Real World Irruption

Irruptive population growth is easy to plot with pen and paper but does it exist in the real world?  The answer is a very definite "Yes" and some predators are experts at it.

In August 2017 we posted about the exponential growth potential of populations that ecologists call an irruption. These generate a characteristic 'J’ shaped growth curve, or “hockey stick” graph of population size over time. While these curves are easy to plot with a calculator and a sheet of graph paper, do they actually exist in nature?

Data from one of our own rodent control programmes demonstrates just how quickly an irruption can occur for fast breeding predators such as rodents. The control site was an area of regenerating coastal kanuka forest in Northland where the abundance of rodents was monitored monthly using 75 double snap trap stations spaced along a 1.5km trapline

The classic "J" shaped curve of irruptive population growth.

Additionally, rodent abundance was being controlled annually using pulse baiting in a 100m x 50m grid of poison bait stations. This was effective at reducing rodent abundance to very low levels and sometimes below the level that could be detected by the monitoring trapline.

Rodent control was undertaken during the winter months when rats and mice were not breeding with the objective being to have low predator populations during the spring breeding season of resident birds. Unfortunately, the arrival of rodents from neighbouring areas could not be prevented, therefore rodent populations increased through summer driven by a combination of birth rate and immigration.

A check of rodent abundance during March provided a late summer trapping index of 13.2 rodents per 100 trap nights. This was up from 8.9 rodents per 100 trap nights in February, almost a 50% increase in just one month. A pulse of poison control was implemented during May that reduced rodent abundance to 1.4 rodents per 100 trap nights in June.

During July, August and September rodent monitoring showed animals moved into the vacant habitat over the winter. However, winter mortality meant that by October the trapping index remained at 1.4 rodents per 100 trap nights, just 11% of the peak population recorded seven months earlier in March.

Low rodent populations during the spring breeding season helps the survival of eggs and chicks in the nest through the reduction of direct predation. However, low rodent populations right through winter also means food competition between rodents and wildlife is reduced for months before the breeding season meaning breeding wildlife will be in better condition to breed and the forest has more food available to support the raising of young.

With no rodent control planned until the following winter increasing rodent abundance was expected right through summer. There was no discernible increase in rodent abundance until January when the trapping index moved up to 2.2 rodents per 100 trap nights. By February it was 4.4 rodents per 100 trap nights, a 100% monthly increase, then in March it was 8.9 rodents per 100 trap nights, another 100% monthly increase.

By June the trapping index was at 35.1 rodents per 100 trap nights and the graph line was showing no sign of slowing down. It was only the commencement of the planned winter poison control operation that stopped the irruption. The peak rodent abundance was almost three times higher than the previous March and rodent abundance had doubled each month for at least six months.
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Population irruptions are real world phenomena. They can result from animals establishing new populations in suitable habitat where few limiting factors exist to check population growth or they can be driven by massive increases in food availability that occur during mast seeding years.

​However, they can also be triggered by management that upsets ecological equilibria. While predator control might sound like a great idea you must be careful what you wish for and be ready to contain the powerful forces of exponential population growth that you may unleash.

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