How might an introduced plant species kill native plant species?

What species of plants have life cycles that include more than one type of plant growth in the same species?

• I remember reading years ago about some plants where the plant grows and makes seeds, then the seeds grow and a plant that looks unrelated grows from the seeds. Then the new plant will grow and make seeds which grow into the plant that you started with. Can anyone please tell me what these plants are called, and where could I find information about them?

• Answer:

  Ferns have this life cycle a protothallus a sexual phase of reproduction that looks nothing like the adult fern. Think the term applied is alteration of generations