Principles of Ecology: Abiotic Factors and Limits
Principles of Ecology: Abiotic Factors and Limits
Adaptations to the Environment: Physiological Adaptations to Thermal Stress
Organisms exhibit two broad categories of responses to thermal stress:
Conformers: Organisms whose internal conditions (e.g., body temperature) change to match external environmental conditions. They do not actively regulate their internal environment, or they have a narrow internal range that is maintained minimally. This means their internal body temperature will largely range with the external temperature.
Regulators: Organisms that maintain their internal body conditions near constant or within set limits, regardless of external environmental fluctuations.
Homeostasis: The process by which regulators maintain a stable internal environment. The internal body condition is kept relatively constant or within a narrow, defined range.
A species might switch from being a conformer in some external conditions to a regulator in others, especially when the external conditions become challenging, requiring the organism to expend energy to maintain internal stability.
Regulation often comes at a cost, involving internal mechanisms that consume energy or resources.
Variation in Tolerance Limits
An organism's tolerance limits (the range of environmental conditions it can withstand) can vary in both location (e.g., a population's optimal temperature) and spread (e.g., a narrow or wide tolerance range).
What shapes tolerance limits?
"Nature" (Natural Selection): Tolerance limits are primarily shaped by natural selection, which acts upon populations based on their environment. The specific range and variability of abiotic factors in an environment influence which traits (and thus which tolerance limits) are favored.
For example, Population A might have a narrow tolerance for high temperatures, while Population B might have a wide tolerance for temperatures, even if they are the same species, depending on their environmental history.
"Nurture" (Prior Exposure/Acclimation): Tolerance limits can also be influenced by an organism's prior exposure to environmental conditions.
Acclimation: An individual's physiological adjustment to challenging abiotic conditions. This involves slow, reversible changes in an organism's phenotype (physiology, morphology, or behavior) in response to environmental shifts. It allows an organism to adjust its tolerances.
Acclimation is generally favored by species and populations living in variable environments, as it allows individuals to cope with changing conditions without genetic adaptation over generations.
The Thermal Environment: Effect of Temperature on Biochemistry
Temperature significantly affects an organism's biochemistry:
Reaction Rates: Most biochemical reactions are temperature-dependent.
Q{10} Value: Represents the rate of reaction at a given temperature divided by the rate of reaction at a temperature 10^{\circ}C lower (Q{10} = R2 / R1, where R2 is the rate at temperature T and R1 is the rate at temperature (T - 10^{\circ}C)).
Low Temperatures: Reaction rates become too slow to meet cellular demands, leading to a