Sagebrush communities, covering millions of hectares in the western United States, are among our most imperiled ecosystems. They are challenged by various anthropogenic stressors, including invasion by nonnative grasses that degrade habitat quality and alter ecosystem function. Sagebrush restoration efforts are underway to improve habitat conditions to benefit a wide range of sagebrush-dependent species. Because birds are good indicators of habitat quality, monitoring avian metrics is an effective way to measure progress of sagebrush restoration. We compared avian community composition and individual species abundance among three sagebrush–steppe habitat types with varying degrees of invasion by nonnative crested wheatgrass Agropyron cristatum at the Camas National Wildlife Refuge in southeastern Idaho, USA. Sagebrush-obligate birds, such as sage thrasher Oreoscoptes montanus and sagebrush sparrow Artemisiospiza nevadensis, were most abundant in sagebrush habitats with an understory of native grass. Community composition was similar between sagebrush habitats with native and nonnative grasses, but quite different from bird communities occupying crested wheatgrass. The Habitats and Populations Strategies database, a conservation planning tool, predicts that restoration of crested wheatgrass sites to sagebrush in poor or fair condition will increase the density of sagebrush-obligate bird species. Taken together, these results suggest that restoration of crested wheatgrass near-monocultures back to sagebrush will improve habitat value for much of the bird community whether or not the understory can be converted to primarily native grasses, or a mix of natives and nonnatives. Of the sagebrush bird species of concern, Brewer's sparrow Spizella breweri occupied sagebrush habitats with native vs. nonnative understory at similar abundances, and this species could serve as a metric of intermediate restoration success. However, sagebrush sparrow and sage thrasher, which were significant indicators of sagebrush with native grasses, will likely benefit most from full restoration of a native herbaceous understory. Grassland-obligate birds such as horned lark Eremophila alpestris and grasshopper sparrow Ammodramus savannarum were most abundant at crested wheatgrass–dominated sites and may not benefit from restoration back to shrubland; managers should understand potential trade-offs.
Alterations to natural sagebrush–steppe ecology, including changes to historic fire regimes, removal of native nomadic grazing ungulates and incompatible livestock grazing, road building, energy development, and other human disturbances have facilitated the establishment and spread of invasive plants on native grasslands in the American West (Chambers et al. 2017). Many western rangelands previously characterized by native sagebrush and perennial bunchgrasses and forbs have been converted to annual grasslands or exotic perennial grasses. Specifically, introduction of Eurasian grass species occurred to improve livestock forage because they are more resilient and grazing tolerant (Wiens and Rotenberry 1985; Lesica and DeLuca 1996; Gunnell et al. 2010). Invasion with nonnative grass is one of the most significant perturbations threatening arid and semiarid ecosystems (Pyke and Knick 2009), representing an important rangeland conservation challenge. Exotic species can compete with and displace native plants, alter the composition and function of native plant communities, negatively affect fire frequency and intensity, and impact soil and water resources—all of which may affect biodiversity at multiple trophic levels by changing habitat suitability (Lesica and DeLuca 1996; Heidinga and Wilson 2002; Henderson and Naeth 2005; Vaness and Wilson 2007).
Crested wheatgrass Agropyron cristatum is a perennial bunchgrass native to the Russian and Siberian steppes introduced to western North America in the early 1900s (Dillman 1946). In the 1930s–1970s, it was considered a vigorous revegetation species which reclaimed retired cropland or strip-mined lands, reduced erosion, and replaced native sagebrush communities to improve forage potential for livestock (Wiens and Rotenberry 1985; Gunnell 2009). Its use is still common in postfire seeding efforts because of its high resistance to invasion by less desirable exotic annual grasses (Arredondo et al. 1998; Davies et al. 2010), and because it establishes well and costs less than native bunchgrasses (Hull 1974; Asay et al. 2001, Epanchin-Niell et al. 2009; Boyd and Davies 2012). Planting of crested wheatgrass is one of the largest- scale management manipulations in the Great Basin (Gunnell 2009). By the mid-1970s, seeding of crested wheatgrass was underway on 650,000 ha of public land in Idaho alone (Reynolds and Trost 1980), and is now established on 6–11 million ha in the American West (Lesica and DeLuca 1996). Despite its perceived benefits, using crested wheatgrass to reclaim disturbed sites promotes monocultures that inhibit the return of native plant and wildlife diversity (Reynolds and Trost 1980; Wilson 1989; Lesica and DeLuca 1996; Henderson and Naeth 2005; Vaness and Wilson 2007), including diversity of landbirds (Reynolds and Trost 1981; Bradford et al. 1998; Sutter and Brigham 1998).
