lecture note 4

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19 Jul 2010
- The ecological efficiencies that lead to the Trophic Transfer Efficiency are:
(1) Consumption Efficiency (CE): the percentage of total production by one trophic
level ingested by the next higher trophic level (this takes into account non-digested
materials; with respect to 10 producers, CE ranges from 2-5% in a temperate forest
[see last lecture], to 25% in grasslands, and 50% in phytoplankton communities)
(2) Assimilation Efficiency (AE): the actually assimilated (e.g. by the gut)
percentage of ingested food energy (as high as 80% in carnivores, 20-50% in
herbivores; in general, AE is higher in homeotherms than poikilotherms)
(3) Production Efficiency (PE): the percentage of assimilated energy converted into
biomass, i.e. growth & reproduction (typical PE values: 0.3–1% in photosynthetic
autotrophs; 30-40% in invertebrate carnivores; as low as 1-2% in homeotherms!)
- Trophic Transfer Efficiency (TTE) = CE x AE x PE (often ~10%)
- The importance of the detrital food chain, as illustrated by Australia’s dung problem:
The point about the dung beetles in Australia was the importance of the detrital food chain; Australia has
no native bovines, and thus no dung beetles that evolved with them. After cows were introduced by British
colonists in 1788 (7 cows!), this situation changed. The number of cows in Australia rose to some 30
million by the late 1950s, and dung production became a nationwide problem - cow excrement
accumulated in the absence of the normal primary consumer of this particular variety of detrital matter, and
everything from smell to the productivity of agricultural lands and the spread of disease became an issue.
The introduction of foreign dung beetles, at first from Africa, and then other places, solved the problem.
The example illustrates the importance of the detritivore members of the food chain.
- James Lovelock’s ‘Gaia’ Hypothesis proposes that the feedback interactions between
living organisms on Earth and Earth’s atmosphere explain the current chemical
composition of the atmosphere (the composition is far from chemical equilibrium; 78%
N2, 21% O2, 0.03% CO2 would not be possible without constant replenishment, and
withdrawal, by living organisms)
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