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

Primary succession

An ecological succession refers to the changes that occur in a biotic community with respect to the changes that occur in the non-living environmental factors such as soil, water, air and others with respect to time. These changes occur until the ecosystem attains the stability. The primary succession occurs in a region which is not previously occupied by any living species. The land is typically barren and in the initial stage of colonization is unable to sustain growth of life forms. The species of organisms that grow or colonize in these lifeless habitat are called as primitive species and are a part of the primary succession.  

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during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.