Conserving biodiversity – what should be prioritised?

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Lack of forest habitat or introduced predators – which is the biggest barrier to native biodiversity in New Zealand’s lowland landscapes? Can they even be considered separately? Is there any point in restoring habitat if you don’t get rid of predators and conversely, are there any benefits from eradicating predators if the habitat available to wildlife is second rate?

A group of people planting along the coastline
Tāwharanui volunteers take part in regular planting days at the sanctuary. Image credit: Supplied

Jay Ruffell and Raphael Didham from the University of Western Australia asked the tough questions in their latest research, published in the New Zealand Journal of Ecology. The answers may help community conservation groups and others prioritise how they use their limited resources and funds most effectively.

Using bird count data from 195 locations across mainland northern New Zealand they looked at abundance and species richness of native forest birds for a range of native forest cover situations and predator control regimes. Most of the sampling sites were selected and surveyed by Auckland Council ecologists as part of the council’s Terrestrial Biodiversity Monitoring Programme (TBMP), which measures biodiversity values by systematically sampling plant, bird, and pest mammal communities in native forest throughout the region.

A kereru on a branch
Tui and kereru were shown to benefit from predator control, but not other bird species. Image credit: Justin Bell (via Wikimedia Commons)

Forest cover was assessed across a gradient of 0-100%, while pest control was categorised as ‘eradication’, ‘high-intensity rat and possum’, ‘low-intensity rat and possum’, ‘periodic possum’ and ‘none’.

“Most response variables were significantly affected by forest cover, and this effect was typically non-linear: response variables declined rapidly below c. 5–10% forest cover, but were relatively invariant to forest cover above this point… Managing forest cover may be relatively unimportant in landscapes with >5–10% forest cover.”

So even a relatively small amount of forest cover – say 10% – benefits wildlife – and if forest cover is less than 5% some restoration work is certainly beneficial. Now for the predator control – how much do you need to do and to what extent will birdlife benefit?

Surprisingly, only kereru and tui were found to benefit from predator control in the study areas. It seems astounding. But when you think about it – if you have introduced predators, then that bird species most vulnerable to those predators have probably already been wiped out. What you’re seeing in your lowland forest remnant are the species which can cope, at least to some extent, with rats, possums and stoats.

“Most species either were not significantly more abundant at pest controlled sites, regardless of the pest control category (fantail, silvereye, and tomtit), or were significantly less abundant (grey warbler). By contrast, kererū and tūī did appear to benefit from pest control, and this effect was consistent across most (kererū) or all (tūī) pest control categories. The size of this effect also corresponded to the intensity of pest control.”

“Species richness and ‘total abundance’ (abundance of all species combined) also increased at pest controlled sites, but effects were largely driven by responses of tui and kereru. Effects of eradication were far larger than effects of other pest control categories, while it was unclear whether ‘low-intensity rat and possum’ or ‘periodic possum’ control had any effects at all.”

It’s great that tui and kereru benefitted, but why not the other native species? The researchers have several potential explanations.

“Invasive mammals are widely believed to be the primary threat to native forest birds in New Zealand, but four of the six species that we modelled (grey warbler, fantail, silvereye, and tomtit) were no more abundant in pest controlled landscapes than in landscapes without pest control. There are several potential explanations for this finding. First, the pest control operations in our study system may have been unsuccessful at reducing pest abundance. Second, we may have been unable to detect real effects of pest control because of insufficient statistical power.

“Finally, invasive mammals may have had negligible impacts on grey warbler, fantail, silvereye, and/or tomtit abundance. Although rats and possums are known to eat the eggs, nestlings, and adults of these species, there is little evidence that this individual-level predation translates into population-level impacts (i.e. reduced abundance or viability). Indeed, Innes et al. (2004) found that the relative abundance of grey warbler, fantail, silvereye, and tomtit did not measurably increase following pest control, while O’Donnell and Hoare (2012) found the same for silvereye and tomtit. It seems likely that predation on some or all of these species is often compensatory, killing individuals but having limited impacts on populations.”

So how exactly can conservation managers interpret the results?

“Our results suggest that both managing levels of forest cover and controlling invasive mammals can benefit native forest birds, but the occurrence and magnitude of these benefits will be context-dependent. Managing forest cover may be relatively unimportant in landscapes with >5–10% forest cover, while benefits of pest control may be limited unless intensive methods are used. Moreover, even intensive pest control may only benefit a small subset of species unless coupled with reintroduction of [predator-sensitive] locally-extinct species.”

Futhermore, the effects of forest cover and introduced predators are not separate and distinct and that can potentially muddy the picture.

“Understanding the relative benefits of managing the impacts of forest loss versus the impacts of invasive mammals is also complicated by the fact that these threats may interact. For example, impacts of forest loss may weaken following pest control if forest loss affects native species by driving changes in the abundance of invasive mammals…”

“…Real-world management decisions will depend on the costs of particular management strategies in addition to their effectiveness, and this is something that we did not account for in our study. For example, eradication appeared to produce much larger increases in bird abundance than other types of pest control, but may be prohibitively expensive at most mainland sites.”

“Social and ethical ‘costs’ may also influence the feasibility of different management strategies. For example, some pest control regimes may not be tenable because of ethical or social issues such as the use of inhumane anticoagulant poisons or the application of toxins onto public land.”

The full research report is published in the New Zealand Journal of Ecology and can be freely viewed online:

Conserving biodiversity in New Zealand’s lowland landscapes: does forest cover or pest control have a greater effect on native birds? (2017)