Bottom‐up effect of eradications: The unintended consequences for top‐order predators when eradicating invasive prey

Abstract

The eradication of invasive species from islands yields significant conservation returns. However, novel challenges continue to arise as projects expand in their scope, complexity and scale. Prey‐loss and secondary poisoning were historically considered to have limited impact on native top‐order predators when planning eradications, but this has rarely been tested quantitatively. We used a 10‐year timeseries of Brown Skua Stercorarius antarcticus lonnbergi breeding surveys and isotopic dietary analysis on Macquarie Island to investigate how prey‐loss and secondary poisoning deaths resulting from the eradication of an abundant invasive prey species, European rabbits Oryctolagus cuniculus, affected a top‐order predator. Skua nest density declined from 7.14 nests/km2 (95% CI: 6.01–8.27) in the presence of rabbits (pre‐eradication) to 3.73 nests/km2 (95% CI: 2.96–4.51) in the first 3 years after the eradication of rabbits, before showing signs of recovery in the 4 years thereafter. However, breeding success dropped from 1.01 chicks/nest (95% CI: 0.76–1.26) to as low as 0.38 chicks/nest (95% CI: 0.23–0.53) with little evidence of recovery. Secondary poisoning affected a greater number of skuas than anticipated prior to the eradication, including skuas nesting in areas where rabbits were not typically hunted as prey. We highlight that invasive prey often replace native prey in the diet of native predators rather than provide an additional source of food, and rapid eradication of non‐native prey can have long‐term impacts for predators, particularly when recovery of native prey is slow. Synthesis and applications. Monitoring programmes that complement large‐scale eradication projects and address (a) trophic‐driven declines in predator populations and (b) population‐level impacts of secondary poisoning are integral to ensuring bottom‐up effects of eradications are anticipated and adequately quantified. If prey deficits caused by eradication of invasive prey are expected to be severe but short‐lived, supplementary feeding programmes may buffer against increased predation pressure on native prey and reduced breeding success of native predators. Alternatively, if the rapid recovery of native prey is not expected to occur naturally, breeding programmes and translocation of native prey prior to assist recovery of native predators should be considered to support ecosystem restoration.

Nesting Success and Nesting Height in the Critically Endangered Medium Tree Finch (Camarhynchus pauper)

“…The Galapagos National Park Directorate with support of Island Conservation is planning to eradicate invasive black rats and house mice (Mus musculus) from Floreana Island using a rodent bait formulation (Conservation 25D), a cereal pellet bait containing the rodenticide brodifacoum in 2023 [81]. Planning for non-target mortality is a cornerstone risk management component of eradication projects [82], and the mitigation plan includes a safeguard population (i.e., a small genetically representative number from different island zones) for all Darwin's Finch species and also Short-eared Owls held in captivity until traces of poison have disappeared from the environment [83,84]. Invasive rats have been eradicated from 447 islands [2,85] and, in general, the local flora and fauna have shown a rapid recovery response [81,[86][87][88][89][90][91][92].…”

The EICAT+ framework enables classification of positive impacts of alien taxa on native biodiversity

“…More generally, an EICAT+ assessment will provide stakeholders (including managers, policymakers, and the public) with evidence-based information that can guide the implementation of adequate countermeasures and avoid or minimize undesirable outcomes in 2 principal ways. First, the benefits of removing a certain deleterious alien species can be compared to the costs that the removal imposes on other native species [ 49 , 54 , 93 , 94 ]. The necessity of conducting such a cost–benefit analysis has been recently stressed by Rees and colleagues [ 95 ], who suggested that the choice of removing dingoes ( Canis lupus dingo ) in some parts of Australia should be assessed case by case, as these apex predators increase the abundance of native small mammal species and a native owl species by suppressing 2 alien mesopredators ( Vulpes vulpes and F .…”

“…2) Informing predictions of indirect, and potentially adverse, effects of alien taxa management Improving the understanding of the positive impacts that an alien taxon causes on native biodiversity attributes can help to identify possible undesired effects that its removal might have on native communities [47]. For instance, the removal of aliens may release herbivorous pests or meso-predators controlled by alien predators [48], decrease the density of predators feeding on alien preys [49], or reduce the availability of predator-free refugia provided by an alien plant and exploited by endangered species [50].…”

Hanging out at the club: Breeding status and territoriality affect individual space use, multi‐species overlap and pathogen transmission risk at a seabird colony

“…Our study suggests that environmental changes that increase the number of nonbreeding skuas would also increase interspecific interactions, with potentially deleterious effects on other seabird species of conservation concern. For example, ongoing or planned eradications of introduced rodents in mixed-species seabird colonies such as Amsterdam Island could reduce supplemental food availability and limit the number of nonbreeding skuas the island can support; however, it could also increase predation pressure on other seabirds (Travers et al, 2021), with potentially conflicting effects on pathogen circulation.…”