Because of exotic grass invasion as well as other types of habitat degradation or loss, high-quality sagebrush Artemisia spp. habitat now represents an imperiled ecosystem in North America (Noss et al. 1995; Davies et al. 2011; USFWS 2013). Sagebrush ecosystems have lost more than 40% of their historic range, with significant alterations to the ecological processes and components of those that remain (Chambers et al. 2017). More than 350 plant and animal species highly associated with sagebrush ecosystems are of conservation concern (Wisdom et al. 2005), although some authors suggest that this number is much higher (630 species; Rich et al. 2005). In recent decades, there have been shifts in rangeland management away from emphasizing forage and toward restoring native sagebrush communities to improve plant species diversity and wildlife habitat (Mitchell 2000; Gunnell 2009). Accordingly, many restoration efforts are underway in the Great Basin and other parts of the western United States (e.g., Davies et al. 2011).
Monitoring programs to evaluate the success of such management activities at achieving desired ecological conditions are critical, but restoration monitoring often focuses on plant community composition and cover (Herrick et al. 2006). Broader indicators of ecological function may be better predictors of overall success than short-term plant establishment targets alone (Bradford et al. 1998). Birds are effective monitoring tools of ecological restoration for the following reasons: 1) ease of detection and cost effectiveness of surveys using standardized protocols, 2) different bird species associate with various attributes of functioning habitat, and 3) birds respond quickly to habitat change at multiple scales (Hutto 1998; Altman and Holmes 2000; Alexander et al. 2007; Stephens et al. 2019). Avian habitat choice is an integrative response to multiple environmental gradients (Wiens 1989). Thus, bird use of restored habitats can provide a more meaningful assessment of restoration success, when conducted in conjunction with vegetation surveys. Recently developed conservation planning tools use birds as effective indicators of progress toward restoration objectives. For example, the Habitats and Populations Strategies (HABPOPS) database is a novel planning tool developed by the Intermountain West Joint Venture, American Bird Conservancy, and Point Blue Conservation Science to explore the probable effects that habitat management actions may have on sagebrush-associated bird populations (Intermountain West Joint Venture, 2021). This tool can help land managers develop restoration targets and then provide abundance metrics for evaluating outcomes with data from bird monitoring.
The Camas National Wildlife Refuge in southeastern Idaho, USA (hereafter referred to as the Refuge) includes almost 400 ha of crested wheatgrass. The Refuge's comprehensive conservation plan (USFWS 2014) establishes goals for native habitats, including management objectives for restoring crested wheatgrass stands to sagebrush with a native grass understory. Although current management objectives in the comprehensive conservation plan are tied to habitat conditions and not directly to bird populations, maintaining habitat for migratory birds is part of the mission of the USFWS Refuge System; therefore, they are a priority for the Refuge (USFWS 2014). The analysis presented here intends to help develop more quantitative wildlife objectives connected to sagebrush habitat objectives, as a component of a long-term plan to monitor progress toward upland habitat goals through cycles of adaptive management. To inform restoration design and monitor the effectiveness of restoration actions on the Refuge, we studied birds in three types of sagebrush–steppe habitat before restoration: sagebrush with a native grass understory (i.e., the reference condition), sagebrush with a nonnative grass understory, and crested wheatgrass–dominated sites. Our objectives were to 1) document the prerestoration avian community and species–habitat relationships; 2) identify a suite of bird species most highly associated with each habitat type, including sagebrush birds of conservation concern, and evaluate their potential as management indicators; and 3) use prerestoration data and a conservation planning tool (i.e., HABPOPS) to inform avian restoration targets and make predictions regarding future avian response. We hypothesized that prerestoration bird communities would differ between sagebrush and crested wheatgrass habitats and that sagebrush-obligate bird species would be more abundant in areas with a sagebrush shrub component compared with more open, crested wheatgrass–dominated sites.
