Mostly we get rid of introduced predators by, well – killing predators. It works, up to a point. But if you can’t get rid of every single rat or stoat then the few survivors suddenly find themselves with ample food supplies and very little competition. They breed and they breed very successfully. Numbers climb rapidly again, particularly in the case of rats.
So what’s the solution?
The answer may be in what they eat. In many, if not most cases, predator numbers are thought to be controlled by food availability. Prey is the limiting factor. It’s what’s known as a bottom up system. Predators don’t control prey. It’s the other way around. Prey controls predator numbers.
Feral cats, ferrets and stoats all prey on native wildlife and we certainly don’t want to start limiting the number of native wildlife we have. But for each of those predators, their main and ‘natural’ prey is actually another introduced mammal. In open grassland such as Central Otago, feral cats and ferrets prey mainly on rabbits and are limited by the rabbit population. Stoats are limited by rat availability. Rats and mice in turn, thrive where there is plenty of seed.
Predator numbers get high when there’s plenty of food and then, when numbers are high, the occasional variation in diet by way of native wildlife starts to have a high, cumulative effect. Competition at high predator levels probably also pushes predators to broaden their diet options.
Removing the natural prey (rabbits, rats) will eventually lead to a decline in feral cat, ferret and stoat numbers too – but in the short-term, those predators will turn in even greater numbers to alternative native prey if their natural prey is removed. So simply killing off rats to get rid of stoats isn’t the answer either.
It’s a complex problem that needs a multi-pronged attack. But Landcare Research scientist Grant Norbury believes there is a benefit to be gained by including bottom-up predator control in a wider top-down predator control regime. He outlines his reasoning and recommendations in an article just published in the New Zealand Journal of Ecology.
Cost to economy
“Plant and animal pests are a drain on New Zealand’s economy, costing up to NZ$3.3 billion per year (or 1.86% of GDP) in control and production losses for the primary sector alone… I suggest adopting an ecologically-based approach to pest control that breaks the interactive linkages between invasive mammalian pests at high trophic levels and the other introduced species that they rely on as their primary food resource. Some of these primary food species are, in turn, affected by land management practices and land-use policy… I do not suggest that this wider ecological approach replaces conventional control. Targeting pests directly at high trophic levels will always be the primary focus, but I do argue that a more sustainable, and potentially more effective, solution to management of introduced mammalian pests should include greater focus on bottom-up control.”
Experience has shown that having large number of rabbits living in nearby pastures, can put native birdlife at risk.
“In North Island forests adjacent to pastures that support rabbits, ferrets kill adult kiwi.”
Land management practices can affect rabbit density and controlling rabbit numbers has economic advantages.
“Rabbit control using aerial poisoning costs approximately NZ$70 per ha every 5 or so years, whereas the maintenance cost of trapping predators intensively (mixture of daily checks of leg-hold traps and weekly checks of lethal traps) is approximately NZ$60 per ha every year. Furthermore, numbers of ferrets, cats and rabbits could be controlled by management actions targeted even further down the food chain in this system. Rabbit abundance is affected by vegetation structure and composition. Woody ecosystems and tall tussock grasslands that are modified by clearing and burning to enhance agricultural productivity result in short green pastures (preferred rabbit food) within mosaics of intact vegetation (rabbit shelter). This enrichment of food and shelter favours rabbits and is used as a form of habitat management to restore rabbit numbers in their native Iberian Peninsula.”
So some of our farming practices may actually be ‘conserving’ rabbits! Land management is particularly important where farmland adjoins conservation reserves.
“A key question is how can farmland adjacent to conservation reserves be managed in a productive way, but at the same time minimise the impacts of rabbits and predators? This question has never been adequately addressed in New Zealand. In addition, the indirect effects of landscape supplementation on indigenous biodiversity are usually unrecognised in land-use policy. The Resource Management Act 1991 is the legislation designed to minimise the adverse effects of land management. However, local government authorities charged with enacting the Act rarely, if ever, consider the wider ecological perspectives described here when considering consent applications. Consents are regularly issued for land developments that happen to favour rabbits.”
Sometimes even well-intentioned land restoration projects can have unintended consequences.
“Other ecosystems that could potentially benefit from bottom-up pest management are modified lowland ecosystems undergoing restoration by removal of livestock. These ecosystems are usually well-endowed with introduced pasture species that produce large seed crops every year. Removal of livestock results in a super-abundance of seed, which boosts mouse populations. Mice are also predators of indigenous invertebrates and lizards. Some researchers have implied that well-intentioned restoration projects involving cessation of livestock grazing have led to lizard declines due to greater predation by mice or by top-order predators that have benefited from more mice. Recent experiments have confirmed the links between cessation of grazing, profusion of seed from introduced grasses, rodent increases, and elevated impacts on lizards.”
So what do you do when even your well-intentioned restoration project has unexpected consequence? A carefully monitored compromise may be the answer, rather than sudden, all-out change.
“A bottom-up method of mitigating these impacts is to judiciously graze livestock to crop introduced grasses and therefore reduce seed production, perhaps combined with adding refuges for native species. Close monitoring would be required to ensure that the anticipated positive effects are balanced against the potentially negative effects of grazing.”
Another example involves deer, possums, fungi and wildling trees.
“A different example from those above is an interaction between herbivores and wilding trees. The dung of red deer (Cervus elaphus) and brushtail possum contain ectomycorrhizal fungi that are mutualistic with wilding trees (Pinus contorta, Pseudotsuga menziesii). Therefore, deer and possums could facilitate the spread and establishment of these wilding species. A bottom-up approach to wilding tree control may be to reduce deer and possum populations, or to restrict their movements between infected and uninfected areas. Research is required to understand how effective this approach might be.”
These are just some of the examples given by Grant Norbury. There are many other situations, where a better understanding and incorporation of ‘bottom-up’ predator control could enhance traditional ‘top-down’ lethal predator control programmes.
The full paper is published in the New Zealand Journal of Ecology and is freely available.