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Federal Register / Vol. 86, No. 243 / Wednesday, December 22, 2021 / Proposed Rules habitat AECOM 2011, p. 13.
Additionally, a wide area from the international border in Nogales, through Tucson, Phoenix, and north into Yavapai County called the Sun Corridor Megapolitan Area is projected to have 11,297,000 people by 2050, a 132 percent increase from 2005
AECOM 2011, p. 13. If build-out occurs as expected, it will encompass a substantial portion of the current and historical distribution of the pygmy-owl in Arizona.
In Texas, the pygmy-owl occurred in good numbers until approximately 90
percent of the mesquite-ebony woodlands of the Rio Grande delta were cleared in 19101950 Oberholser 1974, p. 452. Currently, most of the pygmyowl habitat occurs on private ranch lands and therefore the threat of habitat loss and fragmentation of the remaining pygmy-owl habitat due to urbanization is reduced. However, urbanization and agriculture along the United StateMexico border are likely to continue to isolate the Texas population of pygmyowls by restricting movements between Texas and northeastern Mexico.
The United States-Mexico border region has a distinct demographic pattern of permanent and temporary development related to warehouses, exports, and other border-related activities, and patterns of population growth in this area of northern Mexico has accelerated relative to other Mexican States Pineiro 2001, pp. 12.
The Sonoran border population has been increasing faster than that States average and faster than Arizonas border population; between 1990 and 2000, the population in the Sonoran border municipios increased by 33.4 percent, compared to Sonoras average 21.6
percent and the average increase of Arizonas border counties 27.8
percent. Urbanization has increased habitat conversion and fragmentation, which, along with immigration, population growth, and resource consumption, were ranked as the highest threats to the Sonoran Desert Ecoregion Nabhan and Holdsworth 1998, p. 1. This pattern focuses development, and potential barriers or impediments to pygmy-owl movements, in a region that is important for demographic support immigration events and gene flow of pygmy-owl population groups, including movements such as dispersal. When looking specifically at the United StatesMexico border region extending from Texas to California, the human population is approximately 15 million inhabitants and this population is expected to double by 2025 HHS 2017, p. 1.

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Significant human population expansion and urbanization in the Sierra Madre foothill corridor may represent a long-term risk to pygmyowls in northeastern Mexico. From 2010
to 2015 the population in Tamaulipas increased by 8 percent to 3,527,735 and the population in Nuevo Leon increased by 24 percent to 5,784,442 DataMexico 2021, unpaginated. Such increasing urbanization results in the permanent removal of pygmy-owl habitat reducing habitat availability and, more significantly, increases habitat fragmentation affecting the opportunity for pygmy-owl movements within northeastern Mexico and between Mexico and Texas. Habitat removal in northeastern Mexico is widespread and nearly complete in northern Tamaulipas Hunter 1988, p. 8. Demographic support rescue effect of pygmy-owl population groups is threatened by ongoing loss and fragmentation of habitat in this area. Urbanization has the potential to permanently alter the last major landscape linkage between the pygmy-owl population in Texas and those in northeastern Mexico Tewes 1993, pp. 2829.
Human population growth in Sinaloa, Nayarit, Colima, and Jalisco, Mexico are relatively slow compared to Sonora and northeastern Mexico. From 2010 to 2015, the population in Sinaloa grew at a rate of 9.3 percent, Nayarit grew at a rate of 13.9 percent, Jalisco grew at a rate of 13.6 percent, and Colima grew at a rate of 12.4 percent DataMexico 2021, unpaginated. These areas of Mexico are not experiencing the very high growth rates of Sonora and other border regions of Mexico, but will likely have some concurrent spread of urbanization. In addition, most of the growth is taking place in the large cities, and rather than in the rural areas that likely support pygmy-owl habitat Brinkhoff 2016, unpaginated. However, these Mexican states have other threats to pygmy-owl habitat occurring such as agricultural development and deforestation that, in combination with habitat lost to urbanization, represent threats to the continued viability of the pygmy-owl in this area.
Invasive Species The invasion of nonnative vegetation, particularly nonnative grasses, has altered the natural fire regime over the Sonoran Desert ecoregion of the pygmyowl range Esque and Schwalbe 2002, p.
165. In areas comprised entirely of native species, ground vegetation density is mediated by barren spaces that do not allow fire to carry across the landscape. However, in areas where nonnative species have become
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established, the fine fuel load is continuous, and fire is capable of spreading quickly and efficiently Esque and Schwalbe 2002, p. 175. As a result, fire has become a significant threat to the native vegetation of the Sonoran Desert.
Nonnative annual plants prevalent within the Sonoran range of the pygmyowl include Bromus rubens and B.
tectorum brome grasses, Schismus spp.
Mediterranean grasses, and Sahara mustard Brassica tournefortii Esque and Schwalbe 2002, p. 165; ASDM
2021, entire. However, the nonnative species that is currently the greatest threat to vegetation communities in Arizona and northern Sonora, Mexico is the perennial Cenchrus ciliaris buffelgrass, which is prevalent and increasing throughout much of the Sonoran range of the pygmy-owl Burquez and Quintana 1994, p. 23; Van Devender and Dimmit 2006, p. 5.
Buffelgrass is not only fire-tolerant unlike native Sonoran Desert plant species, but is actually fire-promoting Halverson and Guertin 2003, p. 13.
Invasion sets in motion a grass-fire cycle where nonnative grass provides the fuel necessary to initiate and promote fire.
Nonnative grasses recover more quickly than native grass, tree, and cacti species and cause a further susceptibility to fire DAntonio and Vitousek 1992, p. 73;
Schmid and Rogers 1988, p. 442. While a single fire in an area may or may not produce long-term reductions in plant cover or biomass, repeated wildfires in a given area, due to the establishment of nonnative grasses, are capable of ecosystem type-conversion from native desertscrub to nonnative annual grassland. These repeated fires may render the area unsuitable for pygmyowls and other native wildlife due to the loss of trees and columnar cacti, and reduced diversity of cover and prey species Brooks and Esque 2002, p. 336.
The distribution of buffelgrass has been supported and promoted by governments on both sides of the United States-Mexico border as a resource to increase range productivity and forage production. A 2006 publication estimates that 1.8 million ha 4.5
million ac have been converted to buffelgrass in Sonora, and that between 1990 and 2000, there was an 82 percent increase in buffelgrass coverage Franklin et al. 2006, pp. 62, 66.
Following establishment, buffelgrass fuels fires that destroy Sonoran desertscrub, thornscrub, and, to a lesser extent, tropical deciduous forest; the disturbed areas are quickly converted to open savannas composed entirely of buffelgrass which removes pygmy-owl nest substrates and generally renders
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