Ecology Lecture Sept. 9th
Physiological Ecology: Temperature, Tolerance, and Adaptation
Two interrelated concepts when thinking about survival conditions
Survival conditions: where organisms live long enough to reproduce (reproduction and survival across the range)
Persistence: how well organisms persist under varying conditions within their range
Plants as a starting point: immobile; tolerance tightly linked to location
Because plants don’t move, they’re good indicators of the physical environment
Aspen example and climate envelopes
Aspen trees are widespread in North America and parts of Europe (e.g., around London)
Distribution is limited by factors that affect reproduction and survival, especially temperature (low temperatures) and dryness (aridity)
Physiological tolerance of a species (e.g., drought tolerance, cold tolerance) can be mapped across grid cells with known temperature and water availability to predict distribution
The dark-green areas on a predicted map indicate where you expect to find the species based on climate tolerance (the climate envelope)
Why actual distribution is often smaller than the predicted baseline climate tolerance:
Disturbance, competition, and other biotic factors can shrink realized distribution
The actual distribution is shaped by species interactions, disturbance regimes, and other limitations
Core concepts: climate envelope vs potential vs actual distribution
Potential distribution (blue): based on climate tolerance alone across environmental gradients (temperature, precipitation, salinity, etc.)
Actual distribution is often smaller due to species interactions, disturbances, and other ecological factors
Over the course, focus shifts from potential distribution to actual distribution and to the roles of biotic interactions and disturbances
Physiological ecology and ecophysiology
Field focuses on how physiological processes constrain survival and persistence under environmental variation
The terms physiological ecology and ecophysiology are closely related and sometimes used interchangeably
Conceptual binary for a single individual (e.g., an axiom in the lecture): survival vs non-survival varies across the range; performance (e.g., photosynthesis, growth) can be optimal in some microhabitats and suboptimal elsewhere
Stress and performance along environmental gradients
Define a simple framework: an environmental variable (e.g., temperature) increases from low to high; a physiological process (e.g., metabolic rate, photosynthesis) responds with an optimum at some middle value
When the environment deviates from the optimum, performance is suboptimal; this deviation is interpreted as stress
Simple schematic: environmental gradient from low to high, process P with maximum at x* (optimum), suboptimal on either side of x*
Consequences: deviations from optimum affect growth, survival, and reproduction; also influence competitive dynamics
Practical takeaway: real-world performance is a function of both abiotic tolerance and biotic interactions, and is seasonally dynamic as organisms migrate, acclimate, or adapt
Acclimation vs adaptation: a continuum of responses
Acclimation: short-term, reversible physiological adjustments within an individual's lifetime (e.g., acclimating to high elevation)
Adaptation: evolutionary change in populations across generations; driven by natural selection under environmental stress
What acclimation looks like:
Example: Mount Everest base camp acclimatization
Physiological changes: faster breathing, higher production of red blood cells, higher blood pressure over time
Acclimation is reversible: return to baseline when environment returns to previous conditions (e.g., returning to London reduces acclimated changes after a period)
What adaptation looks like:
Example: a rabbit with larger ears in desert environments; larger ears improve heat exchange
Across generations, individuals with beneficial traits are selected, leading to population-level changes
Ecotypes: Populations with Adaptations to unique environement
can eventually become separate species as populations diverge and become reproductively isolated
Key point: adaptations for one environment often come with costs in other environments; trade-offs are central to why a single universal solution is rare
Conceptual continuum: from acclimation (short-term) to adaptation (long-term) with potential costs and constraints in between
Temperature tolerance: internal vs external temperatures; broad ranges and trade-offs
Organisms tolerate a wide range of temperatures; internal body temperature is maintained by different mechanisms depending on endothermy vs ectothermy
Internal vs external temperature regimes
Endotherms (mammals, birds): generate metabolic heat to maintain a relatively constant internal temperature; this can extend geographic range
Ectotherms (reptiles, amphibians, many invertebrates): body temperature tracks external temperature more closely; temperature of the body moves with the environment
Internal temperature ranges and extremes
Internal range of many organisms spans roughly from near 0°C to above 100°C (illustrative range in lecture; not all species reach these extremes)
Enzymes and membranes under temperature stress
Enzymes have specific shapes and charges; increasing temperature can denature enzymes, disrupting essential metabolic processes
