The range of tolerance in biology describes the span of environmental conditions, from minimum to maximum, within which an organism can survive and function. Every living thing has limits for factors like temperature, salinity, pH, and oxygen availability. Between those limits lies a narrower “optimal zone” where the organism thrives, flanked by wider stress zones where it hangs on but struggles. The concept, formalized over a century ago, remains one of ecology’s most useful frameworks because it applies to virtually every species and every environmental variable.
Shelford’s Law and the Bell-Shaped Curve
The idea was crystallized by the American ecologist Victor Shelford in 1913. Shelford’s law of tolerance holds that an organism’s performance in response to any environmental factor follows a bell-shaped curve: performance rises from zero at the lower lethal limit, peaks at an optimum, then falls back to zero at the upper lethal limit. The curve is not always symmetrical; many species tolerate cold better than heat, or tolerate mild acidity but not even slight alkalinity. Still, the bell shape captures the general pattern remarkably well across species and stressors.
The curve can be broken into zones. At the center sits the optimum range, where growth, reproduction, and metabolic efficiency are highest. Moving outward in either direction, organisms enter “pejus” zones, a term borrowed from Latin meaning “getting worse.” In animals, these pejus thresholds mark the point where the body’s capacity for aerobic work starts to decline even though the animal is still alive and active.1PubMed. Climate variations and the physiological basis of temperature dependent biogeography: systemic to molecular hierarchy of thermal tolerance in animals Beyond the pejus zones lie the critical zones, where survival is possible for a limited time but normal function is not. Beyond those, the organism dies.
An important companion idea is Liebig’s Law of the Minimum, which says that the factor in shortest supply, not the average of all factors, is what limits growth. A plant might sit comfortably within its tolerance range for temperature, light, and pH, but if soil nitrogen is below its minimum threshold, it will not thrive. In practice, ecologists often combine Shelford’s tolerance framework with Liebig’s limiting-factor concept to understand real ecosystems. A recent study of algal blooms in Taihu Lake, for example, identified ammonia nitrogen and dissolved oxygen as the acute bottlenecks limiting algal growth, while total phosphorus and water temperature were the strategic management targets for improving the lake overall.2Ecological Indicators. An improved habitat suitability model with dual diagnostic outputs for identifying limiting factors and management priorities: Application to algal blooms in Taihu Lake In other words, tolerance ranges and limiting factors work together to determine where a species actually shows up in nature.
Temperature as the Classic Example
Temperature tolerance is probably the most studied example, largely because temperature affects everything from enzyme speed to membrane fluidity. Every organism has a critical thermal maximum and a critical thermal minimum. Between those endpoints, there is an optimal temperature where performance peaks and a broader range where the organism functions adequately.
In cold-blooded animals, whose body temperatures track their surroundings, these limits are especially consequential. Research across a wide range of ectotherms shows that the temperature at which performance peaks and the maximum rate of performance both rise with an animal’s maximum heat tolerance, but they rise more slowly than you might expect. Species that can tolerate very high heat do not gain a proportional boost in how fast they can grow or reproduce at their optimum.3bioRxiv. Temperature-dependent performance scales with maximum heat tolerance across ectotherms The payoff for being heat-tough is real, but it flattens out at the high end.
Warm-blooded animals face the same basic constraints but buffer themselves internally. A human, for instance, has a core body temperature range of roughly 36 to 38°C under normal conditions, and internal temperatures much above 42°C or below 28°C are typically fatal. The tolerance range for ambient temperature is much wider because of sweating, shivering, and behavioral choices like seeking shade. This distinction between internal tolerance and environmental tolerance is easy to blur, but it matters. A desert lizard and a polar bear both rely on keeping their cells within a livable window; they just use different strategies to get there.
Salinity, pH, and Oxygen
Temperature gets most of the attention, but organisms face tolerance curves for every environmental variable. Three of the most ecologically important are salinity, pH, and dissolved oxygen.
