Complex sentence: When she began observing birds, there were about 8,500 known species, compared to about 10,000 in the year of her death; she had seen and identified over 8,400, so nearly 85% of the known species in the world.
Split into simpler sentences: When she began observing birds, there were about 8,500 known species, compared to about 10,000 in the year of her death. By the time of her death, she had added to that list, having seen and identified over 8,400, so nearly 85% of the known species in the world.