Maarg Manthan · Topic 1.12

Range of Tolerance and Maximum Range

Environment & Ecology › Basic Concepts of Ecology · Topic 1.12

The range of tolerance is the span of values of an environmental factor, such as temperature, salinity, light or moisture, within which a species can survive, grow and reproduce. It extends from a minimum limit, through an optimum, to a maximum limit. Within the range, the zone of optimum has the best performance, the zones of physiological stress on either side have declining performance, and beyond the limits lies the zone of intolerance where survival is not possible. The maximum range, also called ecological amplitude, is the full span of each factor over which the species can persist. Victor Shelford's law of tolerance, of 1913, says that each species has such a range for every factor and that its distribution and abundance depend on it. Species with wide ranges are called eury-species, and those with narrow ranges steno-species.

5 min read · UPSC Prelims · Environment & Ecology · Basic Concepts of Ecology

At a Glance

Law Shelford, 1913
Curve zones Optimum, stress, intolerance
Wide range Eury-species
Narrow range Steno-species
Also called Ecological amplitude
Exam link UPSC Prelims, Environment & Ecology: Basic Concepts of Ecology

Where Does It Fit?

This topic follows the account of the climax community. It returns to the idea of limits set out in the ecological principles and describes the tolerance curve in more detail, ahead of the last topic of the macro. Previous topic: Climax Community. Next topic: Distinction Between Ecology, Environment and Ecosystem.

What Does the Tolerance Curve Show?

Zone	Meaning
Optimum zone	The middle part where the species grows and reproduces best and is most abundant
Zones of physiological stress	On both sides of the optimum, where the organism survives but growth and reproduction decline
Zones of intolerance	Beyond the limits, where the species cannot survive
Minimum and maximum limits	The lowest and highest values that the species can endure, set by its physiology
Ecological amplitude	The whole span between the limits, also called the maximum range
Two factors together	A species may tolerate different ranges of each factor, and factors interact, so tolerance to one can depend on another
Life stages	Eggs, larvae and breeding adults often have narrower ranges than adults, so the most sensitive stage limits the species

What Are Eury and Steno Species and Why Does Tolerance Matter?

Eurythermal and stenothermal: wide versus narrow temperature tolerance, for example the common carp versus a coral polyp
Euryhaline and stenohaline: wide versus narrow salinity tolerance, for example salmon and eels versus freshwater fishes
Euryphagic and stenophagic: wide versus narrow diet
Euryhydric and stenohydric: wide versus narrow water requirement
Eurytopic and stenotopic: wide versus narrow habitat range
Distribution: a species occurs only where all factors lie inside its range of tolerance
Indicator species: steno-species often indicate environmental quality, such as lichens for clean air and stoneflies for clean streams
Climate change: if conditions move out of the range, species must migrate, adapt or disappear

What Else Should You Remember?

Thermal tolerance of corals: corals bleach if sea temperatures stay 1 to 2 degrees above the usual summer maximum for a few weeks
Salinity in estuaries: animals there tolerate big changes
Acclimation: shifts in the tolerance range after exposure
Adaptation: genetic change that widens or shifts tolerance over generations
Extremophiles: organisms that tolerate extreme heat, cold, salt or acid
Tardigrades: tiny animals with extreme tolerance
Hot spring bacteria: Thermus aquaticus lives above 70 degrees Celsius
Camel and desert plants: adaptations that widen tolerance to dry conditions
Cold tolerance: antifreeze proteins in polar fish
Pollution tolerance: some species such as tubifex worms tolerate heavy organic pollution
Invasive species: often have wide tolerance ranges
Conservation use: predicting where species can live under climate change
Agriculture: crop varieties are bred for tolerance of drought, salt and flood
Oxygen tolerance: fish need dissolved oxygen above certain levels
Law of limiting factors: see Ecological Principles
Niche link: the range of tolerance shapes the fundamental niche, as in Habitat and Ecological Niche
Caution: limits vary between populations of the same species
Plants and tolerance: halophytes tolerate salt and xerophytes tolerate dryness, which shows different ranges
Thermal limit examples: most fish die above their upper thermal limit, which is why warm water discharge from power plants harms rivers
Range shift: species move upslope in mountains as temperatures rise
Phenotypic plasticity: some individuals can change body form or physiology within limits
Tolerance and invasion: alien species with wide tolerance can colonise many new regions
Study methods: laboratory gradients and field records are used to measure limits
Fish and oxygen: cold water species like trout need high dissolved oxygen, so they live in cool mountain streams
Seed germination: seeds of many plants have a narrow temperature tolerance for germination
Tolerance of eggs: sea turtle eggs hatch into males or females depending on sand temperature within a narrow range

Why Does It Matter for the Exam?

