Applications from food web and ecosystem theory - Ecological Applications: Toward a Sustainable World - page 245

(a) Forest (b) Grassland

Respiration

Respiration

Respiration Respiration

Grazer
system

Decomposer
system

Grazer
system

Decomposer
system

NPP DOM NPP DOM

(c) Plankton community (d) Stream community

Respiration Respiration Respiration

Respiration

Decomposer
system

Grazer
system

Decomposer
system

GS

NPP DOM

NPP DOM

From terrestrial catchment

Fig. 9.2 General patterns of energy fl ow for: (a) a forest; (b) a grassland; (c) a marine
plankton community; (d) the community of a stream or a small pond. The relative sizes of
the boxes and arrows are proportional to the relative magnitudes of compartments and fl ows.
DOM, dead organic matter; NPP, net primary production. (After Begon et al., 2006.)

moves chemical elements between the various living and nonliving (abiotic) compartments of the
ecosystem. Some abiotic compartments are in the atmosphere (carbon in carbon dioxide, nitrogen
as gaseous nitrogen), some in the rocks of the lithosphere (calcium, potassium) and some dis-
solved in the hydrosphere - the water of soil, stream, lake or ocean (nitrogen in dissolved nitrate,
phosphorus in phosphate).
Nutrient elements, of which carbon, phosphorus and nitrogen are of critical importance, are
available to plants as simple inorganic molecules or ions and are incorporated during photosyn-
thesis into complex organic carbon compounds in biomass. Ultimately, however, when the carbon
compounds are metabolized to CO 2 (via the plant's own respiratory activity or after use by herbiv-
ores, bacteria, fungi or detritivores) energy is lost as heat and the mineral nutrients are released
again in simple inorganic form. Another plant may then absorb them, and so an individual atom of
a nutrient element may pass repeatedly through the ecosystem. By its very nature, each unit of
energy in a high-energy compound can be used only once before being lost as heat, whereas
chemical nutrients can be used again and again. However, nutrient cycling is never perfect and a
terrestrial ecosystem may lose nutrients via drainage to an aquatic ecosystem. Natural ecosystems
tend to be more nutrient-tight than plantation forestry with its clear-felling approach (Figure 9.3)
or agroecosystems. Managers of waterways need to be wary of the input of excess nutrients
because this can fuel undesirably high primary productivity by phytoplankton, causing the water
to become turbid and eliminating large rooted plants, and eventually, via decomposition of the
excessive primary production, reducing oxygen levels and killing invertebrates and fi s h . T h i s
process is known as eutrophication .

Ecological stoichiometry - a complex tale of nutrient dynamics
Ecological stoichiometry is concerned with the consequences for ecological interactions of the
relative availability of different elements - and particularly the ratio of carbon to nitrogen (Elser &
Urabe, 1999). The rate at which dead organic matter decomposes depends strongly on its bio-
chemical composition. This is because bacterial and fungal tissues have very high nitrogen