Invasive mammals and habitat modification interact to generate unforeseen outcomes for indigenous fauna

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Abstract:In 2003, a review of how introduced mammals were managed as pests in New Zealand was published. Since then trends for the control of these mammals include moves from pest-by-pest prioritisation towards site-based and multiple-pest management, extension of large-scale aerial control of predators to include beech forests, increasing intensive management of sites by private and non-government agencies, and increasing effort by regional councils and managers of vectors of bovine tuberculosis. The current deployment is the sum of largely independent decisions made by different agencies and individuals with different priorities. National agencies might optimise their own decisions, but overall deployment will always be nationally suboptimal when objectives are set at regional or local scales. This system includes both small areas where most of the mammals present are managed intensively and large areas where fewer species are managed. We discuss how this deployment might evolve as a network of smaller core areas aiming to achieve zero or low densities of all or most pests, with surrounding halos with lesser control effort against fewer species, enough to allow for at least 'safe passage' of native animals between the cores. This system might work best along the more-or-less contiguous native habitats along the axial ranges of the North and South Islands, especially if agencies with a national mandate are prepared to act in the extensive areas between core sites. Some have asked whether the successful eradication of mammals on small islands can be scaled up to the main islands of New Zealand. The current version of this predator-free model aims at the ubiquitous ship rat, brushtail possum and stoat. A key need for the predator-free model is a tool that kills 100% of each of the three main target species, ideally with one large-scale, socially-acceptable application. Without this tool the cost to find and kill survivors would raise the costs of national eradication to about $32 billion, assuming current costs of multiple-species eradications on islands. For sustained control options, research is needed to inform the frequency and intensity of control, and thus the best control tools. The ability to detect target pests at low densities and management of reinvasion are essential for achieving zero or low pest density goals. We suggest that the core/halo model of pest management, within a network of assets to be protected, will best protect New Zealand's biodiversity for the foreseeable future. The national scale pest-or predator-free aspiration is not currently (and may never be) feasible and risks diverting resources from more optimal solutions, as occurred with the 'last rabbit' and 'last deer' programmes promoted last century.

Section: Which Pest Species Should Be Targeted?mentioning

confidence: 99%

Past, present and two potential futures for managing New Zealand’s mammalian pests

Parkes¹,

Nugent

Forsyth

et al. 2017

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“…combined; Norbury et al 2013) and larger endangered skinks (grand skinks and Otago skinks; Reardon et al 2012) at Macraes Flat. The gradient in our skink density estimates, from c. 1200 per ha at the experimental control site to c. 4000 per ha at the fence and removal treatments may therefore result in part from variation in predator management.…”

Section: Discussionmentioning

confidence: 99%

“…More recently, studies on the New Zealand mainland have shown that removing or excluding mammals can increase the survival and/or abundance of skinks (Oligosoma spp.) (Lettink et al 2010;Reardon et al 2012;Jones et al 2013;Norbury et al 2013Norbury et al , 2014.…”

Section: Introductionmentioning

confidence: 99%

Assessing and comparing population densities and indices of skinks under three predator management regimes

Wilson¹,

Mulvey²,

Clarke³

et al. 2017

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Introduced mammalian predators threaten populations of endemic New Zealand skinks. Their effects on skink populations have been not often quantified on the mainland and are known primarily from skink population increases on islands from which mammals have been eradicated. Estimating skink population density with capture-recapture trapping is time-consuming and costly. Counting skinks in artificial retreats in specific weather conditions may be a useful and relatively quick way to index population density, but needs calibration for different habitats and species. In 2007 and 2009, we estimated the population density of small terrestrial skinks (McCann's skink Oligosoma maccanni, southern grass skink O. aff. polychroma clade 5 and cryptic skink O. inconspicuum), based on spatially explicit capture-recapture (SECR) in pitfall traps in three mammal-management treatments at Macraes Flat, Otago. The treatments were eradication of large predators and near-eradication of rodents inside a mammal-resistant fence, suppression of mammalian predator populations through continuous trapping (two locations within an extensive area), and no mammal management. We tested for relationships between the estimated population densities and dawn and late-morning counts of skinks in artificial retreats. Skink density (three species combined) ranged from c. 1200 per ha at the experimental control site to c. 4000 per ha at the fenced site. These treatment differences in skink density may be the combined effect of predator management and pre-existing differences due to habitat characteristics and farming practices. Skink counts done in late morning (2009) were related to estimated skink densities but did not differ significantly between treatments. Skink counts done at dawn (both years) were not related to densities. Counts, but not densities, were significantly higher at locations with a more northerly aspect. We recommend further investigation of the utility of skink counts in artificial retreats for monitoring skink density at this location, with careful control of ambient temperature during sampling, and of aspect, habitat and device placement.

“…Both of these ecological release mechanisms can result in increased predation on indigenous taxa in the managed ecosystem (e.g. Rayner et al 2007;Ritchie & Johnson 2009;Norbury et al 2013).…”

Section: Introductionmentioning

confidence: 99%

“…Norbury et al (2013) demonstrated mouse population growth after experimental predator removal, but only where grass seed provided ample food for mice. Other studies have attributed mouse population increases to relaxation of interference and/or exploitative competition after removal of ship rats (Rattus rattus) (e.g.…”

Section: Introductionmentioning

confidence: 99%

Population dynamics of house mice without mammalian predators and competitors

Wilson¹,

Innes²,

Fitzgerald³

et al. 2018

Mesopredator and competitor release can lead to population increases of invasive house mice (Mus musculus) after larger introduced mammals are controlled or eradicated. In New Zealand, mammal-resistant fences have enabled multi-species mammal eradications in order to protect indigenous species. When house mice are the only mammals remaining in these biodiversity sanctuaries, they may reach a high population density, with potential consequences for their indigenous prey. We studied mouse populations in the absence of other mammals for 5 years at mammal-resistant fenced forest sites at Maungatautari, Waikato. We used spatially explicit capture-recapture (SECR) to estimate mouse population density quarterly in two independently fenced sites, with contrasting levels of mouse management that were switched half-way through the study. In the absence of mouse control, mouse population density reached 30-46 ha -1 at one site each year after summer breeding, and 23 ha -1 at the other site. Mouse tracking rates in inked footprint tunnels were positively related to numbers of mice captured in each session, but not significantly to mouse density. The highest mouse densities were similar to estimates in New Zealand forest and alpine ecosystems after mass seeding (masting) events, but lower than estimates in another sanctuary and on some islands lacking larger terrestrial mammals. We suggest that in the absence of competition and predation from other mammals, food limitation may have prevented mouse populations from attaining very high densities in this mainland forest location.