Predation impedes recovery of Sierra Nevada bighorn sheep

Abstract

Translocation of animals into formerly occupied habitat is a key element of the recovery plan for Sierra Nevada bighorn sheep (Ovis canadensis sierrae), which are state (California) and federally listed as endangered. However, implementing Sierra bighorn translocations is a significant conservation challenge because of the small size of the extant population and the limited number of herds available to donate translocation stock. One such herd, the Mt. Langley herd, recently became unusable as a translocation source following a substantial population decline. At the time of listing in 1999, predation by mountain lions (Puma concolor; hereafter lion) was considered a primary threat to Sierra bighorn, and since then lion predation may have continued to limit the ability of source herds to provide translocation stock. We evaluated the relationship between lion predation and ewe survival rates within three source herds of the Southern Recovery Unit, compared lion abundance and ewe survival among years of varying predation levels, provided a range of estimated times for the Mt. Langley herd to recover to its former status as a translocation source, and determined if the rates lions have been removed to mitigate Sierra bighorn predation exceeded sustainable harvest guidelines. We found compelling evidence that lion predation has impeded the recovery of Sierra bighorn by reducing survival rates of adult ewes (and consequently, population growth) and by preying upon individuals that could have otherwise been translocated. Ewe survival was poor during years of extreme predation but even during years of typical predation, survival rates were below a level needed to ensure population growth, indicating that years with little or no lion predation may be necessary for the population to grow and meet recovery goals. Because the intensity of predation was related to lion abundance, monitoring lion populations could provide managers with advance warning of periods of extreme predation. We found that following a period of particularly extreme predation, the Mt. Langley herd decreased in abundance far below the threshold needed to be considered a source of translocation stock, resulting in the loss of approximately 25% of the recovery program’s capacity for translocations. It is unclear how many years it will take for this herd to recover, but management actions to reduce lion predation are likely needed for this herd to grow to a size that can afford to donate individuals to translocation efforts in the near future, even when optimistic growth rates are assumed. We found that lion removal may also be needed to prevent predation from leading to Sierra bighorn population decline. Lion removal rates that have been implemented thus far are well below what would be needed to reduce the abundance the eastern Sierra lion population itself. We recommend continued monitoring of Sierra bighorn and sympatric lions and note that lion removal may be required to facilitate bighorn recovery for the foreseeable future.

Predator avoidance influences selection of neonatal lambing habitat by Sierra Nevada bighorn sheep

“…Prey switching and increased occurrences of mountain lions becoming bighorn specialists can lead to sudden increases in predation rates (Mooring et al 2004, Rominger 2018. Stochasticity in predator occupancy and increases in mountain lion abundance can greatly increase the risk of extinction for small and threatened populations (Wehausen 1996;Johnson et al 2010Johnson et al , 2013Conner et al 2018;Gammons et al 2021). It remains unclear how maternal Sierra Nevada bighorn select habitat to mitigate risk of mortality for themselves and their neonates.…”

“…In addition to being a leading cause of mortality among wild populations of ungulates, predation is a potential threat to long‐term viability for many populations (Bangs et al 2005, Smith et al 2014, Karsch et al 2016, Rominger 2018, Gammons et al 2021). Mountain lions ( Puma concolor ) are the predominant predators of bighorn sheep ( Ovis canadensis ) across subspecies (Hayes et al 2000, Stephenson et al 2012, Johnson et al 2013, Conner et al 2018, Rominger 2018).…”

“…High mountain lion predation has been a leading cause of adult Sierra Nevada bighorn mortality in numerous subpopulations for decades (Wehausen 1996, Stephenson et al 2012, Johnson et al 2013, Conner et al 2018, Gammons et al 2021). Apparent competition between populations of mule deer ( Odocoileus hemionus ) and Sierra Nevada bighorn influences patterns of mountain lion predation on Sierra Nevada bighorn (Johnson et al 2013).…”

Seeing Is Be-Leaving: Perception Informs Migratory Decisions in Sierra Nevada Bighorn Sheep (Ovis canadensis sierrae)

“…Lions consume migratory SNBS as an alternative prey source on common winter range, making lion depredation one of the primary causes of adult mortality in SNBS (Davis et al, 2012;Conner et al, 2018). As an index of predator abundance, we used annual counts of adult lions (≥18 months) obtained following the methods described in Gammons et al (2021). Lion counts were conducted at a recovery-unit scale, which encompasses multiple herd units.…”

“…Group membership also reduces predation risk through confusion and dilution effects (Pulliam, 1973;Krause and Ruxton, 2002). While there is limited predation risk on high-elevation range during the winter, mountain lion predation events on lowelevation range are the greatest source of adult mortality for SNBS (US Fish and Wildlife Service, 2007;Johnson et al, 2013;Gammons et al, 2021). Migration to low-elevation range may become safer as the number of migrants increases for which proportion of migrants serves as a proxy.…”