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Federal Register / Vol. 86, No. 185 / Tuesday, September 28, 2021 / Proposed Rules and rodents are reported as being susceptible USDA Wildlife Services 2010, p. 15. The prevalence of antibodies of Y. pestis was reported for 17 species of mammals from the western United States Abbott and Rocke 2012, p. 26; of those, feral hogs had the highest prevalence rate at 74%.
Although the sample size for this assessment was relatively low 18 out of 23 were positive, these data demonstrate that feral hogs in both the Sacramento Mountains and White Mountains could contribute to disease dynamics in the small mammal communities in these mountain ranges Abbott and Rocke 2012, p. 26.
Impacts from feral hogs may include rooting, predation, spreading diseases and parasites, spreading invasive weed species, and competition with native species for water and food resources Service 2018, p. 48. We lack specific data demonstrating overlap of feral hog occurrence with Penasco least chipmunk occurrence; however, feral hogs are known to occur in the vicinity of Penasco least chipmunk habitat or areas formerly known to be occupied by the Penasco least chipmunk Service 2018, p. 48.
Small Population Size and Lack of Connectivity Compared to large populations, small populations are more vulnerable to extirpation from environmental, demographic, and genetic stochasticity random natural occurrences, and unforeseen natural or unnatural catastrophes Shaffer 1981, p. 131.
Small populations are less able to recover from losses caused by random environmental changes Shaffer and Stein 2000, pp. 308310, such as fluctuations in reproduction demographic stochasticity, sweeping losses from disease events, or changes in the frequency or severity of wildfires environmental stochasticity.
Another type of random fluctuation, genetic stochasticity, results from: 1
Changes in gene frequencies due to the founder effect, which is the loss of genetic variation that occurs when a new population is established by a small number of individuals Hedrick 2000, p. 226; 2 random fixation, or the complete loss of all but one allele at a locus Hedrick 2000, p. 258; or 3
inbreeding depression, which is the loss of fitness or vigor due to mating among relatives Hedrick 2000, p. 208.
Additionally, small populations generally have an increased chance of genetic drift, or random changes in gene frequencies from generation to generation that can lead to a loss of variation, and inbreeding Ellstrand and
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Elam 1993, p. 225. Allee effects, when there is a positive relationship between any component of individual fitness and either numbers or density of conspecifics Stephens et al. 1999, p.
186, may also occur when a population is in decline Dennis 1989, pp. 481
538. In a declining population, an extinction threshold or Allee threshold Berec et al. 2007, pp. 185
191 may be crossed, in which adults in the population either cease to breed or the population becomes so compromised that breeding does not contribute to population growth. Allee effects typically fall into three broad categories Courchamp et al. 1999, pp.
405410: Lack of facilitation including low mate detection and loss of breeding cues, demographic stochasticity, and loss of heterozygosity. Environmental stochasticity amplifies Allee effects Dennis 1989, pp. 481538; Dennis 2002, pp. 389401. In Penasco least chipmunks, random fixation and loss of heterozygosity have been observed Sullivan 1985, pp. 431433. The extinction risk for a subspecies represented by few small populations is magnified when those populations are isolated from one another, as is the case for the White Mountains and the Sacramento Mountains Service 2018, p.
50.
It is suspected that the White Mountains and Sacramento Mountains populations may have been physically separated over a long time period with little to no genetic interchange, based on morphometric differences in collected specimens Sullivan 1985, pp. 424425.
However, connectivity could play an important role as it relates to the overall viability to the subspecies if it is found to be present in the Sacramento Mountains in the future. Connectivity between White Mountain and Sacramento populations would contribute to the number of reproductively active individuals in a population; mitigate the genetic, demographic, and environmental effects of small population size; and recolonize extirpated areas Service 2018, pp. 48
49. Additionally, the fewer the populations a species or subspecies has, the greater the risk of extinction. The combination of a very small population in the White Mountains, a likely extirpated population in the Sacramento Mountains, and no population connectivity between the mountain ranges, synergistically interacting with the other stressors and potential stressors described above, greatly increases extinction risk for the Penasco least chipmunk Service 2018, p. 50.
Because of this combination, the
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stressor of small population size is included in our analysis of future subspecies viability.
Conservation Actions The White Mountains Wilderness Area within the Lincoln National Forest is currently closed to grazing and will remain closed for the recovery and protection of the Penasco least chipmunk Williams pers. comm. 2020.
As part of the SSA, we also developed multiple future scenarios to capture the range of uncertainties regarding future threats and the projected responses by the Penasco least chipmunk. Our scenarios included a continuing conditions scenario, which incorporated the current risk factors continuing on the same trajectory that they are on now.
We also evaluated an optimistic scenario and a scenario with increased stressors. Because we determined that the current condition of the Penasco least chipmunk was consistent with an endangered species see Determination of Species Status, below, we are not presenting the results of the future scenarios in this proposed rule. Please refer to the SSA report Service 2018
for the full analysis of future scenarios.
Determination of Species Status Section 4 of the Act 16 U.S.C. 1533
and its implementing regulations 50
CFR part 424 set forth the procedures for determining whether a species meets the definition of an endangered species or a threatened species. The Act defines an endangered species as a species that is in danger of extinction throughout all or a significant portion of its range, and a threatened species as a species that is likely to become an endangered species within the foreseeable future throughout all or a significant portion of its range. The Act requires that we determine whether a species meets the definition of endangered species or threatened species because of any of the following factors: A The present or threatened destruction, modification, or curtailment of its habitat or range; B
overutilization for commercial, recreational, scientific, or educational purposes; C disease or predation; D
the inadequacy of existing regulatory mechanisms; or E other natural or manmade factors affecting its continued existence.
Status Throughout All of Its Range The range of the Penasco least chipmunk once included the Sacramento and White Mountains in Lincoln and Otero Counties in New Mexico. The Penasco least chipmunk is now found in only one isolated
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