Arboreal activity of invasive rodents: conservation implications for the control of an island pest

Context Invasive rodents pose a substantial threat to biodiversity and are a leading cause of species decline and extinction on islands. Population suppression using ground-based methods is common practice, though arboreal behaviour of rodents may render control programs less effective.Aims We aimed to quantify species-specific invasive rodent activity (Rattus rattus, R. exulans, Mus musculus) across three forest strata to determine the prevalence of arboreal rodent behaviour in a system that has undergone extensive long-term rodent baiting, and therefore assess the suitability of solely ground-based baiting in this system.Methods We calculated rodent presence at the ground, mid-storey, and canopy using three detection methods (camera traps, chew cards and tracking tunnels) deployed for 30-day periods across three structurally distinct forest types (canopy heights ranged from 3.5 to 16.7 m). We developed continuous rodent activity indices for each method, which we paired with density estimates. Strata-specific species composition was determined using camera trap images.Key results Rodent presence was recorded equally across all strata, with R. rattus dominating above-ground strata. Rodent densities differed significantly between forest types, which was largely consistent with activity indices.Conclusions Our findings offer an additional explanation for reduced efficacy of long-term ground-based control programs: arboreal behaviour may exacerbate the reduction in efficacy often associated with long-term control.Implications Effective management of invasive rodent species on islands is a global conservation challenge. Our findings suggest above-ground control may be required in some long-term suppression programs or eradication campaigns, particularly in the presence of the black rat. Introduced rodents are highly destructive island invaders the world over. Control programs are usually ground-based, however our study found that black rats (Rattus rattus) dominated the forest mid-storey and canopy, casting doubt on the effectiveness of solely ground-based programs. Including above-ground control in the conservationist's rodent control arsenal may improve outcomes for island biodiversity.