In the first step of Primary Succession there is only bare rock. There are no organisms present.#160;
Fourth Step of Succession
In the second step of Primary Succession pioneer species begin to grow. Lichens are usually the pioneer species. Lichens are fungi and algae growing together. They form a symbiotic relationship called mutualism. Following lichens, moss begins to grow. The pioneer species break down the rock into soil. 
Fifth Step of Succession
In the third step of Primary Succession small grasses begin to grow.  Insects and small rodents also begin to populate the area. 
Fifth Step of Succession
In the fourth step of Primary Succession small shrubs and bushes begin to grow. Also, small animals like squirrels begin to appear. 
In the fifth step of Primary Succession small trees begin to grow.  Larger animals also appear. 
In the fifth step of Primary Succession large trees grow, and large animals appear. The ecosystem reaches climax community in the fifth step of Primary Succession.