Fish illustrate salinity tolerance vividly. Species living in stable-salinity environments, such as open ocean or deep freshwater lakes, tend to be stenohaline, meaning their tolerance range for salt concentration is narrow. Species living in estuaries or tidal zones, where salinity swings with the tides, are euryhaline and tolerate a wide range.4PubMed. Physiological mechanisms used by fish to cope with salinity stress This pattern holds beyond fish: among mosquito species in the same evolutionary group, some are stenohaline freshwater breeders while close relatives have evolved euryhaline tolerance to saltwater, likely driven by the habitats available during their radiation across Africa.5PubMed Central. Transcriptomic differences between euryhaline and stenohaline malaria vector sibling species in response to salinity stress
For pH, most bacteria need to keep their internal environment close to neutral even when the world outside is acidic or alkaline. Acid-tolerant microbes do this by letting their internal pH drop somewhat with the external environment, but always maintaining it higher than what surrounds them. This costs energy. Once the acid concentration crosses a threshold, pH regulation collapses, proteins and DNA suffer damage, and the cell dies.6PubMed Central. Microbial response to acid stress: mechanisms and applications Bacteria have evolved diverse sensing and homeostasis mechanisms that allow many species to grow at external pH values well outside the internal range they actually need.7PubMed Central. Molecular aspects of bacterial pH sensing and homeostasis The tolerance range here is really about how wide that buffering capacity can stretch before breaking.
Dissolved oxygen is a third critical factor, especially for aquatic life. Fish have a measurable critical oxygen level below which they can no longer sustain normal aerobic metabolism. Below that threshold, survival depends on how efficiently they can switch to backup energy pathways and suppress their metabolic demand.8PubMed Central. A new analysis of hypoxia tolerance in fishes using a database of critical oxygen level (Pcrit) Interestingly, fish that spend time in low-oxygen water can extend how long they tolerate hypoxia, suggesting a degree of physiological adjustment rather than a fixed boundary.9Conservation Physiology. Prolonged exposure to low oxygen improves hypoxia tolerance in a freshwater fish
Tolerance Is Not Fixed
One of the most important refinements to the original tolerance concept is that the curve can shift. When an organism is exposed to a stressor for a period, its tolerance limits often move outward through a process called acclimation (over days to weeks) or hardening (over hours). This is phenotypic plasticity: the same genetic individual performing differently under different conditions.
Prior exposure to a warmer temperature, for instance, can raise an organism’s upper thermal limit.10PubMed Central. Limited plasticity in thermally tolerant ectotherm populations: evidence for a trade-off How quickly and by how much this shift happens depends on circumstances. Well-fed individuals adjust their thermal tolerance roughly twice as much as starved ones, suggesting that reshaping tolerance is energetically expensive.11PubMed Central. Energy availability influences the dynamics of thermal phenotypic plasticity This makes intuitive sense: rewiring proteins, producing heat-shock molecules, and altering membrane composition all take resources.
There is a catch. Species that already have high baseline tolerance tend to show less plasticity. Getting to the maximum hardening response takes them longer, and the overall gain is smaller compared with species that start at a lower baseline.12PubMed Central. Threshold shifts and developmental temperature impact trade-offs between tolerance and plasticity This tolerance-plasticity trade-off shows up across both heat and desiccation stressors. In simple terms, you can be tough or you can be flexible, but doing both at once seems to be biologically difficult.
Evolving New Tolerance Ranges
Plasticity covers short-term adjustments within a lifetime. Over generations, natural selection can permanently shift a population’s tolerance curve. This has been documented in real time. Populations of a damselfly expanding into cooler regions rapidly evolved improved cold tolerance as they colonized new territory.13PubMed. Evolution of cold tolerance and thermal plasticity in life history, behaviour and physiology during a poleward range expansion Experimental work has confirmed the same pattern under controlled conditions, showing that both heat and cold tolerance can evolve within just a few generations when populations face novel thermal environments.14PubMed Central. Maladaptive plasticity facilitates evolution of thermal tolerance during an experimental range shift
This evolutionary capacity matters enormously for predicting how species will handle climate change. Research on mosquitoes found heritable genetic variation in acute heat tolerance, and a simple evolutionary model estimated that the maximum rate at which mosquito heat tolerance could evolve typically exceeds the projected rate of climate warming.15PubMed Central. Evolutionary adaptation under climate change: Aedes sp. demonstrates potential to adapt to warming That sounds reassuring, but it comes with caveats: the model assumed idealized conditions, large populations, and no competing selective pressures. Real populations face habitat loss, disease, and pollution simultaneously. A phenotypic trade-off also emerged: mosquitoes better at surviving acute heat bursts were worse at surviving prolonged heat exposure, and vice versa. Evolution does not hand out unlimited tolerance.
Extremophiles and the Outer Edges of Life
Most of the tolerance discussion centers on organisms living in moderate environments, but extremophiles push the boundaries of what biological tolerance can look like. These are microbes and a few multicellular organisms that not only survive but actively thrive under conditions lethal to most life. Archaea in deep-sea hydrothermal vents grow at temperatures above 100°C. Acidophilic bacteria flourish at pH values below 2. Halophiles live in salt concentrations that would pickle most cells.
