Fast and Slow Processes that may lead to Thresholds

1. Definition of a threshold

• a region in which the state of a system can change greatly
• may not always be predictable
• there may also be a surprise (when the system changes qualitatively)

examples:

small increments of habitat disruption may not cause any big problems until the fragmentation threshold is crossed

loss of biodiversity may go almost un-noticed until there is a shift in the community (such as from fish to jellyfish dominated)

 

2. Is the threshold due to changes in external drivers or intrinsic changes?

good question

 

3. Examples of slow processes that cause a shift

• fast processes such as photosynthesis and water cycling may be working fine
  • adapting to environmental changes (such as temperature or decreased water availability)
  • "resiliency" of the current structure of the system to handle these stresses
• slow processes accumulate, un-noticed in many situations
  • build up in some reservoir
  • non-linear rates (that accelerate)

examples:

Phoshporus loading into lakes can go on for a long time. The phosphorus builds up in the sediment and when the population finally switches over to bloom forming algae, these algae maintain their domiance by exploiting the P in the sediments.

Tropical forest loss can creep along incrementally with seemingly healthy remaining forests, until a critical level is reached where the internal cycling of water (evapotranspiration to basin rain to soil) no longer in excess of the forest's needs and the trees start to die back - expanding the forest.

 

4. Many slow processes are not apparent

• monitoring and research may be focused on dominant factors
• scale of research and monitoring
  • size that's managable for individual or team
  • quality of data is better on smaller plots
  • time - Lake Tahoe change took 50 years (10 PhD lifetimes)

example:

In the management of natural resources, such as fisheries, there is a minimum sustainable population below which the population can't sustain itself. Depending on the peculiarities of the organism and life history this population may be quite high.

diagram - minimum replacement population for fish stock

• if some combination of
  • juvenille loss is high
  • maturation is slow
  • recruitment efficiency is low
  • adult natural loss is high
• it can take a large population of adults to maintain this population
• over-fishing can drive this below the minimum sustainable population

It's not as easy as blaming someone who caught the last fish (may have been many years ago) or cut the last tree (such as Diamond does in Collapse).