Abstract
BackgroundInvasive species are recognized as a primary driver of native species endangerment and their removal is often a key component of a conservation strategy. Removing invasive species is not always a straightforward task, however, especially when they interact with other species in complex ways to negatively influence native species. Because unintended consequences may arise if all invasive species cannot be removed simultaneously, the order of their removal is of paramount importance to ecological restoration. In the mid-1990s, three subspecies of the island fox Urocyon littoralis were driven to near extinction on the northern California Channel Islands owing to heightened predation by golden eagles Aquila chrysaetos. Eagles were lured to the islands by an abundant supply of feral pigs Sus scrofa and through the process of apparent competition pigs indirectly facilitated the decline in foxes. As a consequence, both pigs and eagles had to be removed to recover the critically endangered fox. Complete removal of pigs was problematic: removing pigs first could force eagles to concentrate on the remaining foxes, increasing their probability of extinction. Removing eagles first was difficult: eagles are not easily captured and lethal removal was politically distasteful.Methodology/Principal FindingsUsing prey remains collected from eagle nests both before and after the eradication of pigs, we show that one pair of eagles that eluded capture did indeed focus more on foxes. These results support the premise that if the threat of eagle predation had not been mitigated prior to pig removal, fox extinction would have been a more likely outcome.Conclusions/SignificanceIf complete eradication of all interacting invasive species is not possible, the order in which they are removed requires careful consideration. If overlooked, unexpected consequences may result that could impede restoration.
Highlights
Invasive alien species are considered one of the most significant threats to biodiversity and on islands their impacts have been grave [1,2,3]
Functional frameworks based on ecological principles can be used to presage unexpected consequences of removing invasive species, and pre-eradication data and/or ecological modeling coupled with continuous evaluation of program goals can further our knowledge of best practices for ecological restoration [6]
The Laguna Pair had food habits that differed from the other nesting eagles (G = 16.7, d.f. = 4, P,0.005) primarily because they rarely preyed upon birds, consumed more foxes and relied more heavily on feral pigs
Summary
Invasive alien species are considered one of the most significant threats to biodiversity and on islands their impacts have been grave [1,2,3]. We were afforded a unique opportunity to assess how the order of removal of invasive species on the California Channel Islands could have influenced the recovery of the critically endangered island fox Urocyon littoralis. Eagles were lured to the islands by an abundant supply of feral pigs Sus scrofa and through the process of apparent competition pigs indirectly facilitated the decline in foxes. As a consequence, both pigs and eagles had to be removed to recover the critically endangered fox. Removing eagles first was difficult: eagles are not captured and lethal removal was politically distasteful
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