The battle to purge our wilderness of pest predators is helping our threatened species - but some more than others.
Early results of a meta-analysis of conservation data gathered across the country found that while pest control brought big benefits for our native species, there appeared to be "winners and losers" among them.
A Manaaki Whenua-Landcare Research team pulled together information shared by 23 sanctuaries and three "mainland islands" managed by the Department of Conservation, along with data from published studies.
Dr Rachelle Binny, who presented the preliminary findings at a Christchurch seminar today, said the work involved comparing a range of measures used to monitor native plants and animals, such as bird call counts, nesting success and forest canopy health.
Unsurprisingly, pest control tended to bring greater benefits within fenced sanctuaries where mammal pests had been wiped out, compared to unfenced mainland islands where some pests were being kept at low levels.
"However, our results also suggest that while some species do indeed respond very positively to pest control, other species show only small benefits or, in some cases, even decline."
The research threw up another interesting trend: those birds that benefited most were typically deeply endemic species, such as kiwi, North Island kōkako and hīhī, with weaker benefits among more recently endemic species, like tui and tomtit.
The losers, in this case, appeared to be introduced species or "recent natives" - including grey warbler, silvereye and fantail - whose populations often show little effect or even decline in response to control.
"These trends are not necessarily a bad thing, but rather a natural restructuring of ecological communities when the threat of predation by pests is reduced."
Binny noted that ecosystems were incredibly complex, and predators weren't the only factor hurting threatened populations.
"Habitat, competition and other factors also play key roles and it is becoming increasingly important that we fill the knowledge gaps around the effects of these factors alongside predation so that realistic conservation objectives can be set and achieved," she said.
"For example, very little is known about the naturally-occurring densities of native populations in the absence of pests, and this is something we hope to shed light on through this work."
The team would be digging deeper to better understand the early findings.
"Combining this data has been challenging because it is so variable – different people want to measure different things in different ways - and has really highlighted the need for clear leadership on the use of tried-and-tested standardised techniques and measures for monitoring."
Sanctuaries, particularly, played a critical role in bringing threatened species back to our mainland, but despite decades of work, there was a surprising deficit of published work from these sites.
"Monitoring is often time-consuming and costly, and sanctuaries rarely have the resources to employ scientists to collect and analyse data, meaning that only a few, such as those situated close to universities, are able to publish their work," Binny said.
"In addition to sanctuaries, considerable credit is owed to staff at the Department of Conservation's Mainland Islands, where extensive long-term biodiversity and pest monitoring has been conducted for over two decades to test different control techniques and conserve native biota."
A consistent monitoring approach and better sharing of data would make the national picture clearer.
Ultimately, Binny said the mission to make New Zealand pest-predator free by 2050 wasn't mainly about killing pests, but conserving our unique biodiversity before it was lost forever.
"As New Zealand scales up pest control efforts, our indigenous ecological communities will reorganise themselves and we can expect to see both winners and losers in this restructuring," she said.
"Our work helps set realistic expectations around what a predator-free New Zealand might actually look like."