Primary Succession
Picture a barren volcano in Hawaii. Lava has streamed down over the lush landscape for thousands of years, leaving smooth, black rock behind. However, life can still take hold here through primary succession, which starts in a barren environment. Primary succession usually takes a long time, since the ecosystem has to start from scratch.
Primary succession starts from barren rock
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Secondary Succession
Secondary succession is like primary succession fast forwarded. It doesn't start from a barren location, but rather an ecosystem that has been through a natural or man-made disaster, such as a fire, clear-cutting, or a flood. The soil and seeds present aren't destroyed, so species that appear much later in primary succession, like shrubs and trees appear rapidly. Secondary succession skips the early stages where rock needs to be broken down into soil.
Secondary succession starts with existing plants
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Species Colonization
Now, let's look at the different types of species that grow, starting with the first species to appear in primary succession.
Pioneer Species
Picture a barren rock quarry. Humans have destroyed all forms of life, digging up valuable minerals in the area. Surely this disaster of an ecosystem couldn't support life. Well, think again because some plants will persevere. These species are called pioneer species, and they take hold even on empty rock to make a life when wind or animals bring seeds into the area.
Pioneer species are generally low laying plants with simple structures, such as lichen, moss and even some types of fungi. These plants change the structure of the rock, breaking it down into smaller pieces and eventually into soil for other plants to take hold.
Lichen and moss are the first species to take hold
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Grasses
Now that those pioneer species have created soil, grass can take hold. The grasses continue to change the soil, adding more nutrients. The grasses reproduce and bring with them bugs, small birds and mammals. These animals continue to help the grass reproduce, shaping the environment into a grassland. Secondary succession starts here and proceeds through the next steps much more rapidly than primary succession.
Shrubs
So, we have some grasses and lichen. We're still pretty far away from the beautiful forests that occurred before the ecosystem was destroyed. We need some larger plants to take hold with the nutrients the grass provides. The first to come in are shrubs that love the hot, baking sun present in the grassland. The shrubs bring new animals that bring more seeds and continue to change the ecosystem.
Young Forest
Young forests are next in succession and contain large amounts of young trees, called saplings. As the new ecosystem grows it becomes vertically stratified. Different species live at different heights, each needing different amount of light and catering to different species. As trees take hold, shade on the forest floor increases. Grasses and shrubs that are shade-intolerant will die out, with shade-lovers taking their place. This also affects the animal population in the forest. New animals that primarily live in trees and prefer the canopy begin to appear.
Sapling regrowth in secondary succession
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Old Forest
Old forests contain the mature trees that started off as saplings in the young forest. The exact definition of old forests is somewhat debated by ecologists, but they all agree on a few characteristics that describe old forests. First, there needs to be an accumulation of dead wood and bark on the forest floor. This enriches the soil when the matter is decomposed, making a more suitable environment for plants and animals. There is also significant vertical and horizontal stratification, where the type of species and prevalence change based on nutrient and water availability in the soil. However, old and young forests can be mixed together, with ecosystems exhibiting growth of both.
Finally, the ecosystem becomes relatively stable in a climax community. Like the name sounds, a climax community is the end point for succession. Plant growth and the type of species present remain constant. Although, there can still be small changes in a climax community, such as a tree falling from a storm, or human activity. These things create tiny pockets of succession within a fully developed ecosystem.
Lesson Summary
Primary succession starts from barren rock and secondary succession starts when plants are still present in the ecosystem. Both go through a series of steps where plant and animal life gradually colonizes the area. Primary succession starts with pioneer species that break down rock into soil. Grasses enter next, which occurs in both primary and secondary succession. The grasses make room for shrubs, and eventually saplings in a young forest which experiences vertical and horizontal stratification. The young forest matures into an old forest and eventually becomes stable in a climax community until another disruption starts the sequence again.
