a. The biomass of an ecosystem has been called a "standing crop," as one trophic level harvests another (Figure 28.1).

b. Ecosystems obtain their energy as phototrophs transform sunlight into chemical energy and convert carbon dioxide and water into organic molecules.

c. Matter moves through the ecosystem from one trophic level to another as energy is lost through metabolism.

d. The gross primary productivity of an ecosystem is the amount of incoming light captured by photosynthesis.

e. The net primary productivity is the amount of energy actually incorporated into the biomass of the phototrophs, and does not include the energy they use for their own maintenance.

f. In a typical field ecosystem, plants have a gross primary productivity of only 1.3—1.6 percent, and leave only 1.0—1.2 percent as net productivity.

g. Aquatic ecosystems have gross primary productivities in the 1—2 percent range.

h. Primary productivity figures can be used to estimate growth rates.

i. Table 28.1 shows net primary productivities and biomasses of several typical ecosystems.

a. Heterotrophs use much of their food for respiration, and relatively little biomass and energy actually get transferred to higher trophic levels.

b. Homeotherms convert about 1—3 percent of their food into biomass.

c. Poikilotherms have higher efficiencies, such as about 10 percent for fish, 25—35 percent for some invertebrates, and 50—60 percent for insects.

d. The overall efficiency of conversion from one trophic level to another is shown as a pyramid in Figure 28.2.

e. A 1942 study of a Minnesota lake (Figure 28.3) showed how biomass was converted from each trophic level to another.

f. In the oceans, the pyramid for plankton looks inverted, as zooplankton outweigh phytoplankton in biomass; however, the energy flow from each trophic level to higher levels still forms the expected pyramid shape.

a. Turnover is the process by which materials move into and out of organisms, as new molecules are made and old ones are lost.

b. Each whole ecosystem is also turning over.

c. Residence time is the amount of time a material spends in the ecosystem.

d. The turnover rate is the time taken by the ecosystem to process the material.

e. Complex systems have different subsystems, or compartments, and materials can flow from one compartment (e.g. soil, water, tree wood, tree leaves, flowers, fruit) to another.

f. The average residence time and turnover rates are thus a more meaningful measurement of turnover.

g. Turnover is faster in summer than in winter in every ecosystem.

h. Figure 28.4 shows the flow of nutrients and turnover rates in different systems.

a. The hydrologic cycle (Figure 28.5) involves enormous amounts of material when one considers the amount of precipitation that falls on the earth's surface.

b. About 95 percent of the earth's water is bound up in rocks, and 97 percent of the remaining 5 percent that is free is found in the oceans.

c. The hydrologic cycle is driven by evapotranspiration, the combination of evaporation and respiration, and the whole cycle is powered by the sun.

d. In a terrestrial ecosystem, precipitation and percolation are the most important parts of the cycle.

e. Plants in such a system use some water in their own structure, but a much larger amount is merely cycled through them (Figure 28.6).

f. Animals also participate in the hydrologic cycle, as they perspire, drink water, urinate, and otherwise exchange water with their surroundings.

a. Ecosystems are always open and matter is always flowing through them, creating a series of biogeochemical cycles, including the carbon cycle, nitrogen cycle, sulfur cycle, and phosphorous cycle.

b. The nitrogen cycle (Figure 28.7) is most significant, after the carbon cycle.

c. Nitrogen enters food webs through nitrogen fixation, the conversion of free N2 to ammonia, which is accomplished mainly by bacteria living symbiotically with plant roots.

d. Denitrification, the conversion of nitrate and nitrite back into atmospheric nitrogen, is also carried out by bacteria.

e. Many bacteria convert nitrogen into compounds to obtain energy; these include anaerobic bacteria that reduce nitrite to nitrate.

f. Other bacteria oxidize ammonia to nitrite, or nitrite to nitrate.

g. The sulfur cycle is important because sulfur is a component of biomolecules, and also because sulfur in the atmosphere has a major affect on many ecosystems in the form of acid rain.

h. Bacteria carry out most of the biological transformations of sulfur (Figure 28.8).

i. Atmospheric sulfur is mainly H2S, the main sources of which are sea-spray aerosols, respiration by sulfate-reducing bacteria, and volcanic activity.

j. Accumulated sulfides can inhibit the growth of most organisms, and some bacteria can oxidize sulfide back to sulfur, but free oxygen is required for this process.

k. The burning of sulfur-bearing fossil fuels has released large amounts of sulfur dioxide into the atmosphere, and its conversion to sulfuric acid, which falls in rain, has caused the breakdown of plant structures, and the leaching of nutrients from the soil.

a. The biomass of an ecosystem is constantly being converted into necromass that takes the form of detritus (Figure 28.9).

b. Feces, shed skin, scales, hair, dead organisms, and other materials accumulate as detritus.

c. A community of animals is supported by detritus (Figure 28.10).

d. The bulk of detritus is plant material that is difficult to digest, but bacteria, yeasts, and fungi live off the progressive breakdown of these materials and their products.

