A call to make New Zealand Predator Free by 2050 is an ambitious aspiration that is a complex problem with many hidden traps for the unwary. Forewarned is forearmed.
To help conserve our unique biodiversity, the New Zealand government has taken an explicit stand against predators with the Predator Free 2050 initiative. This was announced in 2016 and is structured as a government and community partnership to advance predator control to the landscape level and eradicate predators from the country within the 34 years to 2050.
This lofty ambition has caught many people's imagination and has galvanised action across the nation in towns and cities, in the countryside and in the wilderness. However, despite the clamour for results it is worth pausing a moment to consider and better understand what this catch cry actually means. The ‘why' is pretty obvious - predators kill our native wildlife - but the ‘what’ and ‘how’ are less clear.
At face value, Predator Free 2050 means no predators in New Zealand by 2050. Clearly this isn't the intention as we have native avian predators that need protection, including morepork, harriers and endemic New Zealand falcon.
The ‘what’ focus is actually on introduced mammalian predators, but this only includes five species, including three species of rats, brush-tailed possums and stoats. It does not include mice, weasels, ferrets, hedgehogs or feral cats and it certainly doesn't include other biologically destructive introduced mammals such as goats, pigs and rabbits that can all impact native wildlife by being food competitors and habitat modifiers. So Predator Free 2050 is actually Some Predator Free 2050.
This leaves the ‘how’ question. Specific control methodologies of trapping and poisoning are pretty well the only tools in the toolbox with variations on these being different kinds of traps and ground based versus aerial distribution of toxins. Scientists have identified novel gene technologies, however the reality is these are so far only theoretical and their practical application lies in the distant future.
How we move towards a predator free future should also include a consideration of the target species. Viewed ecologically, the suite of invasive mammalian predators introduced to New Zealand form a guild, a group of species that exploit the same type of environmental resources in a similar way.
These species need not be closely related, although mice, black rats, brown rats and kiore are all rodents while weasels, stoats and ferrets are all mustelids. However, the species within a guild often compete with each other for common resources. Therefore, the density of one species can be a limiting factor for another species and rats for example can suppress mice.
Therefore, to become predator free, the objective is actually to disassemble this guild of invasive mammalian predators. Accordingly, the question then becomes, is targeting rats, possums and stoats the best way to achieve this? Is it the best ‘how’?
In New Zealand forests during favourable summers the forest trees produce huge quantities of seed. This is called a mast year and with prolific food supplies rats and mice numbers explode. In response, stoat numbers also rapidly increase and when rodent numbers collapse during the following winter the stoats switch to alternative food supplies and significantly impact bird populations. Does this pattern provide any insight for the Predator Free 2050 programme?
Because rats can suppress mice and feral cats prey on rats, a successful eradication of rats without addressing mice or feral cats risks removing a limiting factor from mice, allowing their numbers to increase substantially, while also driving prey switching in feral cats. Significant disturbance to the predator guild by removing rats and stoats could significantly hamper regeneration of forest plants as their seeds are consumed by higher mouse numbers and significantly impact wildlife as feral cats target native animals to replace rodent prey in their diet.
Upsetting guild relationships by hollowing out the centre could generate damaging collateral effects. Instead, disassembling the guild starting with the apex predator may produce a more orderly outcome, as happened on Lord Howe Island where the removal of cats first led to significant recovery and increases in nesting seabird populations.
Predator Free is indeed an aspirational goal, however, you must be careful what you wish for. How this is achieved may be more important than what is achieved as without carefully considering the ecological changes that will result from the removal of selected species from a guild may produce a result that is contrary to what is anticipated.
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