The first organisms to colonize a newly formed, lifeless habitat are part of

The first organisms to colonize a newly formed, lifeless habitat are part of "a primary succession".

Primary succession is going to be par of the biological and the ecological succession, but there is another type as well.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.