Introduction: Imagine a volcano has just erupted and the lava flowing down it is wiping out all of the trees, grass, rocks, houses, and everything else that gets in its way. Once the lava cools there is nothing left but a barren wasteland, devoid of life. However, in a few thousand years, that volcanic desert will be a flourishing ecosystem. How? Ecological succession.

There are two main types of succession. The key difference is that one relies upon soil already being present, while the other creates its own soil. In primary succession, some form of disturbance has occurred. This may be a volcano eruption, as described above (see image), or a glacier retreating. When this happens, it leaves behind rocks. Over time, lichens start to grow on the rocks. These organisms are called pioneer species because they are the first ones to live in this new habitat. Lichens secrete special acids that breakdown the rocks. Once the lichens die, the organic matter within their bodies is added to the rock dust forming an early soil (the making of good soil can take thousands of years). Next, seeds from small,grassy plants floating by settle out and start to grow. Once these die, their nutrients are added to the soil, which allows larger shrubs to settle in. Over time, these shrubs will be outcompeted by small trees, and then larger trees. Ultimately, a climax community will be reached.

Secondary succession occurs when soil is already present. Imagine a farmer has been harvesting the same fields for years. When he retires, there is no one to take over for her, so the fields go fallow. Since there is already soil present, and the plants from the farm have added nutrients to it, it will not take long before grassy plants start to grow (to see this in action, do not cut the grass for a few weeks during the summertime). Like with primary succession, the grasses will be replaced by shrubs, and ultimately by trees.

Once the climax community has been reached, the hope is that the new ecosystem will become stable and be able to support all of the organisms living within it until the next big disturbance occurs.

Directions for this Lesson: Answer the practice questions and then watch the video to learn more about how ecosystems change over time after a disturbance.