Observations placeholder

Foster and Kreitzman - Seasons of Life - The importance of understanding dependencies

Identifier

014201

Type of Spiritual Experience

Background

Unless we understand the dependencies we may take actions ourselves which have a huge and detrimental impact upon the whole of the system.  We could collapse the whole system. 

Here is just one example which could have resulted in an  ‘unwise naïve act of interference’ without the work of Peter Yodzis.

Yodzis found that all the food webs in the sea are dependent on the amount of phytoplankton, and populations decline or increase depending on the amount of phytoplankton.  As you move up the food chain or web, the biomass declines by about 90% per level, so, for example there is 10% as much zooplankton as there is phytoplankton.  The mass of the phytoplankton thus sets an upper limit on the size of the entire web.  Phytoplankton depend on light and nutrients, the nutrients depend on the ocean currents and the ‘upwelling’.  And so it continues….

A description of the experience

Seasons of Life – Russell G Foster and Leon Kreitzman

 These webs can become incredibly complex, particularly in the seas.  When Peter Yodzis examined one 29 species food web to determine whether culling Cape fur seals would increase hake biomass, he noted the entire interaction is so complicated that even if he counted only paths with eight links, there were 28,722,675 distinct simple open pathways (paths that never pass twice through the same species ) through the food web from seals to hake.  His counter intuitive conclusion was that culling fur seals would probably do more harm than good to commercial fish such as hake.

Cape fur seals live in the waters off the southwestern coast of Africa.  The Benguela ecosystem, as the area is known, is the site of a huge seasonal increase in phytoplankton that supports a major fishing industry.  A Cape fur seal consumes approximately its own mass annually in hake.  So common sense would suggest that regularly culling seals so as to reduce seal biomass by a given amount will permit an increase by the same amount in the annual yield of hake biomass to the fishery.  But it does not work like that.

Although seals eat hake, they also eat species that the hake feed on and several predators of hake as well as several competitors of hake.  The hake predators and the species on which the hake feeds, as well as its competitors are in turn predators and prey of other species.  So what seems simple turns out to be fiendishly complicated as the interactions come into play.

The source of the experience

Foster and Kreitzmann

Concepts, symbols and science items

Symbols

Science Items

Activities and commonsteps

Activities

Commonsteps

References