Complex sentence: The conservation potential of the island was seen as early as 1870, and it was reserved as a bird sanctuary in 1897 but it was not until 1987 that the New Zealand Department of Conservation took over the island.
Split into simpler sentences: The conservation potential of the island was seen as early as 1870. It was reserved as a bird sanctuary in 1897 but it was not until 1987 that the New Zealand Department of Conservation took over the island.