The study was conducted at the Camas National Wildlife Refuge, in the high desert of Idaho's eastern Snake River Plain. The Refuge is located at an elevation of approximately 1,463 m and comprises 4,373 ha of a diverse mosaic of sagebrush–steppe, grasslands, wetlands, and seasonal-to-ephemeral wet meadows (USFWS 2014). Climate is typical of the Intermountain West: relatively arid with mild summers and cold winters. Precipitation averages less than 25 cm/y. Upland vegetation generally corresponds to basin big sagebrush Artemisia tridentata subsp. tridentata, green rabbitbrush Chrysothamnus viscidiflorus shrubland and steppe, needle-and-thread Hesperostipa comata semidesert grassland, or western wheatgrass Pascopyrum smithii grassland alliance (USFWS 2014). Crested wheatgrass, and more recently cheatgrass Bromus tectorum, are common invasive plants. There has been no grazing in the Refuge since 1993. Land use in the Beaver-Camas subbasin outside of the Refuge is primarily agriculture with the majority (64%) of the watershed used for rangeland (USFWS 2014).
In 2011–2012, the Refuge mapped major vegetation groups following a field-based vegetation classification methodology, remote sensing methods including image segmentation and RandomForest classifiers, photographic interpretation, and field validation (see details in Miewald ). This mapping effort identified 23 vegetation classifications; from these 23 classifications, we chose 5 that were relevant to this study and combined them to form 3 basic types of sagebrush–steppe habitat. The reference condition was sagebrush with native grasses as the dominant understory (“intermountain dry tall sagebrush shrubland and steppe group, native” in Miewald ) and was present on approximately 708 ha (16%) of the Refuge. Nonnative grasses could be present in this habitat type, but comprised a minority of the understory cover. Sagebrush with a nonnative understory, primarily crested wheatgrass, had little-to-no native grasses present (“Great Basin and intermountain ruderal dry shrubland and grass” or “intermountain dry tall sagebrush shrubland and steppe group, ruderal” in Miewald ) and comprised approximately 130 ha (3%). Crested wheatgrass sites were essentially monotypic stands of crested wheatgrass with little-to-no shrub cover (“crested wheatgrass ruderal grassland alliance” in Miewald ) and covered approximately 398 ha (9%) of the Refuge. Next, we used the geographic information system ArcMap to randomly place 12 point count stations, a minimum of 300 m apart, in each of these three habitat types. Observers on the ground then confirmed the habitat categorizations at each point count station. Habitat type assignment of each station did not change over the course of the study.
Trained observers completed 5-min point count surveys at each of the 36 stations annually from 2012 to 2016, following a standard protocol (Knutson et al. 2008). During each survey, the observer recorded the number of each bird species detected and their distances from the observer in categories of less than 50 m, 50–100 m, and greater than 100 m (Data S1, Supplemental Material; Text S1, Supplemental Material). Surveys were conducted during the songbird breeding season from May 19 to June 25. There was one survey visit per season to all points in 2012 and 2016 and two visits in 2013–2015. To help control for detectability, we excluded birds recorded more than 100 m from the observer from analyses (Hutto 2016). We chose this distance because we expected vocalizing birds to be consistently detectable up to 100 m in this open habitat type. Because the study focus is specific to upland restoration, we excluded taxa not closely associated with the habitat types of interest (e.g., waterbirds, shorebirds) as well as those species not well surveyed by point count methodology (e.g., owls, raptors). We also excluded birds recorded as flyovers. Of the 54 bird species detected during point count surveys, we included 28 species in the analysis dataset.
For years with multiple visits, we first calculated mean annual abundance at each point count station by summing the number of detections of each species and dividing by the number of visits to each station in each year. We then calculated mean abundance per station by averaging across years. To compare individual species abundance, we examined the six songbirds with the greatest number of detections; all six songbirds are Key Species Supported by the Refuge (USFWS 2014). Sage thrasher Oreoscoptes montanus and western meadowlark Sturnella neglecta are additionally considered Priority Species of Concern for the Refuge, whereas sage thrasher and Brewer's sparrow Spizella breweri are also species expected to benefit from sagebrush restoration actions (USFWS 2014). In addition, we analyzed sagebrush sparrow Artemisiospiza nevadensis as a species of elevated conservation concern (USFWS 2008; IDFG 2017). The sage sparrow (the now defunct Amphispiza belli) was split into two species in 2013, including sagebrush sparrow, the species present at our study sites. However, we use “sage sparrow” when referencing research published before 2013. Mean abundances of individual species followed a nonnormal, right-skewed distribution, except for western meadowlark. Accordingly, we analyzed differences in species abundance among habitat types by using generalized linear models with habitat type as the independent variable and a quasi-Poisson error distribution family (Seavey et al. 2005). Because our dependent variables were means of bird abundance, and not integers, a Poisson error distribution was not appropriate. We considered tests with a P value of less than 0.05 to be significant.
