268 ECOSYSTEM THEORY
FIGURE 5 Pollution of a stream with untreated sewage and the subsequent recovery as reflected in
changes in the biotic community. As the oxygen dissolved in the water decrease (curve to the left), fishes
disappear and only organisms able to obtain oxygen from the surface (as in Culex mosquito larvae) or those
which are tolerant of low oxygen concentration are found in zone of maximum organic decomposition.
When bacteria have reduced all of the discharged material the stream returns to normal. (After Eliassen,
Scientific American, Vol. 186, No. 3, March, 1952.)C005_003_r03.indd 268C005_003_r03.indd 268 11/18/2005 10:21:04 AM11/18/2005 10:21:04 AM