The organisms that first move into an area after a primary disturbance are called pioneer species. These species are typically lichens, mosses, and some types of bacteria and fungi.

Here's why they're the first to arrive:

* They can survive in harsh conditions: Primary disturbances, like volcanic eruptions or glacial retreats, leave behind barren landscapes with little to no soil, few nutrients, and extreme temperatures. Pioneer species have adapted to these harsh conditions and can thrive where other organisms cannot.

* They help create soil: Lichens, for example, can break down rock through chemical processes, gradually creating a thin layer of soil. This soil then allows for other, less hardy plants to take root.

* They fix nitrogen: Some bacteria, like nitrogen-fixing bacteria, can convert atmospheric nitrogen into usable forms for other organisms. This helps build up the fertility of the soil, making it more suitable for further plant growth.

Over time, as pioneer species die and decompose, they contribute to the development of a richer and more diverse ecosystem. This process is known as ecological succession.