ENV100Y5 Lecture Notes - Lecture 19: Food Web, Climax Community, Invasive Species
Module 2: May 22-June 1
Date: Tuesday, May 29
SESSION 20: SPECIES INTERACTIONS AND COMMUNITY ECOLOGY (III)
Readings: Chapter 5
Generally biomass increases and individuals, but energy declines (it is less)
● Fewer individuals, less biomass, less stored energy (move from top)
Why A Pyramid and Not A Column?
● Most of the energy stored in one level does not make it to the next
● Several types of inefficiencies
Inefficiencies
● In a prey population (P): some are eaten/ingested (I), but some will survive
● Exploitation Efficiency = I/P
● From the ingested prey (I): some energy is assimilated (A) and some energy is excreted
● Assimilation Efficiency = A/I
● Of the Assimilated Energy (A): some energy lost through respiration and some energy
converted to biomass (growth and reproduction = G)
● Net Production Efficiency = G/A
● Ecological Efficiency = G/P; usually never more than 10%
Ecological Efficiency:
● Energy content of higher trophic level/energy content of lower trophic level
● Varies across communities (no general pattern)
● Net production efficiency of ectotherms (cannot warm bodies) > endotherms (part of
energy budget devoted to thermoregulation)
● Lower at higher latitudes, low ecological efficiency
Implications of Low Ecological Efficiency
● Takes a lot of grass to make a hamburger
● If you eat high on the food chain, it takes a lot of plants for the animals you eat
● Biomagnification: the concentration of contaminants increases as you go up the food
chain
Links = trophic levels, Average size of links is 3.5; some with view links
Three Hypotheses:
1. Energy Transfer: a large fraction of energy is lost moving up a food chain; thus,
productive ecosystems should have more trophic levels. Supported? Not really. ex.
swamp = 2500 g/m2/yr vs. temperate grassland = 500 g/m2/yr
2. Stability: long food chains take a long time to recover from disturbance, so stable
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ENV100Y5 Full Course Notes
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Document Summary
Session 20: species interactions and community ecology (iii) Generally biomass increases and individuals, but energy declines (it is less) Fewer individuals, less biomass, less stored energy (move from top) Most of the energy stored in one level does not make it to the next. In a prey population (p): some are eaten/ingested (i), but some will survive. From the ingested prey (i): some energy is assimilated (a) and some energy is excreted. Of the assimilated energy (a): some energy lost through respiration and some energy converted to biomass (growth and reproduction = g) Ecological efficiency = g/p; usually never more than 10% Energy content of higher trophic level/energy content of lower trophic level. Net production efficiency of ectotherms (cannot warm bodies) > endotherms (part of energy budget devoted to thermoregulation) Lower at higher latitudes, low ecological efficiency. Takes a lot of grass to make a hamburger.
