Describe how new rock surfaces formed after a volcanic eruption became colonised by a variety of plants.

The bare rock surfaces formed after a volcanic eruption lack soil and are exposed to harsh weather conditions. In this lithosere the change is ocurring via primary succession, meaning that there has been no previous decomposition of plant material, and so there are no nutrients in the rock. There is no soil to retain water and the rock surface would most likely be exposed to the sun meaning that any moisture to settle there would quickly evaporate. A dry rock surface also reflects and absorbs heat, which could result in the temperature becoming too high for any but the most resilient pioneer species to colonise there. 

Lichens (the pioneer species) are able to survive because they are specialised to cope with the hostile abiotic conditions. Lichens can grow in cracks to avoid the wind and they secrete acids that erode rock to release minerals.

The pioneer species have now changed the abiotic conditions. As they die and decompose the lichens help form a thin soil, which thickens as more organic material is formed. This allows other species, such as mosses, to grow.

The basic soil helps to retain more water, allowing larger plants to colonise the area as the soil deepens (e.g. grasses and small flowering plants). The soil continues to become deeper and richer in nutrients as the larger plants die and are decomposed.

In the new, no longer hostile conditions, shrubs, ferns and small trees begin to grow. They are better adapted to the new environment and so can out-compete the grasses and smaller plants for space and nutrients.

In the final sere, the soil is rich and deep enough in nutrients to support large trees. These become the dominant species, and the climax community is formed.

Key definitions:

Pioneer species – the first species to grow on newly formed or exposed land (primary succession), or land that has been cleared of all plants, but where the soil remains (secondary succession)

Abiotic conditions – non-living chemical and physical factors, e.g. temperature, light intensity, moisture content of the soil, pH of the environment

Sere – stage of ecological succession

Lithosere – ecological succession beginning on a newly exposed rock surface, e.g. after a glacial retreat or volcanic eruption

Climax community - an ecological community in which populations of plants or animals remain stable and exist in balance with each other and their environment