In this post Eeva Soininen, author of the paper ‘Under the snow: a new camera trap opens the white box of subnivean ecology’, tells us how the subnivean camera traps they have developed facilitate studies on the population dynamics of small mammals, even in thick snow cover.  

Every year, snow covers large parts of the Northern hemisphere. Life does not stop at the first snowfall; arctic small mammals are active below snow through the winter. Winters last for up to nine months, and what we know about northern ecosystems is mostly based on summertime observations. We assume that wintertime processes are important for small mammal population dynamics but deep snowpacks present an obstacle to research on these mammals. Small rodent populations often crash during winter – or, more precisely, we see a difference between autumn’s high and spring’s low population densities.

Every below-snow data point takes some effort in the Arctic. Image © Eeva Soininen

It is not that easy to find out what happens under the snow. Digging through three metres of snow can take several hours, resulting in just one sample and quite a lot of destruction of the subnivean habitat, as the tunnels in the snow are demolished. Fieldwork in snowy conditions is also somewhat tricky for us humans and the focus is often on staying warm…

Within The Climate Ecological Observatory for Arctic Tundra (COAT) we aim to understand how arctic food web dynamics work, and so we need to get good wintertime data. To achieve this, we developed a camera trap that can be left in the field in the autumn to get covered in snow, and is checked again in the spring.

The camera trap is a box with a camera attached in the ceiling, aimed downwards. The box has a hole in each end, forming a sort of a tunnel. Because small mammals like tunnels, they readily enter this artificial tunnel. When they enter, the movement sensor of the camera notices them and the camera takes a picture. We position the camera trap boxes along a rodent runway, so that it becomes part of their environment; a little like if someone constructed a bridge over a road.

Photo courtesy of Climate-Ecological Observatory for Arctic Tundra / UiT – The Arctic University of Norway.

The fixed distance of the camera from the animals makes it possible to quantify their size and to determine population reproductive status; here are images of an adult and a juvenile Norwegian lemming (Lemmus lemmus). Photo courtesy of Climate-Ecological Observatory for Arctic Tundra / UiT – The Arctic University of Norway.

Camera traps provide views we rarely otherwise see, such as these stoats (Mustela erminea).

Photo courtesy of Climate-Ecological Observatory for Arctic Tundra / UiT – The Arctic University of Norway.

With these camera traps, we can collect data on fine-resolution activity patterns of small mammals throughout the whole winter. We gain simultaneous and continuous data on both small rodents and their mustelid predators, which is exciting, as data on mustelids is difficult to obtain. The camera traps also provide temperature data, so that we can reconstruct the length of snow cover period. We hope this data will help us to better understand the winter ecology of small mammal populations.

For example, to date we have very little firsthand information on the winter crashes of small rodent populations: both predators and poor snow conditions can in principle cause rodent population crashes – but what their respective roles are in different conditions is unknown. Another interesting question is the extent of winter reproduction which we are now able to quantify based on the presence of juvenile individuals in the photos. Find out more by reading our paper in the first issue of the journal.

Eeva Soininen
Department of Arctic and Marine Biology
UiT The Arctic University of Norway