Ecological succession is the change within an ecosystem over time. This occurs as rocks give way to soil, which gives way to vegetation, which turns into forest. Some species will become less abundent due to different food availability while others may become more abundent, and new species may come into the ecosystem.
Pioneer species are the first species to colonize areas which were previously devoid of life. For the most part pioneer species will be small, simple organisms that can produce their own food. This would include things like lichen, algae, bacteria, and herbaceous plants. Pioneer species will typically move into a hostile environment where no other form of life could survive. They will then, simply through living their life, eventually create an ecosystem in which other species are able to survive.
Example of Pioneer Species:
-Lichen will fasten themselves to bare rocks where they will begin to break the rock down into soil. Although the process is slow and tideous, eventually there will be enough soil for a small plants or mosses to develop. These plants will attract insects and herbivorous animals which will eventually attract predators eventually creating an ecosystem of primary succession.
Lichen come in many different shapes and sizes, and due to their nature it is almost impossible to identify them. Lichen are actually made up of two different organisms, a fungus and an algae. They live together in a symbiotic relationship where they depend on one another. Lichen have a unique way of breaking down rock into small granules of sand and soil.
Autotrophic bacteria, along with all of the plants and algaes in the world, create all of the Earth's oxygen. Without the oxygen made by these bacteria the Earth would not be able to harbor any life at all. These bacteria come into a barren area and bring the oxygen and glucose neccessary to make way for other organisms.
Ferns are some of the simpler plants on Earth. Although they have stems, leaves, and root systems like most other plant families, ferns reproduce through the use of spores, much like fungi and mosses. A lot of people have the misconception that ferns are only found in moist, cool places. This is completely untrue, as ferns can grow in high alpine areas as well as in dry desert biomes.
Mosses are even simpler than ferns, as they lack veins, stems, roots, seeds, and flowers. In order to stay anchored to the ground they depend on small fibers called rhizoids that anchor them to their home.
Going hand in hand with pioneer species, primary succession is the transformation from a barren, lifeless environment into one that supports life. The example above is a great example of primary succession. When the lichen first settled on that bare rock, it was the only thing there. Eventually with the assistance of the lichen the ecosystem was able so support much more biodiversity.
Now that the Earth is so old, you may not think that there is anywhere left to pioneer. This is not true, however, as the Earth is constantly changing and shifting. Events like lava flow, mud slides, land slides, and other forms of disturbance provide the perfect chance for pioneer species to begin their work all over again. Fire and logging are other events that may pave the way for pioneer species to start a new.
Secondary succession is the changing of one community to another. Unlike primary succession, secondary succession occurs only in places where there is already life present. A good example of this would be to look at a fallow field (a field that once grew crops, that has since been left to nature.) Witin a fallow field, grasses and weeds will start to grow, attracting wildlife that was never present when the crops were growing. Thus bringing in a new food web with a lot of new characters (plants, animals, etc...) In the picture above, you can see the trees in the background, if this field was left to fallow long enough, it would eventually revert back to the forest stage you see behind it.
Why Does Ecological Succession Occur?
Ecological succession occurs because all species have a special set of needs within their environment (food, water, shelter, etc...) Once a species has established itself within an ecosystem it will go about its daily life eating, drinking, sleeping, and reproducing. During the organisms everyday activities, however, it is changing the environment around it. Eventually the changes will become more obvious, and the environmental conditions in the area, which were originally perfect for the species, have now become perfect for another type of species. This fact will either force the original species to move elsewhere, or kill them off altogether, depending on the species individual ability to adapt and cope.
The climax stage of ecological succession occurs when an ecosystem has finally stabilized. This means that the Net Primary Production (organic material produced vs. organic material consumed) is at zero. This means that all of the oxygen produced is consumed, and all of the glucose produced through photosynthesis is used up. In the climax stage almost all ecological niches have been filled, and there is the greatest amount of biodiversity. Rainforests are a great example of ecosystems in their climax stage.
The Order of Succession:
Although the order of succession may be slightly altered depending on the climate and location, there is a standard method to the madness that is ecological succession. 1) Pioneer species 2) Grasses 3) Shrubs and Brush 4) Trees begin to grow 5) Eventually the area will reach the climax stage and stabilize until some other force turns it into a barren land all over again (fire, clearcutting, etc...)