Tundra ecosystems are considered to be fragile because of

Low biodiversity

Explanation:

Tundra biome is characterized by presence of a very few species. Low biodiversity in Tundra is the reason for presence of only a number of species in the food chains. Less diversity in food options makes the system prone to severe unintended complications due to elimination or reduction of any species from the system.

For example, Lichens in Tundra of Alaska accumulated radioactive materials (strontium 90, etc.) after nuclear testing and affected the population size and health of their predator species.