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Federal Register / Vol. 86, No. 41 / Thursday, March 4, 2021 / Proposed Rules redundancy, and representation, we describe the species levels of viability over time. For the Arizona eryngo to maintain viability, its populations or some portion thereof must be resilient.
A number of factors influence the resiliency of Arizona eryngo populations, including occupied area, abundance, and recruitment. Elements of the species habitat that determine whether Arizona eryngo populations can grow to maximize habitat occupancy influence those factors, thereby influencing the resiliency of populations. These resiliency factors and habitat elements are discussed in detail in the SSA report and summarized here.

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Species Needs Abundance Larger plant populations have a lower risk of extinction than smaller populations Menges 2000, p. 78. Small populations are less resilient and more vulnerable to the effects of demographic, environmental, and genetic stochasticity and have a higher risk of extinction than larger populations Matthies et al. 2004, pp.
481, 485. Small populations may experience increased inbreeding, loss of genetic variation, and ultimately a decreased potential to adapt to environmental change Matthies et al.
2004, p. 481. When rare plant populations are very small fewer than 100 individuals, they may suffer from inbreeding depression Maschinski and Albrecht 2017, p. 392. Furthermore, fewer pollinators visit plants in small and isolated populations, which may lead to reduced pollination and lowered fecundity Matthies et al. 2004, p. 482.
For populations of Arizona eryngo to be resilient, abundance should be high enough that local stochastic events do not eliminate all individuals, allowing the overall population to recover from any one event. A greater number of individuals in a population increases the chance that a portion of the population will survive. The necessary abundance or minimum viable population MVP size for Arizona eryngo is unknown; however, estimations can be attained from literature. For example, Pavlik 1996, p.
137 recommends MVP sizes ranging from 50 individuals to 2,500 individuals for the conservation of rare plants, depending on various life history characteristics of the taxon. Some of the Arizona eryngos life history characteristics indicate that an MVP
may require higher abundance, while other characteristics indicate that lower abundances may be sufficient. For
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example, the species is a perennial and commonly produces ramets, which means that fewer individuals are needed to achieve an MVP. Conversely, it is an herbaceous plant, which means that an MVP may require higher abundance.
The other characteristics are unknown for this species. Based on our current understanding of the species life history, we conclude that an initial MVP
in the middle of the spectrum provided by Pavlik 1996, p. 137 is appropriate.
Therefore, a population size of 1,225
may be needed to achieve high resiliency for the Arizona eryngo.
Determinations of MVP usually take into account the effective population size, rather than total number of individuals; 10 genetically identical individuals for example, clones or ramets would have an effective population size of one. In the case of the Arizona eryngo, we have estimates of abundance of individuals for each population, but we do not know the ratio of ramets to genetically unique individuals, although evidence indicates the species is highly clonal. In cases like this, Tependino 2012, p. 946
suggests adjusting the stem counts of rare clonal species to adjust for the inflated population size from the inclusion of ramets. Therefore, to account for the clonal nature of the Arizona eryngo, to estimate our final MVP we added 50 percent to the estimated MVP, which resulted in a total of about 1,840 plants needed to be a highly resilient population.
Recruitment Arizona eryngo populations must also reproduce and produce sufficient amounts of seedlings and ramets such that recruitment equals or exceeds mortality. Ideally, we would know key demographic parameters of the plant i.e., survival, life expectancy, lifespan, the ratio of ramets to genetically unique individuals to estimate the percentage of juveniles required in a population to achieve population stability or growth.
Because we currently do not know any of these parameters, we are using the presence of juveniles as an important demographic factor influencing resiliency.
Current population size and abundance reflects previous influences on the population and habitat, while reproduction and recruitment reflect population trends that may be stable, increasing, or decreasing in the future.
For example, a large, dense population of Arizona eryngo that contains mostly old individuals may be able to withstand a single stochastic event over the short term, but it is not likely to remain large and dense into the future,
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as there are few young individuals to sustain the population over time. A
population that is less dense but has many young individuals may be likely to grow denser in the future, or such a population may be lost if a single stochastic event affects many seedlings at once. Therefore, the presence of young individuals is an important indicator of population resiliency into the future.
Occupied Area Highly resilient Arizona eryngo populations must occupy cienegas large enough such that stochastic events and environmental fluctuations that affect individual plants or colonies do not eliminate the entire population.
Repopulation through seed dispersal and germination and ramet production within the cienega can allow the population to recover from these events.
Larger functional cienegas are likely to support larger populations of Arizona eryngo and are more likely to provide patches of suitable habitat when small stochastic events and environmental fluctuations occur. For example, during drought years, areas closer to spring seeps and possibly areas with natural depressions i.e., topographic variation may retain more moisture throughout the year than areas farther away from seeps and slightly higher in elevation.
Conversely, during years with heavy rainfall, slightly higher elevation areas may retain moist soils that are not inundated year round, providing suitable habitat for the species.
Areas currently occupied by Arizona eryngo range from about 0.04 hectares 0.1 acre to 0.9 hectares 2.2 acres.
Based on historical and current estimates of cienega size and area occupied by Arizona eryngo, we approximate that a resilient Arizona eryngo population should occupy greater than 1 hectare 2.5 acres within a functional cienega.
Soil Moisture Resilient Arizona eryngo populations need moist to saturated soils year round.
Arizona eryngo has been documented in standing water up to two centimeters to soil that is dry at the surface but saturated several centimeters into the soil Stromberg et al. 2019, pp. 6, 8. It is hypothesized that flowering is determined, in part, by soil moisture availability i.e., plants do not flower in drier conditions when the plants are more stressed and that ramets are produced during drier periods Li 2019, p. 8; Stromberg et al. 2019, p. 8.
Seedling recruitment may be episodic, with greater recruitment success in wetter years. Soils must remain
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