Mini Safe Havens for population recovery and reintroductions 'beyond-the-fence'

Abstract

In response to the ongoing decline of fauna worldwide, there has been growing interest in the rewilding of whole ecosystems outside of fenced sanctuaries or offshore islands. This interest will inevitably result in attempts to restore species where eliminating threats from predators and competitors is extremely challenging or impossible, or reintroductions of predators that will increase predation risk for extant prey (i.e., coexistence conservation). We propose 'Mini Safe Havens' (MSHs) as a potential tool for managing these threats. Mini Safe Havens are refuges that are permanently permeable to the focal species; allowing the emigration of individuals while maintaining gene flow through the boundary. Crucial to the effectiveness of the approach is the ongoing maintenance and monitoring required to preserve a low-to-zero risk of key threats within the MSH; facilitating in-situ learning and adaptation by focal species to these threats, at a rate and intensity of exposure determined by the animals themselves. We trialled the MSH approach for a pilot reintroduction of the Australian native New Holland mouse (Pseudomys novaehollandiae), in the context of a trophic rewilding project to address potential naïveté to a reintroduced native mammalian predator. We found that mice released into a MSH maintained their weight and continued to use the release site beyond 17 months (525 days) post-release. In contrast, individuals in temporary soft-release enclosures tended to lose weight and became undetectable approximately 1-month post-release. We discuss the broad applicability of MSHs for population recovery and reintroductions 'beyond-the-fence' and recommend avenues for further refinement of the approach.

Supplementary information: The online version contains supplementary material available at 10.1007/s10531-022-02495-6.

Keywords: Predator-proof fence; Refuge; Soft-release; Spillover effect; Translocation tactics; Trophic rewilding.

Fig. 1

Mini Safe Haven and 16 soft-release sites (in three groups) in Mulligans Flat Woodland Sanctuary (MFWS). Experimental kangaroo exclusion fences impede, but do exclude, large macropods (Manning et al. ; Shorthouse et al. 2012). Similarly, herbivore exclosures function as an obstacle, rather than a barrier, to any species capable of jumping, climbing, digging under, or passing through, the fence. Location of the 2013 release sites (near the centre of MFWS) retrieved from Abicair et al. (2020)

Fig. 2

Ring-tanks were made from sheet metal (~ 1.2 mm thick), dug ~ 100 mm into the ground and internally supported by metal stakes secured to the walls. The ring-tank was accessible through a join held together by a bolt (stars) at the top and near the bottom (above ground level). All other joins were held together with screws (black dots). The two mouse holes and single large hole each had a removeable door (wider than the measurements given) made from 0.5 mm sheet aluminium held on with multiple bolts. Leafy branches and mouse houses were added to the ring-tanks for shelter. For Trial 2 only, we covered ring-tanks with fine fruit-netting held taut with fold-back clips. These clips were not strong enough to stop possums from jumping in. We suggest the use of screws (as illustrated) to hook the netting over the walls, instead of fold-back clips. All measurements are approximate. Dotted lines illustrate transparency. Illustration not to scale. See Online Resource 1 for additional construction notes and photos

Fig. 3

Our retrofitted Mini Safe Haven (MSH) fence with floppy top (to stop quolls climbing over; adapted from the design in Moseby and Read 2006), and 35 × 25 mm aperture wire mesh (permeable to mice but not quolls) secured over the pre-existing larger mesh. All measurements are approximate. The fence footing (preventing animals from digging underneath) was more easily pinned flush to the ground when grass was cut short before construction. We recommend constructing MSHs away from drainage lines and soggy soil, and suggest the footing be covered in gravel or buried vertically to reduce maintenance effort. Gravel may be more suitable where corrosive soils would damage the fence. Illustration not to scale. See Online Resource 1 for additional construction notes and photos

Fig. 4

Monthly rainfall (BOM 2022) and total monthly survey effort (i.e., sum of ‘trap nights’ for all survey types) deployed for each trial. Trial 1 commenced during the final months of a three-year drought (Commonwealth of Australia 2020). Refer to Online Resource 1 for detailed survey effort and methods

Fig. 5

A Kaplan-Meier survival curves with confidence intervals based on B the minimum number of New Holland mice known alive post-release in each trial from live trapping, remote cameras, and microchip scanners. Censoring (i.e., last detection of individuals that we did not detect mortality for) is indicated in A by a vertical dash ‘|’. A horizontal dash ‘-’ in B indicates periods for which there was no survey effort. Cameras confirmed at least one individual was still present in the MSH at 525 days post-release. The survival probability should be interpreted as the probability of the population still being present

Fig. 6

Change in New Holland mouse body mass over time post-release for each trial (predicted values with standard error)

Fig. 7

Frequency of New Holland mouse (NHM) detections on baited cameras (grid squares) in and around the Mini Safe Haven in Mulligans Flat Woodland Sanctuary (MFWS) over 162* nights between March and September 2020. Dates indicate the first night a New Holland mouse was detected at that location. Cameras without dates indicate locations where New Hollands were never detected. New Holland mouse detections in the August 2020 seed survey (stars) included one outside the western boundary of the MFWS fence. *The camera located in the north-west corner malfunctioned and was operational for 17 fewer nights

Fig. 8

For a given length of fencing, the total area beyond-the-fence that is within the dispersal ability of a focal species is smaller for a fenced sanctuary (A) than for multiple Mini Safe Havens (B) in a landscape. A Mini Safe Haven network is likely to disperse predation risk for the focal species while minimally impeding the migration of other species. Silhouettes from phylopic.org, Public Domain Dedication 1.0