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

Abstract

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.

Total response models as a conceptual management framework for conserving vulnerable secondary prey

“…For some prey species, any degree of predator reduction may have some benefit (i.e., linear relationships), while for some vulnerable prey species there may be no response unless predator densities are reduced to very low levels. The empirical evidence for most secondary prey species supports the latter (Figure 2)-positive responses generally occur below threshold predator densities, but note that some prey populations show little or no response even at low predator densities (see Figure 2 in Binny et al, 2020;Figure 5 in Carpenter et al, 2021; Figure 2 to Figure 5 in Norbury et al, 2022;Figure 3 in Spencer et al, 2017). There are many explanations for this (see Doherty & Ritchie, 2017), including compensatory effects of other pest predators (e.g., Courchamp et al, 1999a;Norbury et al, 2013), or persistence of factors that are more limiting than predation, such as inadequate food supply or shelter (e.g., Fischer et al, 2020;Lavers et al, 2010).…”

“…In these cases, prey can potentially increase to density "c" or "c*" but note that densities below "b" or "b*" can still decline to extinction (in the case of Figure 3g) or to a low-density domain "a" (in the case of Figure 3i). These predictions are evident in the empirical predator density-impact functions for birds, lizards, and invertebrates in Figure 2-positive prey outcomes occur only when predator densities are reduced below a low critical level, but below this level some prey populations still fail to respond (as shown in Binny et al, 2020;Carpenter et al, 2021;Norbury et al, 2022;Spencer et al, 2017). While there are many potential mechanisms for this type of response (outlined earlier), the models in Figure 3 offer an alternative explanation based on total response predation theory.…”

“…Examples of empirically derived predator density‐impact functions for vulnerable secondary prey species, showing relationships between: percentage trap catch of brushtail possums ( Trichosurus vulpecula ) and percentage of pairs of North Island kōkako ( Callaeas cinerea ) fledging young (a) (modified from Innes et al, 1999); mouse printing rates of tracking tunnels and numbers of southern grass skinks ( Oligosoma polychroma ) (b) and ground wētā ( Hemiandrus spp.) (c) in pitfall traps (modified from Norbury et al, 2022); ship rat ( Rattus rattus ) tracking index and number of kererū in 5 min bird counts (d) (reproduced from Carpenter et al, 2021); and residual ship rat tracking index (RTI) and predicted rat control effect size on recovery rates of deeply endemic New Zealand bird species for each endemicity level 7 years after the onset of control (e) (solid lines, predicted mean effect size; marker sizes depict relative weight each study contributed to overall effect size; shading, 95% confidence band; dotted lines, 95% prediction interval upper and lower bounds) (reproduced from Binny et al, 2020, with permission from Wiley). Approximate predator thresholds are apparent where prey recruitment or population size rapidly increase where predator density indices are less than about 10%.…”

Rodent management in Aotearoa New Zealand: approaches and challenges to landscape‐scale control

“…2014a; Norbury et al. 2022; Watts et al. 2022) that hide from the larger rats and mustelids in rock or vegetation crevices but cannot hide from mice.…”

“…2015; Norbury et al. 2023). Some taxa that were extirpated from the mainland and survived only on pest‐free islands have now been successfully reintroduced only inside pest‐fenced ecosanctuaries, despite the frequent presence of mice, demonstrating their need for residual abundance of mustelids, black and brown rats, and possums to be zero or near‐zero.…”