TASK DEFINITION: Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
PROBLEM: Fires, mainly caused by lightning, are a natural disturbance in temperate grasslands.

SOLUTION: Fires, mainly caused by what, are a natural disturbance in temperate grasslands?

PROBLEM: A change of season and less food usually cause certain animals to migrate.

SOLUTION: Which two environmental changes usually cause certain animals to migrate?

PROBLEM: Fresh water biomes are defined by low salt concentration.

SOLUTION:
What are fresh water biomes defined by?