Before an ecosystem is established, biological change occurs in stages in the environment. This Process is called succession.
When rocks, or the bedrock, came to the surface following the latest ice age, the first communities to colonise on the rocks were the lichens. Lichens are not plants but communities of algae or bacteria that co-habit symbiotically with fungi. Once lichens become established, the mosses come very soon afterwards and as they decay biomass is produced, namely soil. Once a density of biomass, or soil, is established, many more plants are able to colonise these rocks to form habitats.
Following the last ice age, Arctic-alpine plants established on the rough slopes and some of these are seen in the northern extremeties of Snowdonia to this day – the Snowdon Lily, purple saxifrage, starry saxifrage and Moss Campion are such examples. As the valleys and cwms warmed, oak, elm and birch arrived, trees that had spread gradually northwards from Europe and the arctic-alpine plants migrated to the higher slopes. The forest ecosystem reached is climatic maximum around 5000 BC when it reached 600m in Snowdonia. Since then, there has been a decrease because of climatic change and the introduction of grazing animals. The remains of trees and their history can be seen in the peaty bogs of Cwm Idwal, which reminds us that there were once plenty of trees here. The remains of the odd branch that can be seen, along with the pollen that is in the peat is a record of the change that took place here over time.