e. A host of scavengers in every ecosystem also remove carcasses and use them in various ways (Figure 28.11).

f. Feces is a source of food for some organisms (Figure 28.12) and one beetle became personified as a god by the ancient Egyptians (Figure 28.13).

g. One fly, Chironomus lugubris, eats particles of peat in bogs, and the fly's feces are eaten by a crustacean, Chyodorus sphaericus (Figure 28.14) in a mutualistic association with the fly.

a. The materials in an ecosystem are in a dynamic state, and move through the system as they also recycle with it.

b. A study of bedrock in New Hampshire found that significant annual losses of water and nutrients occurred, even though chloride and nitrogen accumulated, as plants grew.

c. The study was repeated after a clear-cutting of one of the watersheds, and the outflow of nutrients increased about nine times.

d. In an undisturbed forest, about 40 percent of the input water escapes by evapotranspiration; in a clear-cut forest, most of it runs off as surface water.

e. These figures have clear implications for human interference with ecosystems.

a. Rachel Carson's 1962 book Silent Spring eventually led to the banning of certain insecticides, including DDT.

b. DDT, a halogenated hydrocarbon, and PCBs (polychlorinated biphenyls) are extremely stable and have half lives of a decade or more.

c. These compounds are also very mobile and are poorly soluble in water, but accumulate in the fatty tissues of organisms in a process called bioaccumulation.

d. Biomagnification (Figure 28.15) can then occur, as the compounds accumulate to higher concentrations at higher trophic levels.

e. Increased cancer rates and disruptive affects on the reproductive cycles of many organisms are thought to be associated with the biomagnification of these compounds and other pollutants.

a. No ecosystem is likely to stay the same for a long time, and may change in two ways as materials move through it:

b. Communities are also affected by physical events such as fire (Sidebar 28.1), or geological disturbances, and by interactions among community members.

c. One change in a community may facilitate another, as when small plants secure the soil for large plants to take hold later.

d. Some changes inhibit the growth of species, as when a plant changes the pH of the soil around it.

e. Tolerance may also be shown by organisms that appear unaffected by change.

f. Terrestrial ecosystems are generally identified by their plants, which form the base of the community, but their fauna also affects the plant community structure.

g. Herbivores that prefer some plant foods to others can affect the plant community, as can birds that have nesting or feeding preferences.

h. The series of plant types that replace one another in succession is called a sere, and any particular plant community is a seral stage.

i. It has been theorized that a sere culminates in a climax community, a plant association that lasts indefinitely and will never be replaced by a more stable type.

j. A number of climax communities have been cataloged, mapped, and named for their dominant plant species.

k. Some large ecosystems may not be climax communities, but may be in an equilibrium condition between periodic disruptions and regrowth, as shown by the Balsam fir forests of the northeastern U.S. (Figure 28.16).

a. As an ecosystem moves from one seral stage to another, its productivity and metabolism change.

b. One trend is toward a balance between photosynthesis and respiration, as early communities move from autotrophic stages (Figure 28.17) to heterotrophic stages.

c. A second general trend is toward more biomass and lower productivity.

a. As a new community begins with only a few species, seral stages replace one another and create more and more possible niches.

b. Diversity increases quickly in the early stages of succession, then levels off and may even fall slightly.

c. At the same time, the community's biomass increases, as is apparent in the transition from a community of small herbaceous plants into a forest.

d. Later communities tend to last longer than earlier ones, which implies that later communities are more stable than earlier ones.

a. G. Evelyn Hutchinson and Robert H. MacArthur proposed a simple argument that a community with only a few species and few trophic links is unstable and more likely to fail if one species is removed or affected dramatically in some way.

b. By the same argument, more diverse communities have more checks and balances that can minimize the effects of a change in one species.

c. Conservationists have thus argued that it is important to maintain diversity in order to increase stability.

d. The definition of stability in these arguments is important, and in this text, it means the tendency of an ecosystem to return to its original state if it is disturbed.

e. An ecosystem is resistant if it is not easily disturbed in the first place, and is resilient if it returns quickly to an equilibrium condition after being disturbed.

f. Studies in the lab and in the field have supported the argument that a more complex community is more stable.

a. A secondary succession begins with an ecosystem that has been disturbed, as by fire, flood, or human intervention.

b. Primary successions are rare, and begin with a nearly lifeless environment, as one that exists after volcanic eruption.

c. The succession of communities that followed the Mt. St. Helens volcano in Washington state (Figure 28.18) began with a primary succession.

d. Human intervention efforts to assist in the succession process on Mt. St. Helens did not succeed, but the natural process did.

a. Even in a stable system, one species may be replacing another, and the community structure may shift.

b. Observations on islands have shown that removal of one species is balanced by introduction of new species.

c. E.O. Wilson and Daniel Simberloff studied four mangrove islands in Florida, and observed that the islands were recolonized by new arthropods (Figure 28.19) after they killed off the previously existing organisms.