Double Allee Effects and Extinction in the Island Fox

Abstract

An Allee effect (AE) occurs in populations when individuals suffer a decrease in fitness at low densities. If a fitness component is reduced (component AE), per capita population growth rates may decline as a consequence (demographic AE) and extinction risk is increased. The island fox (Urocyon littoralis) is endemic to six of the eight California Channel Islands. Population crashes have coincided with an increase in predation by Golden Eagles (Aquila chrysaetos). We propose that AEs could render fox populations more sensitive and may be a likely explanation for their sharp decline. We analyzed demographic data collected between 1988 and 2000 to test whether fox density (1) influences survival and reproductive rates; (2) interacts with eagle presence and affects fox fitness parameters; and (3) influences per capita fox population trends. A double component AE simultaneously influenced survival (of adults and pups) and proportion of breeding adult females. The adult survival AE was driven by predation by eagles. These component AEs led to a demographic AE. Multiple-component AEs, a predation-driven AE, and the simultaneous occurrence of both component and demographic AEs in a mammal are all previously unreported processes. Populations below 7 foxes/km(2) could have suboptimal population growth rates due to the demographic AE, and AEs may have contributed to the dramatic declines in three fox populations. Because fox densities in critically endangered populations are well below this level, removing Golden Eagles appears necessary to prevent a predation-driven AE. Conservationists should also be aware of AEs when planning the release of captive foxes. More generally, our findings highlight the danger of overlooking AEs in the conservation of populations of rare or threatened species.

Fatal disease and demographic Allee effect: population persistence and extinction

“…Examples of species that exhibit the Allee effect and suffer from fatal diseases include the endangered African wild dog Lycaon pictus [4,5,8] and the island fox Urocyon littoralis [1,7]. Studies of systems that exhibit the Allee mechanism seem to be focused on the role of the Allee effect at small population densities.…”

“…A strong Allee effect occurs in these models when there is a positive equilibrium density, Allee threshold, and the species goes extinct whenever population densities fall below the threshold [1,[7][8][9][10][11][13][14][15]. The presence of a strong Allee effect in host demographics makes the population vulnerable to extinction as a fatal disease may tip it below the Allee threshold.…”

Roe and fallow deer: are they compatible neighbours?

“…The spatiotemporal variation of densities of wild populations is an important parameter to evaluate their ecological needs (Apps et al 2004) and dynamics (Kruuk et al 1999;Forchammer et al 2002;Angulo et al 2007), as well as their potential impacts on ecosystems (Putman and Moore 1998;Stockton et al 2005). A comparison of densities of species living in sympatry can help in understanding interspecific interactions (Latham et al 1997;Forsyth and Hickling 1998).…”

Incorporating ecological drivers and uncertainty into a demographic population viability analysis for the island fox

“…Fish andWildlife Service 2004). The island fox has been the subject of numerous prior demographic analyses (Roemer et al 2001a, 2002, Angulo et al 2007), including at least two studies that use one or more PVA methods (Roemer et al 2001a, Kohlmann et al 2005. However, none of these prior analyses has been able to use the amount of data available to us to test and refine model structures and parameter estimates, and hence to build as demographically realistic a model for this species as possible.…”