Factors Limiting Distribution: Temperature, Moisture, And Other Physical-Chemical Factors– Chapter 6

Climatology• Large temperature differentials over Earth are

the result of two basic variables: incoming solar radiation and distributions of land and water.

Insolation – Solar RadiationAmount of heat delivered to the poles is only about 40% that at the equator.

• Land heats and cools much more rapid than does water, so land-controlled climates have large daily and seasonal temperature fluctuations.

Average Annual Temperature Range (°C):

Monthly mean warmest – monthly mean coldest (15C = 27F)

World Distribution Of Mean Annual Precipitation

Equator

Moisture Circulation• Of the water that falls on land, about 30% is

returned via runoff.

• The rest (70%) of the moisture returns back to the atmosphere by evaporation and transpiration from plants.– Transpiration is the loss of water through plant

leaves

• Evaporation and transpiration (combined - evapotranspiration) depends primarily on temperature.

• Evaporation is high in arid areas.

Evapotranspiration

• Potential evapotranspiration - Usually measured as a function of temperature.– Amount of water that would be lost given an

unlimited amount of rainfall

• Actual evapotranspiration – the evaporative water loss from a site covered by a standard crop, given the precipitation

Moisture LimitingTemperature Limiting

Climatic Diagrams: Relationship between temperature and precipitation

Tolerating Temperature and Moisture• Organisms either tolerate the conditions as they are

or escape via some evolutionary adaptation.

• Organisms have upper and lower lethal limits.– Shelford’s Law of Tolerance– Organism’s can acclimate to some conditions

Po

pu

lati

on

siz

e

Low High Temperature

Zone ofintolerance

Zone ofphysiological stress

Optimum range Zone ofphysiological stress

Zone ofintolerance

Noorganisms

Feworganisms

Lower limitof tolerance

Abundance of organismsFew

organismsNo

organisms

Upper limitof tolerance

• Temperature and moisture may act on any stage of the life cycle and can limit the distribution of a species through their effects on one or more of the following:– Survival

– Reproduction

– Development of Young Organisms

– Interactions of other organisms (competition, predation, parasitism, disease) near the limits of temperature or moisture tolerance

• What aspect is limiting distribution?– Maximums, minimums, averages, or the level of variability

• No rule of thumb.– Plants and animals respond differently during different stages

of their life cycle

• How to show that temp or moisture is limiting:– Determine which phase of the life cycle is most sensitive to

temperature or moisture– Identify the physiological tolerance range of the organism for

this life cycle phase– Show that the temperature or moisture range in the

microclimate where the organism lives is permissible for sites within the geographical range, and lethal for sites outside the normal geographic range

Drought

• How can plants resist drought (evolutionarily)?– Improvement of water uptake by roots

– Reduction of water loss by stomatal closing, reduction of cuticular respiration, reduction of leaf surface area

• Photosynthesis: Solar Energy + CO2 + H2 O C6H12O6 + O2

• Respiration: C6H12O6 + O2 CO2 + H2 O + Energy

– Storage of water

• Xerophytes – plants that live in dry areas.– Most have the above adaptations

Temperature and Moisture Interaction

• Soil drought – deficiency in soil moisture– absolute shortage of water in soil

• Frost drought – present water is unavailable due to low temperatures– Relative shortage of water– Low temperatures can produce the symptoms of

drought

• Many of the distributional effects attributed to temperature may in fact operate through the water balance in plants.

The rate of water uptake in loblolly pine decreases rapidly at lower temperatures.

Predicted range based on temperature and moisture strongly correlates with actual range

Timberline• Moving up a mountain, there is an altitudinal limit

to the presence of trees – this is called the tree line or timberline.– Strong illustration of physical limitations to plant

growth

• Nine factors that may affect the location of a timberline:– Lack of soil, desiccation of leaves in cold weather, short

growing season, lack of snow (exposing plants to winter drying), excessive snow lasting through the summer, mechanical effects of high winds, rapid heat loss at night, excessive soil temperatures during the day, drought

• Above can be boiled down to three factors: temperature, moisture, and wind.

