Stable States and Thresholds

Disturbances can result in the collapse of the relationships between adaptive cycles at different scales. The collapse occurs when the relationship is pushed past a threshold. Systems reorganise themselves and produce a new identity as new relationships are formed. Thresholds therefore determine the system identity.

As previously mention different identities yield different benefits and values for stakeholders. Port Phillip Bay is in a state that offers good desirability, so much so that economic and population growth are seen as reasonable goals (Harris et al., 1996; DSE, 2002). The current state of the system is summarised in Table 6 along with some stakeholder views.

Identifying future states allows managers to identify the components and relationships that change in order to produce a new identity. The thresholds of the components can then be assessed. Managers can then make decisions that can shift the system away from a new state and identify where existing practices are moving it towards one.

Table 5 shows some of the controlling thresholds on the Port Phillip Bay System.

Threshold Components


Nutrient Concentrations

Controls eutrophication and water productivity. Could lead to a reduction in tourist and recreational activities, aesthetic values, land values etc.


Controls water depth and thus routes that ships can sail through. If water becomes too shallow, all shipping trade will cease.


Controls breeding, colonisation and migration patterns of marine species. Decreased pH reduces range of species that can exist. Reduced biodiversity.

Oxygen Profile

Oxygen needed for respiration and water metabolism. Reduced oxygen profiles decrease biodiversity and could have tourism ramifications

Secchi Depth

Light penetration needed for photosynthesis and thus oxygenated waters


Suspended sediment can release nutrients into the water column, decreases secchi depth and decreases aesthetic values

Exchange Rate

Too little return reduces tourism numbers and spending ability,  will determine if trade is viable in Port Phillip

Above: Table 5: Port Phillip Bay threshold components

Thresholds are important as many systems have feedback mechanism. Positive feedback is most common in natural systems. As a SES, Port Phillip Bay has a large environmental component. Managers will need to take the self-reinforcing nature of components into account in order to mitigate any change to the system. Often only the initial extrinsic disturbance is dealt with believing that the system will absorb any resulting change. In fact systems may through the feedback mechanisms sustain the changes even with the initial influence removed. As a lake these feedback mechanism include: bioturbation, acidification, phosphorous, nitrogen and other toxic heavy metal release (Bronmark & Hansson, 2005).


Current Attribute


Partially Dredged Bay With Some Suspended Sediment

Oligotrophic Bay

Low Primary Productivity and Nutrient Concentrations in Bay

Subject to Periodic Droughts and Forest Fires

Diverse Range of Ecosystem Services

Healthy Agricultural and Aquaculture Sectors

Subject to Sewage Dumping

Storms Force Mixing of Bay Waters

High Levels of Biodiversity

Forested and Cultivated Catchment


Increasing Land Values

Active Tourism Industry

System Subject to Global Economic Crisis

System Used as a Shipping Hub


Aesthetic and Recreational Value

Increasing Urbanisation and Growing Population

Not Subject to Major Incidents (Swine Flu  or Terrorism)

Large Number of Institutions Exist to Maintain the System

Elections in 2010 Could Have Implications

Stake Holder


Individuals and Householders

Current state has seen a desirable house price increase. Very little social unrest and high aesthetic and recreational values are welcome

Business and Industry Leaders

Current state is desirable. Would like to see an increase in opportunities for economic growth, particularly after the global crisis. Expansion of the docks would be very welcome

Environmental Groups

See the current state as relatively good compared to the possibilities. Can identify damage to the system and would like to see a reverse. Less environmental degradation at the expense of growth in other areas would be welcome


Priorities shift depending on scale and institution. Some argue for the expansion of growth opportunities through the port and expansion of Melbourne. Some institutions concerned with environmental damage such as the Victoria Environmental Protection Agency

Above: Table 6: Current system state attributes and stake holder views

Managers may find it useful to identify multiple pathways into different system futures. Theorising the change in land use, economic activities, environmental conditions and societal functions can provide a method of conceptualising how systems can respond to change and how their identities change.

Scenario thinking is not a means of prediction. It is merely a tool to promote the understanding of how multiple system state identities could function (Gunderson, 2002).

Table 7 portrays the identity of the Port Phillip Bay system after undergoing a sudden and dramatic nutrient input into the bay water based on the effects such an input has on shallow lakes (Bronmark & Hansson, 2005; Alabaster & Lloyd, 1982; Lane & Lau, 2001).




Eutrophic Bay with High Productivity

Collapse in tourism – ugly bay and reduced activities

Rising unemployment

Low secchi depth and deoxygenated waters

Reduction in land values

Pressures of welfare state

Increased sewage dumping leads to eutrophication

Tax revenue falls from collapsed tourist industry

Social unrest

Reduction in biodiversity and a simplified food web

Disposable income falls

De-urbanisation possible

Acidification through phosphorous release in sediments

Marina and shipping industry fail – recued demand for goods

Reduced taxation income

Reduction in aquaculture

Possible reduction

Reduced aesthetic value

High levels of bioturbation

Above: Table 7: Ecological, social and economic component changes after a nutrient induced regime shift

Previous: System Disturbances

Next: Management and Governance


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