Th ecosystems that exist today did not always exist. Thy have developed from other previous systems by succession. And many of them began on completely bare ground. Bare rock does not remain bare for long. Very soon, lichens can be seen growing on the surface of the rock. These extremely resilient organisms are able to colonize harsh environments and reproduce there. Thy are pioneer species.
Though the natural recycling processes the very presence of the lichens must change the abiotic conditions, making them less harsh. Th living lichens grow into the rock causing it to crumble. When the lichens die, decomposers act on the remains to release mineral ions into the crumbled rock. Th mixture of dead remains, crumbled rock and mineral ions forms a primitive soil. This less harsh environment is suitable for mosses (provided that there is sufficient water). So, spores of mosses that land there can now ‘germinate’ and the mosses grow, out competing the lichens in the changed environment.
This is the essence of succession:
• Organisms colonize an area.
• Thy change the abiotic (physical) conditions in the area.
• Th changed abiotic conditions allow other species to colonize the area.
• Th new species compete with the ones there before and become dominant.
• Thy also then change the abiotic conditions, more species enter and the process continues.
Th various stages in a succession are called seres. As successive producers colonize the area, they create more and different habitats and niches for other organisms to occupy. As a consequence, succession usually involves an increase in the complexity of food webs. The final, most complex, state of a succession is the climax community.
In the example shown in the above figure, you can see that as different types of vegetation enter the area, they affect the amount and depth of soil. This, in turn, allows other types of plant with more complex root systems to enter. The increasing complexity of the plant community will create more and more ecological niches and so more animals will also enter the area. Th species diversity will rise through the succession, until the climax is reached. Th climax is the most complex community that can exist under the prevailing environmental conditions.
Th following trends occur in any succession:
• Th total biomass of the community increases.
• Th species diversity increases.
• Th number of ecological niches increases.
• Food webs become more complex.
• Th community becomes more stable – can accommodate small changes/losses more easily.
A woodland climax can arise through a totally different succession. A lake or pond can undergo a succession that results in the water being replaced by sediments allowing land plants to grow and giving rise to a succession that results in woodland. Figure above shows how.
An open water area with little obvious vegetation can support animal life because of plankton in the water and smaller animals in the mud at the bottom.
Submerged plants become established in the sediment formed by the dead animals. Thy increase the amount of sediment as they die and larger plants
As time passes, more and more sediment fills the lake and larger ‘emerging’ plants become established. Eventually there is sufficient sediment to support deep-rooted trees and the climax woodland stage of the succession is reached. Both successions end with the same climax. Because the fist takes place from rock it is called a xerosere. Th second, starting from water, is a hydrosere.
Why do different areas have different climax communities?
Forest climax communities in Europe do not become as complex as tropical rain-forest because of the climate. Because of this, they are said to be climatic climax communities. Grassland in many areas would revert to woodland or forest if it were not grazed. Th grazing animals nip of the growing points at the tips of young tree shoots, preventing them from growing. Grasses grow from ground level, not from the tips of shoots, and so can regrow. These grasslands are grazing climaxes. Other factors that could influence the type of climax community formed include: temperature, precipitation (rainfall), soil type and soil depth.
Where a succession starts from bare, previously uncolonized ground, or from a newly formed pond with no life, the succession is a primary succession. Sometimes, communities are destroyed by fie or by a farmer ploughing a filed or by some other human intervention. When a new succession begins in such an area it is a secondary succession. This may result in the same climax as was
originally present or in a very different one. Although the process is still essentially the same (with pioneers colonizing and then being succeeded), secondary successions to the original climax are usually much quicker than primary successions because:
• the succession is not starting from bare rock/open water
• there is a seed bank of many of the climax plant types available in remaining undamaged plants
• the soil is already present