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Federal Register / Vol. 86, No. 185 / Tuesday, September 28, 2021 / Proposed Rules
connectivity between populations, and number of subpopulations within populations. The habitat factors we analyzed include suitable habitat size to support population viability, habitat availability trends, and habitat. For each of these demographic and habitat factors, we characterized the condition
High, Moderate, Low, and Very Low/
Extirpated of each factor for each population Table 1 to assess overall population resiliency. Where more data were available, we assigned scores High = 1, Moderate = 0, Low = 1, and Very Low/Extirpated = 2 to each demographic and habitat factor and
calculated an overall score for each population. We averaged all of the demographic and habitat condition category scores for each population to determine the overall resiliency score for that population Service 2018, p. 64.

TABLE 1POPULATION RESILIENCY CATEGORY DEFINITIONS FOR PEN ASCO LEAST CHIPMUNK
High 1

Moderate 0

Low 1

Very low/extirpated 2

density or relative abundance is high.
population is increasing over time.
there is connectivity between the populations.
the number of subpopulations is high, spatially dispersed, and able to withstand or recover from stochastic events.
large, contiguous areas of increasing availability of suitable habitat with no detectable impacts from land use or management.

density or relative abundance is moderate.
population is stable over time
populations are adjacent to each other, but unsuitable habitat precludes dispersal.
multiple subpopulations, allowing for some ability to withstand or recover from stochastic events.
areas of moderately sized habitat with some isolated habitat patches.
land use or management occurs but does not significantly limit chipmunk resources.

density or relative abundance is low.
population is decreasing over time but still extant.
populations are extremely isolated from one another.
two subpopulations allow for some, but limited, ability to withstand or recover from stochastic events.
habitat occurs as small isolated patches.
land use or management reduces chipmunk resources.

abundance decreases over time, such that population may be extirpated completely.
no connectivity with other populations exists.
if extant, no subpopulation structure occurs.
little to no suitable habitat is available.
if patches exist, they are small and isolated and will lead or have led to high probability of extirpation.
land use or management removes chipmunk resources.

The current condition of each demographic and habitat factor and the overall condition of each population of the Penasco least chipmunk is displayed in Table 2. Historically, there were two known populations of Penasco least chipmunk, the Sacramento Mountains population and the White Mountains population. Based on the demographic and habitat factors discussed in detail in the SSA Service 2018, pp. 6062, the Sacramento Mountains population is considered to be in Very Low/Extirpated overall condition. There have been no detections of Penasco least chipmunk in the Sacramento Mountains since 1966, despite extensive survey effort, indicating that this population is likely extirpated. Even if it is still extant, it has no connectivity with other populations and likely no subpopulation structure Service 2018, p. 11. The Sacramento Mountains have little to no remaining suitable habitat, and land use and management have severely decreased the condition of the resources upon which Penasco least chipmunk depends.
For the White Mountains population, current habitat availability is moderate.
Habitat has experienced a moderate change from historical conditions, and land use or management is not known to significantly reduce Penasco least chipmunk resources. However, in terms of demographic factors, the White Mountains population has a low density and decreasing population trend. The
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population is the only remaining population of the subspecies, and the White Mountains population has no known subpopulation structure. Given these Low and Very Low condition demographic factors, the White Mountains population is in Low overall condition. The current resiliency of Penasco least chipmunk is low to very low, with one population likely extirpated and the remaining population isolated with no subpopulation structure.
Maintaining representation in the form of genetic or ecological diversity is important to preserve the capacity of the Penasco least chipmunk to adapt to future environmental changes. Because one of the two populations of Penasco least chipmunk is likely extirpated, and the extant population persists in extremely low numbers, genetic diversity is likely extremely low.
Penasco least chipmunks in the White Mountains showed the lowest levels of within-population genetic variation out of nine least chipmunk populations in New Mexico, Arizona, and Colorado Sullivan 1985, pp. 431433. In addition, the subspecies has a historical distribution in two very different ecological settings: One in a highelevation subalpine meadow zone in the White Mountains, and one in a lower elevation ponderosa pine zone in the Sacramento Mountains. Because the Sacramento Mountains may no longer support the subspecies, the Penasco
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least chipmunk has already lost ecological representation across its range. Low genetic variation and the loss of one ecological setting results in low representation for the Penasco least chipmunk Service 2018, p. 65.
To be robust in the face of stochastic events, the Penasco least chipmunk needs to have at least two resilient populations Service 2018, p. 64.
Historically there were only two known populations, one each in the White and Sacramento Mountains. Generally, the more populations a species has, and the wider the distribution of those populations, the more redundancy the species will exhibit. Redundancy reduces the risk that a large portion of the species range will be negatively affected by a catastrophic natural or anthropogenic event e.g., wildfire at a given point in time. Species or subspecies that are well-distributed across a wide geographic range are less susceptible to extinction and more likely to be viable than taxa that are confined to small areas where stochastic events are likely to affect all of the individuals simultaneously Carroll et al. 2010, entire. Because one of the two populations of Penasco least chipmunk is likely extirpated, the Penasco least chipmunk currently lacks any redundancy Service 2018, p. 65.

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