What will eventually happen to mouse populations in Australia as they run out of food?

Answer 1

When food is scarce, mice produce fewer offspring; animals that eat mice either decline in numbers or move to some other prey; then that prey decreases, affecting other predators (including, sometimes, people). Eventually the mouse population recovers, but by then new food pyramids may have been established.

In other words, species aren't affected in isolation. When something changes for one group, other things change for many groups.