Forest Black Rats

The annual population cycle of forest rodents is one half of a predator prey cycle between rats and stoats that catches native wildlife in the crossfire.

The most common rodent in New Zealand forests in the black rat, also called the ship rat, house rat or roof rat. Despite its name, it is often grey in colour and is identified by its tail being as long, or longer than its body. In contrast, the brown, or Norway rat has a tail that is shorter than its body.

In forests, ship rat populations cycle in numbers throughout the year. They breed rapidly during the warm days of summer before numbers decline through the cold weather of winter due to natural mortality and predation by stoats.

Rodent population levels can be assessed using a number of techniques. Run tunnels are commonly used where rat population levels are judged by the percentage of tunnels that record rat footprints. Capture rates of rodents caught in snap traps are also informative and can be used to quickly index rodent population levels during the year.

Our rodent trapping programme has just completed a full annual cycle of trapping and the results are plotted in the graph below. The trapline was established in an area of recovering native forest on the East Cape in June 2018 and over the year to June 2019 a total of 520 trap nights were set.

The June 2018 trap results have been discarded as rats are notoriously suspicious of new objects in their environment. As expected, rat population levels declined steeply through late winter to reach a low point in September. The population then began a slow increase through the spring and into summer.

Snap trap capture rates of black rats in an East Cape recovering native forest.

However, the 2018/19 summer turned out to be a long drawn out season and with regular rain the East Cape did not experience drought. As a result, the black rat population continued to breed well into the autumn and by early winter it had reached a staggering level of 27 rats per 100 trap nights.

News reports are now carrying stories of 2019 being a mega-mast as forest trees have abundantly seeded and fruited in unison to provide a massive food supply for all forest inhabitants. However, because rodents can breed so quickly their population can rapidly respond to this food supply and the trap results prove they have done exactly that.

What the trap results don't show is that stoat numbers will also have increased in the forest as their rodent prey food supply has exploded over the summer. Both rats and stoats will be directly impacting native wildlife through predation, however the worst is yet to come.

As the 2019 winter progresses through July, August and September the high rodent numbers will naturally decline as the graph recorded in these months last year. Stoats will be a factor in this decline, however as the stoat population faces declining prey abundance these predators will switch their attention increasingly to native wildlife just as preparations begin for the 2019 spring breeding season.

This cyclical predator-prey relationship between rodents and mustelids in New Zealand forests catches native wildlife in the crossfire. In mast years these impacts are magnified and the grim reality is that native wildlife in our East Cape native forest faces a very bleak immediate future.

​The trapping results of the past year illustrate a pattern that will have repeated itself throughout the country. They are alarming and they refocus our mission; "To build a platform of pest and predator control to shift the paradigms of biodiversity protection".