Avalanches and the ecosystem

10 December 2019

Interview with 

Christian Rixen, Institute for Snow and Avalanche Research

CARIBOU

A caribou in a snowy forest.

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Avalanches are giant forces of nature with enormous destructive power; they kill plenty of people, around a hundred every year in the Alps alone. But with that destruction seems to come new creation, because research from Swiss scientists shows that whole ecosystems may in fact rely on avalanches. Christian Rixen told Amalia Thomas how he compared biodiversity outside versus inside an avalanche zone...

Christian - In our plant study we found, in the darker forest next to the avalanche tracks, about 10 different plant species; and in the central zone, with the highest disturbance of the avalanche tracks - with a frequency of an avalanche every year - we had around 30 plant species.

Amalia - That's three times the number of plant species in a place where avalanches occur regularly. But how can there be higher biodiversity where an avalanche has been? You would think that they wipe out the ground as well as plants and animals in their path.

Christian - The effect is not that disastrous that all the soil would be cleared. It's simply that the continued disturbance by avalanches keeps out the competitive taller plants like trees and also taller shrubs. So from a tree perspective an avalanche is clearly a disaster. However, for the smaller, less competitive plants, there's more space, more room, more light to grow. And that results in more species, higher biodiversity in avalanche tracks.

Amalia - So where the avalanche has happened around once a year, only the taller plants are uprooted without really disturbing the soil, allowing a wider variety of shorter plants to flourish instead.

Christian - So the plants that grow in avalanche tracks are very different ones. You also have some high alpine specialists, which would normally only grow at high elevation, because with the snow you can also have seeds and parts of plants going downhill, and then possibly growing and germinating at low elevation. Again, simply because there's space.

Amalia - And a wider variety of plants means more healthy and varied diets for animals that live in these ecosystems. Several species of animals, including bears and deer, think that those delicious plant salads in the tracks are worth the risk of getting caught in an avalanche.

Christian - Where you have a lot of plants growing, that is also interesting for food for animals. So for example in the Rocky Mountains, mountain caribou feed in avalanche tracks and not only in summer but also in winter. So when the snow goes downhill you have clear patches of grass and the animals need to find food, so they go to these grassy patches where they can eat. So that's a good thing for them in winter. However, that also comes at a certain risk. So while feeding in the avalanche, there could be a new avalanche starting and taking the caribou down. And 10-15% of mountain caribou feeding in these avalanche tracks might in the end become victims of avalanches themselves.

Amalia - But what is bad news for the herbivores like the caribou may be good news for other animals.

Christian - For example, the elusive and rare wolverine, a predator - or a scavenger - actually spends a lot of time in avalanche tracks searching for carcasses. And probably much of the food in winter for the wolverine comes from animals in avalanche tracks. So that is obviously a disaster for the individual caribou, but for the ecosystem, it is a very important process and an important natural disturbance.

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