Positive feedback between invasive species in a sub‐Antarctic ecosystem

Invasional meltdown, where interactions between non‐native species amplify their community‐level impacts, is often evidenced through relatively weak cases in which one invader directly benefits another. However, stronger evidence for invasional meltdown – involving reciprocal benefits with effects at the community level – is rarely reported, particularly in extreme environments. Here, we examine inter‐trophic facilitation, encompassing direct and indirect effects, in a sub‐Antarctic ecosystem on South Georgia Island. A previous study showed that the invasive beetle Trechisibus antarcticus preys on the larvae of the native beetle Hydromedion sparsutum , which feed on the invasive grass Poa annua . Building on this trophic link, we hypothesised that P. annua directly enhances the performance of T. antarcticus by ameliorating microclimatic conditions and reducing physiological stress. Our results show that, relative to the native grass Poa flabellata , P. annua increases the weight and size of T. antarcticus individuals and diminishes their synthesis of heat shock proteins. This direct facilitation provides empirical support for a plausible mutual facilitation between P. annua and T. antarcticus , consistent with processes that may contribute to community‐level invasional meltdown. To our knowledge, this is the first study to document such dynamics in a sub‐Antarctic ecosystem. These findings underscore how cascading inter‐trophic interactions might drive invasion success in ecologically sensitive and climatically challenging systems.