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Double Allee Effects and Extinction in the Island Fox

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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/km2 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.
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Keywords: Allee effect; Aquila chrysaetos; Golden Eagle; Urocyon littoralis; denso dependencia inversa; denso dependencia positiva; efecto Allee; efecto Allee debido a la depredación; inverse density dependence; island fox; positive density dependence; predation-driven Allee effect; águila dorada

Document Type: Research Article

Affiliations: 1: Department of Fishery and Wildlife Sciences, New Mexico State University, P.O. Box 30003, MSC 4901, Las Cruces, NM 88003-8003, U.S.A. 2: Department of Theoretical Ecology, Institute of Entomology, Biology Centre ASCR, Branišovská 31, 37005 České Budějovice, Czech Republic 3: School of Ocean Sciences, University of Wales, Bangor, Menai Bridge, Anglesey, LL59 5AB, United Kingdom 4: Ecologie, Systématique et Evolution, UMR CNRS 8079, University of Paris-Sud, Orsay Cedex 91405, France

Publication date: 01 August 2007

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