Exodus 23:29 says that Hashem promised not to drive out the indigenous peoples of Canaan too quickly lest the wild animals become too plentiful. This is repeated in Deuteronomy 7:22. However the previous verse states that Hashem will send hornets to wage the battles for them. I assume Hashem has equal control over hornets and all other animals. So why not chase the people out quickly and keep the animals from overrunning the land?
Ba'al Ha Turim quotes a midrash, saying that Gd knew that Israel would sin and cause His Providence to depart. This would leave them vulnerable to the feral beasts of the field if they conquered the land too quickly. So future sins would keep Gd from preventing the animals from overrunning the land.