Tipping Points & Resilience (HL IB ESS OLD COURSE - IGNORE)

A tipping point is a critical threshold within a system

• If a tipping point is reached, any further small change in the system will have significant knock-on effects and cause a system to move away from its average state (away from equilibrium)

• In ecosystems and other ecological systems, tipping points are very important, as they represent the point beyond which serious, irreversible damage and change to the system can occur

• Positive feedback loops can push an ecological system towards and past its tipping point, at which point a new equilibrium is likely to be reached

  • This is sometimes known as a regime shift to an alternative stable state

  • Eutrophication is a classic example of an ecological system reaching a tipping point and accelerating towards a new state

The diagram above shows the following:

• (A) The system is subject to some kind of pressure

• (B) This pressure pushes the system towards a tipping point

• (C) The system’s tipping point (critical threshold) is reached—like a ball balancing on a hill, at this stage even a minor push is enough to cross the tipping point

• (D) Positive feedback loops accelerate the shift into a new state (E)

• The change to the new state is often irreversible or a high cost is required to return the system back to its previous state, which is illustrated in the figure as a ball being in a deep valley (E) with a long uphill climb back to the previous state (F)

Tipping points can be difficult to predict for the following reasons:

• There are often delays of varying lengths involved in feedback loops, which add to the complexity of modelling systems

• Not all components or processes within a system will change abruptly at the same time

• It may be impossible to identify a tipping point until after it has been passed

• Activities in one part of the globe may lead to a system reaching a tipping point elsewhere on the planet (e.g. the burning of fossil fuels by industrialised countries is leading to global warming, which is pushing the Amazon basin towards a tipping point of desertification)—continued monitoring, research and scientific communication are required to identify these links

The melting of polar ice caps and glaciers is another example of how human activities can push the Earth's systems beyond their limits and towards environmental tipping points

Any system, ecological, social or economic, has a certain amount of resilience

• This resilience refers to the system’s ability to maintain stability and avoid tipping points

Diversity and the size of storages within systems can contribute to their resilience and affect their speed of response to change

Systems with higher diversity and larger storage are less likely to reach tipping points

• For example, highly complex ecosystems like rainforests have high diversity in terms of the complexity of their food webs

  • If a disturbance occurs within one of these food webs, the animals and plants have many different ways to respond to the change, maintaining the stability of the ecosystem

  • Rainforests also contain large storages in the form of long-lived tree species and high numbers of dormant seeds

  • These factors promote a steady-state equilibrium in ecosystems like rainforests

  • In contrast, agricultural crop systems are artificial monocultures, meaning they only contain a single species

  • This low diversity means they have low resilience—if there is a disturbance to the system (e.g. a new crop disease or pest species), the system will not be able to counteract this

• A simple example of how the size of storage affects the relative stability of a system could be demonstrated by the relative instability of a small pond compared to the relative stability of a lake

  • In a larger storage system, such as a lake, changes in input or output have less immediate impacts on the overall system compared to a smaller storage system like a pond

  • For example, if pollutants enter the lake, they may become more dispersed and diluted due to its size, reducing the overall impact on water quality, whereas in a smaller storage system like a pond, pollutants can quickly accumulate, leading to more immediate and concentrated pollution

  • While evaporation still occurs in a lake, the impact on the overall water level is less noticeable due to the lake's size, providing a buffer against rapid drying, whereas for a pond, evaporation can quickly deplete the water volume, leading to more rapid drying and decreased stability of the system

Humans can affect the resilience of natural systems by reducing the diversity contained within them and the size of their storages

• Rainforest ecosystems naturally have very high biodiversity

• When this biodiversity is reduced, through the hunting of species to extinction or the destruction of habitat through deforestation, the resilience of the rainforest ecosystem is reduced, making it increasingly vulnerable to further disturbances

• Natural grasslands have high resilience, due to large storages of seeds, nutrients and root systems underground, allowing them to recover quickly after a disturbance such as a fire (especially if they contain a diversity of grassland species, including some which are adapted to regenerate quickly after fires)

• However, when humans convert natural grasslands to agricultural crops, the lack of diversity and storage (e.g. no underground seed reserves) results in a system that has low resilience to disturbances such as fires