What, in your opinions is the most sucessful complex organism on the earth?

My vote would go to  the decapods as they seem to be able to inhabit virtually every environment on earth with the singular exception of dry land - and even then some crabs travel surprisngly far inland!

No matter how extreme the environment, there always seems to be a shrimp or prawn that exists there - including environments which are not powered by the sun!

I'm no expert though and I'd love to hear other opinions!

From sheer ability to sequester resources, that would be Homo saipiens. Estimates exist that we sequester 40-50% of net primary productivity. With our technology we can also occupy a very wide range of environments. We also move huge amounts of soil and replace complex environments with our simple agricultural monocultures.

The deep ecologist in me is not happy about all of this but that is my answer.

"Hope is a duty from which palaeontologists are exempt."
David Quammen