Some organisms produce isozymes (different forms of the same enzyme) optimized for different temperatures, e.g., warm-weather vs cold-weather enzyme variants
Membrane fluidity is temperature-dependent; maintaining proper fluidity is crucial for protein function
Plants can adjust membrane lipid composition (increase unsaturated lipids) to maintain membrane fluidity at lower temperatures
Temperature tolerance: endothermy, ectothermy, and insulation
Endothermy advantages
Maintains near-optimal internal temperature for metabolic processes
Extends geographic range; can operate in colder environments than ectotherms of similar size
Endothermy costs
High energy demand; requires more food intake and energy budgets
Ectothermy strategies and trade-offs
Body temperature tracks environment; opportunities to exploit microhabitats (sun, shade) to regulate temperature
Insulation and its role in cold tolerance
Fur, feathers, and fat minimize heat loss; insulation can lower the lower critical temperature and affect the slope of the metabolic response to cold
Insulation is largely about trapped air; in mammals and birds, thicker fur or fat reduces heat loss more effectively
Trade-offs of insulation
Thick fur/feathers incur costs in hot environments and during activity; seasonally shed or molt reduces drag and heat load but has energy and timing costs
Heat management and activity patterns across taxa
Small endotherms face high energy demands due to large SA:V ratio; have limited capacity to store energy as fat; may rely on torpor/hibernation to survive energy-poor periods
Examples of thermoregulatory strategies in mammals
Marmots (groundhogs): hibernation with dramatic drops in body temperature and metabolic rate for energy savings; body temperature can drop from ~35°C to ~<15°C for days to weeks; metabolic rate declines from ~1000 to <50 (arbitrary units in lecture) while inactive
Behavioral cooling: panting in mammals to increase evaporative cooling; evaporative cooling helps regulate body temperature
Heat generation and specificity: tuna as an unusual ectotherm with endothermic-like properties
Muscular heat production; large muscle mass generates heat which warms nearby blood
Countercurrent heat exchange: vessels run in close proximity to warm incoming blood and cooled by the hot muscular heat, maintaining a higher core temperature in swimming muscles
External body temperature may be low (e.g., ~19°C) while swimming muscles run at ~32°C, enabling rapid bursts without full endothermy
Thermoregulation in reptiles and amphibians
Many rely on basking to raise body temperature; basking increases mobility but increases predation risk
Camouflage helps mitigate predation risk while basking
Microhabitat use and avoidance of freezing
Organisms migrate or select micro-sites to avoid freezing (e.g., burrows)
Micro-sites offer more moderate temperatures than the ambient environment
Freezing avoidance vs tolerance strategies
Avoid freezing (behavioral or physiological): staying above freezing; avoiding ice formation in cells
Tolerance to freezing: physiological mechanisms to tolerate ice formation (e.g., glycerol or other solutes in cells, or extracellular ice nucleating proteins to control extracellular freezing)
Ice management in vertebrates vs invertebrates
Vertebrates generally avoid freezing; few exceptions with extreme cryoprotectants (e.g., certain frogs and invertebrates)
Frogs with extracellular ice formation and intracellular protection via cryoprotectants (glucose, glycerol) can survive partial freezing in burrows where ambient temperatures are moderated
Specific mechanisms of freezing and cryoprotection
Extracellular freezing and intracellular protection in some frogs
Ice nucleating proteins outside cells initiate slow, controlled extracellular freezing; intracellular water remains unfrozen due to cryoprotectants like glucose and glycerol
These frogs may freeze up to about 60% of body mass outside cells; inside cells, water remains unfrozen to avoid intracellular ice damage
In some conditions, temperatures can push intracellular freezing, beyond which survival becomes unlikely
Endothermy vs freeze tolerance in vertebrates
Mammals and birds can stay active at subfreezing ambient temperatures, but at the cost of high metabolic energy requirements
General problem: freezing damages cells due to ice formation; organisms must avoid this or manage it through specialized adaptations
Water balance, desiccation, and resuscitation strategies
Desiccation tolerance ranges
Some air-dwelling organisms tolerate extreme water loss (desiccation) and can tolerate losing ~80–90% of their water while remaining viable
Desiccation tolerance in microorganisms
Bacteria and fungi can enter suspended animation and revive when water becomes available
Resurrection plants and tardigrades
Resurrection plants (desiccation-tolerant vascular plants) can dry out completely and rehydrate to resume metabolism
Tardigrades (water bears) exhibit extreme desiccation tolerance and can survive long periods in a dried state
Vertebrate water loss tolerance and structural constraints
Vertebrates generally tolerate less water loss due to skeletal and tissue integrity constraints; examples show humans and rodents lose much less water without lethal consequences
Structural adaptations to reduce water loss in deserts and temperate zones
Lizards and snakes often have thick, dead cell layers of skin and fat-rich tissues that reduce water loss
