Coping with Environmental Variation

Learning goals:
* understand how stress influences organismal function and distribution
* know the difference between acclimitization and adaptation

Each species has a range of environmental tolerances that determines its potential geographic distribution; i.e. the geographic ranges of species are related to constraints imposed by the environment
Some physical environmental limitations are obvious: e.g. freezing temperatures, very low water availability; others are more subtle: e.g. soil and water chemistry

The physical environment influences an organism’s success in two main ways:
1) Affects ability to obtain energy and resources, maintain metabolic functions, grow and reproduce
2) Extreme environmental conditions affect survival

A species’ abundance varies across environmental gradients, reflecting its tolerances to the environment.  Actual distribution reflects additional constraints due to biotic interactions (e.g. competition, predation).
A species’ climate envelope is the range of condition over which it occurs- useful tool for predicting responses to climate change

Physiological processes (e.g. photosynthesis, metabolic respiration) have a set of optimal conditions for functioning.  Deviations from the optimum reduce the rate of the process.
Stress—a decrease in the  rate of important physiological processes due to environmental change, lowering the potential for survival, growth, or reproduction.

Many organisms can adjust to stress through behavior or physiology—called acclimatization; usually a short-term, reversible process
Over multiple generations of an organism, natural selection can result in adaptation to environmental stress, which is a unique, genetically-based solution to the stress
Individuals with traits that make them best able to cope with stress are favored, increasing the frequency of the unique adaptations in the population

Both adaptation and acclimatization enhance survival and reproductive success, but adaptation is a long-term, genetic response of a population to environmental stress; acclimatization can be an adaptation
Populations with unique adaptations are called ecotypes

Acclimatization and Adaptation are not “free;”  they involve tradeoffs with other functions, and require investment of energy and resources that could be used for other functions such as reproduction

Coping with variation in temperature

Learning goals
* know what impact temperature extremes can have on organisms
* understand the exchanges in energy that influence the temperature of an organism (primarily plants and animals)
* learn some of the ways plants and animals can alter the exchange of energy to warm or cool themselves
 

Fluctuations in temperature varies considerably among different environments-  e.g. seasonal temperature swing in a boreal forest can exceed 80 ºC (144 ºF); tropical rain forest 15º C (22 ºF), and tropical oceans as little as 3 ºC (5 ºF)
Life is usually restricted to organismal temperatures between 0 and 50 ºC, but has been recorded in organisms at -5 and 90 ºC.
Some organisms can survive periods of extreme heat or cold by entering a state of dormancy, in which little or no metabolic activity occurs

Temperature controls physiological activity through its effects on chemical reactions, especially enzyme function
Enzymes are proteins that enhance biological reactions
* structure degrades (denature) at high temperature
* their activity decreases at low temperature
Temperature also affects the properties of cell and organelle membranes; composed of two layers of lipid molecules.  At low temperatures, the lipids solidify, embedded proteins can’t function, and the cells leak metabolites

The temperature of organisms is determined by exchanges of energy with the external environment;  The balance between inputs and outputs of energy determines whether the temperature of any object will increase or decrease.
Organisms can adjust their exchange of energy with the environment through behavioral, morphological, and physiological modifications to avoid adverse temperatures
Energy exchange with the environment can be by:
Conduction—transfer of energy from warmer to cooler molecules.
Convection—heat energy is carried by moving water or air.
Latent heat transfer - water absorbs heat as it changes state from liquid to gas

Energy gains and losses for plant leaves
Temperature change in a plant can be expressed as:
∆ (change in) H_plant= SR + IR_in- IR_out +/- H_conv +/- H_cond – H_et (eq. 4.1)
where:
SR = Solar radiation
IR = Infrared radiation
Hconv = Convective heat transfer
Hcond = Conductive heat transfer
Het = Heat transfer by evapotranspiration

Plants adjustment of energy exchange
1) Transpiration (H_et)- rate controlled by specialized guard cells surrounding stomate;
2) changing albedo- reflection of solar radiation, by producing hairs on the leaf surface
3) altering convective heat loss- altering boundary layer (zone next to an object where the velocity decreases in response to frictional drag and turbulence)
depth of the boundary layer is dependent on wind speed and leaf dimensions: small leaves have a smaller boundary layer than large leaves- lose