Temperature Relations: Biotic Response to the Abiotic

Temperature Relations: Biotic Response to the Abiotic

Introduction to Temperature and Heat

• Temperature & Heat: A measure of the average kinetic energy of the molecules in a mass of a substance.

• Kinetic Energy: The energy of motion.

• Temperature of Life: Heat is not only produced as a byproduct of chemical reactions (metabolism) but also significantly influences the metabolic rate of living organisms.

Ecological Significance of Abiotic Temperature

• Abiotic temperature is an ecologically significant environmental factor.

• It directly influences the rates of chemical reactions within living organisms.

• Organisms demonstrate various adaptations to the temperatures of their specific environments.

• The relationship between the abiotic environment (temperature) and the biotic organisms (their responses and adaptations) is a core concept in Ecology.

The Big Picture of Temperature Regulation

• Environmental temperature is highly varied, primarily due to microclimates.

• Organisms possess diverse strategies for body temperature regulation to compensate for environmental temperature fluctuations within their specific microclimates.

• The biotic component (organisms) adapts to the abiotic component (environment) through general patterns, as well as with extraordinary exceptions.

• Organisms are typically adapted to a relatively narrow range of environmental temperatures, which can reduce a population's fitness when exposed to other environments or microclimates.

Microclimates: Local Climatic Variation

• Macroclimate:

  • Refers to the prevailing weather conditions in a region over a long period of time.

  • Influenced by large-scale geophysical properties such as latitude, the rotation of the Earth, and global air and water currents.

• Microclimate:

  • Refers to climatic variation on a much smaller scale, typically a few kilometers, meters, or even centimeters.

  • Usually measured over short periods of time.

  • Influenced by features of the regional landscape.

Factors Influencing Microclimates

• Altitude:

  1. Decreasing atmospheric pressure at higher elevations allows the air to expand, leading to a lesser density of air molecules and consequently cooler air.

  2. There is less atmosphere above to trap heat and radiate it back to the ground.

• Aspect:

  • Defined as the cardinal direction that a slope faces.

  • Northern Hemisphere Examples:

    • North vs. South-facing slopes: North-facing slopes are generally colder, while South-facing slopes are warmer due to more direct sun exposure.

    • East vs. West-facing slopes: East-facing slopes are colder when the morning sun shines, whereas West-facing slopes are warmer when the afternoon sun shines.

• Vegetation:

  • Plants shade the landscape, significantly altering ground temperatures.

  • Temperature Examples (Fictional, but illustrating the point):

    • Soil surface in full sun heats to high temperatures: e.g., $48^ ext{o}C$ in bare soil away from shrubs.

    • Maximum temperatures are lowered by the shading of the soil surface by low shrubs: e.g., $27^ ext{o}C$ in soil under a low shrub.

    • A layer of leaf litter lowers maximum temperatures even more: e.g., $29^ ext{o}C$ in litter under a low shrub.

    • Greater leaf area and numerous twigs of tall shrubs intercept more light, creating the coolest temperatures: e.g., $23^ ext{o}C$ in soil under a tall shrub and $21^ ext{o}C$ in litter under a tall shrub.

• Color of the Ground:

  • The color of the ground greatly influences temperature absorption and reflection. For example, white sand will create a different microclimate than black sand even under the same macroclimate, with black sand absorbing more heat.

• Boulders and Burrows:

  • These features can create stable microclimates by moderating temperature fluctuations.

  • Example from Sevilleta ITER data (University of New Mexico):

    • Air temperature fluctuated $14^ ext{o}C$ over a day.

    • At a depth of $22.5 ext{ cm}$, soil temperature varied by only $2^ ext{o}C$, demonstrating the insulating effect of soil.

• Riparian Vegetation & Depth of Water:

  • Water bodies, especially deeper ones, can stabilize temperatures, and surrounding vegetation adds to this effect.

  • Daily Temperature Variation Examples (from Ward 1985):

    • Air: $2.5^ ext{o} - 28^ ext{o}C$

    • Aquatic reed bed: $7^ ext{o} - 20^ ext{o}C$

    • Shallow riffle: $8.5^ ext{o} - 16^ ext{o}C$

    • Deep pool: $10^ ext{o} - 14^ ext{o}C$

Biotic Strategies for Regulating Body Temperature

Classifications Based on Thermal Stability

• Poikilotherm: An organism whose body temperature varies greatly with the environmental temperature.

• Homeotherm: An organism whose body temperature maintains thermal stability, regardless of environmental fluctuations.

Classifications Based on Heat Source

• Ectotherm: An organism that relies predominantly on external sources of energy for regulating its body temperature.

• Endotherm: An organism that relies on internally derived metabolic heat energy to regulate its body temperature.

Behavioral and Physiological Adaptations

• Inactivity:

  • Organisms may become inactive during periods of extreme temperatures to avoid thermal stress.

  • Example (Hadley, Savill, and Schultz 1992 data):

    • In the morning, when air temperature is $25^ ext{o}C$ and sand temperature is $35^ ext{o}C$, desert beetles are active in the sun.

    • As sand temperatures approach $70^ ext{o}C$ later in the day, most beetles move into the shade to avoid overheating.

• Torpor:

  • A resting state, also known as temporal heterothermy, in which body temperatures drop, metabolic rates slow significantly, and reactions to external stimuli are diminished.

  • Example (Broad-tailed hummingbird):

    • The amount of nectar available determines whether it goes into torpor during the night.

    • If nectar is scarce: The hummingbird enters torpor, with its body temperature dropping to $12^ ext{o} - 17^ ext{o}C$. This reduces its metabolic rate, conserving significant energy.

    • If nectar is adequate: The hummingbird does not enter torpor, maintaining a body temperature of approximately $39^ ext{o}C$. This requires consuming large quantities of nectar just before roosting to meet its high energy demands.

• Hibernation: A prolonged state of torpor occurring in response to cold temperatures and food scarcity, typically during winter.

• Estivation: A prolonged state of torpor occurring in response to hot temperatures and arid conditions, typically during summer or dry seasons.