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Submodule 3: Economic and Ecological Impacts

Spotted Knapweed

Spotted knapweed (Centaurea stoebe) is an alien invasive biennial or perennial herbaceous plant. Native to Europe, it is thought to have first arrived in the U.S. in alfalfa in the late 1800’s. It was first reported in the northwest regions of the U.S. Because it can grow under a variety of soil and climatic conditions, it is now found throughout most of the U.S., with concentrations in Montana, Washington, Idaho, and Oregon. There are also major infestations in western Canada. As expected, it is commonly found on disturbed land and along roadways. It is found growing at elevations from 90 feet to 9,000 feet and in areas receiving between 10 to 30 inches of precipitation annually. As with most invasive plants, it easily invades disturbed areas. Unfortunately, spotted knapweed can also invade relatively stable landscapes, making it even more problematic. Unlike some other knapweeds, this plant can invade riparian areas or other areas prone to flooding. Although it has perennial characteristics and can live for up to 9 years, it reproduces primarily from seeds setting up to 1,000 seeds per plant. These seeds can remain viable for many years and can be distributed by wind and animals, and by water flowing in streams.

 

Economic Impacts of Spotted Knapweed

• Spotted knapweed reduces land and forage values since most grazing animals find it undesirable for consumption, although wild and domesticated grazers will eat the plant when other food is not available.
• Sheep and goats have been observed readily eating spotted knapweed and are useful as a biological control for the weed.
• Current stands or skeletons from past years can impede access to grazing land by livestock and wildlife. Infestations have been reported to decrease grass production on valuable grazing lands by up to 90%, which in turn decreases total nutritional value of available forage.

• In 1984, it was estimated that there was a 63% reduction in forage for cattle in Montana due to spotted knapweed. This translates into a loss of about $155.5 million in livestock income if all susceptible lands are invaded.

• In British Columbia, the monetary loss in 1979 was estimated at $58 million. These numbers are based on net returns for forage land.
• Spotted knapweed infestations have been shown to decrease land values from $40/acre to $26/acre in some areas. These are rough estimates since they are quite difficult and ambiguous to calculate.

• Estimated monetary values for wildlife, recreation, and environmental loses are usually harder to estimate. In Montana, recent estimates for losses due to spotted knapweed infestations were set at about $42 million. This includes losses due to decreased economic activity.

• More recent estimates for the entire western U.S. for wildlife, recreation, and environmental loses exceed $164.5 million based on infestation area totaling 7 million acres.

• On high-value rangeland (based on nutritional and total biomass estimates), control of spotted knapweed is more cost-effective than on marginal grazing land.

• There have been some positive uses identified for spotted knapweed. It is a nectar source for some native butterflies and provides nectar and pollen for bees. Extracts from the plant have been shown to have some anti-cancer activity.
• For many people, exposure to airborne spotted knapweed pollen can cause serious allergic reactions including asthma, skin irritation, and hay fever. This is especially problematic in July and August in seriously infested areas.

 

Ecological Impacts of Spotted knapweed

• Spotted knapweed chokes out native plants by out competing them for valuable resources such as space and water.
• Spotted knapweed has an allelopathic effect due to chemicals excreted by the plant. For this reason, other plants are unable to germinate and grow near these weeds.
• In a survey conducted in 2000, 15% of western wilderness areas reported the presence of spotted knapweed.
• Infestations tend to grow at a rate of up to 24% annually. There is concern that much valuable rangeland in the western U.S. is at risk for infestation.
• Spotted knapweed infestations can interfere with the production of forage for livestock and wildlife, and with access to available forage. While some wildlife will graze this weed, most herbivores will avoid the plant, perhaps due to bitter chemicals present in the leaves. This tends to influence herbivore distribution and wildlife migration patterns.
• Elk seem to be particularly negatively affected by the presence of spotted knapweed because they graze more on the grasses that are displaced by the weed while deer graze primarily on shrubs and trees that are less affected by this knapweed.
• Spotted knapweed infestations are known to decrease species diversity by altering rangeland characteristics. The weed out competes native plants for resources, especially when natives are in the critical seedling stage.
• Biological diversity is also negatively influenced by changes produced by spotted knapweed infestations.
• One of the natural stabilizing factors for rangelands is lichens and mosses that cover exposed soil creating a crust. This crust increases moisture retention and nitrogen fixation. Studies have found that lands infested with spotted knapweed are almost devoid of this protective crust.
• In some areas, native species have become endangered or even extinct due to spotted knapweed infestations.
• Spotted knapweed increases fire frequency contributing to a decrease in biodiversity. The weed seems to recover from fire more quickly leading to more of the weed and less native plant cover.
• Because spotted knapweed has a taproot, it decreases erosion control and soil retention when it replaces native grasses that have fibrous roots. Taproot plants also leave more soil surface exposed and, therefore, increase soil crusting. All of these factors can increase rain runoff and soil erosion.

(photo courtesy Larry Howery)

 

 

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