Sentinel Plants

purple coneflowerWhat motivated the idea of studying sentinel plants (primarily forbs and shrubs) on the Charles M. Russell NWR was recognizing that some refuge lands were being evaluated by biologists as in good or excellent condition for wildlife because the dominant plant species (primarily grasses) were present and abundant. This method of evaluation does not take into consideration that many species of plants that are of the highest food value to wildlife are not the abundant grass species – they are the less abundant (non-dominant) forbs (flowers) and shrubs. Due to prior land uses, these highest food value plants may have already been reduced to remnants scattered across the refuge. In some cases these forbs and shrub species may have actually been eliminated in some parts of the refuge. By sampling the non-dominant plants such as forbs and shrubs, biologists can help make management recommendations that will benefit both the refuge and its wildlife.

In nature, it has been commonly observed that individual plant species react differently to fire as well as grazing/browsing by domestic animals such as cows and wildlife such as deer and elk. There are plant species that increase with grazing and other plant species that decrease with grazing. The same is true with fire – some plant species are killed by fire and others bounce right back and are healthier following a fire. When grazing and fire are combined on a given area, individual plant species again will have different responses to this new combination.

Sentinel plants are “diagnostic” because they are the first species to be affected by fire, grazing or climate change. Sentinel plants can be used by biologists as the “canary in the coal mine” when evaluating refuge lands for wildlife. They can provide an early warning signal of changes in the environment – changes that may negatively affect the very lands and wildlife a refuge is trying to protect.

                   salt bush

How are sentinel plant species chosen? On CMR we have selected plant species as sentinels because of their individual responses to changes in our environment. Grazing is one of the big changes affecting our environment. Large herbivores both domestic and wild can survive on a wide variety of plant species – we call that being a dietary generalist. But much like us humans, large herbivores first select and eat the highest quality foods they like the best. Only when these favorite foods are gone do they move to less favorite foods. Remember – for most dietary generalists – the highest quality foods they like the best are the non-dominant forbs and shrubs and not the dominant grasses. Because they are eaten first and continue to be eaten when they begin to recover, forbs and shrub numbers eventually decline and can eventually disappear completely from an area. Therefore by studying the distribution and abundance of sentinel plants, biologists can better evaluate refuge lands for wildlife. 

In A Sand County Almanac published in 1949 Aldo Leopold writes of Silphium (a large perennial native sunflower preferred by wildlife and domestic animals):

“Why does Silphium disappear from grazed areas? I once saw a farmer turn his cows into a virgin prairie meadow previously used only sporadically for mowing wild hay. The cows cropped the Silphium to the ground before any other plant was visibly eaten at all.” Here Leopold mentions a species that is first to vanish – a sentinel plant.

He continues: “One can imagine that the buffalo once had the same preference for Silphium, but he brooked no fences to confine his nibblings all summer long to one meadow. In short, the buffalo’s pasturing was dis-continuous, and therefore tolerable to Silphium.” Here Leopold writes about the changes to important wildlife plants that can occur on a refuge.

Sentinels are about maintaining or restoring the complete and complex group of plant life including the all-important forbs and shrubs of an area. This can be accomplished by managing for the plants species that are most likely to vanish first. A complete and complex plant life provides thousands of wildlife habitats for the smallest of insects to the largest grazers such as elk.