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Federal Register / Vol. 86, No. 103 / Tuesday, June 1, 2021 / Proposed Rules
converted to row crop agriculture or fragmented by oil and gas exploration and urban development Peterson and Boyd 1998, p. 22. Additionally, honey mesquite Prosopis glandulosa encroachment within this ecoregion has played a significant role in decreasing available space for the lesser prairiechicken. Technological advances in irrigated row crop agriculture have led to more recent conversion of shinnery oak prairie habitat to row crops in Eastern New Mexico and West Texas Grisham et al. 2016a, p. 316.
The Sand Sagebrush Ecoregion occurs in Southeast Colorado, Southwest Kansas, and a small portion of Western Oklahoma McDonald et al. 2012, p. 2.
The vegetation community in this area primarily consists of sand sagebrush and the associated mixed and tall grass species that are usually found in the sandier soils adjacent to rivers, streams, and other drainages in the area. Lesser prairie-chicken from the Sand Sagebrush Ecoregion form a distinct genetic cluster from other ecoregions but have likely contributed some individuals to the Short-Grass/CRP
Ecoregion through dispersal OylerMcCance et al. 2016, p. 653.
Historically, the Sand Sagebrush Ecoregion supported the highest density of lesser prairie-chicken and was considered the core of the lesser prairiechicken range Haukos et al. 2016, p.
282. A single flock detected in Seward County, Kansas, was estimated to potentially contain more than 15,000
birds Bent 1932, p. 281. The population size is estimated to have peaked at more than 85,000 males in the 1970s Garton et al. 2016, p. 62 and has been in decline since the late 1970s.
More recent survey efforts estimate a 5year average population size of 1,215
birds 90% CI: 196, 4,547. Less than 5
percent of all lesser prairie-chicken occur in this ecoregion Service 2021, pp. 6678. Most of the decline has been attributed to habitat deterioration and conversion of sand sagebrush to intensive row crop agriculture due to an increase in center pivot irrigation Jensen et al. 2000, p. 172.
Environmental conditions in this ecoregion can be extreme, with stochastic events such as blizzards negatively impacting lesser prairiechicken populations.
The Short-Grass/CRP Ecoregion falls within the mixedand short-grass prairies of Central and Western Kansas McDonald et al. 2012, p. 2. As the name implies, much of this ecoregion historically consisted of short-grass prairie interspersed with mixed-grass prairie as well as sand sagebrush prairie along some drainages Dahlgren et al.

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2016, p. 260. By the 1980s, large expanses of prairies had been converted from native grass for crop production in this ecoregion. After the introduction of the CRP in 1985, landowners began to have enhanced incentives to convert croplands to perennial grasslands to provide cover for the prevention of soil erosion. The State of Kansas required those enrolling in the CRP to plant native mixedand tall-grass species, which is notable because the grasses in this area historically consisted largely of short-grass species, which generally do not provide adequate habitat for the lesser prairie-chicken. For more information on the CRP, see the SSA
report Service 2021, pp. 5254.
Prior to the late 1990s, lesser prairiechickens in this ecoregion were thought to be largely absent or occurred sporadically in low densities Hagen and Giesen 2005, unpaginated; Rodgers 1999, p. 19. We do not know what proportion of the eastern Short-Grass/
CRP Ecoregion in Kansas was historically occupied by lesser prairiechicken Hagen 2003, pp. 34, and surveys in this ecoregion only began in earnest in 1999 Dahlgren et al. 2016, p.
262. The CRP is an idle lands program, which requires establishment of grass cover and precludes tillage or agricultural commodity production for the duration of the contract, and has contractual limits to the type, frequency, and timing of management activities, such as burning, haying, or grazing of the established grasses. As a result of these factors, CRP often provides the vegetative structure preferentially used by lesser prairie-chickens for nesting. In the State of Kansas, the availability of CRP lands, especially CRP lands with interseeded or original seed mixture of forbs, resulted in increased habitat availability for the lesser prairie-chicken and, thus, an expansion of the known lesser prairie-chicken range and an increase in the abundance of the lesser prairie-chicken Rodgers 1999, pp. 18
19; Fields 2004, pp. 11, 105; Fields et al. 2006, pp. 931, 937; Sullins et al.
2018, p. 1617.
The Short-Grass/CRP Ecoregion is now estimated to contain the majority of lesser prairie-chickens compared to the other ecoregions, with recent survey efforts estimating a 5-year average population size of 16,957 birds 90% CI:
13,605, 35,350, representing approximately 62 percent of the rangewide population Service 2021, pp.
6678. Recent genetic studies indicate that lesser prairie-chickens have moved northward largely from the Mixed-Grass Ecoregion and, to a lesser extent, the Sand Sagebrush Ecoregion into the
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Short-Grass/CRP Ecoregion OylerMcCance et al. 2016, p. 653.
The northern section of this ecoregion is the only portion of the lesser prairiechickens range where co-occurrence with greater prairie-chicken occurs.
Hybridization rates of up to 5 percent have been reported Pitman 2013, p. 5, and that rate seemed to be stable across multiple years, though sampling is limited where the species co-occur Pitman 2013, p. 12. Limited additional work has been completed to further assess the rate of hybridization. There are concerns about the implications of genetic introgression dilution of lesser prairie-chicken genes, particularly given that potential effects are poorly understood Dahlgren et al. 2016, p.
276. Unresolved issues include whether hybridization reduces fitness, alters behavior or morphological traits in either a positive or negative way and the historical occurrence and rate of hybridization.
The Mixed-Grass Ecoregion for the lesser prairie-chicken lies in the northeastern panhandle of Texas, the panhandle of northwestern Oklahoma, and south-central Kansas McDonald et al. 2012, p. 2. The Mixed-Grass Ecoregion is separated from the ShortGrass/CRP Ecoregion in Kansas by the Arkansas River. The vegetation community in this ecoregion consists largely of a mix of perennial grasses and shrubs such as sand sagebrush, sand plum Prunus angustifolia, yucca Yucca spp., and sand shinnery oak Wolfe et al. 2016, p. 300. Based upon population reconstruction data, the mean population estimate was around 30,000 males in the 1970s and 1980s followed by a decline in the 1990s Hagen et al. 2016, pp. 67. The mean population estimate peaked again in the early 2000s at around 25,000 males, before declining to and remaining at its lowest levels, <10,000 males since 2012
Hagen et al. 2016, pp. 67. Although historical population estimates in the ecoregion reported some of the highest densities of lesser prairie-chicken in the range Wolfe et al. 2016, p. 299, recent aerial survey efforts estimate a 5-year average population size of 6,135 birds including males and females; 90% CI:
1,719, 11,847. The recent survey work estimates about 22 percent of lesser prairie-chicken occur in this ecoregion Service 2021, pp. 6678. Lesser prairie-chicken from the Mixed-Grass Ecoregion are similar in genetic variation with the Short-Grass/CRP
Ecoregion, with individuals likely dispersing from the Mixed-Grass Ecoregion to the Short-Grass/CRP
Ecoregion Oyler-McCance et al. 2016, p. 653.

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