Community composition and indicator species analysis
We populated a community composition matrix with mean abundance per station for the 28 bird species in the analysis dataset. We then excluded six bird species because of rarity in the dataset (detected at only one station) and an additional eight species because they do not hold traditional territories (e.g. swallows, brown-headed cowbird Molothrus ater), or because they are more associated with marsh and riparian habitats than the upland habitats of interest (e.g., red-winged and yellow-headed blackbirds Agelaius phoeniceus and Xanthocephalus xanthocephalus, song sparrow Melospiza melodia). This left 14 species in the community composition analysis. We ordinated the point count stations in species-space by using nonmetric multidimensional scaling (Mather 1976). We calculated similarities in breeding bird community composition among stations by using the Sørenson (Bray–Curtis) distance measure in PC-ORD version 7 (McCune and Mefford 2016) and random starting configurations with 250 iterations of real data and 250 iterations of randomized data. We used a Monte Carlo randomization test to determine whether the final axes generated were stronger than those obtained by chance. We used a multiresponse permutation procedure (MRPP) in PC-ORD to test for differences in community composition among habitat types. The MRPP's chance-corrected within-group agreement (A) provides a measure of effect size: if heterogeneity within groups is equal to that expected by chance, A = 0; when all items are identical within groups, A = 1 (McCune et al. 2002). We considered P < 0.05 to be the threshold for statistical significance.
We used an indicator species analysis (Dufrêne and Legendre 1997) within PC-ORD to examine the degree to which each species was uniquely associated with different habitat types, to evaluate their potential value as management indicators and metrics of future restoration progress. Although the Refuge's comprehensive conservation plan puts forth species considered Priority Resources of Concern and an example list of those expected to benefit from upland habitat management actions (USFWS 2014), empirical testing with local data is an important step in refining the suite of focal species to be used as management indicators (Stephens et al. 2019). We generated indicator values for each species based on a synthesis of their relative abundance and frequency within the different habitat groups. Indicator values range from 0, if a species never occurs in that habitat type, to 100; a perfect indicator of a particular habitat type (indicator value = 100) would be present on each survey conducted in that habitat and never present outside of that habitat. We evaluated statistical significance of each indicator value by using a Monte Carlo randomization test and then applying a Benjamini and Hochberg (1995) correction to the resulting P values to control the family-wise error rate across these 14 species' tests.
We used HABPOPS to predict individual bird species' responses to restoration of sites invaded by or dominated by crested wheatgrass. The HABPOPS tool is a Microsoft Access database that combines estimates of current habitat extent and condition with the best available empirical data regarding species occupancy rates and density to derive population estimates at relatively large scales (Bird Conservation Region or state). It relies on several qualitative assumptions; thus, its best use is to estimate magnitudes of change rather than specific point estimates of abundance or density. HABPOPS requires input parameters of the state (Idaho), Bird Conservation Region 9, 1 of 36 habitat associations, condition of the habitat (poor, fair, or good), and size of restoration area. The tool provides estimates for four of the bird species that we included in our abundance analyses: sagebrush sparrow, sage thrasher, Brewer's sparrow, and grasshopper sparrow Ammodramus savannarum. We chose “Intermountain Basins big sagebrush steppe” as the most appropriate available habitat association for our sagebrush sites with crested wheatgrass understory and “ungrazed invasive perennial grassland” as the habitat association for crested wheatgrass–dominated sites, both as pretreatment scenarios in HABPOPS. We predicted, based on expert opinion of Refuge staff, that restoration treatments at the Refuge will likely improve invaded sagebrush habitats from poor condition as defined by the tool (<10% shrub cover, low forb diversity, few natives, and high invasives) to fair condition (10–20% shrub cover, moderate native grass–forb mix, and some invasives) and that current invasive perennial grassland sites will convert to sagebrush habitat in fair condition. We examined pre- and posttreatment outputs from the tool at areas of approximately 40 and 405 ha (100 and 1,000 acres), which are consistent with short- and long-term restoration objectives for the Refuge.
