Primary succession is succession that starts on newly formed or newly exposed land where there is no soil, for example bare rock after a volcanic eruption, rock exposed by a retreating glacier, or newly formed sand dunes.The first organisms to colonise are pioneer species such as lichens, mosses and algae (marram grass on sand dunes). They are adapted to harsh abiotic conditions: they tolerate drought, extreme temperatures and a lack of nutrients, and they produce many easily dispersed spores or seeds. Lichens also release acids that help weather rock.When pioneers die and decompose, their dead organic matter (humus) mixes with weathered rock fragments to form a thin, simple soil. This soil holds more water and nutrients, so the abiotic conditions become less hostile.Mosses, grasses and small plants (sometimes called intermediate species) can now grow. Each stage is a seral stage. The new species change the environment further: soil becomes deeper and richer, and they may outcompete the species that came before. Shrubs follow, then trees.Eventually a stable, self-sustaining climax community forms, with high biodiversity and a large biomass. In most of the UK the climax community is deciduous woodland, such as oak woodland.