Physiological Optima and Critical Limits

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University of Toronto St. George
Paul Thompson

Physiological Optima and Critical Limits NotesOrganism distribution limits and responses to climate change depend on how physiological performance varies as environment shifts bw optimal and extreme conditions o Dependant on biotic and abiotic environmental factorsMaximize their fitness at an optimal range survive only short periods in conditions that exceed a threshold in their critical tolerance limits o Range of environments bw critical limits vary depending on physiological adaptation and energeticsCritical limits define species distribution community structure and how communities respond to enviro changesPerformance curvediagrams organisms critical limits which we can interpret as the functionality of mechanisms at the organismal and cellular levelThermal Performance CurvesIllustrate an organisms performance across a range of enviro conditions temperaturePerformance maximized under a range of optimal temps and declines to 0 just beyond upperlower critical temps that define the organisms thermal tolerance range httpwwwnaturecomscitablecontentne0000ne0000ne0000ne00004590275412jpgCritical thermal limits define range of temps over which organism can tolerate at least short exposure are indexed by 2 metrics upper and lower LT50 or CTmax and CTminEurythermal organisms able to tolerate a wide range of temps can inhabit wide range of thermal envirosStenothermal restricted to narrow thermal enviros or must behaviorally thermo regulateEndotherm has constant and minimized metabolismat temps above and below TNZ thermoneutral zone they must elevate metabolic rate while inducing physiological responses to maintain constant body temp shivering to keep warm sweating to coolEctotherms have no TNZ but do have an optimal temp which they work most efficient o Mobile ectotherms behaviorally thermo regulate a specific body temp by moving bw enviros but doing so takes a lot of energy and could reduce fitness in foodlimited conditionsIncreasedecrease from the optimal performance is reduced o Pejus Temperaturestemps where performance begins to decline and are less extreme than critical limitsRepresent thermal transitions that are more ecologically relevant than critical thermal limitsMechanistic Bases of Thermal Performance Curves Lower Critical LimitsOrganisms that can survive in conditions where temps are below their thermodynamic freezing point employ many physiological mechanisms to prevent formation of intracellular ice o thermodynamic freezing point temperature at which ice is predicted to form Organisms cant survive formation of ice in cells but ice can form outside of cells as long as ice grows in controlled and directed mannerEctothermic terrestrial vertebratesfrogs and invertebrates insects and marine intertidal zone invertebrates mussels that experience temps below freezing are freeze tolerantallow extracellular ice to form not intracellular ice formation o Usecolligative properties of watercolligative propertiesproperties of solutions that depend on the concentration of dissolved solutes but not the identities of the solutesFirst compatible solutes or osmolytes such as glycerol and glucose are concentrated inside cells lowering freezing point of cytosolic fluid
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