Does Invasive Mammal Exclusion Restore an Ecosystem Function Mediated by Invertebrates?

The removal or exclusion of invasive mammals is a crucial strategy for protecting vulnerable biodiversity, particularly on islands. While eliminating invasive mammals has been shown to increase native species abundance and diversity, it remains unclear whether the ecosystem functions those species provide are also being restored. In particular, the restoration of ecosystem functions performed by invertebrates, such as litter decomposition, pollination and herbivory, is poorly understood. We test whether the removal of invasive mammals restores invertebrate-mediated decomposition by measuring decomposition of a common native tree (<i>Melicytus ramiflorus</i>) within three paired ecosanctuary (all invasive mammals eradicated except mice) and non-ecosanctuary sites across Aotearoa/New Zealand. Results demonstrated that ecosanctuaries had increased decomposition compared to non-ecosanctuaries, but this was dependent on litterbag mesh type. Coarse mesh (which allowed access by some larger meso- and macrofauna) had greater decomposition in ecosanctuaries compared to non-ecosanctuaries, but fine mesh had similar decomposition across both site types. Based on other studies of invertebrate diversity inside and outside ecosanctuaries, we suggest this result is largely due to more diverse and abundant detritivore communities within ecosanctuaries. We also detected fine-scale differences in vegetation composition between ecosanctuaries and non-ecosanctuaries, probably due to the exclusion of invasive browsing mammals from ecosanctuaries, which also may have influenced decomposition. Our findings suggest that the eradication of invasive mammals can restore an important ecosystem function driven by invertebrates, and we recommend that future studies examine whether other critical invertebrate-mediated functions (e.g., predation and pest control, pollination) are also being restored.