Thursday, January 27, 2011
More on sage-grouse, horses, and cattle
The greater sage-grouse once had large populations on rangelands in North America. Then they declined in the early twentieth century following the alterations to their habitat that occurred in the shrub steppe biome. Many studies have since been conducted in order to learn about their habitat requirements and what can be done to prevent the extinction of this species. Based on studies of sage-grouse nesting habitat, including those done by Gregg et al. (1994) and Connelly et al. (1991), it was found that sage-grouse need sagebrush and a certain height, no less than eighteen centimeters (Gregg et al., 1994) of herbaceous cover to be successful. Despite the importance of vegetation to the greater sage-grouse’s survival, there is a dirth of literature exploring whether grazing by cattle and feral horses, two controversial ungulates on rangelands, have direct impact on sage-grouse. A few papers synthesize effects of these grazers on the sagebrush habitat used by sage-grouse. Beck and Mitchell (2000) hypothesize that known grazing impacts, which directly affect vegetation, might also have a direct effect on sage-grouse populations. Connelly et al. (2000) and Crawford et al. (2004) claim there is no empirical evidence to support the direct impact of grazing on sage-grouse populations. Though my study is to examine cattle and feral horse grazing impacts in sage-grouse breeding sites, it might also provide evidence linking them to declines in population.
Connelly and Braun (1997) stated that there are three common factors contributing to habitat alteration that occur across all regions: livestock grazing, weather, and fire. Though there is no empirical evidence to support it, grazing by domestic and feral livestock, may have a direct influence on sage-grouse populations and their breeding sites. Since sage-grouse populations have been suffering widespread decline since early in the 20th century (Connelly and Bruan, 1997), it is important to further study the potential effects of grazing on their breeding sites. In addition to livestock grazing and the clearing of sagebrush to make room for forage, sage-grouse habitats have also been altered by agriculture (Braun et al. 1976).
Leks, which are strutting sites where male sage-grouse attract females, are central to nesting sites (Crawford et al., 2004). The leks are open areas surrounded by sagebrush, in which the hens find suitable habitat to create their nests. Several studies, including one done by Gregg et al. (1994) on nest sites in Idaho and on by Connelly et al. (1991), who focused on sites in Wyoming, have found that most successful sage-grouse nests were concealed under shrubs, and a high percentage of this shrub overstory was sagebrush.
A significant herbaceous understory was also found to be a major factor in the success of nesting sites (Sveum et al., 1998, Holloran et al. 2005). Connelly et al. (1991), which studied nest-site use in Idaho, hypothesized that some nests they found in nonsagebrush sites might have been due to lack of herbaceous cover in sagebrush sites. This was later confirmed by Gregg et al. (1994) during a study of vegetational cover of sage-grouse nests in Oregon. Their study found that successful nest sites characteristically had tall grasses. Sveum et al. (1998) had similar results in south-central Washington, as did Holloran et al. (2005) during a study of nesting habitat selection in Wyoming. Sage-grouse prefer sites with more sagebrush cover and taller grass cover to conceal their nests from predators (Gregg et al. 1994, Sveum et al. 1998).
Because the height of grasses is important to the success of sage-grouse nesting, Hollaran et al. (2005) suggested that the height of nest-site grasses should be a minimum of ten centimeters in Wyoming big sagebrush habitats. Grazing can reduce these heights and impact the success of nests sites, which can in turn impact the sage-grouse population. Gregg et al. (1994) suggests that any grazing that reduces grass cover to less than eighteen centimeters will decrease the value of habitat for concealing nests. Crawford et al. (2004) also offers propositions for preventing too much reduction in vegetative cover. The paper stated that shrub utilization should not exceed more than 50 to 60% during the growing season, and that at least half of the ground cover should remain (Crawford et al. 2004).
Through vegetation surveys and measurement of herbaceous understory, my study will determine the amount of damage caused by cattle and feral horses to sagebrush and the height of grasses. I will make observations to determine if grazing the understory too low has an impact on greater sage-grouse nest sites.