Timberline

Wind Beaten Trees

MontaneConiferous

Forest

DeciduousForest

AlpineTundra

TropicalForest

Tropical Forest Temperate Deciduous Forest

Northern Coniferous Forest

Arctic Tundra

LowHigh Moisture Availability

Lo

wH

igh

Elevatio

n

Latitude and Altitude

Adaptation

• We know that genetic and physiological uniformity does not occur throughout a species entire range.

• Ecotype – subspecies or race that is especially adapted to a particular set of environmental conditions.– describes genetic variability within a species.

– For example, Plantago maritima grows tall in marshes and as a dwarf on the coast

• Are ecotypes the result of environmental or genetic differences, or a mixture of both?

Common Garden

• A common garden experiment can be used to separate the phenotypic (environmental) from the genotypic (genotype) components of variation.

• Plants of the same species but growing in a diversity of habitats are grown in the same environment.– Any differences in phenotype can then be

attributed to genotype differences

• Common garden experiments are also used for animal studies.– Temp tolerance, salinity tolerance, reproductive

differences etc.

Plantago maritima

• Marsh normal height: 30 – 40 cm

• Cliff normal height: 5 – 10 cm

• Each grown in a common garden:

Plantago maritima Source

Mean height (cm) in garden

Marsh Population 31.5

Cliff Population 20.7

Morphological and Physiological Differences

• Diverse phenotypes can be explained three ways:– All differences are phenotypic, and seeds

transplanted from one situation to the other will respond exactly as the resident species

– All differences are genotypic, the mature plants will retain the form and physiology typical of their original habitat

– Some combination of phenotypic and genotypic determination produces an intermediate result

Light As A Limiting Factor

• Photoperiod – presence of light during a 24 hour period – Can influence seasonal physiology / behavior

– Increasing/decreasing day length

• Essential for photosynthesis – conversion of CO2 to organic compounds

– Shade tolerant versus shade intolerant plants• Metabolic rate differences

Important principle in evolutionary ecology:

Individuals of a species cannot do everything in the best possible way!

Adaptations to live in one type of habitat make it difficult or impossible to live in another type of habitat – there are no superanimals or superplants!

Seaweeds

• Light is attenuated very fast in water (less light available the deeper you are).

• Two general types of seaweed occur:– Flat wide monolayered thalli

– Highly dissected narrow multilayered thalli

Proposed model for the effects of light intensity.

Relationship of thallus morphology to depth for seven species of sargassum.

C3, C4, and CAM Photosynthesis

• Plants have evolved different types of photosynthesis as an adaptation to different habitat types.

• During photosynthesis CO2 is ‘fixed’ to an organic molecule.– C3 – CO2 is fixed to a three carbon molecule

– C4 – CO2 is fixed to a four carbon molecule

– CAM – crassulacean acid metabolism

• CO2 taken in at night and stored as malic acid, which is then used to complete photosynthesis during the day.

C3 Versus C4

• C4 plants do not reach saturation levels even under the brightest sunlight.

C3

C4

C3 Versus C4

• C4 plants physically separate CO2 fixation and the Calvin cycle

• They are more efficient at recycling CO2 through respiration and and use a different enzyme to fix CO2

C3 RuDP Carboxylase to fix CO2, is inhibited by O2

C4 PEP Carboxylase to fix CO2, not inhibited by O2

C3 Versus C4

• C3 plants are much more common in cooler habitats

% C4 plants

CAM Plants

• Open their stomata at night to take up CO2, presumably to conserve water.

• Segregate photosynthesis by time.

• These plants are found in very dry areas like deserts (cacti are CAM plants).

Climate Change And Species Distribution

• If temperature and moisture are the master limiting factors for the geographical range of animals and plants, the climatic warming that is now occurring will have profound effects on the Earth’s biota.

4.5°C 6.5°C