As shown in the figure above, if the value of an environmental variable falls outside the species' physiological range of tolerance, the species cannot survive in that area.
Exam Tip: Remember that "stress" is not just a feeling; in physiological ecology, it is a measurable decrease in the rate of a vital process, such as hypoxia (low oxygen) at high altitudes.
Strategies for Coping: Tolerance vs. Avoidance
When faced with environmental variation or stress, organisms generally employ one of two main strategies: tolerance or avoidance.
Tolerance
Tolerance involves using physiological or morphological mechanisms to endure a challenge.
- Animals: The wood frog is a classic example; it can survive freezing temperatures by managing ice formation in its cells. Similarly, polar bears possess thick fur and fat to tolerate extreme cold.
- Plants: Drought-tolerant plant species invest energy into repairing xylem (water-conducting tissue) that has been damaged during dry periods.
Avoidance
Avoidance involves moving away from or bypassing the stressor entirely.
- Animals: Migratory birds avoid harsh winters by flying to warmer latitudes.
- Plants: Drought-avoiding plants may shut down their stomata (leaf pores) and enter a state of dormancy or quiescence until water becomes available again.
Time Scales of Response: Acclimation and Adaptation
Organisms respond to environmental shifts on different time scales, ranging from immediate individual adjustments to long-term evolutionary changes.
Acclimation (Short-term)
Acclimation is a short-term, reversible physiological or behavioral adjustment made by an individual to mitigate stress. For example, humans can acclimate to high altitudes over a period of days or weeks by increasing the production of oxygen-carrying red blood cells.
Adaptation (Long-term)
Adaptation is a long-term, genetic process occurring at the population level through natural selection. Over generations, individuals with traits that allow them to cope with stress are more likely to survive and reproduce, making those genetic traits more frequent in the population. Unlike acclimation, adaptation is irreversible for an individual.
Different populations may adapt in different ways to the same stressor. For instance, while people in the Andes may develop higher red blood cell counts, people in Tibet may evolve faster breathing rates and higher blood flow.
The Cost of Survival: Trade-offs
Both acclimation and adaptation require investments of energy and resources. This often leads to trade-offs, where energy allocated to one function limits another. For example, an organism might increase its red blood cell count to survive at high altitudes, but the metabolic cost of maintaining those cells might result in slower overall growth.
Energy and Water Balance
The physical environment influences an organism through the availability of resources and the management of energy and water.
Temperature and Energy Balance
Temperature controls metabolic reactions via enzymes, which have narrow optimal ranges. If temperatures are too high, proteins can denature (unfold and lose function), much like a raw egg turning into a cooked egg.
Organisms manage their temperature through energy exchange, which can occur via conduction, convection, radiation, or latent heat transfer (like evaporation).
- Plants manage energy balance through transpiration (evaporative cooling) and morphological traits like pubescence (hairs on leaves). In deserts, hairs reflect solar radiation to cool the plant, whereas in cold climates, hairs can trap heat and increase the boundary layer to prevent convective heat loss.
- Animals are categorized by how they handle heat:
- Ectotherms: Regulate heat through exchange with the environment (e.g., lizards basking in the sun).
- Endotherms: Generate heat internally (e.g., mammals and birds). While endothermy allows for wider geographic ranges, it requires a much higher food supply to fuel metabolic heat production.
Water Balance
Water availability is intrinsically linked to temperature. Terrestrial organisms must balance the rate of water loss against the need for cooling. For plants, this is managed through the control of stomata, which regulate the rate of transpiration.
Niche Theory and Geographic Distribution
The actual geographic distribution of a species is often narrower than its physiological potential due to biological interactions like competition and predation. To describe this, ecologists use the concept of the "niche":
- Fundamental Niche: The entire range of environmental conditions under which a species could potentially survive and reproduce in the absence of competition or predation.
- Realized Niche: The actual observed distribution of a species, which is limited by biological interactions, dispersal abilities, and disturbances.
Because plants are sessile (cannot move), they are excellent indicators of these environmental constraints. By modeling a species' climate envelope—the range of climatic conditions in which it occurs—scientists can predict how species distributions might shift in response to global climate change.
