Pioneer & Climax Communities (SL IB Environmental Systems & Societies (ESS))

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

Feature	Pioneer Communities	Climax Communities
Stage in succession	Early stages	Later Stages
GPP	Low	High
NPP as a % of GPP	High	Low
Species Richness and Diversity	Low	High
Niches	Fewer, wider	Many, narrow
Size of organisms	Small	Large
Species composition	Fewer species, adapted to harsh conditions	More species, adapted to stable conditions
Total biomass (amount of organic matter)	Low	High
Soil depth	Shallow	Deep
Soil quality	Poor (little nutrients and organic material)	High (nutrient-rich and full of organic matter)
Growth rate	Rapid	Slower
Energy flow	Simple and linear	Complex and cyclic
Nutrient cycling	Less efficient, open system (external inputs)	More efficient, closed system (nutrients are recycled)
Dominant organisms	Lichens, mosses, algae, bacteria, and fungi	Woody plants, trees, and shrubs
Stability	Unstable, prone to disturbance and colonisation	Stable, resistant to disturbance and colonisation
Examples	Pioneer species like lichens and mosses on rocks	Ancient oak forests

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 on Unsplash 

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 on Unsplash

Large mammals such as rhinos are K-strategists

 

Comparison of r- and K-strategist Species

Feature	r-strategist species	K-strategist species
Reproductive rate	High	Low
Body size	Small	Large
Maturity	Early	Late
Lifespan	Short	Long
Growth rate	High	Low
Investment in offspring (parental care)	Low	High
Survival rate	Low	High
Level of specialisation	Generalist species	Specialist species
Controlled by	Density-independent factors	Density-dependent factors
Adapted to	Pioneer communities	Climax communities
Examples	Annual plants, insects, small mammals	Large mammals, trees, some reptiles