Extremophiles achieve this through specialized proteins, modified membrane lipids, and unusual metabolic pathways.16PubMed Central. The Extremophiles: Adaptation Mechanisms and Biotechnological Applications Their enzymes remain stable and functional at temperatures or pH values that would denature ordinary proteins. These adaptations are not just biological curiosities. Extremophile-derived enzymes and molecules are already used in industrial applications ranging from agriculture to pharmaceuticals to cosmetics, precisely because they work in conditions too harsh for conventional biological tools.17Current Research in Microbial Sciences. Perspectives on the microorganism of extreme environments and their applications
Among animals, a handful of species show extreme tolerance capacities through mechanisms like desiccation tolerance and freeze tolerance. Tardigrades and certain nematodes can enter a state called anhydrobiosis, surviving with so little water that even the hydration layer around their cellular machinery is gone.18Nature Communications. Life on the dry side: a roadmap to understanding desiccation tolerance and accelerating translational applications Some frogs and insects survive internal ice formation that would kill most vertebrates.19PubMed Central. Mechanisms and evolution of resistance to environmental extremes in animals These organisms have tolerance ranges so wide, or tolerance mechanisms so dramatic, that they challenge our intuitions about what counts as livable.
Why Tropical Species Are More Vulnerable Than You Might Think
A common assumption is that species living in hot environments are adapted to heat and therefore safe from warming. The evidence points the other way. Tropical species tend to live closer to their upper thermal limits than temperate species do. Their “thermal safety margin,” the gap between the hottest conditions they normally experience and the temperature that starts damaging them, is narrower.20Ecological Indicators. Upper thermal limits and warming safety margins of coastal marine species – Indicator baseline for future reference This pattern shows up across animal groups, from marine invertebrates to terrestrial insects.
Plants face the same problem. In the Amazon, researchers found that acclimation in woody plants is generally too weak to maintain safe thermal margins at high growth temperatures. In other words, the trees can shift their tolerance curves a little, but not enough to keep pace with the warming they are already experiencing.21PubMed. Leaf thermal safety margins decline at hotter temperatures in a natural warming ‘experiment’ in the Amazon For species that evolved under relatively stable equatorial conditions, even modest warming can push them past their pejus thresholds into the critical zone, with cascading effects on reproduction, growth, and survival.
This has practical consequences for conservation. The species most threatened by climate change are not necessarily those facing the largest absolute temperature increases (which tend to be at high latitudes) but those with the smallest tolerance margins relative to the warming they face. Prioritizing protection for narrow-margin tropical species is a direct application of the range-of-tolerance concept.
Tolerance to Pollution and Heavy Metals
The range of tolerance extends beyond natural environmental gradients. Organisms also have tolerance ranges for pollutants, heavy metals, and other anthropogenic chemicals. Some species evolve increased tolerance when populations are chronically exposed to contaminated environments.
Plants have several biochemical strategies for dealing with heavy metals. They can reduce uptake at the roots, shuttle metals into specialized compartments inside cells, or bind metals to small molecules like phytochelatins and metallothioneins that neutralize their toxicity.22Plant Stress. Molecular Mechanisms and Genetic Basis of Heavy Metal Toxicity and Tolerance in Plants This capacity is not evenly distributed. Some plant species, called hyperaccumulators, can soak up concentrations of zinc, cadmium, or nickel that would kill their neighbors. These plants are the basis of phytoremediation, using living plants to clean contaminated soil.
In terrestrial invertebrates, tolerance to heavy metals often involves ramping up existing physiological machinery for metal storage and excretion rather than inventing entirely new mechanisms.23Comparative Biochemistry and Physiology Part C: Pharmacology, Toxicology and Endocrinology. Heavy-metal adaptation in terrestrial invertebrates: A review of occurrence, genetics, physiology and ecological consequences Among butterflies, species with larger geographic ranges tolerate significantly higher tissue concentrations of lead, arsenic, and cadmium. Species whose larvae evolved feeding on chemically toxic host plants also tolerate more lead, suggesting that a history of dealing with one kind of chemical stress can pre-adapt organisms to another.24PubMed Central. Geographic Range Size Predicts Butterfly Species’ Tolerance to Heavy Metals More Than Evolutionary History With Toxic Larval Diets
When Stressors Stack Up
Real environments rarely present organisms with a single stressor at a time. A fish in a warming estuary may simultaneously face rising temperature, falling oxygen, increasing salinity, and pesticide runoff. Each stressor has its own tolerance curve, but they interact. Being near your upper temperature limit can narrow your oxygen tolerance, and vice versa. Climate change intensifies disease susceptibility through stress-mediated immunosuppression and altered pathogen dynamics in groups like amphibians and reptiles.25Frontiers in Physiology. Physiological responses and adaptive mechanisms of amphibians and reptiles to multiple interacting environmental stressors: an integrative review An animal that could handle a warming pond or a fungal pathogen alone may fail when both arrive together.
