phates, and other inorganic components of 

 the pond water and partly by the organic 

 detritus that settles to the bottom. Here it is 

 utilized by bacteria and other fungi, which 

 are the decomposers; and, of course, energy 

 for the support of the system comes from the 

 sun. The producers are partly the higher 

 plants, cattails, water lilies, etc., which grow 

 along the edges of the pond, but mainly they 

 are the phytoplankton — various unicellular 

 and minute colonial algae that float near the 

 surface in easy access to the light. Then come 

 the consumers: primary, secondary, and ter- 

 tiary. Primary consumers are represented 

 mainly by the zooplankton. These various 

 protozoans and semimicroscopic crustaceans, 

 such as Daphnia, feed directly upon the 

 phytoplankton, although some larger forms, 

 such as tadpoles, may nibble at the larger 

 plants along the margins of the pond. The 

 secondary consumers — larger arthropods, par- 

 ticularly water beetles — feed upon the zoo- 

 plankton; and finally the largest animals — 

 fish, frogs, etc. — serve as tertiary consumers, 

 which prey upon the secondaries. Mean- 

 while, organic detritus (excreta, remnants of 

 dead organisms, etc.) keeps silting to the 

 bottom of the pond. Here it may be eaten by 

 scavengers, such as insect larvae. But mainly 

 the organic matter is returned to inorganic 

 form by action of bacteria and other decom- 

 posers. Thus each of the various species has 

 its niche in any system. Each plays some 

 unique and essential role in the complex 

 business that maintains the economy of the 

 whole community. 



Imbalance in an Ecosystem. Sometimes the 

 delicate balance that supports the economy 

 of an ecosystem may be disturbed by some 

 enterprise of man. A case in point is that of 

 the Long Island duck farms bordering the 

 shores of Great South Bay. This has been 

 studied carefully by several ecologists, in- 

 cluding particularly John Ryther of the 

 Woods Hole Oceanographic Institution. 



Shortly after a group of large duck farms 

 was established along the estuaries at the east 

 end of Great South Bay, the oyster beds, 



Ecology and Evolution - 589 



which for years had provided a valuable an- 

 nual crop of blue point oysters, began to fail. 

 Analysis showed that there had been a pro- 

 found change in phytoplankton of the bay 

 and that the substituted producers in this 

 ecosystem did not provide a suitable food for 

 oysters. The oysters were literally starving 

 to death, despite the fact that they fed amply 

 on the newly flourishing phytoflagellates 

 (mainly Nannochloris and Stichococcus). Ap- 

 parently the enrichment of the bay water 

 by the organic excreta from the many birds 

 had favored the growth of a new flora of 

 phytoflagellates to the exclusion of the old 

 (mainly other phytoflagellates, dinoflagel- 

 lates, and diatoms). Further experiments 

 showed that the substituted phytoplankton 

 could utilize the orga?iic forms of nitrogen, 

 whereas the original flora required nitrates. 

 Thus, even though the total productivity of 

 the bay water had been greatly increased, a 

 shift in the species composition of the pro- 

 ducer part of the community had spelled 

 death to the oysters (and certain other shell- 

 fish). 



Ecological Succession. The community 

 composition of most ecosystems is not static, 

 although sometimes a very stable balance 

 exists such that changes in community struc- 

 ture are exceedingly slow. Frequently, how- 

 ever, an orderly succession of communities 

 can be observed and predicted. Such a suc- 

 cession, called a sere, may pass through early 

 (pioneering) serai stages, before reaching a 

 comparatively stable stage, which is called a 

 climax. 



A typical serai succession, observed after 

 the abandonment of a large area of cotton 

 farm land in the Piedmont section of the 

 southern United States, is shown in Figure 

 30-14. The easiest and most accurate observa- 

 tions on ecological successions, of course, are 

 those that deal with plant communities. 

 Census taking among animal populations is 

 sometimes very difficult, because many ani- 

 mals tend to flee and hide from man. It must 

 be remembered, however, that each time a 

 change occurs in the plant part of a commu- 



