Chapter 14 - Succession

Obj. Assessment Statement Notes2.6.5 Describe the concept and processes of succession in a

named habitat.Page 265-270

Students should study named examples of organisms from a pioneer community, seral stages and climax community. The concept of succession, occurring over time, should be carefully distinguished from the concept of zonation, which refers to a spatial pattern.

2.6.6 Explain the changes in energy flow, gross and net productivity, diversity and mineral cycling in different stages of succession.Page 265-274

In early stages, gross productivity is low due to the initial conditions and low density of producers. The proportion of energy lost through community respiration is relatively low too, so net productivity is high, that is, the system is growing and biomass is accumulating. In later stages, with an increased consumer community, gross productivity may be high in a climax community. However, this is balanced by respiration, so net productivity approaches zero and the production:respiration (P:R) ratio approaches one.

2.6.7 Describe factors affecting the nature of climax communities.Page 266-267

Climatic and edaphic factors determine the nature of a climax community. Human factors frequently affect this process through, for example, fire, agriculture, grazing and/or habitat destruction.

Ecological Succession

• Succession is the gradual change in species composition of a given area

• Change occurs in response to changing environmental conditions

• Succession can also be describe as a sequence of communities (a sere) with each transitory community as a seral stage.

• Two types of succession: Primary and Secondary • Primary succession – Establishing communities on

nearly lifeless ground. • Secondary succession – Changing species

composition in an area where a biotic community already exists

Primary Succession

• Places where primary succession occurs: bare rock; cooled lava flows; abandoned pavement.

• Before a biotic community can be established, there must be a soil.

• Terrestrial pioneer species can attach themselves to bare rock/barren areas and begin the soil formation process

• Examples: Moss, lichens, certain grasses

• Pioneer species extract nutrients from precipitation, dust, bare rock.

Soil Formation

• Trap wind blown soil particles and detritus • Produce small amounts of organic matter• Secrete acids that weather the rock to break it down

• Once it has begun to break down, physical weathering (freezing) breaks it down even more

• As patches of soil build up, the pioneer species is replaced by early successional plant species.

• This process takes hundreds to thousands of years

Early Successional Species• As soil begins to form, small grasses and ferns begin

to grow. • Seeds dispersed to the area by wind or birds

• Early successional species grow quickly and have short generations and life spans.

• Early successional species continue to break rock by growing roots

• Their short life span means dead organic material quickly builds on the small layer of soil.

Mid-successional Species• Early successional species continue to add

layers to the soil horizon. • When the soil is deep enough (hundreds of

years) and can hold enough moisture and nutrients, mid-successional species take root. These would include taller grasses and ferns, and low shrubs. Eventually these are replaced by trees species that need plenty of sunlight (shade intolerant).

Late Successional Species• As shade intolerant trees grow, shade is produced. This allows

for shade tolerant tree species to begin to grow. • The shade tolerant trees (late successional species) will

outcompete the shade intolerant species and take over. • This is the end stage of succession and an equilibrium is

reached. A climax community is established. • Climax community – A community of organisms that is in

equilibrium with natural environmental conditions; the end point of ecological succession.

• Primary Succession

Secondary Succession• A disturbance to an already established biotic

community allows for the succession process to start again.

• Examples: an abandoned field; burned forests; heavily polluted streams

• Secondary succession can proceed much faster because the soil has already been prepared by the previous community

• Primary and Secondary Succession

Communities in Succession• The actual climax community found in an ecosystem depends

heavily on the climatic factors and soil (edaphic) factors at that location.

• Leading up to the climax community, it is interesting to note that the presence of the pioneer, early, mid-successional organisms essentially alters the environment (eg. shade, pH, moisture, temperature, chemical composition) sufficiently so that it is no longer suitable for their own offspring to continue

• That is, the populations change the environment so drastically, both biotically and abiotically that it is now more suitable for other populations and less suitable for itself.

• This leads to the succession of one sere after another until a climax is reached and the changes in the environment are now not so drastic as to produce any further major changes.

Succession and Productivity

Ecosystem characteristic

Trends in ecological succession

Gross productivity (GP)

Increasing during early stages of primary succession then little or no increase during final stages of secondary succession

Net productivity (NP) Decreasing

Respiration (R) Increasing

Trends in SuccessionEcosystem

characteristic

Trends in ecological succession

Food chains Simple food chains becoming more complex food webs

Relative species abundance

Changes rapidly first, changes slower in the later stages.

Total biomass

Increasing

Humus (non-living organic matter)

Increasing

Species diversity

Low diversity in the early stages, then increasing in the intermediate stages and then stabilizing in the final stages as an equilibrium is approached

Trends in Succession

Ecosystem characteristic

Trends in ecological succession

Mineral cycles Becomes more self-contained in later stages

Nutrient recycling Increases in later stages

Pioneer Communities vs. Climax Communities

Pioneer Community Climax Community

Unfavorable environment Favorable environment

Biomass increases quickly

Biomass is generally stable

Energy consumption inefficient

Energy consumption efficient

Some nutrient loss Nutrient cycling and recycling

r – strategists K - strategists

Low species diversity, habitat diversity, genetic diversity

High species diversity, habitat diversity, genetic diversity