Pioneer & Climax Communities (DP IB Environmental Systems & Societies (ESS)): Revision Note

Pioneer & Climax Communities

What are pioneer communities?

• In pioneer communities (i.e. in the early stages of succession), gross productivity is low due to the unfavourable initial conditions and low density of producers (low gross primary productivity)

• However, the proportion of energy lost through community respiration is also relatively low

• This means that net productivity in pioneer communities is relatively high

• This allows the pioneer community system to grow and accumulate biomass

What are climax communities?

• In climax communities (i.e. in the later stages of succession), gross productivity may be relatively high, due to a high density of producers (high gross primary productivity) and consumers (high gross secondary productivity)

• However, this relatively high gross productivity is balanced by the large amounts of energy lost from the climax community system through respiration

• This causes the net productivity of a climax community to approach 0

• As this happens, the productivity–respiration (P:R) ratio approaches 1
  

  • This ratio reaches 1 when biomass and energy is being produced by the system at the same rate as it is being used

  • If the ratio >1, then excess energy and biomass is being produced

  • If the ratio <1, then more biomass and energy is being consumed than is being produced

  • To reach a stable (climax) community, there has to be an equilibrium between the community production and the community respiration

• There is no one cl­­imax community, but rather a set of alternative stable states for a given ecosystem
  

  • What the climax community eventually looks like depends on a large variety of factors, including climate, the local soil properties, and a range of random events that can occur over time (e.g. extreme weather events, human interventions) 
    
    

    
    

Changes occurring in a community as it develops from a pioneer community into a climax community through the process of succession

Comparison of Pioneer and Climax Communities

Reproductive Strategies

Density-dependent and Density-independent Factors

• In ecology, population growth and regulation are influenced by a range of biotic and abiotic factors. Some of these factors are influenced by the population density, while others are not

• Density-dependent factors include factors such as competition, predation, parasitism, and disease

• As the population density increases, the impact of these factors becomes more significant, resulting in a decline in the population growth rate

• In this way, density-dependent factors acts as negative feedback mechanisms, leading to the stability and regulation of populations

• Density-independent factors include natural phenomena such as floods, fires, hurricanes, and droughts, as well as anthropogenic activities like pollution, deforestation, and climate change

• These factors affect the population growth rate irrespective of the population density, so their impact is similar across all populations regardless of their density

r-strategist Species

• r-strategists are characterised by having a high reproductive rate, small body size, early maturity, and short lifespan

• They are adapted to unstable and unpredictable environments and tend to be found in pioneer communities

• These species tend to have a high growth rate and reproduce quickly, producing large numbers of offspring with little investment in each

• They have a lower survival rate, but their high reproductive rate enables them to quickly recolonize and establish themselves after disturbances

• Examples of r-strategist species include cockroaches, flies and some small mammal species

• Populations of r-strategists are controlled by density-independent factors

Photo by MOHD AZRIEN AWANG BESAR (opens in a new tab) on Unsplash (opens in a new tab) 

Flies are r-strategists

K-strategist Species

• K-strategists are characterised by having a low reproductive rate, large body size, late maturity, and long lifespan

• They are adapted to stable and predictable environments and tend to be found in climax communities

• These species tend to have a lower growth rate but invest more in each offspring, resulting in a higher survival rate

• They are better able to withstand disturbances, allowing them to persist in stable environments

• Examples of K-strategist species include large mammals

• Populations of K-strategists are controlled by density-dependent factors

Photo by Glen Carrie (opens in a new tab) on Unsplash (opens in a new tab)

Large mammals such as rhinos are K-strategists

Comparison of r- and K-strategist Species