Elimination vs Suppression. How does Covid-19 compare to NZ’s predator elimination projects?

There are some really good comparisons between NZ’s Covid-19 response and its predator elimination strategy. Most of the world has no chance of a successful Covid-19 elimination and hence are using methods for Covid-19 suppression. For Covid, NZ was applying an elimination strategy that acknowledges there will still be cases but they can be controlled. With predators, NZ has great success with elimination on islands and in fenced sanctuaries. There are now numerous projects attempting elimination on the mainland. An analogy with Covid is useful as it helps understand the key tools needed for an elimination strategy. 


Before we launch into this discussion, we need to have a shared definition of some key terms;

  • Eradication - complete elimination with virtually no chance of it coming back (e.g. Smallpox).  Not a real possibility for Covid or for predators (even if we achieve zero across all land masses we will always have the risk of rats arriving on ships etc.)
  • Elimination - aiming to have zero uncontrolled Covid cases in the community or zero predators breeding in an area. If there is a breach it can be detected in a reasonable time span and taken back down to zero. Most of the time, there is no need to do anything other than monitor, jumping into action only when an incursion is detected.
  • Suppression - acknowledging that there will always be Covid in the community at some level or predators in the environment at some level and doing what you can to suppress it/them to an acceptable level. There is a constant need to continue suppressing.


How Covid-19 and predator elimination tools and settings compare



Predators (stoats, possums, & rats)

Working Environment

World where Covid-19 is in most parts under various levels of control but nothing approaching elimination

Country where predators are roaming in most parts under various levels of control but nothing approaching elimination

Target area for elimination

New Zealand

Specific predator free project

How do you know if elimination has been successful?

No community cases of Covid. All new cases are at the border

No predators detected in target area. All new predators caught or detected are at the border of predator free project

Do you have the tool to get the area to zero to start with?

For Covid we know we didn’t have it initially and had sensitive tests to measure new cases. No country has gone from wide spread Covid to be able to eliminate it

Intense poison (usually aerial) is currently the only proven method at scale. Methods used on remote islands not generally acceptable on mainland so for predators it is much harder. 

Sensitivity of testing tools

99% sensitive - with testing we can know with high accuracy that it is under control

10-30% sensitivity. The most sensitive commonly used tool for knowing if predators are there that is currently used is a trail camera and that is designed for pigs and deer not for fast moving small mammals.

Tools to stop re-invasion

Border controls, quarantine

Expensive predator fences are a proven solution. The next best we have so far are rivers and mountains for only some projects. Virtual and/or Active fences have some promise.

Tools to stop a localised outbreak

Tight lockdowns have been shown to get target area to zero by stopping transmission

Need tools that can eliminate to functional extinction so predators can’t breed. Yet to prove if this is actually possible. 

Tools to stop growth and/or reduce harm

Vaccines stop the spread and they also reduce the harm

There is currently no tool available with the same level of effect as a vaccine. Gene editing may get us there (we hope).


Why predator detection sensitivity matters for elimination strategy

Imagine if we had a Covid test that was designed for flu and missed 90% of Covid cases - would we have any chance of elimination with that? Well this is what we are currently doing with predators in NZ.  Despite years of evidence that the current tools miss most predators we still monitor using tools that miss most of them.  We continue to hope that we are doing complete predator elimination but we are not using the right tools to confirm whether this is actually the case.  The detection tools must be sensitive and efficient enough (near real time alerting) to be able to respond to outbreaks in time. 

Why a complete predator free barrier is essential 

If you are only catching Covid cases at the border then elimination within those borders is most likely working.  If you are catching predators only at the border of a predator free area then your predator control within that area is most likely working. Is this the case currently for any of the “predator free” projects?

Data we would love to see that would prove the current approaches are working

  • Annual number of predators caught over time. If you are catching predators all the time then you are clearly executing a suppression project with high ongoing costs, not an elimination or eradication project
  • Where you catch the predators - if predators are ALL caught at the edge of a predator free zone then this would show elimination is working.

Why elimination is more desirable than suppression

The NZ Covid elimination strategy has shown that done right it can have much better outcomes at a much lower long term cost than countries that have had to do suppression. 

NZ’s goal is to be predator free by 2050. Continuing high quality suppression will not achieve elimination. I don’t see any evidence that even with incremental improvements on suppression will get us anywhere near elimination.

The Covid example has made us familiar with the difference between elimination and suppression. We hope this analogy shows that at the moment most mainland projects are doing high quality suppression rather than true predator elimination. We do continue to be optimistic about the long term prospects for complete predator elimination but believe we need dramatically better tools than the current ones we are using for suppression.

As a country, we should be rightly proud of all the efforts we are expending towards making NZ predator free.  We should be rightly proud of the funds we are investing in these efforts. As responsible citizens and custodians of our wildlife, it is our responsibility to question whether those funds are achieving their goals.  If they are not, we need to find ways to target those funds more accurately.  Without developing the tools we need to achieve elimination, we risk having to commit those funds in perpetuity. 

As always, we welcome your feedback so don't hesitate to get in touch - leave a comment below or email us at blog@cacophony.org.nz.


Publication Date: 
Wednesday, 26 January 2022