Ecological Succession

Community Development Over Time


Communities which have reached a state of equilibrium are in the climax stage of succession. They are characterized by the following:

  1. each individual of a species that dies is replaced by another of the same species and
  2. the primary productivity minus the community's respiration rate is close to zero.

 

Communities change with time under geological and climatic pressures and well as biological influences including human intervention.

 

Change brought about by the communities own members is called autogenic succession while change resulting from outside forces is allogenic succession.


During succession the number and types of species within a community change often bringing about their own replacement. Each new step is called a seral stage. The entire series of sequential changes involved in succession is a sere.

 

Ecological succession includes primary succession and secondary succession

 

  1. Primary succession
    1. Primary succession occurs where their is little or no soil and no living organisms have become established. Examples include lava flows, newly formed volcanic islands, river deltas after flooding. Hardy pioneer organisms, such as mosses and lichens, are first to get a foothold. They are followed by grasses and sun-tolerant plants.

 

  1. Secondary succession
    1. Secondary succession occurs after a climax community has been disturbed. Soil and a variety of plants are already present. Examples include deserted farms and orchards, burns or clearings. The first plants to appear are once again pioneer species, often opportunistic, fast-growing "weeds".

      Periodic fires that restart secondary succession are not considered an autogenic factor because fire is not a factor intrinsic to the functioning of a community, it is an outside factor.


Forces Driving Succession.

Three models have been proposed to explain the progression of stages (seres) that appear during succession. They are:

 

  1. The facilitation model of succession proposes that one species or group makes physical changes that favor the establishment of another. This normally occurs in primary succession. Pioneer species change the environment enriching it so other organisms may find their niche.

 

  1. The inhibition model of succession is so called because it posits that a pioneer species inhibits the establishment of other species. Some plants are known to do this by producing chemicals that retard others. Because most pioneer plant species have short life spans they can be replaced by longer-lived species.

 

  1. The tolerance model of succession states that pioneer plants retard the growth of others because their rapid growth rate and development smothers their competitors out at first. such plants are likely to be less efficient in other ways and eventually give way to stronger more durable species.

 

  1. According to the nonequilibrium model of succession, chance events such as dispersal and disturbance play major roles in succession, and species composition continues in flux until a stable climax community is reached.


Climax Communities

Climax communities are those whose net production and utilization are in equilibrium. and whose cycling of nutrients occurs directly between the organisms and decomposing material via exchange pools (rather than from hard to get at environmental reservoirs).

Biodiversity is greater in climax communities and food webs become more elaborate.

Finally climax communities are much more stable than the transition stages leading up to them.


Modified Nov. 7, 2002