lecture note 3

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19 Jul 2010
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LECTURE 3:
- Gross Primary Productivity (GPP = energy fixation by an ecosystem’s primary
producers): tropical rainforests, estuaries, mangrove forests, kelp forests, marshes, and
coral reefs reach a maximum (~20,000 kcal m-2 yr-1); only some agricultural systems
come close to this value (such as irrigated rice fields in Asia)
- Net Primary Productivity (NPP): GPP minus respiration by autotrophs (NPP is what is
available to consumption by heterotrophic organisms)
- Net Primary Productivity (NPP): typically, NPP is at least 50% below GPP, on account
of respiration by autotrophs; peak NPP values: ~9,000 kcal m-2 yr-1
- Energy passes through the biosphere in trophic levels: producers (autotrophs)
consumers (heterotrophs: 10, 20, 30 etc.) detritivores = food chain
- The detrital (decomposition) food chain, as opposed to the herbivory-based food chain,
supports the more substantial flux of biomass (and, thus, energy) in most of Earth’s
ecosystems (e.g. experiments show that in temperate deciduous forests, such as those
around Toronto, only ~2-5% of NPP are directly consumed in herbivores, as opposed to
~35% going directly to decomposition)
- As a rule of thumb, only ~10% of energy remains in the passage from one trophic level
to the next (this is known as the trophic transfer efficiency, see next lecture); because of
this energy dilution through the food chain (energy literally “runs out at the highest
trophic levels), no more than 4 to 5 trophic levels are usually possible in a given
ecosystem (however: up to 7 trophic levels have been documented in some aquatic
ecosystems)
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