Powerpoing: Feb 18 Ecological Succession

Learning Objectives 

• Describe the process of ecological succession
  Define pioneer species and climax community
  Define primary and secondary succession
• Describe the characteristic of species during early and late succession

Ecological Succession is the gradual process by which ecosystems change and develop over time.

Ecological succession can be described something like a story of an ecosystem. Lets start from the beginning.

Once upon a time, there was an ecosystem. Everything was well,  until catastrophe struck (DISTURBANCE)! A volcanic eruption had wiped away all life, and even the soil had been covered and burned (PRIMARY SUCCESSION). There was nothing left but bare rock. There was not a trace of soil to be found. It didn’t look like life was ever going to return.

Yet, there is hope.

Only a few months after the volcanic eruption, life was beginning to come back. Lichens and other hardy species, took root. They produced acids that broke down the rock, producing a thin layer of soil. Other organisms which need soil, such as weeds. Moss grasses then began to grow. As they died, they were broken down by microbes into soil and nutrients. The soil thickened. eventually shrubs and other longer lived, plants were able to grow. Animals that fed on these shrubs returned as well. Eventually, mature oaks, fir trees and other long lived trees came back. They outcompeted and shaded out the weeds and grasses that were there before. Eventually, the community stabilized (CLIMAX COMMUNITY).

Although the process of ecological succession itself may seem random, there are actually some predictable changes that occur.

1. Disturbance: the disturbance can be in many forms, from the fall of a log to an asteroid destroying 90% of all life on earth.
   • Primary succession: when the disturbance destroys or leaves no soil. Succession begins with bare rock. e.g. lava flow, a newly formed island and an asteroid
   • Secondary succession: when the disturbance destroys most of the life, but leaves the soil intact. Succession happens much more rapidly, since some seeds and life may still remain in the soil.
2. If it is primary succession, lichens are the first to colonize. We call these species that colonize first pioneer species. Pioneer species are the first to colonize an area that has suffered disturbance.
3. If it is secondary succession, because seeds and most underground life still remains, the speed of succession will be much higher.
4. The process of succession will continue, usually in the order of
   1. Weedy species (e.g. dandelions)
   2. Small shrubs
   3. Shade intolerant plants (e.g. Red Alder)
   4. Shade tolerant plants (e.g. Western Hemlock and Douglas Fir conifers)
5. Until a climax community is reached. A climax community is a community which remains relatively stable over a long period of time. Unlike earlier stages of succession, which may be as short as decades or centuries at the longest depending on local conditions, Climax communities may remain stable for thousands of years… until another disturbance resets the community.

Organisms in the early and late stages of succession have very different characteristics, since very different characteristics are needed for organisms to survive and thrive in early and late stages of succession.

1. Ability to Disperse: since organisms in early succession are in an environment with lots of spaces and little competition, a species that is able to disperse their offspring far and wide is most likely to proliferate and be successful.
2. Shade tolerance: species in the early stages of succession are in an area with few species. Tall trees that would shade the ground have not yet established. Therefore, species in the early stages of succession tend to be shade intolerant (need lots of sunlight). In the later stages of succession, because tall trees begin to shade the ground, species which can tolerate this shade are more likely to grow.
3. Lifespan: organisms in early stages of succession tend to be short lived. As the environment is constantly changing, having a shorter lifespan would mean that natural selection can happen at a faster rate (since each generation is short lived, and reproduction rates are high). This would mean the population as a whole will be able to adapt better to these changes.
4. Competitors: since not many species would have established in the early stages of succession species that establish in early stages would not benefit from being very good competitors, whereas species in later stages of succession need to compete with many other established species for space and resources.
5. Hardiness: the ability of an organism to be “hardy” (to live in an environment with extreme conditions such as low nutrients or blaring sunlight) is much more important in the early stages of succession than in the later stages.