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Federal Register / Vol. 86, No. 243 / Wednesday, December 22, 2021 / Proposed Rules
Karmalkar et al. 2011, entire; Bagne and Finch 2012, entire; Coe et al. 2012, entire; and Jiang and Yang 2012, entire.
Climate influences pygmy-owl habitat conditions and availability through the loss of vegetation cover, reduced prey availability, increased predation, reduced nest site availability, and vegetation community change. The majority of the current range of the pygmy-owl occurs in tropical or subtropical vegetation communities, which may be reduced in coverage if climate change results in hotter, more arid conditions. Additionally, models predict that the distribution of suitable habitat for saguaros, the primary pygmyowl nesting substrate within the Sonoran Desert ecoregion, will substantially decrease over the next 50
years under a moderate climate change scenario Weiss and Overpeck 2005, p.
2074; Thomas et al. 2012, p. 43.
Climate change scenarios project that drought will occur more frequently and increase in severity, with a decrease in the frequency and increase in severity of precipitation events Seager et al. 2007, p. 9; Cook et al. 2015, p. 6; Pascale et al. 2017, p. 806; Williams et al. 2020, p.
317. Drought and changes to the timing and intensity of precipitation events may reduce available cover and prey for pygmy-owls adjacent to riparian areas through scouring flood events and reduced moisture retention. Although the extent to which changing climatic patterns will affect the pygmy-owl is better understood following the past decade of observations in the field, there remains uncertainty with regard to the overall extent and timing of impacts.
Synergistic interactions are likely to occur between the effects of climate change and habitat fragmentation and loss. Climate change projections indicate that conditions will likely favor increased occurrence and distribution of nonnative, invasive species and alteration of historical fire regimes.
Climate change may also affect the viability of the pygmy-owl through precipitation-driven changes in plant and insect biomass, which in turn influence abundance of lizards, small mammals, and birds Jones 1981, p. 111;
Flesch 2008, p. 5; Flesch et al. 2015, p.
26. Decreased precipitation generally reduces plant cover and insect productivity, which in turn reduce the abundance and availability of pygmyowl prey species. Similarly, increased temperatures reduce pygmy-owl prey activity due to increased energetic demands of thermoregulation and a decreased availability of prey and cover Flesch et al. 2015, p. 26. These indirect effects on prey availability and direct
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effects on prey activity affect nestling growth, development, and survival.
When decreased precipitation affects food supply and increased temperature affects prey activity, reduced pygmyowl productivity is likely to result in reduced pygmy-owl resiliency Flesch et al. 2015, p. 26. Climate change can also influence natural events, such as hurricanes and tropical storms, which can modify and fragment habitats, primarily through loss of woody cover.
Historical and ongoing threats to the pygmy-owl from habitat loss and fragmentation as well as from climate change and climate conditions, have shaped the current habitat and population conditions of the subspecies throughout its range.
Current Condition To assess resiliency, we evaluated six components that broadly related to the subspecies population demography or physical environment and for which we had data sufficient to conduct the analysis. We assessed each analysis units physical environment by examining three components determined to have the most influence on the subspecies: Habitat intactness, prey availability, and vegetation health and cover. We also assessed each analysis units demography through abundance, occupancy, and evidence of reproduction. We established parameters for each component by evaluating the range of existing data and separating those data into categories based on our understanding of the subspecies demographics and habitat.
Using the demographic and habitat parameters, we then categorized the overall condition of each analysis unit.
We provide a summary of each of the six factors below and describe them in detail in the SSA report Service 2021, entire.
Demographic Factors Abundance: Larger populations have a lower risk of extinction than smaller populations Pimm et al. 1988, pp. 773
775; Trombulak et al. 2004, p. 1183. In contrast, 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 Trombulak et al. 2004, p.
1183. Small populations may experience increased inbreeding, loss of genetic variation, and ultimately a decreased potential to adapt to environmental change Trombulak et al.
2004, p. 1183; Harmon and Braude 2010, p. 125; Benson et al. 2016, pp. 1
2. The abundance of pygmy-owls within each analysis unit must be high
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enough to support persistence of pygmy-owl population groups multiple breeding pairs of pygmy-owls within relatively discrete geographic areas within the analysis unit. This is accomplished by having adequate patches of habitat to support multiple nesting pairs of pygmy-owls and their offspring, have adequate habitat connectivity to support establishment of additional territories by dispersing young, and supply floaters unpaired individuals of breeding age within each pygmy-owl population group to offset loss of breeding adults and to provide potential mates for dispersing juveniles.
Occupancy: Sufficiently resilient pygmy-owl populations must occupy large enough areas such that stochastic events and environmental fluctuations that affect individual pygmy-owls, or population group of pygmy-owls, do not eliminate the entire population. Pygmyowls are patchily distributed across the landscape in population groups of nesting owls. Each of these population groups must be occupied by large enough numbers of pygmy-owls to enable the population group to persist on the landscape over time. Enough occupied population groups of pygmyowls must also exist on the landscape, with interconnected habitat supporting movement among population groups, so that each population group can receive or exchange individuals with any given adjacent population group.
Pygmy-owl occupancy is an indicator of habitat conditions as well as demographic factors, such as reproduction and survival. Habitats that support large numbers of pygmy-owls are better able to provide floaters and available mates to dispersing pygmyowls from adjacent populations. These floaters are able to serve as replacement breeders if either or both members of an existing breeding pair are lost.
Observations indicate that if a site is occupied by a breeding pair, they will breed. Survival of adults also affects occupancy, as some occupied sites will be abandoned if one of the adult breeders perishes. These sites can be reoccupied in the future when floaters or dispersing birds move into the area.
Evidence of reproduction: Resilient pygmy-owl populations must also reproduce and produce a sufficient number of young such that recruitment equals or exceeds mortality. 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. Adequately resilient populations of the pygmy-owl must have sufficient numbers of
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