Complex sentence: This is the only cactus species naturally occurring outside the New World, although it is believed that "R. baccifera" was introduced to the Old World by migratory birds, long enough ago for the Old World populations to be regarded as distinct subspecies.
Split into simpler sentences: This is the only cactus species naturally occurring outside the New World. It is believed to have been introduced to the Old World by migratory birds, long enough ago for the Old World populations to be regarded as distinct subspecies.