[Ryota]

Some people think the more species a community embrace, the more function the community can perform.
Is that mean the more is better?

I think a community has an optimal species to perform best: A community show the maximum productivity when there are 100 species, not 10 or 100.

How do you think? Do you agree or disagree that the function of a community is simply proportional to its biodiversity?

[Ophiolite]

If the environment can support and sustain more species then there are likely to be more functions carried out by those species, in total. The greater number of species expand the opportunities for interaction and for side effects of activity, which in turn increase the complexity of the environment, which allows for a greater range and numbers of functions.

More is no better unless you are placing a higher value on more functions. Nature doesn't give a toss, you might.

You switch from talking of functions to talking of productivity, as if the two are equivalent. They are not. Perhaps you had best define what you mean by function, productivity and better.