Maybe - but not just yet
Counting every rat, mouse or mustelid trapped may be satisfying, but it is irrelevant in the war against predators and may be lulling us into a false sense of security, guest speaker Professor Carolyn (Kim) King told DU members at the conference.
In her speech, Maximising the Duck Harvest, Prof King, from the School of Science at Waikato University, thanked Ducks Unlimited, which like many hunting clubs around the world, had turned itself into a conservation group, supportive of conservation research organisations and with members who were observant naturalists contributing to conservation efforts.
She said a good harvest operation:
- aims for a sustainable yield
- never takes more individuals than the population can replace
- can be like the constant harvest by rabbit trappers, deer cullers, and predators
- a population can’t be affected if the yield is not declining.
As an example, she said, the winning team in the Great Otago Bunny Hunt in 2012 bagged 1035 rabbits as its contribution to the 10,424 tally from the 24-hour event. During the 21 years of the event, 253,735 rabbits have been killed (mean: 12,082 a year) but it has made no difference to rabbit numbers.
Harvesting is a dynamic target-habitatpeople system in which:
- protection of a valued resource can be ineffective unless broader ecological/ sociological context is understood
- the interaction of resource/habitat/ people defines the outcome
- solutions require strategic organisation, stakeholders agreement and effective methods
- the biology and strategic organisation determine effective policy (which DU does very well).
Using an example from Wiltshire in the UK she demonstrated how a predator control experiment produced a surprising result. In two areas of farmland, 1km apart and with similar habitat, predators were controlled in one area for three years and then in the other area for three years.
The study had been based on the assumption that habitat on UK game estates was the only important factor but the results clearly showed a big increased harvestable yield of grey partridges in whichever area was subject to predator control.
A computer model developed from the study predicted that the highest populations would be found where nesting cover for the partridges was increased and predators were removed, even if shooting was permitted as well. Key is the interaction of habitat (controls productivity) and shooting mortality. If nests are protected, more young survive, so shooting can substitute for nest mortality caused by predators.
Using a NZ example to highlight the relationship between habitat and mortality, Prof King referred to wetlands in the Upper Waitaki area. In 1850, wetlands and swamps covered 71,000 hectares, but the effects of human activities has been devastating with 7300 ha swamps converted to pasture, more fragmented; 22,300 inundated by hydro electric schemes; 4200 ha braided riverbed dewatered (9% loss); 22,300 ha new open water habitat + 300 km more shoreline which doesn’t suit riverbed birds; predation is heavy and breeding rate low on the remaining 41, 700 ha wetlands (40% loss).
Food supply has the greatest effect on bird populations. The braided riverbeds were dewatered or turned into open water habitat, which did not suit the birds adapted to feeding on the riverbed, exposing them to predation and a drop-off in breeding.
Prof King said the deadliest predators for ducks were egg-loving hedgehogs, ferrets, stoats, rats, mice and cats. An Upper Waitaki Basin study of predation on the nests of dotterels, terns and stilts showed that cats, ferrets and hedgehogs did the most harm to nesting birds in that area.
She said, “We know we can kill predators, so what’s the problem?” Counting a pile of dead pests can be enormously satisfying but it does not tell us what we want to know, which is how many are still there, how to account for those that replace those dead ones, and how to know if we have taken out enough predators to benefit the birds we want to protect. That boils down to what actually determines the numbers of animals – both the predators and the birds.
Ferret numbers are controlled by rabbits (their main food source), not trappers. The rabbit yield is not declining: rabbits and ferrets are co-evolved prey.
A pilot trial in 2005 presented another serious problem – trap avoidance. Radio-collared ferrets in a study near Tokoroa were tracked by an aircraft which flew over the area during the day.
Nine of the 15 ferrets were located. Monitoring sites showed that six of the ferrets made 22 approaches to 2 experimental recording sites, which could have been traps, but only three entered the tunnels. On the final extensive trap-out, four of the 15 eluded capture, although their radio signals confirmed they were still there.
The following year collars were put on 30 ferrets west of Lake Taupo and a new toxic bait dispenser was used. Over five weeks, only 12 visited the bait stations and only eight took the bait. The monitoring regulations said every ferret had to be accounted for but of the 13 known survivors, only two could be caught.
In kiwi sanctuaries in Northland, stoats were refusing to go into the bait tunnels and it was only a brief 1080 operation that stopped the decline of kiwi chicks. Though it is awful stuff, we have to use the tools we have until we come up with something better, she said.
Predators are intelligent and quickly learn to avoid new devices presenting danger to them. In Britain, American mink, which escaped from fur farms in the 1950s, had become a serious threat to the native water vole (Ratty in Wind in the Willows). The mink, like rats, are good swimmers which means they can avoid land traps set on river banks. The problem was tackled by using traps placed on floating rafts, which might be appropriate to adapt as a control for Norway rats here, Prof King said.
New technologies are absolutely essential and can transform results. She said at a 1976 conference that she attended senior scientists said rat eradications on islands were impossible, but they were wrong. The invention of brodifacoum in the 1980s plus precise bait placement enabled Breaksea Island to be cleared of Norway rats in three weeks in 1988.
“That’s what we need – some kind of new technology that will break the mould, something different... One of the definitions of insanity is to keep on doing the same thing and expecting different outcomes,” Prof King said.
We can and must increase the intensity of predator control, but to be effective it must add to the natural mortality. Mustelids were introduced to control rabbits but rabbits were breeding at a faster rate and their numbers were unaffected. The mustelids were only substituting for natural mortality.
Only when rabbit breeding is reduced for other reasons (drought, 1080), can mustelids and/or trappers add to their normal losses, and achieve a real effect
Failures were usually due to some combination of:
- Forgetting that nature is on the predators’ side and ignoring the effects of natural selection
- Counting numbers removed, not numbers remaining,
- Lack of coordinated, agreed strategy
- Lack of flexibility in response to experience
- Insufficient stakeholder support.
- Inefficient tools
- Failing to deploy combination of methods
- Having no effect on fertility, immigration.
Prof King said, on the other hand, some of the common features of successful predator control were a combination of:
- Operator confidence, meticulous preparation, sufficient funding
- Never repeating mistakes, never giving up, so accumulating improvements, combining/switching strategies when necessary
- Landscape scale coordination
- Strong community support.
- Adds to natural mortality
- Prevents immigration
- Targets fertility
- Confirms benefit to native species.
Prof King concluded by saying Predator Free NZ was still a long way away but “don’t despair, history is encouraging!”, and DUNZ would play an important role.
In the meantime we must keep using whatever tools we have so we still have native species surviving by the time we develop something better, protecting duck nesting sites is possible and needed now.
PFNZ is going to be much more difficult as it requires development of as-yet-unknown, 100 per cent acceptable methods to control predator replacement rates.