Final answer:
Lichens serve as pioneer species during primary succession by secreting acids to break down rock, facilitating weathering and soil formation, and paving the way for a climax community.
Step-by-step explanation:
Lichens are the pioneer species on a bare rock. In the process of primary succession, these organisms secrete acids to dissolve rocks and help in weathering and soil formation. The species that invade a bare area are called the pioneer community, and the community that is in near equilibrium with the environment is known as the climax community. In primary succession in water, the pioneers are small organisms such as algae and microscopic plants.
The correct fill-in-the-blank answers for the student's question, in order, would be:
- Lichens
- acids
- pioneer
- climax
- algae.