Many researchers seem to equate diversity and complexity. May be most testable. May's tests of diversity and stability were based on randomly derived interaction matrices of various sizes, locations of species, etc. All organisms have to solve this common thermodynamic problem. Thus, the ultimate currency is energy.
Organisms come up with new inventions all the time to convert energy into fitness.
However, this may be an adaptive dynamic within individuals, and not necessarily an evolutionary process. These new inventions lead to increasing complexity and feedback loops between the need for more energy to maintain complexity and the need for more complexity to capture energy. The behavior of ecosystems may be derived from the adaptive behaviors of their components. Organisms must modify their environment to maintain states far from equilibrium. This makes it look as if the system were regulated.