Complex sentence: He speculated that the population had been adversely affected by the advent of extensive cattle grazing and changes in the fire regime since European settlement of the Roper region, and raised the possibility of birds surviving along rivers north of the Roper in eastern Arnhem Land.
Split into simpler sentences: He speculated that the population had been adversely affected by the advent of extensive cattle grazing and changes in fire regime since European settlement of the Roper region. He also raised the possibility of birds surviving along rivers north of the Roper in eastern Arnhem Land.