This is where the simple bell-curve model gets complicated. The tolerance range for any one variable is not truly independent of other variables. Physiologists sometimes describe the overall tolerance space as a multidimensional volume rather than a single curve: imagine a shape in many-dimensional space, where each axis is a different environmental factor. The organism can live inside that volume and dies outside it. Stress on one axis shrinks the livable range on other axes. For conservation and management, this means that reducing one stressor, say nutrient pollution, can effectively expand a species’ tolerance for another stressor like warming.
Gut Microbes and Borrowed Tolerance
A growing body of research shows that an organism’s tolerance range is not entirely its own. The microbes living in and on an animal can extend or constrain its environmental limits. In fruit flies, removing gut bacteria with antibiotics reduced their median survival time under cold stress to about two-thirds of normal. Reintroducing a single key bacterial species restored cold tolerance to normal levels, apparently by fueling a metabolic pathway the flies rely on in the cold.26PubMed Central. Gut microbiota promotes host resistance to low-temperature stress by stimulating its arginine and proline metabolism pathway in adult Bactrocera dorsalis
Microbiomes can also be locally adapted in ways that benefit the host. In water fleas exposed to toxic cyanobacteria, individuals receiving gut microbiomes from their home population generally survived better than those given microbiomes from a different population.27The ISME Journal. Locally adapted gut microbiomes mediate host stress tolerance The microbial community had essentially co-evolved with the host to handle the specific stressors in that environment. This means the range of tolerance for a species is partly a property of the holobiont, the host plus its microbial partners, rather than the host genome alone.
Behavior as a Tolerance Strategy
Before physiology kicks in, behavior is often the first line of defense. Animals routinely extend their effective tolerance range by choosing microclimates. A gecko in an Indian city, for example, thermoregulates by shuttling between wall surfaces and tree bark. Its body temperature stays roughly constant across these different microhabitats, but the thermoregulatory effort is higher on walls, which are thermally harsher than trees.28Frontiers in Amphibian and Reptile Science. Microhabitat level thermal physiology and thermoregulation of a diurnal gecko in an urban landscape By picking favorable perches, the gecko keeps itself within its optimal zone even when the broader environment would push it toward the edges of its tolerance curve.
Behavioral thermoregulation is ancient and widespread: basking reptiles, shade-seeking mammals, burrowing desert beetles, and migrating birds all use movement to stay within livable conditions. When behavioral options shrink, as they do in degraded habitats, fragmented landscapes, or urbanized areas with fewer microhabitats, the organism’s effective tolerance range shrinks too. A species that looks heat-tolerant in a complex natural habitat may not survive the same temperatures in a parking lot with nowhere to hide.
Breeding Crops for Wider Tolerance
Agriculture is, in many ways, applied tolerance biology. Crop scientists spend enormous effort trying to widen the tolerance range of food plants, especially for drought, salinity, and heat. Understanding the physiological and molecular responses that underlie drought tolerance, from osmotic adjustment to antioxidant production, is central to developing crop varieties that can maintain yield in water-limited environments.29PubMed Central. Drought Tolerance in Plants: Physiological and Molecular Responses
Soil salinity is a particularly pressing problem because irrigation and climate change are increasing salt concentrations across farmland worldwide. Combining knowledge of plant salt-tolerance mechanisms with microbial diversity in the soil offers one promising path toward more resilient agriculture.30Plant Stress. Soil salinity and drought tolerance: An evaluation of plant growth, productivity, microbial diversity, and amelioration strategies Salt-tolerant crop varieties, improved irrigation management, and soil microbiome engineering are all strategies grounded in manipulating where a crop’s tolerance curve sits relative to the conditions it faces. The goal is not to make a plant that thrives in salt, but to push the lower edge of its tolerance range far enough that moderate salinity no longer falls outside it.