ECOSYSTEMS

• Consists of all the organisms in a community as well as abiotic factors with which they interact

I. Trophic Levels and Food Webs

• each ecosystem has a trophic structure of different feeding relationships that determines the route of energy flow and pattern of chemical cycling
• divide species in an ecosystem into trophic levels on the basis of their main source of nutrition
• the trophic level that will ultimately support all others is the autotrophic or primary producers
• all other organisms of the ecosystem are consumers
• detritivores-derive energy from organic wastes
• the pathway along which food is transferred from trophic level to trophic level is the food chain (are usually woven together into complex food webs)

II. Energy Flow

1. The global energy budget

1. much of the solar radiation that reaches the biosphere lands on bare ground and bodies of water that either absorb or reflect it
2. only a small amount actually strikes algae and the leaves of plants
3. of the light that does, only about 1% is used in photosynthesis

1. Primary Productivity

1. is the rate at which light energy is converted to chemical energy by autotrophs
2. the total of this productivity is known as gross primary productivity (GPP)
3. net primary productivity (NPP) is equal to the GPP - energy used for respiration

*represents the storage of chemical energy available to consumers in an ecosystem

4. biomass- the dry weight of organic matter comprising a group of organisms in a particular habitat (doesn't count water because water contains no usable energy)
5. Standing Crop Biomass- total biomass of plants present in an area at any given time
6. Tropical rainforests are among the most productive terrestrial ecosystems and contribute a large porportion of the planet's overall productivity
7. The factors most important in limiting productivity depend on the type of ecosystem and on seasonal changes in the environment
8. Inorganic nutrients may also be important in limiting the productivity
9. Limiting nutrient- a specific nutrient that can slow productivity if it is not available
10. Productivity in the seas is usually greatest in the shallow waters near the continents and coral reefs where abundant nutrients, light, and heat stimulate growth

1. Energy Transfers and Ecological Pyramids

1. the rate which on ecosystem's consumers convert the chemical energy of the food they eat into their own new biomass is called the secondary productivity of an ecosystem
2. productivity eventually declines with each transfer of energy through the trophic levels
3. this decline is partially due to the first law of thermodynamics (total energy must be conserved, but working organisms cannot avoid converting some of it into heat, which is dissapated from the ecosystem)
4. thus not all the chemical energy stored as biomass by NPP can be converted into secondary productivity.
5. Ecological efficiency- the ratio of NPP at one trophic level to NPP at the level below (approximately 90% of energy available never transfers to the next)
6. The multiplcative loss can be represented diagrammatically with a pyramid of productivity, in which the trophic levels are stacked as blocks with primary producers at the bottom (See example in book)
7. Biomass pyramids- each tier is proportional to the standing crop biomass in a trophic level at any given time (biomass represents the chemical energy stored in organic matter)
8. Pyramid of Numbers- the size of each block is porportional to the number of individual organisms present in each tropic level (see book for example)

III. Chemical Cycling (see book for diagrams of all of these)

1. Biogeochemical cycles- various nutrient cycles that involve both biotic and abiotic components
2. General Model

1. most nutrients accumlate in four compartments with each compartment defined by two characteristics: whether or not it contains organic or inorganic materials and whether or not the materials are directly available for use by organisms
2. one compartment of organic material is composed of the living organisms themselves

1. The Carbon cycle

1. the reciprocal processes of photosynthesis and cellular respiration provide a link between the atmosphere and the terrestrial environments
2. carbon recycles relatively fast because plants have a high demand for CO2
3. decomposition eventually recycles carbon to the atmosphere
4. the amount of CO2 present in the atmosphere varies with the seasons
5. burning of fossil fuels adds CO2

1. The Nitrogen Cycle

1. the atmosphere is almost 80% N2 but plants can't assumulate this form of nitrogen
2. only certain types of prokaryotes can fix N2 (reduce it from gas to ammonia) in a process called ammonification which can be used to make amino acids
3. N2 is fixed in terrestrial ecosystems by free living soil bacteria as well as some symbiotic bacteria in root nodules
4. Small amounts can be fixed by lightening
5. Although plants can use ammonia directly, most ammonia in the soil is used by aerobic bactera as an energy source (their activity oxidizes ammonia to nitrate and then to nitrate in a process called nitrification
6. Animals can assimlate only inorganic N2 by eating plants or other animals
7. Some bacteria get the oxygen they need from nitrate

1. The Phosphorus Cycle

1. cycling does not include movement through the atmosphere
2. weathering of rocks gradually adds phosphate to the soil