After a primary disturbance, such as a volcanic eruption or glacial retreat, an ecosystem starts from scratch. The first organisms to colonize this barren land are called pioneer species.

Here are two characteristics of pioneer species:

1. Tolerance to harsh conditions: Pioneer species are highly tolerant of extreme conditions like poor soil, lack of nutrients, and intense sunlight. They can withstand these challenges and start the process of soil formation.

2. Fast growth and reproduction: They grow and reproduce quickly to quickly establish themselves in the newly forming ecosystem. This allows them to rapidly spread and take advantage of the available resources.

Examples of pioneer species include:

* Lichens: These symbiotic organisms are able to break down rocks and contribute to soil formation.

* Mosses: These small plants can tolerate harsh conditions and provide a base for other organisms to grow.

* Grasses: Grasses can quickly establish themselves and stabilize the soil.

* Nitrogen-fixing bacteria: These bacteria convert atmospheric nitrogen into a form that plants can use, enriching the soil.

These pioneer species pave the way for other organisms to enter the ecosystem, leading to a gradual process of succession until the ecosystem reaches a stable climax community.