What is the smallest level of organization that includes more than 1 types of organism?

Answer 1

The levels of organization of an ecosystem in order are:

organism/species, population, community, ecosystem

Obviously, a species has one kind of organism.
A population is the number of organisms of one species in the ecosystem (like all of the white oak trees). So they're all the same kind.

A community includes all the populations, or all the living things in the area. So that's the first one with more than one type of organism.

The ecosystem includes all the living and nonliving factors - the community plus the nonliving or abiotic factors.