Freshwater Ecosystems - Freshwater Ecology: Concepts and Environmental Applications - page 449

are so complex that they still must be reduced conceptually to units man-
ageable for study. In this chapter, I discuss general methods of approaching
ecosystems (including trophic energy transfers, nutrient budgets, and the
link between biodiversity and ecosystem function) and then focus on ecosys-
tem properties in groundwaters, rivers, streams, lakes, and wetlands.
GENERAL APPROACHES TO ECOSYSTEMS
Some of the earliest attempts to reduce ecosystems to manageable units
revolved around assigning organisms to trophic levels. The levels are de-
composers, primary producers, primary consumers (herbivores), and higher
levels of consumers (secondary, tertiary, etc.). For example, in Fig. 22.1,
phytoplankton are primary producers, zooplankton are primary consumers,
and zooplanktivores are secondary consumers. Movement of energy
through these trophic levels has been a focal point of ecosystem research
and can be used to illustrate basic ecosystem concepts.
I discuss ecosystem concepts of energy flow using data from an early
ecosystem study on Silver Springs, Florida (Odum, 1957; Odum and Odum,
1959). Biomass of the various trophic levels is considered first (Fig. 22.2A).
The biomass of the primary producers is 10 times greater than that of any
other trophic level. This forms what is traditionally called a biomass pyra-
mid because the base of producers has much more biomass than the top-
level carnivores.
The second community characteristic is production, or flux of carbon or
energy through an ecosystem compartment. It is essential to remember that
production is not equivalent to biomass. Biomass is an amount, and pro-
duction is a rate. A common way to contrast these two values is to use the
ratio of production to biomass (P:B). The problem with assuming that high
biomass is equivalent to high production is well illustrated by the decom-
posers in Fig. 22.2. The biomass of the decomposers is relatively low, but
their production is second only to that of the primary producers. This is be-
cause metabolic activity per unit biomass (P:B is a ratio that reflects this) is
much higher for decomposers than for any of the other trophic levels for Sil-
ver Springs. Thus, P:B can be thought of as an index of relative efficiency.
A central concept of thermodynamics that can be related to efficiency
is the Second Law, which roughly states that all processes must lose some
energy. However, all processes could be 99.999% efficient, or any other
value less than 100%, according to the Second Law. In nature ecological
transformation is considerably less efficient than 100%; organisms may be
able to turn as much as 75% of the mass of food consumed into biomass
or less than 10%, with the remainder lost to respiration. The data in Fig.
22.2B suggest that about half the carbon taken up by producers is trans-
lated into biomass available for the next trophic level, 10% of the decom-
poser's production becomes biomass, and about 30% of the herbivore's
biomass becomes available to predators. For each individual organism, the
amount of energy required to obtain, ingest, and assimilate food; the sum
of efficiencies of all the metabolic pathways; and the energy required for
reproduction and survival determine how efficient the organism is in ob-
taining and utilizing energy. These efficiencies can also vary with abiotic
factors over space and time, such as changes in temperature.