Subject: Loop of the Week

Posted by John Heinbokel on 11/4/2003
In Reply To:Loop of the Week Posted by John Sterman on 11/1/2003

Message:

Mornin' all,

Last week when this was generating some attention I was tied up with other matters, so I just lurked quietly but didn't take the time to reply. I'm always impressed by how easy it is to lose the idea of loops and fall back to linear thinking. I appreciate the good catch that John S used to introduce/illustrate the idea. As a recovering ecologist, HOWEVER, I was also struck by the possibility of confusing a properly linear process with a cycle. The "energy cycle" that came up in one of the responses is such an illustration. Energy does indeed pass through a food CHAIN in a linear (non-cyling) manner, with major 'chunks' being lost irretrievably as heat with each link in the chain (the 'calories' that we so casually use to equate the energy and material components of these processes). That heat is lost to biological processes and doesn't come back to support the plants in beginning a second trip through the linked digestive systems. Nutrients, however, (the mass or material part of the process) DO cycle (carbon cycle, nitrogen cycle, etc comparable to the water cycle).

A second related rant for anyone who still cares is stimulated by the classic and almost inevitably misrepresented food pyramids that clutter up biology/ecology texts through the college level. You know: plants making up the large base, herbivores comprising a smaller layer just above, topped by smaller layers of carnivores and perhaps 'top' carnivores (don't remember ever seeing decomposers represented, but that's another issue). Pyramids of numbers are just plain wrong; think of the aphids that can infest an oak tree. Pyramids of biomass are better; that oak tree probably represents much more biomass than all the herbivores nibbling it. Aquatic systems, specifically the plankton (drifting, typically microscopic organisms) where I once focused my attention, however, are frequently (perhaps normally) characterized by zoo-(animal)-plankton populations that outmass the phyto-(plant)-plankton for long periods of time. Apart from the fact that distinctions between herbivores and carnivores get easily blurred in the plankton, the phytoplankton turn over so rapidly (potentially doubling in hours to days) that a relatively small but actively growing population can produce enough food (fix enough energy) to support significantly larger, but slower growing, animal populations. The proper pyramid here, as elsewhere, is created by comparing the PRODUCTION of biomass (the flow of new mass into a population) by each successive level of organism. In all illustrations, the plant components must, over some reasonable period of time, produce more biomass than do the herbivores that depend on them. In turn, the carnivors will produce much less than the herbivores (the decomposers still get short shrift). The real pyramid is in comparing the flows, not the stocks that characterize each trophic level. I have yet to see that properly emphasized in a text.

Thank you for your patience in allowing my rants. I feel much better now.

john heinbokel