Hi
Interesting ?? Would suppose the reason that there are so many species is that each has adapted to a narrow micro-climate ?? Certainly works that way with temp tolerance. Certainly don't see the extreme adaptations you see in other epiphytic plants such as orchids or epicactus where the leaves are modified to store water
maybe some really wise person will jump in and give us an answer?? lol gary

Thanks for the reply...I'm thinking that maybe the carnivorous aspect of nepenthes and other arborial pitcher species is secondary. Water retention was originally the primary purpose and having a free lunch was just secondary, evolving over time to become primary. Many bromeliads store water and that becomes a nice 'insect soup' that the plant can harvest when need be.