Range managers have long known that overgrazing changes the plant composition of pastures.

As a result, plant species are referred to as increasers, decreasers or invaders. This is a reference to their reaction to years of overgrazing (generally by cattle as opposed to sheep).

“By knowing how a few common species react to grazing and monitoring their abundance in pastures, managers can improve forage quality, quantity and, consequently, profits,” says Chuck Lura, rangeland specialist at North Dakota State University’s Central Grasslands Research Extension Center near Streeter.

Decreasers are generally plant species that are part of the historical climax of an ecological site and, for a variety of reasons, decrease during the course of several years of overgrazing. A historic climax community is a plant community that existed at the time of European immigration and settlement in North America and was best adapted to the unique combination of environmental factors associated with a particular site.

Examples of decreasers would include big bluestem, western wheatgrass and the needlegrasses. These plants are among the most desirable for livestock because of their abundance, nutrient content and palatability.

Increasers are also part of the historical climax community of an ecological site, but are not as palatable and nutritious as the decreasers, so they tend to increase because of overgrazing on other species. However, if overgrazing continues, these plants will begin to bear the brunt of the grazing and eventually decrease. This usually results in lowered forage quality and quantity for the livestock, which also reduces the grazing capacity of a pasture.

Blue grama, prairie junegrass and threeawn are more common increasers.

As the name implies, invaders are plants that become established due to disturbance. They were not part of the historical climax community. Although they can be native plants, many of them are introduced species, such as wormwood sage, leafy spurge and knapweed. Most invaders have little or no forage value, although some do.

“Healthy rangelands are productive rangelands,” Lura notes. “Techniques to monitor increasers, decreasers and, certainly, invaders can provide the producer with valuable information as to the success or failure of management strategies.”

The Central Grasslands Research Extension Center has established a program to assist producers in implementing and maintaining range monitoring procedures. This effort is made possible through funding from NDSU, the North Dakota Natural Resources Trust, and Ducks Unlimited.

To learn more about rangeland monitoring, contact Lura at (701) 424-3606 or chuck.lura@msub.nodak.edu. More information on rangeland plants is available in NDSU Extension Service publication EB-69, “Selected North Dakota and Minnesota Range Plants.” It’s available through the NDSU Distribution Center at (701) 231-7882 or NDSU.DistributionCenter@ndsu.edu.

NDSU Agriculture Communication