The Biosphere

From Chapter 29 and M.C. Molles (1998) Ecology Concepts and Applications, WCB McGraw-Hill Boston.

Major Concepts:

I. Organisms Must Cope With a Varied Environment

The Environmental Challenge

How Environments Vary

Temperature - most organisms are adapted to live within a relatively narrow range of temperatures

Kinetic considerations

Temperature increases the kinetic energy of the system (velocity of motion) and therefore directly influences the velocity of enzyme reactions. At higher temperatures, the increased velocity of motion of the substrates for Rubisco (CO2 and RuBP) means that they are more likely to wind up in the active site of the enzyme (quicker) and thus the reaction speeds up. In general as temperatures increase so does the rate of enzymatic reactions.

Conformational considerations

The functioning of enzymes often depends on its shape and flexibility. This is so that it can initially form the lock and key arrangement with the substrate and then assume another shape after binding with the substrate. At low temperatures most enzymes have rigid predictable shapes and are relatively inflexible. At very high temperatures enzymes lose their shape all together.

Enzymes usually work best in some intermediate range of temperatures, neither too hot nor too cold, where they can maintain the balance between shape, flexibility and molecular motion.

From this mechanistic point of view the activity of the biological system of our planet is sensitive to temperature.

  1. Minimum temperatures

    At low temperature the reaction velocity becomes so slow that the process essentially halts.

    At critically low temperatures (such as freezing) the structure of biological membranes, cells or even organisms can be damaged (water expands when it freezes)

    The solubility of compounds decreases with temperature (can limit substrate availability)

  2. Maximum temperatures

    At higher temperatures reactions tend to run faster, however as temperatures increase beyond a critical level, reactions will slow and damage is done to the living system.

    At critically high temperatures, membranes lose their integrity and fall apart

    Proteins can denature at very high temperatures (putting an ultimate constraint on the temperatures at which life is plausible)

  3. Average temperatures

    Many organisms can adapt or acclimate to the average temperature of their environment isozymes are variations of enzymes that have different temperature tolerances

  4. Temperature variability

    Plant life is extremely sensitive to temperature extremes since they are immobile. Accordingly they must be able to survive the extreme events.

    Animals must also be able to survive extreme events but are able to migrate and or modify their environment to help extend the range of environments they can live in.

    Bacteria have adapted to many temperature extremes

Water - all organisms require water, and sever limitation can exclude life


The movement of water through plants is critical, providing:

  1. Turgor
  2. Nutrient delivery
  3. Evaporative cooling

A lack of water will result in limitation of physiological activity, wilting and ultimately death of the individual.


Many small terrestrial animals can adsorb water from the air, however most terrestrial animals satisfy their need for water either by drinking or by taking in water with food. Metabolism (the consumption of carbohydrates) can also create some water.

The movement of water through animals is critical, providing:

  1. Nutrient delivery
  2. Waste removal
  3. Evaporative cooling
  4. Facilitation for oxygen exchange

A lack of water will result in limitation of physiological activity, dehydration and ultimately death of the individual

Sunlight - to provide the input of energy, constrains the total amount of biomass that can be supported

Soil - consistency, pH, mineral composition

Range and Grain of Environmental Variation

Spatial size of the variation relative to the size and mobility of the organism = environmental grain

patches are large relative to the size and activity of the organisms, allowing selection (e.g. a bee "choosing" which flower to visit)
patches are small relative to the size and activity of the organisms, so small they can be ignored by certain organisms (e.g. cows eat all the grasses and forbs in the pasture)

Temporal variation also exists and can be course or fine grained.

Active and Passive Approaches to Coping with Environmental Variation

organisms that maintain a steady-state internal environment even with external environmental variation (Homeostasis) by employing physiological, morphological or behavioral mechanisms.
conform with the ambient environment by changing the temperature, salinity andother aspects of their surroundings.

II. Adaptations to Environmental Change


Enzyme Examples

Acetylcholinesterase (Fig 4.8) enzyme produce at the synapse between neurons to turn off the signal. Harvested from Rainbow trout (Oncorhynchus mykiss). Two isozymes are formed, one at 2°C and one at 17°C. These trout live in waters that have a large annual variation in temperature, averaging 2° C in the winter and 17°C in the summer.

Photosynthesis (Rubisco and other photosynthetic enzymes) - Temperature experiments can be used to look at the distribution of different plant types, as here where the optimum temperature for photosynthesis of a moss from a boreal forest is 15°C while the optimum temperature for photosynthesis of a desert shrub is 44°C (Fig 4.9). Or can be used to look at the acclimation of a single species to the average environment, as here where clones of one shrub where grown under two different temperature conditions and the optimum shifted by 8°C (Fig 4.10).

