Ecosystem Structure

Ecosystem structure represents the various pools (both sources and sinks) of energy and matter and their relationships to each other (i.e., directions of matter or information flow; e.g., Fig. 1.3). The size of these pools (i.e., storage capacity)

determines the buffering capacity of the system. Ecosystems can be compared on the basis of the sizes and relationships of various biotic and abiotic compartments for storage of energy and matter. Major characteristics for comparing ecosystems are their trophic or functional group structure, biomass distribution, or spatial and temporal variability in structure.

A. Trophic Structure

Trophic structure represents the various feeding levels in the community. Organisms generally can be classified as autotrophs (or primary producers), which synthesize organic compounds from abiotic materials, and heterotrophs (or secondary producers), including insects, which ultimately derive their energy and resources from autotrophs (Fig. 11.2).

Autotrophs are those organisms capable of fixing (acquiring and storing) inorganic resources in organic molecules. Photosynthetic plants, responsible for fixation of abiotic carbon into carbohydrates, are the sources of organic molecules. This chemical synthesis is powered by solar energy. Free-living and symbiotic N-fixing bacteria and cyanobacteria are an important means of converting inorganic N2 into ammonia, the source of most nitrogen available to plants. Other chemoau-totrophic bacteria oxidize ammonia into nitrite or nitrate (the form of nitrogen available to most green plants) or oxidize inorganic sulfur into organic compounds. Production of autotrophic tissues must be sufficient to compensate for amounts consumed by heterotrophs.

Heterotrophs can be divided into several trophic levels depending on their source of food. Primary consumers (herbivores) eat plant tissues. Secondary consumers eat primary consumers, tertiary consumers eat secondary consumers, and so on. Omnivores feed on more than one trophic level. Finally, reducers








111 P=<



gm/mz dry BIOMASS

gm/mz dry BIOMASS

I Biomass pyramid for the Silver Springs ecosystem. P, primary producers; H, herbivores; C, predators; TC, top predators; D, decomposers. From H. Odum (1957) with permission from the Ecological Society of America.

(including detritivores and decomposers) feed on dead plant and animal matter (Whittaker 1970). Detritivores fragment organic material and facilitate colonization by decomposers, which catabolize the organic compounds.

Each trophic level can be subdivided into functional groups, based on the way in which organisms gain or use resources (see Chapter 9). For example, autotrophs can be subdivided into photosynthetic, nitrogen-fixing, nitrifying, and other functional groups. The photosynthetic functional group can be subdivided further into ruderal, competitive, and stress-tolerant functional groups (e.g., Grime 1977) or into C-3 and C-4, nitrogen-accumulating, calcium-accumulating, high-lignin or low-lignin functional groups, etc., to represent their different strategies for resource use and growth. Similarly, primary consumers can be subdivided into migratory grazers (e.g., many ungulates and grasshoppers), sedentary grazers (various leaf-chewing insects), leaf miners, gall-formers, sap-suckers, root feeders, parasitic plants, plant pathogens, etc., to reflect different modes for acquiring and affecting their plant resources.

The distribution of biomass in an ecosystem is an important indicator of storage capacity, a characteristic that influences ecosystem stability (Webster et al. 1975; Chapter 15). Harsh ecosystems, such as tundra and desert, restrict autotrophs to a few small plants with relatively little biomass to store energy and matter. Dominant species are adapted to retain water, but water storage capacity is limited. By contrast, wetter ecosystems permit development of large producers with greater storage capacity in branch and root systems. Accumulated detritus represents an additional pool of stored organic matter that buffers the ecosystem from changes in resource availability. Tropical and other warm, humid ecosystems generally have relatively low detrital biomass because of rapid decomposition and turnover. Stream and tidal ecosystems lose detrital material as a result of export in flowing water. Detritus is most likely to accumulate in cool, moist ecosystems, especially boreal forest and deep lakes, in which detritus decomposes slowly. Biomass of heterotrophs is relatively small in most terrestrial ecosystems, but it may be larger than primary producer biomass in some aquatic ecosystems, as a result of high production and turnover by small biomass of algae (Whittaker 1970).

Trophic structure can be represented by numbers, mass (biomass), or energy content of organisms in each trophic level (see Fig. 11.2). Such representations are called numbers pyramids, biomass pyramids, or energy pyramids (see Elton 1939) because the numbers, mass, and energy content of organisms generally decline at successively higher trophic levels. However, the form of these pyramids differs among ecosystems. Terrestrial ecosystems usually have large numbers or biomasses of primary producers that support progressively smaller numbers or biomasses of consumers. Many stream ecosystems are supported primarily by allochthonous material (detritus or prey entering from the adjacent terrestrial ecosystem) and have few primary producers (e.g., Cloe and Garman 1996, Oertli 1993, J. Wallace et al. 1997, Wipfli 1997). Numbers pyramids for terrestrial ecosystems may be inverted because individual plants can support numerous invertebrate consumers. Biomass pyramids for some aquatic ecosystems are inverted because a small biomass of plankton with a high rate of reproduction and turnover can support a larger biomass of organisms with low rates of turnover at higher trophic levels (Whittaker 1970).

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