Cost-effective pest management relies on knowing where and when pest numbers are high so that management can be focused in the right place at the right time. Understanding how altitude and food availability regulate rat numbers should give conservationists the edge in protecting wildlife from rat plagues, which can lead to localised extinctions of native wildlife.
To tease out these factors, the researchers have been intensively monitoring rat population dynamics at both high and low elevations in forested areas near the lake.
Since the study began 14 months ago, 912 individual rats have been livecaptured and given a microchip and a metal tag in their ear before being released. Rats at high elevation are also being fed to see whether they can survive cold temperatures when they have sufficient food.
The preliminary results have been startling. Following the beech seed mast in 2019 the population density at Lake Alabaster reached a phenomenal 17 rats per hectare. This is one of the highest rat densities ever measured on the New Zealand mainland and reflects the incredible ability of rats to multiply rapidly following beech seeding.
Although food helped sustain the rats through the autumn, during the winter the rats being fed declined as much as the rats that were not fed. This suggests that another factor – perhaps temperature or predation by stoats – is limiting rats. Once this is known, it should be possible to create a ‘weather forecast’ about likely rat numbers for conservation managers, which will involve making predictions about rat numbers based on the climate and forest at a specific site. In turn, this will allow rat control to be done as effectively as possible, resulting in more birds in the bush.