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Lecture 3: Ecological Limiting Factors and Adaptation
Reading: Economy of Nature, pp. 60-70.
Water absorbs light energy and scatters
light
In sea water: At 10m, the energy
of visible light decreases 50%
At 100m, the energy of visible light decreases to <7%
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Red is
absorbed first |
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Blue and
violet scatter easily |
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Green
penetrates water best |
Euphotic zone: Depth to which photosynthesis
exceeds respiration in water.
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Rarely
the compensation point, the bottom of the euphotic
zone, is as deep as 100m. Examples, very clear ocean or
lakes near equator. |
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In highly
turbid waters, the compensation point may be reached
at 1m. |
Major Essential Elements
Calcium (Ca), Iron (Fe), Nitrogen (N),
Magnesium (Mg), Potassium (K), Phosphorus (P), Sodium (Na), Sulfur (S)
Limiting Nutrient Elements
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In aquatic
(freshwater) environments: nitrogen and phosphorus |
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In marine
(saltwater) environments: iron |
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In terrestrial
environments: nitrogen and phosphorus (calcium) |
Other Essential Resources
Carbon Dioxide: Not limiting
Oxygen: Can be limiting in water
Water: Often limiting in terrestrial
environments
Limitations for one essential resources
can influence the availability of other essential resources. This is
the case among terrestrial plants for the relationships between photosynthetic
rates, water loss, and gas exchange.
Photosynthetic Capacity and Water Conservation
Photosynthesis rate varies widely among
species (100x) even with light saturation and all other resources in
abundance. This variation is due in part to differences between plant
species in the biochemistry of carbon fixation in photosynthesis (Calvin
Cycle). Plants can be categorized as having  ,
 , or CAM metabolism.
Photosynthetic Rate, Water Loss and Gas
Exchange Specializations and Compromises Among Plants
- Short life, high photosynthetic rate
when water abundant, dormant
at other times (seed stage) (desert annuals)
- Long life, leaves produced when water
abundant, leaf drop during
droughts (winter or dry season) (deciduous woody plants)
- Leaves long lived, transpire slowly,
tolerate water deficit but have
low photosynthetic capacity (woody evergreens, evergreen desert
shrubs)
-
photosynthesis: increased efficiency of carbon dioxide use per
unit of water loss, but inefficient at low light intensity (not shade
tolerant), high temperature optima, adaptation for water conservation
and efficient nutrient capture (arid, tropical, and saline environments)
(see figures on next page)
- CAM photosynthesis (Crassulacean Acid
Metabolism): control of
water loss by limiting atmospheric carbon dioxide capture to night
hours when water transpiration rates are at a minimum, stomata
are open at night and are closed during the day, good water
conservation but there are limits on photosynthetic capacity (arid,
high elevation, windy environments)
Major steps in the capture of carbon dioxide
in CAM plants. Atmospheric carbon dioxide is brought into the plant at
night, when the stomata are open. In the daytime, stored reserves of malic
acid and oxaloacetate are broken down to release carbon dioxide inside
leaves for use in the Calvin Cycle, but gas exchange to the atmosphere
is minimal. Abbreviation key: RuBP = ribulose bisphosphate (a 5C compound),
PGA = phosphoglycerate (two molecules of a 3C compound), PEP = phosphoenolpyruvate
(a 3C compound), OAA = oxaloacetate (a 4C compound), Malate = malic acid
(a 4C compound). Water loss is controlled in this system by mean of temporal
separation of carbon dioxide capture from the atmosphere and the addition
of one carbon to RuBP in the first step of the Calvin Cycle (Ricklefs,
1996, p 69, Fig. 3.9).
Comparison between
and plants showing
the differences in the physical distribution of chloroplasts in the leaves,
and differences in first steps of atmospheric carbon dioxide capture for
photosynthesis. Abbreviation key: RuBP = ribulose bisphosphate (a 5C compound),
PGA = phosphoglycerate (two molecules of a 3C compound), PEP = phosphoenolpyruvate
(a 3C compound), OAA = oxaloacetate (a 4C compound), Pyr = pyruvate (a
3C compound). Water loss is better controlled in
plants by physically separating carbon dioxide capture from the atmosphere
and the addition of one carbon to RuBP in the first step of the Calvin
Cycle. Water loss is minimized in this system because the atmospheric
carbon dioxide capture step is catalyzed by the enzyme PEP carboxylase
which has a higher affinity for carbon dioxide than does the enzyme RuBP
carboxylase (Ricklefs, 1996, pp 68 and 69, Fig. 3.7 and 3.8).
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