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Federal Register / Vol. 86, No. 185 / Tuesday, September 28, 2021 / Proposed Rules
fragments that happen to lodge in suitable niches. These mats can expand to occupy new habitats while the portion that established earlier dies. An individual remains alive as long as old stems die no faster than new stems develop. The same individual could migrate back and forth through available habitats for an unlimited period of time, and it is not inconceivable that the individuals we see today arose from spores that germinated many thousands of years ago. For the species to persist, the recruitment of new individuals must equal or exceed mortality.
Wyatt and Stoneburner 1980, pp.
519520 estimated that the species range may have been more extensive 10,000 years ago, and subsequently became restricted to this single location as the climate warmed and other springs periodically stopped flowing. To assess the climate changes that could affect this species into the future, we examined the climate parameters using both the representative concentration pathway RCP 4.5 and RCP 8.5
scenarios to provide a range of projected values. These models predict that by 2074 climate changes could result in a reduction of aquifer recharge and an increased duration and severity of droughts and heavy rainfall, thereby increasing the threats of interrupted spring flows and flash floods. Annual precipitation is highly variable in central Texas, and severe, multi-year droughts occurred during the 1950s and from 2006 through 2012. During these historical periods of drought, only the largest springs along the South Llano River, including Seven Hundred Springs, continued flowing, but at lower rates. Prolonged drought in combination with increased pumping from the Edwards-Trinity aquifer could increase the probability of interrupted flows of these springs and, consequently, the extirpation or extinction of the South Llano Springs moss. Despite the frequency of prolonged drought, the region is also subject to extremely heavy rainfall, often resulting from tropical storms in the Gulf of Mexico as well as the Pacific Ocean. All of these factors contribute to flash floods high intensity, low duration floods that can drastically change stream beds and the surrounding vegetation, potentially scouring the South Llano Springs moss from its rock substrate along the edge of the stream, or burying it beneath deposits of silt, sand, and gravel.
The amount of pumping from the Edwards-Trinity aquifer is one of the most important factors influencing storage in the aquifer and spring flows.
Aquifer water levels are stable or have declined slightly over most of the
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Edwards-Trinity aquifer, but in some areas, heavy pumping has led to longterm declines in aquifer levels and diminished or interrupted spring flows George et al. 2011, p. 35; Region F
Water Planning Group 2015, pp. 134, 315; Plateau Region Water Planning Group 2016, pp. 711. These sources project relatively little growth in the human population in Edwards and Kimble Counties during the next 50
years. Conversely, population growth is projected to increase for five central Texas counties, which include the metropolitan areas of San Antonio, New Braunfels, San Marcos, Austin, Round Rock, and Georgetown, by 32 percent between 2017 and 2037, and by 53
percent between 2017 and 2050 Texas Demographic Center 2017, p. 1. It is reasonably foreseeable that increased pumping may occur from the EdwardsTrinity aquifer for transfer to other regions to supply increased municipal water demands. This increased pumping could reduce water storage in the Edwards-Trinity aquifer and spring flows in the South Llano River. Loss of spring flows, even for a short time, would likely reduce or extirpate the only known remaining population of the South Llano Springs moss because the species requires constant immersion in flowing spring water to persist.
The Upper Llano River Watershed Protection Plan Broad et al. 2016, pp.
51, 6466, 86 identifies increased runoff, evapotranspiration, and sediment loading as impacts to the upper Llano River watersheds due to the encroachment of woody species.
Recharge into the Edwards-Trinity aquifer in Edwards County has been reduced during prior periods of vegetation loss from overgrazing, resulting in increased runoff and the drying of some smaller springs Brune 1981, p. 173. Aquifer recharge may also have been reduced by the encroachment of brush into formerly grass-dominated uplands South Llano Watershed Alliance 2012, p. 9; Broad et al. 2016, pp. 4041, 51. Aquifer recharge would also be reduced by an increase in evapotranspiration, due to increased temperatures.
Small populations are less able to recover from losses caused by random fluctuations in recruitment demographic stochasticity or variations in spring outflow environmental stochasticity Service 2015, p. 12. In addition to population size, it is likely that population density also influences population viability, as sexual reproduction, if it occurs at all in the species current situation, requires male and female mosses to be in close proximity. Small, reproductively
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isolated populations are also susceptible to the loss of genetic diversity, to genetic drift, and to inbreeding Barrett and Kohn 1991, pp. 330. The loss of genetic diversity may reduce the ability of a species or population to resist pathogens and parasites, to adapt to changing environmental conditions, or to colonize new habitats. The combined demographic and genetic consequences of small population sizes may reduce population recruitment, leading to even smaller populations and greater isolation, and further decreasing the viability of the species. These factors may already have contributed to the decline of the South Llano Springs moss to its current state of extreme endemism in the upper South Llano River. All of the above stressors are exacerbated by the fact that the South Llano Springs moss likely consists of only one, small population.
We note that, by using the SSA
framework to guide our analysis of the scientific information documented in the SSA report, we have not only analyzed individual effects on the species, but we have also analyzed their potential cumulative effects. We incorporate the cumulative effects into our SSA analysis when we characterize the current and future condition of the species. Our assessment of the current and future conditions encompasses and incorporates the threats individually and cumulatively. Our current and future condition assessment is iterative because it accumulates and evaluates the effects of all the factors that may be influencing the species, including threats and conservation efforts.
Because the SSA framework considers not just the presence of the factors, but to what degree they collectively influence risk to the entire species, our assessment integrates the cumulative effects of the factors and replaces a standalone cumulative effects analysis.
Determination of Status for the South Llano Springs Moss 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 endangered species or 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
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