Primary succession -the sequence of communities that forms in an originally lifeless habitat
-Primary succession occurs in new habitats that are without life, such as the cooled lava field, or the bare rock exposed by a retreating glacier. Primary succession is an orderly process.
-The first steps in primary succession are colonization by new organisms and formation of soil from exposed rocks.
-A lichen is a fungus and an alga living in a mutualistic relationship. Unlike most organisms, lichens can live on bare rock. Lichens secrete acids that break down the rock and form organic material by photosynthesis
-Lichens secrete acids that break down the rock and form organic material by photosynthesis. Rocks are also broken down by weathering caused by wind, rain, and frost.
-Scientists call the lichen community a pioneer community because it is the first to colonize a new area.
-Once soil has formed, grasses and other small plants begin to grow from seeds carried to the habitat by wind or animals. Root growth and the accumulation of dead leaves accelerate the process of soil formation.
-n time, the plants grow dense enough that lichens cannot get enough light. The lichens disappear. The lichen community is replaced by the grasses.
-The grass community survives for many generations and makes the soil deeper and more fertile. Eventually the soil is deep enough to allow the growth of nonwoody plants with deeper roots.
-These plants are taller than the grasses and shade them out in the competition for sunlight. Since both the grass and shrub communities are usually not diverse, a small disturbance may cause drastic changes in the community.
Why is the grass community vulnerable to even small disturbances?
-Since the grass community shows primary succession, it is not a diverse community and, hence, even a small disturbance may cause a drastic change.
-Pines or other trees with shallow roots invade when the soil is deep enough. As trees move in, the community changes again.
- Trees shade out the shrubs and grasses found on what is now the forest floor. The soil continues to deepen.
-Finally, broadleaf trees take root and hardwood trees begin to replace the pines. The hardwood forest is the final stage of succession in many areas.
-In the absence of external disturbances, this mature ecosystem will change very little over time. A community that does not undergo further succession is called a climax community.
-Climax communities are usually highly diverse and can often survive even severe local disturbances.
-Primary succession occurs only on freshly exposed rock or in places where a severe disturbance has destroyed all the organisms and the soil.
-Living things quickly colonize clear patches created by these disturbances.
-Succession that occurs where a disturbance eliminates most organisms but does not destroy the soil is called secondary succession.
-Fires, storms, and human activity are common initiators of secondary succession.
-Secondary succession resembles the later stages of primary succession. Fast-growing grasses and nonwoody plants are the first plants to colonize the area, followed by larger shrubs.
-Fast-growing trees such as pines then crowd out the shrub community, followed finally by slow-growing hardwood trees. Eventually, a climax community forms again, and the process of succession is complete.
-Results from long-term research have shown that many habitats never develop climax communities. If disturbances recur in less time than the several hundred years needed to complete the process of succession, a climax community may only rarely form.
-Periodic disruption maintains the balance in these communities.
-The process of succession also occurs in aquatic habitats.
-Islands undergo succession in much the same way as land on the continents. New islands can form quickly through volcanic eruptions.
-Living things are quick to colonize this new land. Seagulls are observed nesting on some islands