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Lecture 2: Physical controls on the distribution of organisms
Light
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Variation in light intensity is a key control on distribution of plant species within an
ecosystem
Heliophytes (sun plants): grow best in full sunlight and are shade intolerant
Sciophytes (shade plants): grow best in shade (sun intolerant)

C4 plants:
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Examples; Maize, Sugar cane, Savana grasses and sedges
Confined to hot and dry climates
Large bundle sheath cells span width of leaf
Chloroplast concentrated within bundle-sheath cells
More efficient use of carbon dioxide
No photorespiration
1st product of photosynthesis 4C compound (e
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oxaloacetic acid, malic acid)
Efficient at high temperature and low co2
Good water-use efficiency

C3 plants
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All trees and shrubs
Temperate grasses and sedges
Several small bundle sheath cells
1st product of photosynthesis 3C PGA (phosphoglyceric acid)
Inefficient at high temperatures and low co2
Increased photorespiration
Poor water-use efficiency
Physiological drought

Transpiration: evaporation from the leaf
Evapotranspiration: evaporation from leaf plus soil

CAM photosynthesis (Crassulacean Acid Metabolism)
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Crassulacea
Bromeliaceae
Cataceae

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Large variation among plant species in reaching light saturation
Sun plants – 30-65% full light
Shade plants – 5-10% full light
Photoinhibition
Shrinking of chloroplasts, loss of chlorophyll, loss of co2 due to photo-oxidation
Photo-oxidation leads to photo-respiration
Co2 released but not utilisable by plant
Only occurs n C3 plants
Over-heating

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Rate of photorespiration
Positively correlated with light intensity and temperature
C4 plants do not suffer from photorespiration
Therefore, have higher rates of photosynthesis at higher light intensity and temperature
But less efficient at photosynthesis than C3 plants at low light intensity and temperature
Obligate heliophytes (shade intolerant)

Leaf adaptations to different light intensities
Sun leaves: thicker, more cell layers, thicker cuticle, more lobes
Shade leaves: thinner, fewer cell layers, thinner cuticle, less lobes

Life history strategy according to light conditions
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Shade-tolerant plants are perennials- live for more than a year
Not enough light in shady conditions to support annuals – must germinate, grow, flower and
set seed in 1 year
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Ectotherms have
wider range of tolerance
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9°c, lethal above 6°c
Ants in Namib desert can survive 52-55°c for short periods
African rock hyrax survives at 41°c

Polar bear strategies to reduce the lower critical temperature
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11cm thick blubber
Black skin
Hollow hairs
Ambient air temp -50°c
Core temperature 37°c

Daphnia – water flea
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Metabolic rate declines rapidly above 40°c
Enzyme activity slows
Proteins break down
Cell membranes destroyed
Rates of oxygen intake no longer match respiratory needs

Allen’s Rule: the shorter an animals extremities are relative to body mass, the lower the rate of heat
loss
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Many birds have winter
range limits dictated by low temperature

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