Environmental conditions fluctuate over minutes to seasons. To survive, organisms detect internal and external changes and respond through behavioral actions and physiological adjustments that help maintain stable internal conditions while meeting energy and resource demands.

Environmental change and detection

Organisms monitor both external cues and internal states.

• External: temperature, light, water availability, salinity, oxygen, toxins, predator presence

• Internal: hydration level, blood glucose, osmolarity, body temperature, tissue oxygenation

Homeostasis as the organizing principle

Maintaining homeostasis requires coordinated sensing (receptors), integration (nervous/endocrine control), and responses (effectors such as muscles, glands, transport proteins, and stomata).

Negative feedback loops maintain homeostasis by detecting a deviation from a set point, integrating the information in a control center, and activating effectors that counteract the change. The right-hand panel applies the same logic to thermoregulation, showing how a rise in body temperature triggers heat-loss responses (e.g., sweat gland activation) that push temperature back toward normal. Source

Behavioral responses (what an organism does)

Behavioral mechanisms are often rapid, reversible, and can prevent physiological stress from occurring.

• Microhabitat selection: moving into shade, burrows, tide pools, or deeper water to buffer temperature and humidity

• Postural changes: spreading limbs to increase heat loss, curling to reduce surface area, orienting leaves to reduce midday irradiance

• Activity timing shifts: switching to nocturnal/crepuscular activity during heat; reducing activity during drought

• Migration and dispersal: relocating to track seasonal resources, avoid harsh winters, or find water

• Dormancy behaviors: seeking sheltered sites prior to hibernation/torpor; reducing foraging when predation risk rises

Behavior can be triggered by simple cues (photoperiod, temperature) or by internal signals (thirst, hunger), and it often works by changing exposure to environmental stressors.

Physiological responses (what an organism changes internally)

Physiological mechanisms adjust internal function to counter changing conditions, typically via nervous and endocrine control.

Thermoregulation and metabolic adjustment

Responses depend on the organism and the challenge.

• Heat stress:

  • Vasodilation increases blood flow to skin surfaces for heat loss

  • Sweating/panting increases evaporative cooling (with water cost)

  • Heat-shock proteins help protect and refold damaged proteins during acute temperature spikes

• Cold stress:

  • Shivering thermogenesis generates heat via muscle contractions

  • Non-shivering thermogenesis (in some animals) increases heat production via metabolic pathways

  • Antifreeze compounds/proteins (some fish/insects/plants) reduce freezing damage

Osmoregulation and water balance

Changing water availability or salinity drives adjustments in transport and excretion.

• Kidney/ion transport changes regulate salt and water reabsorption in many animals

• Hormonal control (e.g., antidiuretic signals) can reduce water loss by concentrating urine

• In plants:

This diagram illustrates how guard cells regulate stomatal aperture by changing their solute concentration and water content. When guard cells accumulate K+ (lowering water potential), water enters, turgor rises, and the pore opens; when solutes leave, water follows, turgor drops, and the pore closes—reducing transpiration but also limiting CO₂ entry. Source

• Stomatal closure reduces transpiration during drought (trading off CO₂ intake and photosynthesis)

This schematic traces abscisic acid (ABA) signaling during water stress, showing how ion channel changes drive guard cells to lose solutes and then water. The resulting drop in guard-cell turgor pressure closes the stomatal pore, conserving water while constraining gas exchange and photosynthetic carbon uptake. Source

• Root and leaf signalling can shift water allocation and growth patterns during prolonged dryness

Resource limitation and internal energy status

When resources drop, organisms may alter internal allocation.

• Metabolic rate suppression during torpor reduces energy demands

• Mobilising stored fuels (lipids/glycogen) supports short-term survival when intake is low

• Stress physiology can redirect energy from growth/reproduction toward immediate survival functions

How behavioral and physiological mechanisms work together

Many responses are integrated sequences rather than either/or choices.

• Behavioral avoidance often reduces the magnitude of physiological correction needed (e.g., seeking shade reduces reliance on evaporative cooling).

• Physiological limits can constrain behavior (e.g., dehydration reduces sustained activity, forcing rest or sheltering).

• Responses can occur on different timescales:

  • seconds–minutes: reflexes, ventilation changes, stomatal movements

  • hours–days: hormonal shifts, enzyme activity adjustments

  • weeks: acclimation of membrane composition, transporter abundance, or baseline metabolic patterns

Constraints and trade-offs

Effective responses maintain internal stability but have costs.

• Evaporative cooling improves temperature control but increases dehydration risk.

• Concentrating urine conserves water but requires energy and can increase solute load.

• Stomatal closure conserves water but limits photosynthesis and growth.