Climax Communities (HL) (DP IB Environmental Systems & Societies (ESS))

Climax Communities

What is a climax community?

• A climax community is the final, stable stage in ecological succession

  • It is a state of equilibrium where the ecosystem has reached its full potential in terms of species diversity and complexity

• In a climax community, populations of organisms remain relatively stable, and the ecosystem is in balance

  • The species present are well-adapted to the local conditions

  • Competition for resources, such as light, water, and nutrients, is high

  • Examples of climax communities include mature forests, like temperate rainforests, or savannas where large mammals dominate

Changing views on climax communities

• The traditional idea of a climax community being the ultimate stable stage of an ecosystem has been challenged in recent years

  • Ecosystems are dynamic and continually changing, which makes it difficult to define a permanent 'final' stage

  • Human influences (such as agriculture, urbanisation, and pollution) affect what ecosystems may develop naturally

    • This complicates the prediction of true climax communities

  • Some ecologists suggest that multiple stable states can exist depending on the starting conditions and random events

    • These multiple states are known as alternative stable states

Alternative stable states

• An alternative stable state means that an ecosystem can follow multiple pathways and end up in different 'stable' configurations depending on the conditions and random events that influence it

  • For example, a forest ecosystem might be dominated by one type of tree in some areas and a different type of tree in others due to variations in local conditions, fire events, or animal populations

  • These alternative stable states show how ecosystems are not always predictable and are shaped by chance events like fires or storms, which can move them from one developmental path and onto another

The Vera wood-pasture hypothesis

• The Vera wood-pasture hypothesis is a theory that challenges the idea of closed-canopy, forested climax communities

• It suggests that large herbivores, like bison or wild cattle, would have naturally prevented forests from completely closing by grazing and creating open spaces

  • This would have led to a landscape that was more like a mosaic of grasslands and woodlands

• This hypothesis suggests that herbivores play a larger role in shaping climax communities than previously thought

  • Without these primary consumers, we might not see the natural mix of trees, shrubs, and grasses that would otherwise form

  • A local example of this might be how deer populations in parts of the UK impact forest regeneration by eating young tree saplings, which alters the forest structure

Human impact on climax communities

• Human activities can disrupt the natural progression of ecosystems towards climax communities

  • Deforestation, agriculture, and urbanisation all significantly alter the environment and prevent ecosystems from reaching their natural climax stages

• This diversion of succession by human activity can lead to a plagioclimax

• This is where the ecosystem remains in a semi-stable state that is maintained artificially, e.g. through grazing or regular mowing

  • Overgrazing by domesticated livestock:

    • Grazing by cows or sheep can prevent forests from regenerating

    • This maintains grasslands or shrublands instead of allowing trees to grow back

  • The removal of top predators:

    • In areas where top carnivores like wolves or lions have been removed, prey populations (like deer or antelope) can grow unchecked

    • This overpopulation can prevent forests from regenerating as herbivores eat young plants

  • Heathlands in the UK:

    • Historically maintained by grazing and cutting, these areas represent a plagioclimax where human intervention has prevented forests from developing