"Now that the wolves had taken most of the taiga, we were forced to march southward and settle ourselves on the very cold tundra, a place where there were too few monsters to hunt."
That's right, the foxes have been driven out of their former territory, slash, hunting grounds. The location of the biome must be near the south pole, so the temperature there is terribly harsh.
The colder the region, the fewer beings are able to survive that level of chill. Thus, there is a lesser source of food for the foxes, who are forced to leave there.
"And even if we go to the warmer parts, we cannot defend ourselves from the organized assault of the intellectual races, such as the werepeople of the north, the winged people of the east, the monster confederation of the west, and the mighty reptilians of the center."