GRAZING IMPACTS ON VEGETATION
Cattle grazing can impact both sagebrush and grasses. According to Beck and Mitchell’s (2000) synthesis of cattle grazing and its potential impact on sage-grouse habitat, grazing increased big sagebrush cover in grazed communities. Under grazing pressures, the decreasers, which are the more palatable vegetation (grasses) desired by cattle, are decreased and the increasers, vegetation less desired by cattle (forbs and shrubs), increase (Brotherson and Brotherson 1981). As a result, cattle grazing shifts sagebrush-grass communities into a lower successional state that is dominated by shrubs with little grass understory. Though the primary need of sage-grouse is sagebrush, the aforementioned studies by Connelly et al. (1991), Gregg et al. (1994), Sveum et al. (1998), and Holloran et al. (2005) highlighted the additional importance of grasses to nesting sites.
The height of grasses is clearly impacted by cattle grazing. Richard, Uresk, & Cline (1975) conducted a study in south-central Washington to determine the impacts of cattle grazing on three perrenial grasses: Cusick’s bluegrass, Thurber’s neeedle grass, and bluebunch wheat grass. They found that grazing reduced herbage production. Bluebunch wheatgrass showed the most pronounced response to grazing with a 39% decrease in leaf length (Richard, Uresk, & Cline 1975), but all three species expierienced a measurable decrease in production. A decrease in grass height can impact sage-grouse nests, since the most successful nests are found in areas of tall grass.
Feral horses also impact sagebrush plant communities. Several studies conducted by Erik A. Beever have found that they can decrease the percent cover of vegetation, increase soil penetration resistance to water, and increase soil erosion (Beever et al., 2003, Beever et al., 2008). In contrast to Brotherson and Brotherson’s (1981) finding that shrubs increased under grazing, Beever et al. (2008) found that horse-grazed areas had lower shrub cover. They believed it may be due to horses trampling and rubbing sagebrush. They also found that there was lower percent cover of vegetation at horse-occupied sites (Beever et al., 2008, Beever et al. 2003). Like cattle grazing, these impacts may have adverse effects on prime nesting habitat for sage-grouse. Where cattle grazing may increase shrub cover, Beever et al. (2003, 2008) found that horses decreased shrub cover and grasses. This could impact sage-grouse habitat by reducing preferred overstory as well as the understory.
Erik Beever, who has done extensive field work studying the feral equids roaming public rangelands, wrote a commentary as a result of his studies (2003). He highlighted some of the unique differences in the impacts and management strategies of feral horses compared to other ungulates. He stated that horses eat more since they are not as selective of their forage, therefore consuming 20-65% more than cows. Physically, horses can trim grasses closer to the ground because they have front incisors, which cattle do not have. Beever (2003) argued that horses are also managed differently than cattle and wildlife because they have a unique, political status among large mammals of North America. Free-roaming horses are by law managed minimally; they are not fenced, or hunted. Cows, on the other hand, are intensively managed with fences, rotations, and herding. Beever (2003) argued that because of these policies, horse management is constrained.
The similarities and differences between horse and cow use of rangelands may directly impact sage-grouse breeding sites, which include nesting habitat. There is evidence that both species of grazers disrupt the composition of sagebrush habitat by reducing herbaceous cover, increasing forbs, and in some cases increasing sagebrush in cattle-grazed areas (Brotherson and Brotherson 1981), and decreasing sagebrush in horse-grazed areas(Beever et al. 2008). The differences in how these species graze may provide different levels of impact to sage-grouse breeding sites. Horses are less selective in what they eat, therefore have to consume more to make up the caloric intake that they require. Horses also can trim vegetation closer to the ground. So, though there are fewer horses on rangelands than cattle during portions of the year, feral horses may have as much influence as the more-numerous cattle in decreasing vegetative cover in sage-grouse breeding habitats.
My study will add to the body of literature concerning the impacts of grazing on sage-grouse nesting sites. By determining the influences of cattle and horse grazing on vegetation and impacts to nests, my study may supply evidence that grazing has direct affects on sage-grouse populations, thereby filling a data gap.