The Main Points
- Since energy is being "lost" along the food chain, the essential hyerarchy of "feeding levels" produces a kind of pyramid with the producers on the bottom and the carnivores in the top levels. I will be presenting several examples of pyramids of numbers, biomass and productivity to illustrate the variation in shape that these pyramids take in different sorts of communities.
- Here is a classic set of pyramids from Whittaker (1961 data). The levels from Bottom to Top are: Producers, Herbivores, 1st Carnivores and 2nd Carnivores. Notice how the orders of magnitude of the values change with the different kinds of measurements.
Whittaker, R.H. 1970. Communities and Ecosystems. The Macmillan Company. N.Y. 160pp.
- Lindeman felt that the shapes of these pyramids would change during succession. For instance, because decomposition in shallow bodies of water like Cedar Bog Lake is "inefficient", organic matter accumulates in the sediments and fills the basin. Eventually, these basins become part of the terrestrial community, the aquatic organisms gradually disappearing and the site colonized by terrestrial plants and animals.
- In the 1950s and 60s, Eugene Odum (University of Georgia), developed interesting ways to visualize the energy flow patterns along the "food chain". I'll be talking about Fig. 9-4 in lecture. Also study Fig. 9-5.
- Odum's "hydraulic" models ( e.g. Fig. 9-4)depict the community as a series of "reservoirs" connected by pipes through which "energy" flows. The sizes of the "reservoirs" show the relative scales of the biomass accumulated in the different trophic levels. The widths of the "pipes" illustrate the rate of energy flow passing from one level to the next or leaving the system as metabolic losses.
- A comparison of the models for planktonic and forest systems shows nicely how structural and dynamical differences are produced by the differences in efficiency of energy transfer. For instance:
- The standing biomass of the producers in the planktonic system is much smaller than that in the forest. This has to do with relative accessibility and digestibility of the primary production to the herbivores. The planktonic algae are small and easily harvested and digested by the zooplankton whereas the trees have little available to be harvested directly by the grazers. In the short run, wood is relatively inaccessible, so the only materials of primary production utilized by the grazers are in the leaves. The result is that a much larger proportion of the primary production in the plankton than in the forest passes along the grazing food chain. The major pathway for energy in the forest is through the litter.
- Because of this difference, standing biomass of the herbivores is much larger in the planktonic system than in the forest.
- The standing biomass of the producers in the planktonic system is much smaller than that in the forest. This has to do with relative accessibility and digestibility of the primary production to the herbivores. The planktonic algae are small and easily harvested and digested by the zooplankton whereas the trees have little available to be harvested directly by the grazers. In the short run, wood is relatively inaccessible, so the only materials of primary production utilized by the grazers are in the leaves. The result is that a much larger proportion of the primary production in the plankton than in the forest passes along the grazing food chain. The major pathway for energy in the forest is through the litter.
- The connection between the energy flow and the cycle of nutrients can be appreciated in Odum's classic 1960 model presented in Ricklefs (Fig 9-5). Spend some time with this model and notice the "nutrient pool." Here's another model that I used in lecture:
This model illustrates the "Odum laws" that energy flows through a system and the nutrients cycle. Here, the energy fixed in photosynthesis is all accounted for in the respiration of the producers, consumers and decomposers. But most systmes are not that simple. Think of Lindeman's energy budget for Cedar Bog Lake, where incomplete decomposition allowed organic matter (and energy) to accumulate in the sediments. In this case, only at some time in the very distant future would that accumulated energy be released in respiration or combustion. Just now, for instance, we are releasing the energy of the oil produced from organic matter accumulating in ancient shallow seas.
This "pool" is the source of the inorganic molecules plants need in primary production. It is the link between the decomposers and the plants. Shortage of a single nutrient needed by the plants acts as a regulator on production.
The amount of each nutrient in the pool at any one time is the net result of the Inputs (from decomposition and imports) and Outputs (exports and uptake by plants ).
The rate of replacement of molecules in the pool is often called "turnover rate" and the time to remove and replace the pool of a particular nutrient is called "turnover time."
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