Winners and losers in the battle for New Zealand’s biodiversity
New Zealand’s (NZ’s) sanctuaries play a critical role in mainland restoration, as exemplars of what can be achieved when pests are controlled to low numbers. As well as managing pests, reintroducing missing indigenous species and inspiring communities in local conservation, some sanctuaries also monitor biodiversity and present an incredibly valuable opportunity for improving our understanding of biodiversity responses to pest control.
However, despite decades of work in sanctuaries, there is a surprising deficit of published scientific information from these sites. Monitoring is often time consuming and costly, and sanctuary managers rarely have the resources to employ scientists to collect and analyse data, meaning that only a few sanctuaries (such as those situated close to universities) publish their work. In addition to sanctuaries, considerable credit is owed to staff in the Department of Conservation’s (DOC’s) mainland island projects, where extensive long-term biodiversity and pest monitoring have been conducted for over two decades to test different control techniques and conserve native biota.
Recent work to assess the biodiversity outcomes from NZ sanctuaries by Rachelle Binny and her colleagues has involved a huge collaborative effort, with data being shared by 23 sanctuaries and three DOC ‘mainland islands’. Sites range in area from a few hundred hectares to several thousand, and span a range of habitats across the North and South Islands. Monitoring data gathered at these sites over the last two decades on a wide range of flora and fauna are being collated using a diverse array of monitoring methods and measures. By combining information across multiple sites, this work aims to provide new insights that couldn’t be gained by considering any one site alone.
Collating the data has been challenging because it is so variable – different people want to measure different things in different ways – which has highlighted the need for clear leadership on the use of ‘standard operating procedure’ techniques and measures for monitoring. As Predator Free 2050 (PF2050) ramps up and new conservation projects pop up across NZ, a consistent approach to monitoring both pests and native biota, and better sharing of such data, will allow agencies, communities and individuals to readily combine their data in order to contribute to the national picture.
A range of different measures commonly used in New Zealand to monitor birds, lizards, frogs, invertebrates and vegetation (e.g. bird-call counts, nesting success and forest canopy health) are being analysed. Early results from a meta-analysis of these data suggest benefits for biodiversity within NZ sanctuaries, across a range of flora and fauna. As might be expected, greater benefits are seen within sanctuaries surrounded by a pest-proof fence, where all mammal pests (except mice, typically) have been eradicated, compared to unfenced mainland islands where at least some pests remain but are controlled to low levels. However, while the populations of some species respond very positively to pest control, the populations of some other species show only small benefits, or in some cases even decline. In other words there are winners and losers.
Another interesting trend emerging from the data on bird responses is that the ‘winners’, or those that benefit most strongly, tend to include our long endemic species (e.g. kiwi, North Island kōkako and stitchbird/hihi), with weaker benefits observed in more recently endemic species (e.g. tui and tomtit). The ‘losers’ in this case are introduced species or ‘recent natives’ (e.g. grey warbler, silvereye and fantail), whose populations often show little change or even decline in response to pest control. These trends are not necessarily a bad thing, and may reflect a natural restructuring of ecological communities when the threat of predation by pests is reduced. Ecosystems are incredibly complex and predation is not the only factor limiting threatened populations. Habitat, competition and other factors also play key roles, and it is becoming increasingly important to fill the knowledge gaps around the effects of these factors alongside predation so that realistic conservation objectives can be set and achieved. For example, very little is known about the naturally occurring densities of native populations in the absence of pests.
This research will improve understanding of the broad biodiversity outcomes from pest management and is relevant for conservation managers and policy makers. As control efforts expand across NZ, reliable and comparable measures of effectiveness for each of the currently available control strategies are needed. The cost of each regime is, of course, another important factor to consider. Overall we hope this work will highlight the importance of continued support for NZ’s sanctuaries, both pest-fenced and mainland islands, and of ongoing allocation of resources for towards monitoring biodiversity and pests using standardised techniques.
The motivation behind the PF2050 mission is not mainly about killing pests. It is about conserving NZ’s unique biodiversity before it is lost. As PF2050 drives pest control efforts to spread and intensify across the country, it is critical that the resources being invested in achieving the predator-free goal are justified, and that understanding of the biodiversity outcomes from pest control is improved so that realistic expectations can be set around what success would actually look like for PF2050.
This research is funded by Manaaki Whenua and aligned with NZ’s Biological Heritage National Science Challenge. Special thanks to Robbie Price and Ben Ridgen (Manaaki Whenua), Craig Gillies and Oliver Gansell (Department of Conservation, Hamilton), and the 23 sanctuaries that have contributed data for this work.
Andrea Byrom (New Zealand’s Biological Heritage National Science Challenge)
Alex James (University of Canterbury)