Solving the puzzle of Pringleophaga – threatened, keystone detritivores in the sub‐Antarctic

In the globally significant, lowland terrestrial systems of the sub‐Antarctic's South Indian Ocean Province Islands, caterpillars of the flightless moth genus Pringleophaga (Lepidoptera: Tineidae) are typically responsible for much nutrient turnover. On Marion Island, Pringleophaga marioni is a keystone species for this reason. Rising temperatures have led to increasing populations of introduced house mice, which, in turn, feed extensively on Pringleophaga caterpillars. Because of the caterpillars’ keystone role, predation by mice is leading to changes in the functioning of the terrestrial system. Given the estimates of an extended life cycle duration for P. marioni (and its congeners), that is, two to more than 5 years, an ongoing puzzle is why the species has not shown greater population declines on Marion Island than have been recorded (in some habitats 40–97% in 20 years), given extremely high estimates of predation ( c . 1% of standing biomass per day). One reason may be inaccurate previous estimates of life cycle duration for the species. Here, we provide a new, quantitative estimate, by rearing caterpillars at different temperature regimes (5, 10, 15 and 5–15 °C), and combining these results with additional data from a prior study, to demonstrate that the life cycle duration for this species is approximately 1 year – half the previous minimum estimate. The new quantitative data provide grounds for improved models for estimating population persistence of this species, and information for models assessing the costs and benefits of conservation interventions such as the eradication of invasive house mice.