Mammals may also employ thick fur, scales, or other adaptations to minimize dehydration, at the cost of energy and activity
Trade-offs of dehydration resistance and cooling/heat loss
Water conservation may reduce evaporative cooling ability; thus, organisms balance heat dissipation with water conservation
Plant strategies for temperature and water stress
Leaf temperature regulation via transpiration and stomata
Plants transpire to exchange CO2 with the atmosphere; stomata open to allow gas exchange and water loss, which cools leaves via evaporative cooling
When stomata are closed, leaf temperature rises; a thermal image shows higher leaf temperature with stomata closed and cooler leaves when stomata are open
Example: leaf temperatures around 23–24°C when stomata are closed; opening can reduce leaf temperature by ~2–3°C due to evaporative cooling
Water availability strongly influences this mechanism; in dry conditions, stomata closure reduces cooling and can lead to wilting
Deciduousness and leaf shedding as heat management strategies
In hot, dry tropical forests, many trees shed leaves during the dry season to avoid heat load and water loss; leaves regrow in the wet season
Leaf pubescence (trichomes) and heat/light management
Some desert species have dense leaf pubescence, which reflects solar radiation and reduces heat gain
Common garden experiments show desert pubescent species reflect more light and maintain leaf temperatures better under hot, dry conditions
Summer vs winter leaf traits: pubescence and wax layers can vary seasonally; summer leaves may reflect more to reduce heat load, while winter leaves reflect less to maximize light absorption for photosynthesis
Absorption vs reflection trade-offs in leaves
Desert species from hot environments may have leaves that reflect more solar radiation (lower absorption) to stay cooler; temperate desert-adapted species adjust reflectance seasonally
Pubescence and wind/ convection: Himalayas and wind-chill considerations
In windy, cold regions (e.g., Himalayas), high pubescence can help trap a boundary layer and reduce convective heat loss, helping plants stay warm
In still hot environments, pubescence that reflects sunlight helps keep temperatures down, but there is a trade-off with photosynthetic light capture
Common trade-offs in leaf traits
Leaf traits optimized for cooling or warming in a given environment often reduce efficiency of photosynthesis in other contexts; evolution shapes these traits to balance heat load, water status, and energy capture
Water stress across taxa: desiccation tolerance and practical limits
Desiccation tolerance across taxa shows wide variation; many invertebrates and some plants handle extreme water loss
Across larger vertebrates, tolerance to water loss is more limited due to structural and functional constraints
The final takeaway: organisms deploy a toolbox of strategies (behavioral, physiological, anatomical) to manage water loss and temperature stress depending on their ecological niche
Practical takeaways for exam preparation
Understand the distinction between climate envelope (potential distribution) and actual distribution; factors such as disturbance and competition shrink realized distribution
Distinguish acclimation (short-term, reversible) vs adaptation (generational, evolutionary) and recognize their costs and trade-offs
Be able to explain why endothermy allows a wider geographic range but comes at a substantial energetic cost
Describe at least three mechanisms species use to cope with temperature stress (enzymatic adaptation, membrane fluidity, insulation, behavioral strategies)
Explain the role of surface area to volume ratio in heat exchange and why small organisms are especially challenged to maintain body temperature
Use concrete examples from the lecture to illustrate points: Mount Everest acclimation, desert rabbit ears, tuna countercurrent heat exchange, elephant thermoregulation, desert pubescence in leaves, resurrection plants, tardigrades, frog cryoprotection, and beehive heat-ball defense against hornets
n-1 key terms to memorize
Climate envelope, potential distribution, actual distribution, acclimation, adaptation, lower critical temperature, thermal neutral zone (TNZ), endothermy, ectothermy, membrane fluidity, isozymes, pubescence, transpiration, stomata, boundary layer, desiccation tolerance, resurrection plants, tardigrades, ice nucleating proteins, glycerol, glucose, countercurrent heat exchange
Connections to broader ecology and real-world relevance
How climate change could shift climate envelopes and alter potential distributions of many species
The balance of heat retention vs cooling in endotherms has implications for energy budgets, migration patterns, and habitat suitability
Plant leaf traits (pubescence, leaf angle, stomatal regulation) illustrate physiological ecology in terrestrial ecosystems and their responses to drought and heat stress
Behavioral strategies (migrating, basking, shading) show that behavior integrates with physiology to shape organismal performance
Closing reminder for exam readiness
The instructor emphasized that what is covered in class is the core of what may appear on the exam; focus on the concepts, definitions, and examples discussed above, including the trade-offs and the continuum from acclimation to adaptation