Density-impact functions for invasive house mouse (Mus musculus) effects on indigenous lizards and invertebrates

House mice are among the most widely distributed mammals in the world, and adversely affect a wide range of indigenous biota. Suppressing mouse populations, however, is difficult and expensive. Cost-effective suppression requires knowing how low to reduce mouse numbers to achieve biodiversity outcomes, but these targets are usually unknown or not based on evidence. We derived density-impact functions (DIFs) for mice and small indigenous fauna in a tussock grass/shrubland ecosystem. We related two indices of mouse abundance to five indices of indigenous lizard and invertebrate abundance measured inside and outside mammal-resistant fences. Eight of 22 DIFs were significantly non-linear, with positive responses of skinks (Oligosoma maccanni, O. polychroma) and ground wētā (Hemiandrus spp.) only where mice were not detected or scarce (< 5% footprint tunnel tracking rate or printing rate based on footprint density). Kōrero geckos (Woodworthia spp.) were rarely detected where mice were present. A further 9 DIFs were not differentiated from null models, but patterns were consistent with impacts at 5% mouse abundance. This study suggests that unless mouse control programmes commit to very low abundances, they risk little return for effort. Impact studies of invasive house mice are largely restricted to island ecosystems. Studies need to be extended to other ecosystems and species to confirm the universality or otherwise of these highly non-linear DIFs.

Low structural complexity of nonnative grassland habitat exposes prey to higher predation

The structural complexity of vegetation can have profound effects on the hunting efficiency of predators, thereby affecting their intake rate of prey. While studies have shown that vegetation complexity can play an important role in managing unwanted impacts of predators, it is less clear how structural complexity of invasive vegetation affects the vulnerability of terrestrial prey. Short nonnative pasture species bred for agricultural production, for example, are highly invasive and pervade grassland ecosystems worldwide. They generally have low structural complexity compared with taller native vegetation they often displace. We conducted controlled experiments to test whether nonnative pastures expose fauna to greater predation risk. Survival of invertebrates (tethered locusts) subject to predation by invasive mammalian insectivores (European hedgehogs) in nonnative pasture (0.10 per 24 h; 95% CI, 0.08-0.13) was less than one-half that in structurally complex native perennial tussock (bunch) grass (0.24; 95% binomial CI, 0.18-0.31). A significant positive relationship was apparent between structural complexity (grass dry stem density) surrounding each locust and their survival. In a second experiment, survival of locusts placed solely in tussock increased with decreasing locust density in tussock, presumably reflecting fewer resource-rich patches on which predators could focus. These results demonstrate that invasion by structurally simple nonnative vegetation exposes prey to greater risk of predation. This is concerning from a global nature conservation perspective given that conversion of nearly one-half of the world's temperate grasslands to agriculture includes a range of invasive, structurally simple, nonnative, plant species. Minimizing invasion and maintaining and restoring complex habitat structure may be a useful conservation option for reducing unwanted predation.