Primary Succession
Primary succession begins with bare rock exposed to geological activity and ends with an ecosystem. In this case a temperate broad-leaf forest like Wissahickon Valley Park it goes rock-lichen-moss-grass-shrub-tree-oak trees
This photo shows primary succession, the development of a community where none was before. First to appearing on the bare rocks are lichens and algae. These secrete acids which begin to extract nutrients from the rock and which form tiny cracks which are widened by freezing and thawing. As the cracks widen they trap enough organic material and moisture for mosses to take hold. Larger cracks have enough soil to support grasses and small shrubs. The largest cracks come together to form small basins where trees can take root.
This photo shows primary succession, the development of a community where none was before. First to appearing on the bare rocks are lichens and algae. These secrete acids which begin to extract nutrients from the rock and which form tiny cracks which are widened by freezing and thawing. As the cracks widen they trap enough organic material and moisture for mosses to take hold. Larger cracks have enough soil to support grasses and small shrubs. The largest cracks come together to form small basins where trees can take root.
Secondary Succession
Unlike primary succession, secondary succession can proceed much faster because the soil has been prepared by the previous community which was removed by a circumstance such as a fire. If Wissahickon Park had been affected by a fire or another independent-density factor the order of rebirth would be- grass - shrub - trees - oak hickory forest.