But that isn't all there is to this interaction. Rodent populations increase and decrease with the availability of forage. When swards are grazed by ruminants, there is less forage available to support rodent populations. That's how grazers can hurt gophers in the scenario discussed above. But goat grass isn't the only invasive plant rangeland managers battle, woody shrubs can be as bad or worse. In a paper presented to the Ecological Society of America and the Society for Ecological Restoration at their joint 2002 Annual Meeting in Tucson, Arizona, Dennis M. Bramble, University of Utah, noted that ruminants do not generally consume large amounts of brush and woody vegetation, but rodents will consume the roots of these species when other forage is not available.

Range managers can use this interaction to control sward composition and to reduce sward degradation by invasive shrubs. Deferring ruminant grazing in the spring can allow an increase in rodent populations. When the sward is subsequently grazed by ruminants, the large rodent population will switch to eating shrubs. The net result is a reduction of shrubs. It's a balancing act in which the timing of actions matter as much or more than the intensity. In this case both the invasive grass and the invasive shrubs can be controlled by timing grazing pressure, by deferring ruminant grazing in the spring.

There are economic and ecological costs to doing this. Economic losses come not only from lost grazing time but also from sward degradation. The key to a thick, healthy sward is controlling the spring flush when growth is explosive. Early grazing keeps grasses from becoming tall and woody, and so less nutritious, and encourages tillering to thicken the sward. It also allows nutritious species of short perennial grasses and forbs, such as nitrogen fixing clover, to get enough light to grow. Failure to control the spring flush degrades the sward for the whole year and favors taller, more woody and less nutritious annual grasses.

Good rangeland management requires close attention to specifics and variable treatment depending on conditions. It's a perfect example of the old saying that the best fertilizer is the farmer's footprints. The only way to make good decisions is to walk the fields and gather information.

This relates to the earlier post about biological control of pests.