Of the sagebrush obligates (also Species of Conservation Concern; USFWS 2008), Brewer's sparrow and sage thrasher were each more abundant in both sagebrush habitat types compared with crested wheatgrass–dominated sites (Figure 1; Table S1, Supplemental Material). Brewer's sparrow abundance was similar between sagebrush with native or nonnative grasses (mean ± standard deviation [SD] = 1.05 ± 0.40 birds/station and 1.16 ± 0.71 birds/station, respectively; P = 0.68), whereas sage thrasher was more than three times more abundant in sagebrush with native than nonnative understory (0.26 ± 0.34 and 0.08 ± 0.12 birds/station, respectively), but not significantly different (P = 0.08). Sagebrush sparrow was more abundant in sagebrush with native understory (0.11 ± 0.14) than in either sagebrush with nonnative understory (0.01 ± 0.03; P = 0.02) or crested wheatgrass (0.00 ± 0.00; P = 0.007). We did not detect sage thrashers or sagebrush sparrows in crested wheatgrass–dominated sites (Figure 1; Table S1).
Of the grassland-obligate birds, horned lark Eremophila alpestris was more abundant in areas dominated by crested wheatgrass (0.83 ± 0.68 birds/station) than in sagebrush with a nonnative understory (0.28 ± 0.29; P = 0.01). Horned lark abundance in sagebrush with native understory was intermediate between those two habitat types (0.40 ± 0.42) and not significantly different from either (Figure 1; Table S1, Supplemental Material). Grasshopper sparrow was more abundant in crested wheatgrass–dominated sites (0.86 ± 0.39) than in sagebrush with nonnative understory (0.32 ± 0.33; P = 0.002) and more abundant in sagebrush with nonnative understory than sagebrush with native understory (0.08 ± 0.13; P = 0.02; Figure 1; Table S1).
Of the grassland-associated birds that are more shrub tolerant, western meadowlark was more abundant in sagebrush with native understory (1.60 ± 0.72 birds/station) than in crested wheatgrass (0.90 ± 0.37; P = 0.003; Figure 1; Table S1, Supplemental Material). Abundance in sagebrush with nonnative understory was intermediate between those two habitat types (1.30 ± 0.45) and not significantly different from either. Vesper sparrow Pooecetes gramineus abundance was highest in sagebrush with nonnative understory (0.43 ± 0.29), greater than abundance in sagebrush with native understory (0.23 ± 0.14; P = 0.03), but not greater than in crested wheatgrass–dominated grasslands (0.24 ± 0.18; P = 0.051; Figure 1; Table S1).
Community composition and indicator species analysis
The nonmetric multidimensional scaling ordination resulted in a three-dimensional solution that was stronger than expected by chance (Monte Carlo randomization test, P = 0.04), with a minimum stress value of 12.1. The first axis of the ordination captured 56.2% of the variation in bird community composition, whereas all three axes combined accounted for 84.7%. There was a strong gradient along axis 1, with grassland-associated songbirds clustering on the right-hand portion of the graph, including grasshopper sparrow, horned lark, and savannah sparrow Passerculus sandwichensis (Figure 2). Vesper sparrow was located toward the middle of axis 1, differentiating less along this gradient than other species. Sagebrush-obligate birds (sage thrasher, sagebrush sparrow, and Brewer's sparrow) as well as loggerhead shrike Lanius ludovicianus and northern shrike Lanius borealis tended toward the left-hand side of the graph (Figure 2). Results indicated that bird communities significantly differed among habitats (MRPP: A = 0.16, P < 0.0001; Figure 2). Crested wheatgrass sites strongly differed from both sagebrush with native understory (MRPP: A = 0.20, P < 0.0001) and sagebrush with nonnative understory (MRPP: A = 0.14, P < 0.0001). Bird communities in sagebrush habitats with native and nonnative understory were much more similar to each other, though statistically different, but with a very small effect size (MRPP: A = 0.03, P = 0.04). MRPPs tend to be sensitive tests, and this effect size may not reflect biological significance.
After applying a Benjamini and Hochberg (1995) correction to control the family-wise error rate, grasshopper sparrow was a significant indicator of crested wheatgrass–dominated habitats; horned lark was significant before the correction was applied (Table 1). Sage thrasher and sagebrush sparrow were significant indicators of sagebrush with native understory (Table 1). Although the two shrike species (including loggerhead shrike, listed as Priority Resource of Concern and a species to benefit from sagebrush restoration in the comprehensive conservation plan; USFWS 2014) were detected exclusively in sagebrush with native understory, they had insufficient frequency to be significant indicator species (Table 1). Brewer's sparrow was detected with similar abundance and frequency in sagebrush habitats with different understory composition (Table 1). Mourning dove Zenaida macroura was a significant indicator of sagebrush with nonnative understory (Table 1).
