movement of species around the earth and the loss of biodiversity, the earth runs the risk of becoming more homogenous. The natural borders established by evolution, geography, and climate are eliminated when humans move species around the globe, such that unique species in isolated environments are put in direct competition with non-natives that have no natural predators or pests. Eventually, this can lead to “increased biological sameness” (OTA 1993).

The OTA report contends that much of the evidence for the high rates of extinction due to non-indigenous species is indirect and inconclusive, but points out that on islands (Puerto Rico and Hawaii) many direct connections between introduced species and native species extinctions can be made. In the continental US, unique environments, like fragments of forest, lakes, and hotsprings, mimic islands in their isolation and may be just as vulnerable to introduced species. On most of the continent, however, the decline in the abundance of many species has been connected to introduced species, so that while few species have gone extinct because of non-indigenous invaders, there has been a significant alteration in the ecosystem dynamics (OTA 1993).

Habitat Loss

Similar to outcompeting native plants for resources, and outcompeteing the native food or native animals, invasive plants can cause habitat loss for native animals. There are numerous example of habitat loss caused by plant invasions. Arundo donax, or giant reed, is an escaped garden grass that is plaguing parks of Southern California. It has even pushed up a bridge. Its spread destroyed habitat for the endangered bird, the least-billed virio (Raver 1999). Baby's breath (Gypsophila paniculata) destroys habitat for a threatened thistle (Randall and Marinelli 1996, Raver 1999). Melaleuca quinquenervia has invaded the Everglades at such densities that it crowds out native wildlife (Randall and Marinelli 1996, Fairchild Tropical Garden).

Habitat loss is overwhelmingly caused by humans. In 1998, the World Conservation Union and Wilcove et al ranked exotic species as the second biggest threat to biodiversity, only behind habitat loss (Raver 1999, Wilcove et al 1998). Habitat loss and invasive species are intimately intertwined besides just being ranked next to one another (D'Antonio and Vitousek 1992). Because invasive plants are dependent on disturbance and ecosystem fragmentation they increase the impacts of habitat loss alone (Baker 1974, Raver 1999, D'Antonio and Vitousek 1992).

Human disturbances to the environment lead to a more habitable place for invasive exotic plants. Everything from acid rain, pollution, soil compaction, and habitat fragmentation due to development leads to weaker ecosystems that are more susceptible to invasions (Baker 1974, Raver 1999, D'Antonio and Vitousek 1992). Biological invasions are a form of global change that, unlike atmospheric change and climate change, will be irreversible because barriers that once stood between ecosystems will be permanently destroyed (D'Antonio and Vitousek 1992). In fact, the changes brought about by invasions could ultimately affect other components of global change, such as the atmosphere and the climate.