Increases in invertebrate abundance and shifts in assemblage composition following rodent eradication on Lord Howe Island

Abstract

Invasive rodents represent one of the most serious threats to island ecosystems, affecting a wide range of native plants, vertebrates and invertebrates. While nearly ubiquitous on human-modified islands, the last four decades have seen the advent of targeted rodent eradications, which have generally resulted in positive impacts for biodiversity. Invertebrates, which are crucial to the functioning of island ecosystems, are known to be negatively impacted by rodents, but their response to rodent removal is less well understood. The largest rodent eradication on an inhabited island was undertaken in 2019 on Australia’s Lord Howe Island, which successfully extirpated black rats ( Rattus rattus ) and house mice ( Mus musculus ) more than a century after their introduction. To examine the impacts of rodents on invertebrates on Lord Howe Island, we collected arboreal and terrestrial species and identified them to order. Samples were taken from 20 sites across the island’s two main soil types, collected in two annual cycles pre- and post-eradication, respectively. Total invertebrate abundance increased after the eradication of rodents, alongside substantial shifts in assemblage composition, however ordinal diversity slightly decreased, albeit with strong seasonal variation. Orders with large increases in abundance included Isopoda and Blattodea, while the abundance of Coleoptera and Polydesmida did not change. In addition, the abundance of large invertebrates, which are presumably subject to stronger rat predation, rose dramatically following rodent eradication. While we cannot exclude an effect of extraneous environmental changes, our results suggest an ecological rearrangement following the relaxation of predation pressure and augment documented evidence of improved biodiversity outcomes for forest tree species, seabirds and land birds.

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