The HABPOPS tool predicted that there may be no sagebrush sparrow or sage thrasher abundance in the crested wheatgrass–dominated sites (the model assumes that these species do not occupy this habitat type in the Idaho portion of Bird Conservation Region 9), which is consistent with our prerestoration results. However, it also assumes no Brewer's sparrow occupancy, but we did observe them at low densities in this habitat type (0.30 birds/station, or 0.10 birds/ha). The HABPOPS tool predicted that there may be very low densities of the first two of these sagebrush birds in invaded sagebrush sites in poor condition (0.025 and 0.06 birds/ha, respectively), also consistent with results from this study, but it somewhat overestimated prerestoration abundance of Brewer's sparrow (0.67 birds/ha, compared with the observed 0.37 birds/ha) in this habitat type. The output predicts that by improving 40 ha to sagebrush habitat from poor to fair condition, we may expect to see an increase of 8-fold, 7-fold, and 2.7-fold among sagebrush sparrow, sage thrasher, and Brewer's sparrow, respectively. A similar increase in postrestoration density is predicted if 405 ha is restored. The HABPOPS tool predicted no grasshopper sparrow abundance in poor sagebrush conditions (this time because of low predicted occupancy and density in this habitat type), although we observed 0.32 birds/station (0.10 birds/ha). It estimated 1.6–16 grasshopper sparrows in 40–405 ha of prerestoration crested wheatgrass grasslands (0.04 birds/ha, although we observed them at 0.27 birds/ha). The tool predicted that grasshopper sparrow abundance may remain zero in poor sagebrush restored to fair condition and drop to zero when converting invasive perennial grasslands to sagebrush habitat in either poor or fair condition.
The Refuge's management objectives include restoring crested wheatgrass–dominated sites to a native sagebrush plant community in select areas (USFWS 2014). To inform this management action, we examined baseline avian use of sagebrush–steppe habitats with varying degrees of invasion by crested wheatgrass in the Camas National Wildlife Refuge in southeastern Idaho from 2012 to 2016. As predicted, several bird species had similar abundances in sagebrush habitats regardless of understory. However, some sagebrush obligates, such as sage thrasher and sagebrush sparrow, were most abundant in sagebrush habitats with an understory of native grass. Community composition was similar between sagebrush habitats with native and nonnative grasses, but quite different from bird communities occupying crested wheatgrass–dominated sites, supporting our hypothesis. We also identified several avian indicator species and used HABPOPS to inform restoration targets that, in combination, may provide useful metrics to measure restoration progress in future cycles of adaptive management.
Our results largely corroborate those of previous studies regarding sagebrush bird–habitat relationships. Sage sparrow and sage thrasher abundance have been reported to be positively correlated with shrub and bare ground cover and negatively related to grass cover and stem density (Wiens and Rotenberry 1981), and displacement of sagebrush with crested wheatgrass can lead to declines in sage sparrow and Brewer's sparrow, but increases in horned lark abundance (Wiens and Rotenberry 1985). Reynolds and Trost (1980, 1981) found that sage sparrow, sage thrasher, and Brewer's sparrow were more abundant in sagebrush areas, whereas crested wheatgrass plots sometimes supported only horned lark, although we found Brewer's sparrows using crested wheatgrass in low densities. Our results for grassland-associated species are less consistent with those of previous research, but we are lacking sufficiently detailed vegetation data to fully interpret these results at our study site. We found western meadowlarks to be more abundant in sagebrush–steppe habitat with shrubs compared with habitat without shrubs, despite their typical positive association with grass and forb cover (Wiens and Rotenberry 1981; Madden et al. 2000) and negative association with woody vegetation at breeding sites (Bakker et al. 2002; Davis 2004; Grant et al. 2004). However, in British Columbia, Canada, they were found to tolerate shrubs up to 1 m in height (Schwab et al. 2006). Our results for vesper sparrow contrasted with those of a previous study noting that this species did not inhabit native sagebrush sites in southern Oregon, USA, until shrubs were removed and replaced with crested wheatgrass (Wiens and Rotenberry 1985).
The similarity in abundances of several bird species in sagebrush habitats regardless of understory may indicate that the distribution of birds is influenced more by structural vegetation features (horizontal and vertical heterogeneity) than plant species composition (e.g., Johnsgard and Rickard 1957; MacArthur and MacArthur 1961; Rotenberry and Wiens 1980; Alexander 1999). That is, bird species may respond more to the presence and structure of the shrub and herbaceous layers than to whether the grass species are native or introduced. In the community composition analysis, sagebrush habitat with native and nonnative grass understories overlapped considerably in species-space, whereas bird communities in crested wheatgrass habitat were substantially different. This also suggests that the presence of a shrub and grass layer vs. a grass layer alone may be a more important determinant of bird community structure than grass species composition. Future studies should quantify shrub cover as a potential explanatory variable in avian response to restoration and examine differences in horizontal and vertical heterogeneity in vegetation among habitats with different degrees of invasion by nonnative grasses and their influence on bird communities.
