The sage-grouse is considered a keystone species in the sagebrush ecosystems of western North America. As a keystone species, the sage-grouse plays an outsized role in maintaining the health and biodiversity of this ecosystem.
What is a keystone species?
A keystone species is one that has a disproportionately large influence on its environment relative to its abundance. The keystone species concept was first described by ecologist Robert Paine in 1966. Paine studied starfish in the intertidal zone and found that when he removed the starfish, which is a predator of mussels, the overall diversity and abundance of species declined dramatically. This demonstrated the pivotal role of the starfish in maintaining balance in that ecosystem.
Since Paine’s seminal study, the keystone species concept has been applied to species in many different ecosystems. Keystone species exert strong direct and indirect effects on community structure and function. Their removal initiates major shifts in relative abundances of other species and processes. Sage-grouse are considered a keystone species because of their integral role in sagebrush ecosystems.
Why are sage-grouse considered a keystone species?
Sage-grouse depend on sagebrush for food and cover throughout the year. They are considered an “umbrella species” – meaning that protecting sage-grouse habitat also benefits many other wildlife species that use sagebrush ecosystems. Sage-grouse serve important roles both as consumers of sagebrush and as prey for other species.
Some of the key reasons sage-grouse are considered a keystone species include:
- Sage-grouse consume sagebrush leaves and buds year-round, influencing sagebrush growth and distribution.
- Sage-grouse require large, interconnected expanses of sagebrush. Protecting this habitat benefits 350 other species that use sagebrush.
- As prey, sage-grouse provide an important food source for many predators, including golden eagles, coyotes, bobcats, and weasels.
- Sage-grouse nests provide food for snakes, ground squirrels, and ravens when eggs or chicks are preyed upon.
- Loss of sage-grouse leads to decline of sagebrush and negatively impacts sagebrush-dependent species.
How do sage-grouse influence the sagebrush ecosystem?
As a dominant herbivore that feeds exclusively on sagebrush in most of its range, the sage-grouse exerts strong influences on sagebrush growth, distribution, and composition. Sage-grouse consume sagebrush throughout the year. During winter, mature sagebrush leaves comprise 100% of their diet. The rest of the year bugs, forbs, and sagebrush make up their diet.
Sage-grouse feeding reduces the size and density of sagebrush stands. This creates a patchwork of sagebrush interspersed with openings, benefiting other wildlife species. Sage-grouse also influence sagebrush chemistry. As they selectively forage on compounds like monoterpenes, this affects plant defense chemistry.
In addition, sage-grouse dispersal of seeds and fertilization of soil with their droppings influences sagebrush distribution. Sage-grouse dependence on sagebrush means they require large expanses of interconnected habitat. Estimates range from 150,000 acres to over 450,000 acres needed to sustain populations. Protecting this habitat in turn benefits many sagebrush-obligate species.
What is the sage-grouse’s role as prey?
As a ground-nesting bird reliant on cryptic plumage rather than speed or agility to avoid predators, sage-grouse provide an important prey source for many predatory species in sagebrush ecosystems. Documented predators of sage-grouse include:
- Golden eagles
- Red fox
- Bobcats
- Coyotes
- Badgers
- Weasels
- Ravens
Golden eagles are perhaps the most important sage-grouse predator, with grouse comprising a major part of eagle diets in some areas. Other avian predators like hawks and falcons also prey on sage-grouse. Snakes and ground squirrels consume eggs and chicks. Juvenile sage-grouse experience high mortality rates, providing abundant food for predators.
This predator-prey relationship helps support stable populations of these predatory species. Declines in sage-grouse populations can reduce food availability for predators. For example, decreasing sage-grouse numbers in parts of Utah have been linked to decreasing golden eagle reproduction in those areas.
How does loss of sage-grouse impact the ecosystem?
Sage-grouse populations have declined range-wide over the past century, largely due to habitat loss and degradation. sagebrush ecosystems are among the most imperiled in North America. As a keystone sagebrush species, sage-grouse declines can initiate an ecological cascade:
- Reduced herbivory on sagebrush decreases plant species diversity.
- Loss of disturbed openings negatively affects other species like antelope that rely on these spaces.
- Predators lose an important prey source as grouse populations decline.
- Loss of fertilization from sage-grouse droppings may impact soil health.
- Isolated populations lose genetic diversity and become more prone to extirpation.
Fragmentation of sagebrush habitat is a major threat to sage-grouse. Maintaining large expanses of intact habitat is crucial. Where sage-grouse have been extirpated, the overall diversity and abundance of other sagebrush-obligate species decreases.
Efforts to protect sage-grouse
Due to dramatic population declines over the past century, efforts have been undertaken to protect sage-grouse and their habitat. These include:
- In 2010, the U.S. Fish and Wildlife Service designated sage-grouse as a candidate species under the Endangered Species Act. Listing was precluded by higher priority species.
- Federal plans have been developed to guide conservation, including a 2015 Bureau of Land Management Sage-Grouse Plan.
- States within the sage-grouse range have developed conservation plans and 5 have protected the bird under state endangered species laws.
- Landscape-level partnerships like the Sage Grouse Initiative have worked with ranchers to implement grazing systems and conservation easements protecting millions of acres of habitat on private lands.
- Reducing other threats like invasive grasses and conifer encroachment into sagebrush has also been a focus.
While populations remain below historic levels, some recovery has occurred in recent years. Continuing broad-scale efforts to protect intact sagebrush will be important to maintain sage-grouse populations and thus a functioning sagebrush ecosystem.
Conclusion
Sage-grouse serve vital functions in sagebrush ecosystems as consumers of sagebrush, dispersal agents for seeds, fertilizers through their droppings, and prey for many predator species. Their strong reliance on sagebrush means they require large, intact expanses of habitat. Conserving sage-grouse populations and habitat helps preserve the diversity and health of the sagebrush biome. Due to substantial population declines, the sage-grouse is considered an indicator of the overall health of this threatened ecosystem.
