How Communities of Living Organisms Colonize an Ecosystem

2. What are pioneer species? What is the role of pioneer species?

Pioneer species are the first species that colonize places where there were previously no other living organisms, such as algae that colonize bare rocks. In general, pioneer species are autotrophs or maintain harmonious ecological interaction with autotrophic organisms(such as autotrophic bacteria, herbaceous plants, lichens).

A pioneer community is formed of species able to survive in hostile environments. The presence of these species modifies the microenvironment, generating changes in the abiotic and biotic factors of the ecosystem in formation. Therefore, they pave the way for other species to establish themselves at the location through the creation of new potential ecological niches.

3. What is the difference between primary ecological succession and secondary ecological succession?

Primary ecological succession is the changing sequence of communities starting with the first biological occupation of a place where there were no living organisms previously. For example, the colonization and the following succession of communities on a bare rock is a case of primary ecological succession.

Secondary ecological succession is the changing sequence of communities starting with the substitution of a community by a new one in a given place. An example of this is the ecological succession of the invasion of plants and animals on an abandoned crop or land.

4. What is the climax stage of an ecological succession?

The climax stage is the stage of the ecological succession in which the community of an ecosystem becomes stable and does not undergo significant changes. In the climax community, practically all ecological niches are explored and greater biodiversity is possible. In this stage the biomass, the photosynthesis rate and cellular respiration reach their maximum levels and therefore net primary production (NPP = organic material made by the producers – organic material consumed in the cellular respiration of the producers) approaches zero. During the climax, the amount of oxygen released by photosynthesis is practically equal to the oxygen consumed by respiration. (This is one more reason why it is wrong to say that the Amazon Rainforest, an ecosystem at the climax stage, is “the lung” of the earth. Other reasons are: because lungs are not producers of oxygen; and because the algae and cyanobacteria of phytoplankton are the main producers of the molecular oxygen on the planet.)

5. How do biodiversity, the total number of living organisms and biomass vary during ecological succession?

Biodiversity, the number of living organisms and the biomass of an ecosystem tend to increase as the succession progresses and stabilize when the climax stage is reached.

During the initial stage of succession, the use of carbon dioxide and the fixation of carbon into the biomass are high, since the total number of living organism in the ecosystem is increasing. During the climax stage, the use of carbon dioxide by photosynthesis equals its production by cellular respiration and the fixation of carbon into the biomass approaches zero.