Covid-19

While Covid-19 is a human disease sweeping the world, it exhibits similarities with invasive predators and shows the difficulties of eradication and reinvasion prevention.

The rapid establishment, development and acceleration of the global coronavirus pandemic has very quickly introduced people across the world to biological patterns that are all too familiar to predation scientists. And as with predation, for some individuals their interaction with this invasive organism has sadly been fatal, the outcome of most predatory interactions.

New Zealand’s response to Covid-19 has been world-leading and currently the novel coronavirus is considered to be eliminated from Aotearoa as there is no transmission of the virus within the community. Understanding how this was achieved illustrates many parallels between fighting to eradicate a virus and fighting to eradicate predators. So does the eradication of Covid-19 tell us anything about the eradication of invasive predators?

Invasive organisms come from somewhere. As predators colonise new habitats, either by natural invasion, or deliberate introduction, so Covid-19 was first detected in New Zealand when an infected traveller arrived from the Middle East.

Once established, the size of all biological populations is governed by birth rate and immigration that increase numbers and death rate and emigration that decrease numbers. How these two pairs of factors balance each other drives changes in population size.

Numbers of invasive organisms increase slowly at first then accelerate quickly to produce irruptive geometric growth that does not slow down until the effect of limiting factors put on the brakes. These limiting factors “flatten the curve”. And so it was with Covid-19. After 14 days New Zealand had six cases. It then doubled in three days and doubled again in the next two days. Within a month there were more than 500 cases and then this number doubled within a week.

A graph from Waikato University tracking New Zealand's daily number of new coronavirus cases (columns), the total number of cases showing irruptive growth (red line) and the number of active cases (blue line) falling to zero as eradication was achieved. The green line shows recovered cases.

​Initially this growth was powered solely by the immigration factor but increasingly the birth rate, or infection rate factor also kicked in as local infections increased from arriving travellers. New Zealand’s largest coronavirus cluster, a subpopulation of viral infections, established at a wedding that lasted a few hours and was attended by one traveller who likely did not know they were carrying the virus. The origin of New Zealand's second largest cluster is not known and is unlikely to be traced.

As with predators, the key to winning the war of attrition is to force the reproduction rate of the invasive organism below one. This slows the rate of increase to below the death rate and time takes care of the rest.

And so it was with Covid-19. Barely two weeks into the fight large gatherings were banned and arriving travellers had to self isolate for 14 days, a period longer than the incubation period of the virus. This meant that someone infected while waiting for their baggage at the airport could be confirmed ill before they joined the general population.

Three days later the border was closed, except for returning New Zealanders. These border restrictions decoupled the immigration factor from the birth rate factor so they could be attacked separately.

Propagation of the virus within the country was aggressively suppressed when the whole country was ordered into a 28 day lockdown when only essential workers could leave their local area. Just 11 days later, approximately one virus incubation period, the daily increase of infections peaked and began an inexorable slide to reach zero after a further 48 days, or approximately four virus incubation periods.

So it is with predator control. Eradication can only be achieved if you can secure the perimeter of the control area. Islands have natural barriers at the border but pest-proof fencing can also secure the boundary of an area from continual immigration. Without successfully stopping immigration, population suppression is the best possible outcome. Many countries have attempted this with coronavirus with variable success.

Motuihe Island, a predator free island in Auckland's Hauraki Gulf.

Once the perimeter is secure, aggressive control can begin. Masks, social distancing, hand washing and cough etiquette work as limiting factors to slow the spread of Covid-19 making it difficult to find new hosts. When a vaccine is available this can break the chain of disease transmission. Without a control agent, the invasive organism has to be hunted down and eliminated. For disease this is community testing, contact tracing, isolation and patient hospitalisation.  For predators, it is monitoring and traps and toxins. 

For predator populations seasonal food restrictions and winter weather are limiting factors that restrict population size. Predator numbers annually rebound and occasionally skyrocket when food supply and breeding conditions are most conducive during forest mast years. So it is with viral pandemics when a second wave can sometimes be more infectious and debilitating than the first.

So is eradication job done?

With both disease and predators this is a milestone to be celebrated but it is also the beginning of a very dangerous time with the biggest enemy being complacency. New Zealand’s border is still closed and likely to remain so for an indeterminate time. All returning residents must complete a mandatory two-week isolation to preserve the integrity of the perimeter protection. Potential reinvasion is monitored with disease testing in quarantine.

In two months since Covid-19 elimination was achieved, 50 infected arrival passengers have been detected, each one a reinvasion risk. Alongside this, disease testing in the general population is also continuing in the hope that any coronavirus infection that is hiding amongst seasonal cold and flu infections can be quickly found, contained and eliminated.

So it is with predators. With eradication complete, effective perimeter surveillance is crucial but persistent habitat monitoring is vital. Without it a reinvasion may be undetected before significant impacts occur.

Covid-19 is an illustrative fast paced parallel with attempts to eradicate invasive predators. Most usefully it also highlights post-eradication risks and demonstrates the significant ongoing efforts that are required to mitigate these.

Unfortunately, it also demonstrates the potential impacts of falling at this hurdle. The reality of such a failure has already been experienced at a perimeter fenced wildlife sanctuary. This must serve as a “canary in the coal mine” for larger and more ambitious landscape scale predator eradications currently being attempted. Getting post-eradication right is possibly more critical than achieving the eradication in the first place.