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Federal Register / Vol. 86, No. 146 / Tuesday, August 3, 2021 / Rules and Regulations Creek areas Sacks and Quinn 2020, p.
1; CSERC et al. 2020, pp. 23, CDFW
2020, pp. 34 and appear to have been low for many years. Sightings fell considerably in the mid-1900s, for instance, as compared to trapping data reported by Grinnell et al. 1937, p. 389
Schempf and White 1977, p. 44. The low numbers make this DPS more susceptible to deleterious stochastic events such as major fires or diseases.
Loss of a few individuals due to stochastic events would mean the loss of a relatively large proportion of the small Sierra Nevada DPS population.
Additionally, the Sierra Nevada DPSs low population numbers make it vulnerable to inbreeding depression.
Inbreeding depression is caused by the chance loss of beneficial gene variants alleles in small populations, leaving deleterious alleles as the only remaining variants of a given gene Soule 1980, pp.
157158. It can result in lowered reproductive ability, congenital defects, and lowered disease resistance Soule 1980, pp. 157158; Gilpin 1987, p. 132;
OBrien 2003, pp. 6263. To avoid inbreeding depression, a population typically requires an effective population size of at least 100
reproducing adults Frankham et al.
2014, p. 58. The effective size of a population is generally smaller than the actual size, and refers to the number of breeding individuals that would be necessary to produce the level of genetic diversity observed in the population if the members of the population interbred in a manner that was ideal for maximizing genetic diversity Lande and Barrowclough 1987, pp. 8889. So for instance, a population in which few individuals bred, and in which they chose mates from among their geographical neighbors, would have a smaller effective size than a population in which almost all adults bred and chose mates from among the entire population.
The Sierra Nevada DPSs actual population size of 18 to 39 individuals is already well below 100, but based on samples taken from 2015 to 2017 its effective population size was only 6.1
prior to the immigration into the population of two nonnative males in 2012 CDFW 2020, p. 3. Thus, the same level of genetic diversity could have been produced by only about six breeding individuals in an ideal population in which breeding practices maximized diversity. This means the Sierra Nevada DPS had likely been suffering from inbreeding depression prior to the arrival of two Great Basin foxes in 2012 Sacks et al. 2015, pp. 3, 10, 2930 see Genomic Integrity, below. Additional support for this
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p. 2, although we have no information to indicate whether either of these produced young.
While the hybrid pups assist in helping the Sierra Nevada DPS
experience less inbreeding depression as discussed above, there remains the possibility that so many immigrants might enter the population and produce young that the unique heritable characteristics of the Sierra Nevada DPS
are lost Sacks et al. 2015, pp. 1718;
Quinn et al. 2019, p. 573. This loss of genes representative of the diversity of the DPS would initially mean a loss of representation i.e., a diminished ability to adapt to long-term changes due to the lost genes. If such genetic replacement continued to the point where the DPS as a whole was facing replacement by nonnative foxes, then that would represent a loss of resiliency i.e., the inability of remaining members of the DPS in the population to recover from stochastic events. For instance, if the last remaining individuals considered members of the DPS were of an older generation because their pups were all too hybridized to qualify as Sierra Nevada DPS, then any stochastic event that eliminated the last of the older DPS
individuals would also eliminate the DPS as a whole, despite the continuing existence of non-DPS foxes in the area.
The current demographic circumstances of the DPS as a single, small population is also likely to result in low representation, because unique adaptations and genetic variations that DPS members in other portions of the historical range may once have had are likely to be lost now that the DPS no longer includes those areas. The historical range as sketched by Grinnell Genomic Integrity et al. 1937, p. 382 stretched for Prior to spring of 2013, no roughly 460 km 285 mi from the reproduction between native northern to the southern Sierra Nevada individuals of the Sierra Nevada DPS
mountains. The estimated current range, and nonnative immigrant red fox was at only about 188 km 117 mi long, and known to have occurred Sacks et al.
about half as wide, only covers portions 2015, p. 9; Sacks 2017, p. 4. However, of the central Sierras. Examples of two nonnative male red foxes with a differing ecological characteristics mixture of Great Basin montane V. v.
across the historical range include a macroura and fur-farm ancestry arrived north to south pattern of decreasing at the Sonora Pass area in 2012 Sacks annual precipitation, increasing et al. 2015, pp. 3, 10, 2930. By 2014, temperatures for a given elevation, and they had produced a total of 11 hybrid increasing maximum elevations Fitespups Sacks et al. 2015, pp. 2930, and Kaufman et al. 2007, p. 458. Vegetation by 2017, the hybrids had interbred and differences also follow this gradient, produced 13 additional pups Quinn et with whitebark pine more dominant in al. 2019, p. 571. These 24 pups, all with the north, but limber pine Pinus a mixture of Sierra Nevada DPS and flexilis becoming more prominent in Great Basin montane fox ancestry, are the central Sierras and foxtail pine the only pups known to have been Pinus balfouriana in the south Fitesproduced in the population since 2011
Kaufman et al. 2007, 475.
Quinn et al. 2019, p. 571; Sacks and Cumulative or Synergistic Effects Quinn 2020, p. 2. A third nonnative As discussed above, both rodent male was sighted once in 2014, and a population numbers and the incidence fourth in 2017 Sacks and Quinn 2020, conclusion is provided by preliminary results of a study that estimated the inbreeding coefficient of a Sierra Nevada DPS fox that was born prior to the arrival of the Great Basin immigrants Sacks and Quinn 2020, p. 2. The inbreeding coefficient was found to be above 0.4, which is at the high end of the range found in Isle Royal wolves, a population with demonstrated severe inbreeding depression Sacks and Quinn 2020, p. 2.
These data indicate that lowered reproductive success from inbreeding depression may be primarily responsible for the complete lack of pup production documented in the Sonora Pass area from 2011 through 2017 by mated pairs of pure Sierra Nevada DPS
foxes Quinn et al. 2019, p. 571. It is thus likely to have constituted a limiting factor on population size in recent years Sacks and Quinn 2020, p. 3. And while recent interbreeding with foxes from the Great Basin appears to have increased reproductive success, we have no information regarding the extent of other potential effects that are typically associated with inbreeding depression, such as congenital defects and lowered disease resistance, nor whether these potential effects may also have been alleviated. The population also remains small at present, and thus potentially susceptible to renewed impacts from inbreeding depression Quinn et al.
2019, p. 573, or from deleterious chance events such as drought or fire. If inbreeding depression does return, the impacts would likely be worse due to the addition of new alleles from the Great Basin into the population Quinn et al. 2019, p. 573.

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