Complex sentence: The total population was thought to be only about 2,000 in 1989, but in more recent studies it was found to be about 4,000; this increase is due not to population growth, but to improved data collection.
Split into simpler sentences: The total population was thought to be only about 2,000 in 1989, but in more recent studies it was found to be about 4,000. This increase is due not to population growth, but to improved data collection.