Zones: optimum, stress, intolerance
Prefixes: eury versus steno
Law: Shelford
Current affairs link: coral bleaching and species shifts
Common errors: confusing amplitude with the optimum or swapping eury and steno

As an analytical point, species at the edge of their range of tolerance are the first to be affected by climate change. Narrow-ranged steno species, such as corals, are therefore most at risk and make good early warning indicators.

Why Is This Topic Important?

Curve Three zones
Terms Eury, steno
Use Indicators

What Came Next?

The next topic covers the distinction between ecology, environment and ecosystem. Next topic: Distinction Between Ecology, Environment and Ecosystem.

Key People and Terms

Tolerance range Span a species can endure
Optimum Best conditions
Ecological amplitude Total tolerance span
Eurythermal Wide temperature range
Stenohaline Narrow salinity range
Indicator species Species showing conditions

Exam Corner

Points to Remember

Law of tolerance: Shelford, 1913.
Zones: optimum, stress, intolerance.
Eury = wide; steno = narrow.
Amplitude = maximum range.
Corals are stenothermal.

Do Not Confuse With

Optimum and amplitude Best point versus whole span
Eurythermal and euryhaline Temperature versus salinity
Stress zone and intolerance zone Reduced performance versus no survival

Memory Hook

Optimum in the middle, stress on each side, intolerance past the edge; eury wide, steno narrow.

Mains Angle

Questions ask for the curve zones and the prefix terms. These points help.

Concept Tolerance
Species Eury and steno
Climate Risk at edges

A Question You May Face

An original practice question, not a past paper question.

“Explain the concept of range of tolerance and its relevance to the distribution of species and the impact of climate change.”

How to Answer

Define Range.
Zones Curve.
Species types Eury, steno.
Climate Risk.

GS Relevance

Prelims: tolerance curve, eury and steno terms, optimum, indicator species. GS Paper 3: ecology.

Frequently Asked Questions

What is the range of tolerance?

The range of tolerance is the span of values of an environmental factor, such as temperature, salinity or moisture, over which a species can survive and reproduce. It has a minimum, an optimum and a maximum, beyond which the species cannot live. It explains why species occur only in some places.

What is ecological amplitude?

Ecological amplitude is the full range of an environmental factor or set of factors over which a species can live, from the lower limit to the upper limit. A species with a wide amplitude is common and widespread, and a species with narrow amplitude is restricted and more vulnerable to change.

What is the difference between eurythermal and stenothermal species?

Eurythermal species can tolerate a wide range of temperatures, like the common carp or the house sparrow, while stenothermal species can live only within a narrow range, like polar fishes or coral polyps. Similar terms for salinity are euryhaline and stenohaline.

What are the zones in a tolerance curve?

A tolerance curve has an optimum zone in the middle where the species does best, zones of physiological stress on either side where growth and reproduction decline, and zones of intolerance at the two ends where the species cannot survive. The limits at the ends are the minimum and maximum tolerance values.

Why are steno species good indicators?

Steno species have a narrow range of tolerance, so they react quickly when conditions change. For example, lichens disappear where air is polluted by sulphur dioxide, and mayfly and stonefly larvae need clean, oxygen-rich water, so their presence or absence shows the quality of the environment.

PYQ Practice — Statement Analysis

1 Stenothermal species tolerate a narrow range of temperature.

True

Eurythermal species tolerate wide ranges.

2 Beyond the limits of tolerance lies the zone of optimum.

False

It is the zone of intolerance.

3 The law of tolerance was proposed by Shelford.