Arthropod Community Responses to Fire and Grazing

It appears as if insects rule the world when it comes to sheer numbers. A recent study led by Alyssa Vachino, an undergraduate student in the Department of Natural Resource Management at SDSU advised by Dr. Lan Xu, sampled over 24,000 arthropods at the Cottonwood Research Station. Vachino and Dr. Xu investigated the effects of patch-burn grazing and winter-patch grazing on arthropod communities living in mixed-grass prairies.

Historically, the Great Plains were grazed by large, roaming herds of bison. Fire was common place, either set on purpose by Native Americans or by nature from summer thunderstorms. Fire and bison grazing interacted to create a mosaic of grassland structure across the vast prairie. Following the invention of barbed wire, settlers hemmed in their livestock and pastures were heavily grazed. Later, range professionals suggested rotational grazing could help even out the grazing distribution such that land managers could reduce over– and under-grazing their rangeland. This resulted in a more uniform use of the land. In ecology, we call this “managing for the middle”; species that rely on the extremes appear to lose out.

Some methods to encourage a patchy land-use to recreate a shifting mosaic of prairie vegetation are patch-burn grazing and winter-patch grazing. Patch-burn grazing was started by researchers in Oklahoma, Kansas, and Nebraska in tallgrass prairie. The pasture burns are rotated every year; cattle create the focus grazing such as the bison of the past. Winter-patch grazing was an idea created by SDSU professor Dr. Pat Johnson. to use livestock to graze a patch heavily in the winter to remove standing dead grass and litter. Thus, the patch would warm up quicker and consist of greener grass, as well as attract focus grazing by livestock in the summer much like a fire.

Interestingly, these data collected supported their hypothesis. In the figure to the right, patch-burn grazing had the highest arthro-pod abundance, followed by winter-patch grazing, and continuous season-long grazing. The most abundant orders were Hemiptera (true bugs), Diptera (flies), and Hymenoptera (wasps, bees, and ants).

Many of these species play a role in pollination (butterflies), regulate pest insects (wasps), cycle nutrients (beetles), and are food for birds and small mammals. This study demonstrates the im-portance of providing a variety of vegetation structure to support more diversity of life.

We recognize that fire is not practical in many areas of the Great Plains, not to mention its cost. Patch-grazing most likely could be applied to create heterogeneity on the landscape. In fact, most producers would have the capability to do this because of the cross-fencing and water infrastructure. Rotational grazing pastures and purposely varying the degree of use could create a mosaic of structure at the ranch scale rather than at a single pasture as was experimented with winter-patch grazing.

Warren Hammerbeck essentially does this with his rotation near Iona, SD. He grazes his pastures according to four categories: light, moderate, heavy, and “golf course”. However, Warren does not return to those pastures for 15-18 months. This low-frequency, high-intensity style grazing creates a shifting mosaic of tall, medium, and very short structure on his ranch. No matter what technique is used, the point is to provide a variety of habitats at a large scale to maximize diversity. Arthropod diversity is just as important if not more so because it provides so many ecosystem services.

Source: SDGC Newsletter