The natural world of fauna is based around food chains and predators. It is a reality of nature that larger birds, animals, fish and insects eat smaller ones. –Jim Hilton and Roger Childs

by Tony Orman

Avian predators have been around for millions of year

A newspaper report in the Marlborough Express of 29 January told of black-backed gulls preying on other native birds. So what?

Predators in the New Zealand avian ecosystem are nothing new. The native falcon (karearea) preys on small to medium-sized birds, and even occasionally takes prey much larger than itself such as black shags, poultry and pheasants. Pukeho can prey on small ducklings. harrier hawk prey on small birds.

Over millions of years New Zealand has always had predators. Among extinct birds:

• the giant Haast Eagle preyed on moa (picture above), 
• the adze bills preyed on the chicks of nesting birds, 
• the Eyles harrier preyed on kereru, kokako, kaka and smaller moa up to 40 kgs 
• small birds were part of the diet of the laughing owl.

The pointless but persistent war on predators

New Zealand has for many decades waged a war against predators. Yet in other countries, predators are recognised as generally beneficial to their prey species. Predators are part of a healthy ecosystem, removing vulnerable prey such as the old, injured, sick or very young, leaving more food for the survival and success of healthy prey species. Predators in effect, control the size of prey populations.

Predators will catch healthy prey when they can, but catching sick or injured animals is much more likely, and results in healthier prey populations because diseased animals are quickly removed and only the fittest animals survive and are able to reproduce.

But in New Zealand there is a zealous, deep prejudice against predators. Predator Free 2050, and Zero Invasive Predators (ZIP), represent an ill-conceived all-out war against predators.

Championing the fight may be:

• prime ministers
• central and local government politicians, 
• local bodies, 
• naive unquestioning psuedo-investigative journalists, 
• extreme green groups 
• unprincipled “scientists” following the money trail of funding.

Instead of hysterically rushing into sentencing predators to death, wildlife managers, strident green groups and others with a phobia against predators should calm down and consider the causes of increased predation.

The Marlborough scene

Are black-backed gulls referred to earlier, coming inland because their coastal food in fish stocks have been reduced by commercial fishing? Here in Marlborough, seals in recent years have been ascending rivers up to twenty of more kilometers inland preying on trout and salmon. A likely reason is commercial seiners plundering stocks of kahawai, a major food for seals. 

Both gulls and seals are being forced — by man’s interference — to go looking elsewhere for food.

In another case in Marlborough, gulls have been found dead on beaches.  In a parallel to the abnormal behaviour of seals, a reason advanced was again diminished kahawai stocks with the predatory kahawai no longer frequently and now only occasionally, seen in surface feeding frenzies from which gulls benefit with floating scraps. 

Healthy kahawai numbers also drive bait fish into the water’s edge from which gulls benefit. Both serve as a probable example of Man’s greed to over-exploit having adverse consequences for other species in the food chain.

Sensible policies overseas

Wildlife managers overseas are increasingly regarding predators as an important part of a healthy ecosystem. 

For example in 2014, A. S. Glen of New Zealand’s Landcare Research and Christopher Dickman of Sydney University co-authored a book on “Carnivores of Australia” and in a chapter “The Importance of Predators” it says — to maintain or restore functioning ecosystems, wildlife managers must consider the ecological importance of predators.

American author Dr.Caroline Fraser writing for the US’s Yale School of the Environment  said – experts beginning with aquatic experiments, have amassed considerable evidence of damage done to food chains by predator removal and have extended such studies to land.

Interference upsets nature’s balance

 There are many examples of human interference directly or indirectly into Nature’s food chains, resulting in profound consequences. In a classic 1966 experiment, biologist Robert Paine removed the purple seastar Piscaster ochraceus – a voracious mussel feeder from an area of coastline in Washington in the US.  Their predator gone, mussels exploded in numbers, crowding out biodiverse kelp communities with monoculture. 

 A similar scenario is happening right here in New Zealand with the removal of large snapper and crayfish causing sea urchin (kina) “barrens”.

In New Zealand on land, the fervour and haste which the Department of Conservation and local councils spread toxins for predators is reckless and fraught with disruptive ecological danger. Large scale poisoning with eco-toxins such as 1080 and brodifacoum may heavily reduce predator numbers initially, but with a few short years, the outcome is disastrous. The science is there to show the resurgence in predator numbers and subsequent damage to the native ecosystem.

Scientist Wendy Ruscoe in a study published in Landcare Research’s publication 2008 showed aerial dropping of 1080 will temporarily knock back a rat population — but due to the rodent’s amazing reproductive capacity, the surviving rats recover rapidly and within 18 months, their numbers are back to former levels. 

The momentum continues and about the three to four year mark, are two to three times greater than before poisoning began. Another study by Landcare scientists Graham Nugent and Peter Sweetapple showed similar rat population explosions caused by aerial 1080 drops. Stoats whose main prey is rats, then surge in numbers in response to the rat population explosion.

The concept of being ”predator free” or having  “zero predators” makes no ecological sense, except in limited circumstances on smaller offshore islands and “mainland” islands. Even on islands where predators may have been eliminated e.g. Secretary Island in Fiordland, the success is relatively short-lived and temporary as animals can and do swim from the mainland to re-colonise.  How else did these predators get there in the first place?

The sad outcome is the gross misuse of public funds and more tragically the profound ecological damage that often occurs in the pursuit of the “Impossible Dream” of exterminating predators.

Footnote: Tony Orman has spent a lifetime in the outdoors, observing and reading about it and Nature. He has had some two dozen books published, mainly on fishing, deerstalking, conservation and rural life.