With these physiological mechanisms in mind we could constrain life to the maximum and minimum temperature for any critical reaction catalyzed by an enzyme. Yet life exist well outside of these constraints. Many organisms have evolved ways to regulate body temperature.


Temperature Regulation in Plants(Fig 4.14)

Energy balance - Three main options:

  1. Decrease heating by conduction (get leaves off the ground)
  2. Increasing rates if convective cooling (small leaves, open growth form)
  3. Reduce rates of radiative heating (reflective surfaces)
Temperature Regulation in Exothermic Animals
  1. Variation of body size, shape and pigmentation
  2. Behavior to take advantage of these variations
  3. Ex. Liolaemus (Fig 4.17)


Avoidance strategies for extending thermal tolerance

Many organisms avoid extreme temperatures by entering a resting stage

  1. Inactivity (daily)
  2. Reducing metabolic rates (seasonally)

III. Climate Shapes The Character Of Ecosystems

The amount of solar radiation reaching the surface of different parts of the Earth and the circulation of the Earth's atmosphere are tow key physical features determining the distribution of life on Earth.

Uneven heating of the surface of the Earth causes climatic variation.

  1. Spherical shape

  2. Angle of rotation about the Earth's axis

    Molles Figure 2.4

    1. Causes seasonal shifts in temperature and day length
    2. Dramatic near the poles, minimal near the equator
  3. Drives atmospheric circulation
  4. The sun heats the surface of the earth and the atmosphere at the equator. This causes the air to expand and rise. This warm moist air cools as it rises, causing the water to condense, form clouds and leads to rain.

    Molles Figure 2.5a

    Equatorial air masses eventually spread both north and south, as dry air masses (since it left the moisture behind in the tropical rainstorms). As the air masses flow north and south, it cools, causing the density to increase and the air mass to sink. These sinking air masses are quite dry and form deserts at 30° latitude before the air masses return to the topical regions to complete the atmospheric circulation cell. There are three of these cells on either side of the equator

    Molles figure 2.5b

    Since the earth is spinning on its axis (west to east) the circulation patterns on the surface of the earth are not due north and south as suggested. The Coriolis effect causes an apparent deflection of winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

    Molles Figure 2.6

Atmospheric Circulation, Precipitation, and Climate

Rain Shadows

Figure 26.9 from your textbook


Figure 26.11 from your textbook

Solar radiation is most intense when the sun is directly overhead - the tropics!


Figure 26.12 from your textbook

Air temperature falls about 6 °C for every 1000-meter increase in elevation.

In North America a 1000 m increase in elevation = 880 km in latitude!


localized, fine scale variation within ecosystems

Climate Diagrams - a tool to relate terrestrial vegetation and climate.

Molles Figure 2.7

Climate diagrams (like the general one above) summarize the annual variation in climate, and display

  1. January to December for the Northern Hemisphere
  2. July to June for the Southern Hemisphere

Reading the diagram:

  1. Location and Elevation are stated at top
  2. Mean monthly temperature on the Left Axis
  3. Mean monthly precipitation on the Right Axis
  4. Mean annual temperature is given in the upper left corner
  5. Mean annual precipitation is given in the upper right corner

IV. Biomes Are Widespread Terrestrial Ecosystems

Biomes are major communities of organisms that have a characteristic appearance and that are distributed over a wide land area defined largely by regional variations in climate.

Distribution of the Major Biomes

Major Biomes:
  1. Tropical Rainforest
  2. Savanna
  3. Desert
  4. Temperate Grassland
  5. Temperate Deciduous Forest
  6. Temperate Evergreen Forest
  7. Taiga (Boreal Forest)
  8. Tundra
Minor Biomes:
  1. Polar Ice
  2. Mountain Zone (Alpine)
  3. Chaparral
  4. Warm Moist Evergreen Forest
  5. Tropical Monsoon Forest
  6. Semidesert

While Defined by Vegetation, each biome also has a distinct fauna:

Figure 26.13 from your textbook

Biomes and Climate

Moisture and Temperatuer are two key environmental variables defining the distribution of biomes.

Figure 26.14 from your textbook

Other factors such as soil structure, mineral composition and seasonal versus constant climates are also important and can further define the location of biomes.

Topography and the relative location of ocean and land masses further define the distribtuion of the biomes.

Figure 26.15 from your textbook

Survey of The Major Biomes

Lecture by Professor Kevin Griffin.

Updated April 25, 2005
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