Final answer:
The main factor accounting for the longer time frame required for primary succession compared to secondary succession is the initial absence of soil in areas undergoing primary succession.
Step-by-step explanation:
The main factor for the difference in time between primary succession and secondary succession is due to the presence of soil. In primary succession, new land is often formed from volcanic activity or after glaciers retreat, leaving a surface where no soil exists. This demands the slow process of soil formation, starting from the breakdown of rock by pioneer species like lichens and hearty plants. These species contribute to soil formation, allowing other species to establish and evolve the ecosystem over thousands of years.
In contrast, secondary succession occurs in areas where a disturbance has affected an existing ecosystem but left the soil intact. As a result, plant growth can commence almost immediately, and the area can be repopulated within decades. Ecological recovery is much faster when the foundation for plant growth—soil—is already present.