Alternatively, some true differences in bird abundance between habitats with different understory species composition may have gone undetected because of type II error associated with small sample sizes. For example, we only detected 35 sage thrashers within 100 m of our point count stations during the 5 y of the study, providing relatively low power to detect differences among habitat types. Sage thrasher mean abundance in sagebrush habitats with native grass understory was more than three times greater than that in sagebrush with nonnative understory, but with P = 0.08. It is possible that this represents a true biological difference, but we lacked the strength of evidence to detect it statistically.
Taken together, our species abundance, community composition, and indicator species results suggest that birds may be responding more strongly to the presence of the shrub layer than understory species composition. These results indicate that partial restoration, that is, reintroducing a shrub component into crested wheatgrass–dominated grasslands, may be sufficient to support some sagebrush birds without negative impacts on grassland associates such as western meadowlark and vesper sparrow. For example, Brewer's sparrow occupied both sagebrush shrub habitat types in similar numbers, but was much less abundant in crested wheatgrass. It could serve as an indicator of the presence of sagebrush shrubs rather than open grassland and thus be a metric of intermediate restoration success. In contrast, sagebrush sparrow and sage thrasher, both species of concern that were most abundant in sagebrush with native grasses and significant indicators of this habitat type, will likely benefit most from full restoration of a native herbaceous understory. Grassland-obligate birds such as grasshopper sparrow (a significant indicator of crested wheatgrass) and horned lark (a weaker indicator of crested wheatgrass) were most abundant in crested wheatgrass–dominated areas and less abundant in habitats with a shrub component.
This study serves as a model for the process by which novel, data-driven conservation planning tools, such as HABPOPS, can be used to generate hypotheses about the effects of management and set restoration targets that can be measured by future monitoring. Restoration objectives at the Refuge include converting select crested wheatgrass–dominated areas to sagebrush habitat with a native herbaceous understory (USFWS 2014). If this is not possible, especially in the short term, establishing sagebrush with a mix of native and nonnative grasses is a secondary objective. By increasing shrub cover and the proportion of native plants in near-monotypic stands of crested wheatgrass, we expect to enhance vegetation structural and functional diversity and thus improve habitat for sagebrush-associated wildlife (McAdoo et al. 1989; Davies et al. 2013). Although population estimates from HABPOPS are derived at broad scales and are not expected to have a high level of precision at a single site, they are appropriate for establishing the order of magnitude of expected change reflecting increased habitat capacity for sagebrush birds. With restoration that improves crested wheatgrass sites or sagebrush in poor condition to sagebrush in fair condition, HABPOPS predicts that densities of sagebrush species of conservation concern—sagebrush sparrow, sage thrasher, and Brewer's sparrow—will increase by 8, 7, and 2.7 times, respectively. In contrast, grassland-obligate birds such as grasshopper sparrow may decrease to very low densities postrestoration. These predictions can be used to evaluate trade-offs presented by management alternatives and provide a yardstick for measuring progress toward restoration objectives. The suitability of these species as monitoring metrics is further strengthened by the fact that three of them (sagebrush sparrow, sage thrasher, and grasshopper sparrow) were significant in the indicator species analysis. Given differences observed in our study compared with the baseline HABPOPS estimates, particularly for Brewer's sparrows and grasshopper sparrows, one could also use our empirical density estimates to refine the HABPOPS database in future updates to this tool.
Locally derived data appropriate to the management scale, combined with HABPOPS outputs, allow managers to hypothesize future project outcomes, assess trade-offs, and set meaningful and realistic targets to measure success within an adaptive management framework. Our work suggests that restoration of crested wheatgrass near-monocultures back to sagebrush shrubland will improve habitat value for most sagebrush-associated wildlife whether or not the understory can be converted to primarily native herbaceous species, or a mix of natives and nonnatives. Although removing invasive plants is an important aspect of many restoration programs, managers restoring invasive perennial grasslands back to sagebrush–steppe should be aware that these actions may be detrimental for grassland-associated species and ensure that conservation needs for native shrubland and grassland habitat types are balanced across the landscape. Managers may want to consider broader landscape and ecoregional stewardship responsibility contexts (e.g., proportion of a species' total breeding population in a given Bird Conservation Region) to inform how to weigh these potential trade-offs.
