Extinction vortex dynamics of top predators isolated by urbanization

Abstract

Extinction risk is elevated in small, isolated populations due to demographic and genetic interactions. Therefore, it is critical to model these processes realistically in population viability analyses (PVA) to inform local management and contribute to a greater understanding of mechanisms within the extinction vortex. We conducted PVA's for two small mountain lion populations isolated by urbanization in southern California to predict population growth, extinction probability, and loss of genetic diversity with empirical data. Specifically, we (1) provide the first PVA for isolated mountain lions in the Santa Ana Mountains (SAM) that considers both demographic and genetic risk factors and (2) test the hypothesis that variation in abundance and mortality between the SAM and Santa Monica Mountains (SMM) result in differences in population growth, loss of heterozygosity, and extinction probability. Our models predicted 16–21% probability of local extinction in the SAM due purely to demographic processes over 50 yr with current low levels or no immigration. Our models also predicted that genetic diversity will further erode in the SAM such that concern regarding inbreeding depression is warranted unless gene flow is increased, and that if inbreeding depression occurs, rapid local extinction will be highly likely. Dynamics of the two populations were broadly similar, but they also exhibited differences driven by larger population size and higher mortality in the SAM. Density‐independent scenarios predicted a rapidly increasing population in the SMM, whereas growth potential did not differ from a stable trend in the SAM. Demographic extinction probability and loss of heterozygosity were greater in the SMM for density‐dependent scenarios without immigration. However, higher levels of immigration had stronger, positive influences on both demographic viability and retention of genetic diversity in the SMM driven by lower abundance and higher adult survival. Our results elucidate demographic and genetic threats to small populations within the extinction vortex, and how these vary relative to demographic structure. Importantly, simulating seemingly attainable increases in connectivity was sufficient to greatly reduce extinction probability. Our work highlights that conservation of large carnivores is achievable within urbanized landscapes, but requires land protection, connectivity, and strategies to promote coexistence with humans.

Models for Eco-evolutionary Extinction Vortices and their Detection

“…Furthermore, inbreeding depression is often included in population viability analyses, e.g. in a recent study on mountain lions by Benson et al (2019) and many studies using the software VORTEX (Lacy 1993). For mutation accumulation, eco-evolutionary models have been developed to show how time to extinction depends on the population’s carrying capacity (Lynch and Gabriel 1990) and to quantify the strength of the mutational meltdown compared to a scenario without eco-evolutionary feedbacks (Coron et al 2013).…”

Mountain lions, minimum viable populations, and intact ecosystems: a cautionary note

“…To the Editor, I read with interest the paper by Benson et al (2019), wherein the authors noted that conserving mountain lions in the Santa Monica and Santa Ana mountains, both located in the greater Los Angeles area of California, USA, is feasible. The authors further explained that such an accomplishment would be an important step toward maintaining, ".…”

“…The contention that maintaining viable populations of mountain lions in those small geographic areas will yield intact and naturally functioning ecosystems, however, must be interpreted with care. I offer these comments not as a criticism, but as a cautionary note in the spirit of reducing the likelihood that others will misinterpret Benson et al (2019).…”

“…Each biotic or abiotic component plays a role in any ecosystem but, in the absence of some components, the system cannot be considered intact. Benson et al (2019) did not note the extirpation of the grizzly bear (Ursus arctos) from the coastal plains and foothills or mountains surrounding the Los Angeles Basin, which would have included the Santa Monica and Santa Ana mountains (Storer and Tevis 1955), or the near extinction of the California Condor (Gymnogyps californianus), which formerly ranged across the American southwest, including those mountain ranges (Koford 1953, Snyder andSnyder 2005), but now is restricted severely in numbers and distribution (Johnson et al 2010, Cogan et al 2012.…”

“…The role of suppression in altering fire regimes (Heinselman 1981), and subsequent vegetation succession or shifts in composition in the Santa Monica and Santa Ana mountains and proximate ranges also has affected the population dynamics of mule deer, the primary prey of mountain lions in southern California (Holl et Holl and Bleich 2010). Additionally, urbanization and infrastructure development surrounding the Santa Monica and Santa Ana mountains have all but eliminated movements by mountain lions into or out of those areas (Riley et al 2014, Vickers et al 2015, which was the primary impetus for the detailed modeling of Benson et al (2019).…”