Which of the following ecologists has tried to put price-tags on nature's life support services?

Robert Constanza and his colleagues have very recently tried to put price tags on nature’s life-support services. Researchers have put an average price tag of US $ 33 trillion a year on these fundamental ecosystems services, which are largely taken for granted because they are free.

Paul Ehrlich- rivet popper hypothesis

David Tilman- long-term ecosystem experiments

Robert May places the global species diversity at about 7 million.