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Federal Register / Vol. 86, No. 246 / Tuesday, December 28, 2021 / Proposed Rules
segments SSA Report Service 2021, Figure 55, and 3 population level factors such as connectivity, relative risk of decline, and level of threats.
These factors were assessed in terms of their potential influence on the ability of foothill yellow-legged frog metapopulations to survive and recover after a plausible catastrophic event. For example, isolation of occupied stream segments or lack of functional connectivity in an analysis unit, could prevent recolonization of extirpated areas after a massive die-off or temporary habitat destruction.
At the analysis unit scale of redundancy, long-term viability after a catastrophic event would likely be possible in the North Coast clade North Coast California and North Coast Oregon units and might be possible in the North Sierra clade. In the North Coast clade, there are large numbers of occupied streams and there are numerous occupied stream segments that both are in the low risk of decline category and are distributed widely across the geographical area SSA
Report Service 2021, Figure 55.
Furthermore, resiliency is intact in both of the two analysis units that comprise this clade. Resiliency is also intact in the North Sierra clade because there are numerous occupied stream segments that both are in the low risk of decline category and are distributed widely across the geographical area SSA
Report Service 2021, Figure 55.
However, the North Sierra clade has less redundancy than the North Coast clade because the North Sierra clade is small in size and has poor functional connectivity, which could prevent recolonization after catastrophic events.
The North Feather DPS occupies a relatively small area and several streams or occurrences have been extirpated from past impacts eastern portion of range, southwestern area near Lake Oroville, and some occurrences in northern Butte County CDFW 2020, dataset, entire; Service 2021, figure 49, p. 131. The North Feather DPS also has the highest average relative risk of population decline with only 16 15
percent of the 109 analyzed stream segments in the low risk category and 34
stream segments 31 percent in the high risk category. Overall abundance of foothill yellow-legged frogs for the North Feather DPS is largely unknown, but egg mass densities are very low in
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the two regulated stream reaches that have long-term monitoring Rose et al.
2020, pp. 6364, table 1. For example, sections of the Cresta reach of the North Feather River that historically had relatively high numbers of foothill yellow-legged frog egg masses did not have egg masses or were extremely reduced for several years 20062017
CDFW 2019, p. 31; Dillingham 2019, p.
7. As a result, redundancy is limited in the North Feather DPS. The North Feather DPS is not only the smallest clade, but its occupied stream segments are not well-distributed over the geographical area SSA Report Service 2021, Figure 55. The extant North Feather populations occupy an area small enough that a large catastrophic event, such as a high-severity wildfire or drought, could result in functional extirpation. Furthermore, the North Feather DPS has reduced resiliency because of poor occupancy and relatively high risk of population decline.
Redundancy is poor in the South Sierra and Central Coast clades. Both the South Sierra and Central Coast clades have substantially reduced resiliency because of poor occupancy, poor connectivity, relatively high risk of decline, and substantial threats. A single catastrophic event would be unlikely to extirpate the entirety of either unit, but the patchy distribution of occurrences SSA Report Service 2021, Figure 55
and limited connectivity would make it extremely unlikely that extirpated areas would be recolonized naturally.
Redundancy within the South Coast clade is nearly zero. Not only is the resiliency in this clade extensively reduced, but there are only two known populations SSA Report Service 2021, Section 8.2 in the South Coast clade.
These two populations comprised of seven stream segments are also very close in proximity SSA Report Service 2021, Figure 55. These streams are located close to one another, but the foothill yellow-legged frog populations within them appear to have lost genetic connectivity. Although the stream flows are not regulated by dams, the risk of population decline continues to be medium or high under current conditions due to the combination of threats identified above altering habitat and impacting the DPS. Furthermore, the close proximity of the stream segments to each other makes the South
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Coast DPS especially vulnerable to extirpation from a single catastrophic event.
Overall Current and Future Condition As discussed above, we used the information from the PVA to inform both the species current condition Service 2021, chapter 8, pp. 122166
and potential future condition Service 2021, chapter 9, pp. 167193. The PVA
assessed how the following measures would change from current condition:
1 Occupancy and abundance, 2
connectivity, 3 modeled risk of population decline, and 4 status of threats under each future scenario.
Because changes to environmental conditions under the future scenarios were reflected by environmental covariates in the PVA see Service 2021, section 9.2 Scenarios; Table 17, we were able to forecast the magnitudes of changes in resiliency by comparing the modeled risk of decline Rose et al.
2020, entire under current conditions to modeled risk under the three future scenarios. The results of the analysis showed that the average risk of population decline for each analysis unit increased under the three future scenarios Rose et al. 2020, p. 39.
Under current conditions and all future scenarios, the average relative risk of decline was highest in the South Sierra and Central Coast units and was lowest in the North Coast Oregon, North Coast California, and North Sierra units Table 3 below and Service 2021, Tables 18
and 19. Under the lower change scenario, decreases in resiliency, compared to current conditions, were small in most analysis units. However, decreases in resiliency were more dramatic under the mean and higher change scenarios. These dramatic declines in resiliency put several analysis units at risk of unit-wide extirpation or functional extirpation i.e., such extensive reduction in condition that extirpation of the entire unit is likely to eventually occur as remnant populations experience normal environmental and demographic fluctuations under the mean and higher change scenarios SSA Report Service 2021, Table 19. One of the analysis units South Coast unit is at risk of unit-wide extirpation under all three of the future scenarios.

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