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Data S1. Bird abundance data collected via point count methodology in three different habitat types—sagebrush with native understory, sagebrush with nonnative understory, and crested wheatgrass near-monocultures—on the Camas National Wildlife Refuge in southeastern Idaho, USA, from 2012 to 2016. Data include the point count station name, visit number, survey date, survey start and end times, the four-letter alpha species code, common name and scientific name of each species observed, and the number of individuals of each bird species counted within a given time bin and distance bin during a point count survey. For a complete list of data field definitions, see Text S1, Supplemental Material.
Found at DOI: https://doi.org/10.3996/JFWM-20-035.S1 (86 KB XLSX).
Text S1. Metadata information for the avian point count dataset provided in Data S1 (bird abundance data collected via point count methodology in three different sagebrush–steppe habitat types on the Camas National Wildlife Refuge in southeastern Idaho, USA, from 2012 to 2016.
Found at DOI: https://doi.org/10.3996/JFWM-20-035.S2 (48 KB PDF).
Table S1. Mean (± standard deviation) abundance per point count station of the six most abundant upland songbird species in sagebrush–steppe habitats at Camas National Wildlife Refuge, Idaho, USA, in 2012–2016 (limited to individuals observed within 100 m of a point count station), plus sagebrush sparrow Artemisiospiza nevadensis as an additional species of conservation concern. Species are listed in decreasing order of total abundance. The last set of columns indicates significance of the differences in abundance between habitats from quasi-Poisson–generalized linear models; significant differences (P < 0.05) are highlighted in bold. CR = nonnative crested wheatgrass; S = sagebrush with native understory; SC = sagebrush with nonnative understory.
Found at DOI: https://doi.org/10.3996/JFWM-20-035.S3 (20 KB DOCX).
Reference S1. Altman B, Holmes A. 2000. Conservation strategy for landbirds in the Columbia Plateau of eastern Oregon and Washington. Version 1.0. Boring, Oregon: Oregon-Washington Partners in Flight and American Bird Conservancy.
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Reference S5. Mitchell JE. 2000. Rangeland resource trends in the United States: a technical document supporting the 2000 USDA Forest Service RPA assessment. Fort Collins, Colorado: USDA Forest Service, Rocky Mountain Research Station. Report RMRS-GTR-68.
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Reference S7. Rich TD, Wisdom MJ, Saab VA. 2005. Conservation of priority birds in sagebrush ecosystems. Pages 589–606 in Ralph CJ, Rich TD, Long L, editors. Proceedings of the Third International Partners in Flight Conference. Albany, California: USDA Forest Service, Pacific Southwest Research Station. General Technical Report PSW-GTR-191.
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Reference S8. Seavey NE, Quader S, Alexander JD, Ralph CJ. 2005. Generalized linear models and point count data: Statistical considerations for the design and analysis of monitoring studies. Pages 744–753 in Ralph CJ, Rich TD, Long L, editors. Proceedings of the Third International Partners in Flight Conference. Albany, California: USDA Forest Service, Pacific Southwest Research Station. General Technical Report PSW-GTR-191.
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Reference S10.[USFWS] U.S. Fish and Wildlife Service. 2013. Greater sage-grouse (Centrocercus urophasianus) conservation objectives: final report. Denver, Colorado: U.S. Fish and Wildlife Service.
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We appreciate the advice of Jay Carlisle of the Intermountain Bird Observatory regarding breeding season windows in this region. We are also grateful to Dan Casey of the Northern Great Plains Joint Venture (formerly of the Intermountain West Joint Venture) for assistance with the HABPOPS tool. This paper benefitted from scoping and review by Kevin Kilbride of the USFWS. We also thank the anonymous reviewers and the Associate Editor for constructive comments that greatly improved this manuscript.
Any use of trade, product, website, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.
The findings and conclusions in this article are those of the author(s) and do not necessarily represent the views of the U.S. Fish and Wildlife Service.
Citation: Rockwell SM, Wehausen B, Johnson PR, Kristof A, Stephens JL, Alexander JD, Barnett JK. 2021. Sagebrush bird communities differ with varying levels of crested wheatgrass invasion. Journal of Fish and Wildlife Management 12(1):27-39; e1944-687X. https://doi.org/10.3996/JFWM-20-035