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Disturbance: A weed's best friend

Image showing a road cutDisturbance is anything that disrupts the soil and existing plant community, changing resource availability and allowing other plants to grow. This disruption can be small, like an animal track, or large like a fire or road being built. The size of a disturbance, and the number of times it occurs, will determine the effect it has on the land. Large, repeated disturbances pave the way for drastic changes to a landscape.


Farming and Ranching
Old photo of ranchingThen: During the late 1800’s, livestock numbers increased dramatically in the Great Basin. Not understanding the full impact, ranchers competed fiercely for the use of open land. Coupled with severe drought, this led to diminished grass cover and shifted to more shrub dominance. Other western settlers attempted dryland farming in unsuitable areas. When drought and economic depression combined, farmers abandoned huge expanses of tilled land. In addition, alfalfa seed imported from Eurasia was contaminated with weed seeds. This marked the accidental introduction of cheatgrass into the Great Basin. Cheatgrass spread quickly on the overgrazed landscape and abandoned farmland.

Modern photo of ranching

Now: Ranching and farming can still contribute to disturbance in the Great Basin, but landowners and managers are more aware and concerned with their stewardship. Ranchers consider the timing and intensity of grazing practices to minimize disturbance, while maximizing native plant growth. Farmers actively participate in weed management programs and purchase certified weed free seed to significantly reduce new weed introductions.


TransportationOld picture of a railroad

Then: The path of the transcontinental railroad left a scar of disturbed land spanning the Great Basin. Similar clearing occurred as roads were built to connect new western towns. These pathways of disturbance promoted weed establishment. Vehicles and trains quickly transported weed propagules along these corridors.


Close-up of a car tire in a weed patch

Now: Today’s public transportation corridors are still notorious for encouraging weed spread. Road work often removes existing native vegetation and opens places for weeds to thrive along the edges. Developers have divided large acreages into smaller units. Subdivisions and smaller scale ranching operations require numerous roads, and bring in many additional people. Trucks and cars can pick up weed seeds and carry them to new areas. Railroad tracks are also often lined with invasive weeds.

Once the invading weeds are established on human pathways, sparks from trains or a carelessly discarded cigarette may ignite the weedy roadside. Fires extend the disturbance beyond the transportation corridor, opening surrounding areas to further invasion.



Old picture of recreators

Recreation
Then: In the past, there were a handful of “resort” locations in the Great Basin frequented by vacationers. Lake areas and spas with hot springs were attractive getaways for visitors and city dwellers.


Picture of 2 mountain bikersNow: Today, recreation activities have a much bigger influence on the spread of invasive weeds. Biking, hiking, hunting, fishing, horse-back riding and off-road vehicle use in the Great Basin have increased dramatically. These activities can unknowingly contribute to disturbance and the spread of invasive weeds as people enjoy outdoor recreation. To varying degrees, they can disturb areas far from designated roadways, as well as transport seeds.

Many private and public groups have invested in education, monitoring and control practices to prevent and reduce weed infestations. Our understanding of weed ecology and management, and participation in these efforts must increase if we hope to reduce weed invasions.


Images:
-Road cut: Southwest Regional Gap Analysis Project. Field site database. (http://earth.gis.usu.edu/swgap/trainingsites.html, 26 September 2006).
-Drawn image: Maria Fonnesbeck
-Then and now photos: Public domain.

Text references:
-Hobbs, R.J. and L.F. Huenneke. 1992. Disturbance, diversity, and invasion: Implications for conservation. Conservation Biology Vol. 6 No. 3. pp. 324-337.
-National Parks Service, Great Basin National Park. Grazing in the Great Basin: A connection to the land. Available: (http://www.nps.gov/archive/grba/grazing.htm, 2 October 2006).