[pp. 844 - 851; 854 - 859]

A. Ecosystem Participants

1. Ecosystem = a complex of organisms interacting with one another and with the physical environment.
- ecosystems are open systems through which energy flows and materials are cycled (energy cannot be recycled) [Fig. 48.2]
- ecosystems require energy and nutrient input; they generate energy (usually as heat) and nutrient output
2. Participants depend upon energy input from the sun; they process this energy: some goes to other organism (a relatively small amount); the rest is lost as metabolic heat or locked up in organic wastes and remains [Fig. 48.5]
- primary producers (autotrophs) --> consumers (herbivores, carnivores, parasites, omnivores = heterotrophs) --> decomposers, detritovores (extract energy from remains or products of organisms)

B. Ecosystem Structure

1. Trophic levels
- trophic ("feeding") levels are a hierarchy of energy transfers = "Who eats whom?"
- Level 1 = producers; level 2 = herbivores; level 3 and higher = carnivores
- decomposers feed on organisms from all levels; detritivores feed on organic matter produced by organisms from levels 2 and higher (plants don't produce detritus)
2. Food webs
- linear sequence of who eats whom = food chain
- interconnected food chains make up food webs

C. Biogeochemical Cycles

1. Movement of nutrients into and out of reservoirs and into and out of living systems
- Elements are available to producers mainly as ions
- Nutrient reserves are maintained by environmental inputs and recycling activities
- Amount of nutrients being cycled through living organism in most major ecosystems is greater than the amount entering or leaving
- Environmental inputs to ecosystem's nutrient reserves: precipitation, metabolism (e.g. nitrogen fixation), weathering; outputs (land ecosystems): runoff, evaporation
2. Carbon cycle [Fig. 48.10]
- carbon enters atmosphere by aerobic respiration, fossil-fuel burning, volcanic eruptions
- most ends up dissolved in water of oceans; other, large "holding stations" = soil, atmosphere, plant biomass [Fig. 48.10b, see Table p. 855]
- carbon removed from atmosphere (and bodies of water) by photosynthesizers and shelled organisms
- decomposition of buried carbon compounds millions of years ago caused the formation of fossil fuels
- burning of fossil fuels putting more carbon into atmosphere than can be cycled to ocean reservoir and holding stations
3. Nitrogen cycle [Fig. 48.11]
- nitrogen is needed for proteins and nucleic acids; nitrogen is abundant in atmosphere, but not in earth's crust
- of all macronutrients needed for plant growth, nitrogen is the scarcest
- most of the nitrogen assimilated by plants is in the form of nitrate (NO3)
- symplified nitrogen cycle has 5 steps:

1. nitrogen-fixing bacteria convert atmospheric nitrogen into ammonia
2. ammonia is converted to nitrites (NO2) and then into nitrate (NO3) by soil bacteria that use ammonia as an energy source
3. nitrate is the main form of nitrogen absorbed by plants, although they can also use ammonia directly; plants assimilate nitrogen to produce proteins and nucleic acids; animals eat plant proteins and produce animal proteins
4. when plants and animals die, their nitrogen compounds are broken down by ammonifying bacteria, and ammonia is released
5. some nitrogen is returned to the atmosphere by denitrifying bacteria which convert nitrate to molecular nitrogen