Tundra

Why is the tundra considered a fragile and unstable biome

Fragile, yes. Not necessarily unstable unless we humans mess it up.

The reason it's fragile has to do with the cold temperatures and difficult growing conditions, with a very short growing season each year. All organic matter grows - and decomposes - much, much more slowly than in warmer climates. It can take years for a tiny Arctic flower to mature: push its stalk above the ground, slowly develop leaves to protect itself against the cold, slowly slowly develop a flower and then seeds... things that can happen in a matter of weeks in the tropics.

So any environmental damage takes much longer to regenerate - whether it's tracks made by vehicles, bushes crushed, not to mention the danger of oil spills.

The animals and people who live (naturally) in these areas have long since found a balance with their surroundings. They take no more than is necesary, and use everything, carefully. They have adapted, as has the plant life..

Modern technologically-focused societies haven't understood this, and have been rushing in to take out the mineral resources, without paying attention to the fragile natural surroundings.

er...yes ^
xD