Complex sentence: In the mid 1980s gray wolves recolonised the Bow Valley in Banff National Park after they had been absent for 30 years, as a result of systematic predator control hunting which began in 1850.
Split into simpler sentences: In the mid 1980s gray wolves recolonised the Bow Valley in Banff National Park. They had been absent for 30 years due to systematic predator control hunting which began in 1850.