Abstract
Exotic predators are a major threat to native wildlife in many parts of the world. Developing and implementing effective strategies to mitigate their effects requires robust quantitative data so that management can be evidence-based, yet in many ecosystems this is missing. Birds in particular have been severely impacted by exotic mammalian predators, and a plethora of studies on islands record predation of bird eggs, fledglings and adults by exotic species such as rodents, stoats and cats. By comparison, few studies have examined nest predation around mainland urban centres which often act as dispersal hubs, especially for commensal species such as rodents. Here, we experimentally examine nest predation rates in habitat patches with varying black rat (Rattus rattus) densities in Sydney, Australia and test whether these exotic rats have the effects expected of exotic predators using effect size benchmarks. In the case where black rats have replaced native Rattus spp., we expected that black rats, being more arboreal than native Rattus spp., would be a significant source of predation on birds because they can readily access the arboreal niche where many birds nest. We tested this idea using above-ground artificial nests to represent those of typical small bird species such as the New Holland honeyeater (Phylidonyris novaehollandiae). We found that fewer eggs were depredated by rodents on sites where we removed black rats compared to unmanipulated sites, and that the effect size calculated from the total number of eggs surviving beyond the typical incubation period was similar to that expected for an exotic predator. Our results suggest that, although Australian birds have co-evolved with native Rattus species, in the case where black rats have replaced native Rattus species, exotic black rats appear to pose an additive source of predation on birds in remnant habitats, most likely due to their ability to climb more efficiently than their native counterparts. Management of these commensal rodents may be necessary to retain urban birdlife.
Highlights
Exotic species have devastating impacts on wildlife around the globe [1,2,3], with island avifauna having suffered severe declines and extinctions upon the arrival of exoticPLOS ONE | DOI:10.1371/journal.pone.0156180 June 13, 2016Nest Predation by Commensal Rodents predators (e.g. [4, 5]).Nest predation is a major source of reproductive failure in birds [6,7,8,9] and provides strong selective pressure that appears to have considerably shaped bird behaviour and life histories (e.g. [10])
This study is the first to quantify the predatory impacts of black rats on arboreal nesting birds in Australia in a system where black rats have replaced native rodent counterparts
Consistent with our predictions, black rats contributed significantly to nest attacks, and appear likely to contribute an additive source of nest mortality for small birds in urban habitat remnants
Summary
Exotic species have devastating impacts on wildlife around the globe [1,2,3], with island avifauna having suffered severe declines and extinctions upon the arrival of exoticPLOS ONE | DOI:10.1371/journal.pone.0156180 June 13, 2016Nest Predation by Commensal Rodents predators (e.g. [4, 5]).Nest predation is a major source of reproductive failure in birds [6,7,8,9] and provides strong selective pressure that appears to have considerably shaped bird behaviour and life histories (e.g. [10]). Exotic predators often represent functionally novel predator-types, and the predation pressure they exert can lead to the decline or even extinction of susceptible bird species [11]. Exotic mammalian predators such as mustelids, rodents and cats in particular are primary drivers of bird extinctions on islands [4], whereas in more urban regions, other factors such as habitat destruction or fragmentation, and changes to habitat complexity, can cause declines in bird species, as well as exacerbate the threat of nest predation from native and exotic predators [12]. Commensal rodents are among the species that effectively exploit urban resources [21] and, since island birds often suffer severe predation on nests and fledglings from exotic rodents, we could expect that birds inhabiting remnant urban habitats are susceptible to predation from commensal rodents
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