Testing the effectiveness of integrated pest control at protecting whio (Hymenolaimus malacorhynchos) from stoat (Mustela erminea) predation in beech forest (Nothofagaceae)

Abstract

The introduction of mammalian predators, particularly stoats (Mustela erminea), to New Zealand led to the decline in whio (Hymenolaimus malacorhynchos), an endemic riverine duck. Stoat control for whio in the South Island has focused on valley floor trapping along waterway margins but increasing survival and productivity for whio using this method is complicated by irruptive predator dynamics caused by occasional masting of beech species (Nothofagaceae). We investigated the effect of integrating stoat trapping with pulsed aerial 1080 toxin operations to counter predator irruptions on whio survival and productivity in c. 40 000 ha of beech dominated Kahurangi National Park. We collected data on adult female whio survival, probability of breeding, nesting success, and duckling survival for seven years through two full beech mast events. We found a positive relationship between distance into the interior of the treatment block and whio population growth, nesting success, duckling, and adult survival. Heavy female whio were more likely to breed than light ones, and adult female survival was higher during the breeding season than during non-breeding. Nesting success was greatest in the breeding season following a 1080 operation. Duckling survival was higher at lower river flows. Positive population growth was only predicted near the centre of the study area (c. 13 km from the edge) with both traps and 1080. We conclude that in the centre of treatment blocks at least as large as our study block, integrated pest control should be enough to ensure long-term population persistence of whio in a beech-dominated system.

Quantifying the relative contributions of habitat modification and mammalian predators on landscape-scale declines of a threatened river specialist duck

“…Our approach provides a method of quantifying the relative effects of multiple mechanisms of species decline and extinction by using landscape-scale modelling techniques to analyse readily available information about species distributions. We have shown that predation by introduced mammals is the primary mechanism driving historic whio declines, supporting previous research based on contemporary populations [27,[30][31][32]. Indeed, several populations within our occurrence dataset are known to have gone locally extinct between 1979-2016 (e.g., Catlins River), despite no significant habitat modification.…”

“…They also provide guidance to managers by highlighting the large amount of suitable habitat that whio could potentially occupy if the effects of predation can be mitigated. Controlling stoats is possible, with large-scale, low-intensity predator control significantly increasing whio productivity and population viability [27,30,32]. Currently over 1,550 km of contemporary whio habitat (or 1.5% of their likely pre-human range) is actively managed [59].…”

“…However, conservation managers have considerable scope to expand whio populations into new areas using predator control and could prioritize sites based on our predictions of the RLO. Indeed, whio populations are already benefiting from an expansion of predator control networks by both government agencies and local community groups [30,32,59]. To help facilitate prioritisation of future conservation efforts for whio, we have made the predictions of mean RLO for whio under contemporary habitat conditions available via the NZ River Maps webtool (https:// shiny.niwa.co.nz/nzrivermaps), an online platform for visualising environmental data on the New Zealand digital river network [60,61].…”

“…Contemporary whio populations are sparsely distributed throughout New Zealand with small, fragmented populations in a wide range of unmodified habitats, largely protected within conservation reserves [ Fig 1], [20,29]. However, despite habitat protection, whio populations are still declining rapidly in the absence of predator control, with an estimated national population of less than 3,000 individuals [29][30][31][32]. Here, we use landscape-scale models to a) assess the relative influences of habitat modification and mammalian predation as potential drivers of historic whio range contractions and b) identify potential areas for prioritising whio conservation efforts.…”

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

“…Stoat control increases the survival and nesting success of large‐bodied birds such as whio [ Hymenolaimus malacorhynchos (Gmelin, 1789); Steffens et al. 2022], kea ( Nestor notabilis Gould, 1856; Kemp et al. 2018), and kaka [ Nestor meridionalis (Gmelin, 1788)].…”