Proceed with Caution: Unintended Consequences May be Ahead
By Rachel Mealor, UW Extension Rangeland Specialist
The intentional importation and cultivation of plant species from abroad has been occurring since the first settlers came to America. It has contributed to a highly productive agricultural system, beautiful parks and landscapes and hours of enjoyment for the backyard gardener.
Introduced forage species have increased the productivity of many livestock operations and have added increased flexibility in supplemental feeding programs. On the other hand, importation and improvement of plant materials from abroad has also led to the unintentional development of highly invasive weeds.
Some of the most problematic invasive weeds were introduced intentionally, for the purpose of ornamental plantings, forage production or erosion control measures. To be fair in this discussion, most introduced species fail to establish, and only a small portion of those that establish spread and cause ecological or economic problems.
There are many biological barriers that must be overcome upon introduction into a new environment. Is the introduced species adapted to the new climate and soils? Are other members of the same species near enough to allow for reproduction? Is pollination possible with local insects? Is genetic variation sufficient to allow for adaptation to new conditions? Although this is not an exhaustive list, it illustrates the point.
Species that have been intentionally introduced (rather than accidentally introduced) often have higher genetic diversity, which may contribute to a higher probability of overcoming potential genetic barriers to successful invasion. Multiple introductions from various source populations, which may occur during plant selection and breeding for desirable traits, can result in genetic combinations that display a high level of invasiveness. For example, eight to 12 species of Tamarix (saltcedar) were brought to the U.S. from Eurasia in the 1800s for shade and erosion control. A subset of these species (including hybrids of the originals) has invaded and impacted well over one million acres of riparian and wetland habitats in the Western U.S.
Another invasive plant, kudzu, was intentionally introduced into the southeastern U.S. in the early 1900s for forage and soil stabilization. Millions of cuttings were distributed by government agencies for landowner use. Today somewhere around 10 million acres are infested with kudzu, many of those acres where it was not intentionally planted, but are now dominated by the invasive weed. Cogon grass and Johnson grass are two more examples of forage species that have become widespread and problematic invasives.
It must be noted that each of these species was chosen for desirable characteristics such as low pathogen susceptibility, palatability to livestock, perennial growth habit, easy establishment and vigorous growth, extensive root systems, etc. These characteristics, although desirable from a forage or erosion control standpoint, may also contribute to the species’ ability to invade and dominate intact vegetation communities. The impacts of such species go far beyond their ability to reduce erosion or act as a forage source. They can alter other important ecosystem properties such as species diversity, nutrient cycling, and physical structure, thereby having a net negative impact on the system from both an agricultural and wildlife habitat standpoint.
One of the difficulties in predicting invasiveness or stating whether a species has invasive tendencies is a phenomenon called a lag phase. Very few, if any, species introductions have immediately resulted in rapid spread and impact across a region. Most introduced species undergo a relatively long period of spread latency, where their population stays localized and may not move into surrounding areas. Invasion ecologists call this the lag phase. It has been hypothesized for many plant species to be somewhere around 20 to 75-plus years, depending on the species. After the lag phase, true invasive species enter into an increase/spread phase where they actively move into surrounding areas and experience exponential growth.
Like many issues in life, the introduction, improvement and use of imported plants for reclamation, habitat improvement, environmental engineering and forage sources is not as clear-cut as we might hope. I have received numerous requests for information regarding forage kochia (Kochia prostrata) over the past several weeks.
Forage kochia may fall squarely into this ambiguous category. “Immigrant” forage kochia, the most widely planted cultivar, was released in the U.S. in 1984 (first research planting in the ‘60s) and has been planted on 200,000-plus acres of semi-arid rangelands. It is a perennial half shrub with an erect growth form that can develop an extensive tap root and fibrous root system. Attributes that make it desirable include: it can prevent accelerated soil erosion on degraded sites, it provides livestock and wildlife with forage and cover, and it may suppress invasive species like cheatgrass, halogeton and Russian thistle. It has also been described as having the ability to increase plant species diversity and improve the establishment of other desirable species, while reportedly not being invasive.
Given the characteristics possessed by forage kochia and its time since introduction, an alternative explanation for its reported lack of invasiveness may be that it is still in the lag phase of invasion. Recent research in Idaho (M.S. thesis by Erin C. Gray) suggests that this may be the case. An evaluation of 28 forage kochia plantings ranging in age from three to 24 years documented spread to adjacent, unseeded areas in 89 percent of the sites. Spread ranged from zero to over 2,200 feet away from the seeding boundary, with a mean distance of around 650 feet. Additionally, forage kochia plantings had decreased species diversity, but an increase in abundance of exotic annual forbs.
We cannot generalize too much given this study, because the observed tendencies likely depend on factors such as forage kochia seeding rate, the ecological sites where it was planted, management of the surrounding rangelands, etc. Also, these observations were in lower elevation habitats than we deal with in Wyoming, but they indicate forage kochia has a higher probability of invading than what has been previously stated by proponents of forage kochia.
Am I making a clarion call to prevent the planting of forage kochia in Wyoming? No, but I do recommend that we proceed with caution when deciding to perform large-scale plantings of a species that may have the potential of becoming an ecological problem in the future. Ongoing research at UW is investigating the utility of forage kochia for reclaiming difficult-to-reclaim areas, and into its potential as a supplemental forage for livestock producers… both commendable goals for such a species. We should also balance these and other efforts with a thorough understanding of how to manage the species, should it have unintended consequences, as well.