Ecological Succession
In 1980, Mount St. Helens in Washington State blew its top. Whole hillsides were buried under ash and rock. Yet a few years later, tiny plants were poking through. Decades later, parts of the mountain are green again. How does life come back to a place that looks completely dead, and why do some areas recover faster than others? By the end of this page you will be able to answer that.
What Is Ecological Succession?
Ecosystems are not frozen in time. A disturbance such as a volcanic eruption, a wildfire, a flood, or a bulldozer can wipe out or change a community of living things. After that, new organisms move in, and the community changes step by step.
Each group of organisms changes the place a little. Their changes make the area suitable for new species, and those species may crowd out the earlier ones. Over time, the number of species (the species diversity) usually goes up, and the community becomes more complex and more stable.
There are two main types of succession. The difference is whether soil is present at the start.
| Primary succession | Secondary succession | |
|---|---|---|
| Starting point | Bare rock or new land with no soil | Soil is still there, often with seeds and roots |
| Typical causes | Lava flow, volcanic eruption, retreating glacier, new sand dune | Wildfire, abandoned farm field, flood, logging, storm |
| First organisms | Pioneer species: lichens and mosses | Grasses, weeds, and wildflowers growing from seeds already in the soil |
| Speed | Very slow (hundreds to thousands of years) | Faster (decades to a century or so) |
Define ecological succession in your own words. Then name the two main types of succession.
Define ecological succession in your own words. Then name the two main types of succession.
A wildfire burns a forest in California, but the soil stays in place. Decide whether the recovery that follows is primary or secondary succession. Give one reason for…
A wildfire burns a forest in California, but the soil stays in place. Decide whether the recovery that follows is primary or secondary succession. Give one reason for your answer.
Primary Succession: Starting from Bare Rock
Imagine lava flowing from a volcano in Hawaii. When it cools, it becomes bare rock. There is no soil, so no seeds, roots, or decaying matter. How can anything live there?
The first organisms are called pioneer species. Lichens and mosses can grow on bare rock. Lichens release substances that slowly break the rock into tiny pieces. When pioneers die and decay, they add organic matter to those pieces. This is the beginning of soil.
As soil builds up, larger plants can take root. Each new group changes conditions: shade, more soil, more moisture. The community goes from lichens and mosses to grasses, then shrubs, then trees. Species diversity rises along the way.
After a volcano erupts and leaves only bare rock, living things slowly begin to return. Decide whether this is primary or secondary succession, and name one type of…
After a volcano erupts and leaves only bare rock, living things slowly begin to return. Decide whether this is primary or secondary succession, and name one type of organism that is usually the first to arrive.
The table shows the number of plant species found on a lava flow in Hawaii at different times after the lava cooled. | Years after lava cooled | Plant species |…
The table shows the number of plant species found on a lava flow in Hawaii at different times after the lava cooled.
| Years after lava cooled | Plant species |
|---|---|
| 5 | 2 |
| 25 | 9 |
| 60 | 18 |
| 150 | 30 |
Describe the trend in species diversity. Then explain how early organisms such as lichens and mosses help make it possible for new species to arrive.
Secondary Succession: Recovering with Soil
Now picture a wildfire in a California forest. The trees burn, but the soil stays. It still holds seeds, roots, and nutrients. Within weeks, grasses and wildflowers can sprout. Later come shrubs and young trees.
Because the soil is already there, secondary succession skips the slowest step: building soil from rock. That is why it is much faster.
Typical disturbances that lead to secondary succession include fires, floods, hurricanes, logging, and abandoned farms or lots. Humans cause some of them. Notice that who or what caused the disturbance does not decide the type. Only the starting condition matters: soil or no soil.
A farmer in Kansas stops mowing a field. Over the next 30 years, the field changes from grasses to shrubs to small trees. Identify the type of succession, and explain…
A farmer in Kansas stops mowing a field. Over the next 30 years, the field changes from grasses to shrubs to small trees. Identify the type of succession, and explain why it is usually faster than succession on bare rock. Mention what is already present in the soil.
Mount St. Helens erupted in 1980. Some areas were buried under thick ash and rock, while other areas kept some soil and surviving plants. Explain why the two kinds of…
Mount St. Helens erupted in 1980. Some areas were buried under thick ash and rock, while other areas kept some soil and surviving plants. Explain why the two kinds of areas may recover at different speeds. Name the type of succession in each area.
Populations, Diversity, and Stability
As succession moves along, two things change: the populations (how many of each species) and the species diversity (how many different species).
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Early on, a few pioneer or fast-growing species have large populations.
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As conditions improve, new species arrive and diversity rises.
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Pioneer species often decline as shade and competition increase.
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Eventually the community becomes fairly steady. Many ecologists call this a stable (mature) community. It keeps changing slowly, but its overall makeup stays about the same unless another disturbance hits.
A graph of species versus time usually rises quickly and then levels off.
A second disturbance sets the community back, often to an earlier stage. Diversity drops, and the recovery restarts.
A town clears a forest to build a parking lot. Years later the lot is abandoned, and cracks in the pavement slowly fill with weeds, then shrubs. Marcus says this is…
A town clears a forest to build a parking lot. Years later the lot is abandoned, and cracks in the pavement slowly fill with weeds, then shrubs. Marcus says this is primary succession because humans caused the change. Aaliyah says it is secondary succession. Evaluate both claims. Decide who is correct, and justify your answer using what defines each type of succession. Then predict how the number of species and the kinds of populations will change over many decades.
Two sites in the same region were disturbed. Site 1 was scoured down to bare rock by a glacier-fed flood. Site 2 was a pine forest destroyed by a wildfire. After 20…
Two sites in the same region were disturbed. Site 1 was scoured down to bare rock by a glacier-fed flood. Site 2 was a pine forest destroyed by a wildfire. After 20 years, Site 2 has 25 plant species and Site 1 has 6. After 200 years, both sites have mature communities with similar numbers of species. Compare how populations and species diversity changed at each site, and explain why Site 2 recovered faster. Then describe how a second disturbance, such as another fire at Site 2 after 10 years, would likely affect its diversity compared with no second disturbance.
Back to the start: at Mount St. Helens, areas buried under thick ash and rock began with no soil (primary succession, slow). Areas that kept soil and surviving plants recovered by secondary succession, which was much faster.