Digging into dirt: Rewilding with threatened mammals shapes soil-emerging insect assemblages.

Digging mammals function as ecosystem engineers by altering soil structure, influencing nutrient cycling and shaping vegetation communities. The widespread decline of these taxa globally, driven by habitat loss and introduced predators, has triggered cascading ecological effects, yet the consequences for soil-dwelling insect communities remain poorly understood. Insects, many of which have subterranean larval stages, provide essential functions such as pollination, decomposition and nutrient cycling, making them ideal indicators for evaluating the restructuring of ecological communities following mammal reintroductions. Here, we used data from a long-term experimental mammal exclusion study within a predator-free sanctuary (Scotia Wildlife Sanctuary) in south-eastern Australia to test how the reintroduction of digging mammals affects the structure and composition of insect communities emerging from soil. We sampled insects using emergence traps across replicated plots of mammal reintroduction, exclusion, and procedural control in 2010 and in 2018, 8 years after fence installation. While variation in digging activity (indexed by pit density) did not significantly affect richness or biomass of soil-emerging insect taxa, areas with digging showed reduced insect abundance. Hierarchical modelling of taxa communities revealed that parasitoid wasps (Hymenoptera) and predatory robber flies (Diptera: Asilidae) were strongly associated with plots without digging activity, likely reflecting sensitivity to direct predation or nest disturbance. No herbivorous beetle taxa showed a statistically supported association with treatment, indicating that negative associations with digging activity were restricted to specific parasitoids and predatory groups, rather than representing a consistent trophic-wide response. Our findings therefore indicate that the reintroduction of ecosystem engineers alters insect assemblages, potentially cascading through to multitrophic interactions and ecosystem functioning. This is important because it (1) suggests there may have been profound effects of the widespread loss of ecosystem engineers on ecosystems across the Australian continent; and (2) highlights that whole-of-ecosystem knowledge is critical to getting rewilding right. We emphasise the importance of thorough, long-term ecological monitoring of invertebrate assemblages to inform mammal reintroduction and restoration efforts, ensuring they align with broader ecosystem management objectives.