What range of conditions can an organism survive, and how does this set the limits of where it can live?
Topic 2.4 Ecological Tolerance: describe the range of tolerance of organisms and explain how tolerance limits determine the distribution and survival of species.
A focused answer to APES Topic 2.4, covering the range of tolerance, optimum range, zones of stress, limits of tolerance, the law of tolerance and how tolerance varies between species and life stages, with a worked tolerance-curve question.
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What this topic is asking
The College Board (Topic 2.4) wants you to describe an organism's range of tolerance: the span of an environmental factor (such as temperature, salinity or pH) within which it can survive, and to explain how the optimum range, zones of stress and limits of tolerance determine where a species can live. You should be able to read a tolerance curve and predict how a change in conditions affects a population.
The range of tolerance
A tolerance curve plots the abundance or performance of a population against an environmental factor, peaking in the middle and falling toward the edges.
Zones of the tolerance curve
This pattern reflects the law of tolerance: the existence and abundance of a species are controlled by whether environmental conditions stay within its tolerance range for every factor it needs.
Tolerance and distribution
Because each species can only live where conditions stay within its tolerance range, tolerance helps set a species' geographic distribution. Species with a wide range of tolerance (generalists) can survive a broad span of conditions, so they tend to be widely distributed and adaptable. Species with a narrow range (specialists) are restricted to particular environments and are more vulnerable to change.
Variation in tolerance
Tolerance is not fixed. It can differ between species, between populations of the same species in different regions (local adaptation), and between life stages: young, eggs and larvae are often far less tolerant than adults, so a condition that adults survive may still kill the population by killing its offspring. Tolerance can also shift seasonally through acclimation. The practical importance of Topic 2.4 is in predicting the effects of environmental change: as climate warming, pollution or salinity change push conditions toward and beyond a species' limits, the species must either tolerate the change, move to a suitable area, adapt over generations, or face local extinction. Tolerance therefore links biodiversity (Topic 2.1) to the disruptions and adaptations covered later in the unit, and explains why specialists with narrow tolerance are usually the first species lost when ecosystems are disturbed.
Try this
Q1. Identify the part of the tolerance curve where a species cannot survive. [1 point]
- Cue. The zone of intolerance (beyond the limits of tolerance).
Q2. Explain why a specialist species with a narrow range of tolerance is more vulnerable to climate change. [2 points]
- Cue. A small shift in conditions can push it past its limits of tolerance, so it cannot survive in its current location and must move, adapt or die out.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2021 (style)4 marksSection II (FRQ). A fish species survives in water between 10 and 28 degrees C, grows and reproduces best between 18 and 22 degrees C, and dies outside 8 to 30 degrees C. (a) Identify the optimum range for this species. (b) Describe what happens to individuals in the zones of physiological stress. (c) Explain how a sustained 4 degrees C rise in water temperature could affect this population. (d) Explain why species with a wide range of tolerance are often more widely distributed.Show worked answer →
A 4-point FRQ on ecological tolerance.
(a) Identify (1 point): the optimum range is 18 to 22 degrees C, where growth and reproduction are best.
(b) Describe (1 point): in the zones of physiological stress (between the optimum and the limits, here roughly 10 to 18 and 22 to 28 degrees C), individuals can survive but are stressed, so growth and reproduction decline and fewer individuals persist.
(c) Explain (1 point): if temperatures rise toward or beyond 30 degrees C, the species moves into stress and then beyond its tolerance limit, so the population declines, must move to cooler water, or dies out locally.
(d) Explain (1 point): a wide tolerance lets a species survive a greater range of conditions, so it can occupy more habitats across a larger geographic area.
Markers reward the optimum range, the reduced performance in stress zones, the prediction of decline or range shift with warming, and linking wide tolerance to wide distribution.
AP 2019 (style)1 marksSection I (multiple choice). On a tolerance curve, the range of an environmental factor where a population is most abundant and individuals thrive is called the (A) zone of intolerance (B) optimum range (C) limit of tolerance (D) zone of physiological stress. Justify your choice.Show worked answer →
A 1-point MCQ on the tolerance curve. The answer is (B).
The optimum range is the part of the range of tolerance where conditions are best, so the population is most abundant and individuals grow and reproduce well. The zone of intolerance (A) is beyond survival; the limit of tolerance (C) is the boundary of survival; the zone of physiological stress (D) is where individuals survive but are stressed. The trap is confusing the optimum (best) with the limit (edge of survival).
Related dot points
- Topic 2.1 Introduction to Biodiversity: describe the three levels of biodiversity and explain how genetic and species diversity contribute to ecosystem resilience.
A focused answer to APES Topic 2.1, covering genetic, species and habitat diversity, species richness and evenness, the value of genetic diversity, bottlenecks and resilience, with a worked diversity-comparison question.
- Topic 2.6 Adaptations: explain how natural selection produces adaptations and how environmental change shifts which traits are favored over time.
A focused answer to APES Topic 2.6, covering adaptations, natural selection, the role of genetic variation, structural, physiological and behavioral adaptations, specialists and generalists, and how environmental change drives evolution, with a worked selection question.
- Topic 2.3 Island Biogeography: explain how island size and distance from the mainland determine species richness, and apply the theory to habitat fragments.
A focused answer to APES Topic 2.3, covering the theory of island biogeography, the effects of island size and distance, immigration and extinction rates, endemism, and its application to habitat fragmentation, with a worked island-comparison question.
- Topic 2.5 Natural Disruptions to Ecosystems: describe natural disruptions to ecosystems and explain their short-term and long-term effects on populations and biodiversity.
A focused answer to APES Topic 2.5, covering periodic, episodic and random natural disruptions, fire, drought, storms, volcanism, plate tectonics and climate change, their short- and long-term effects, and ecosystem recovery, with a worked disturbance-analysis question.
- Topic 1.2 Terrestrial Biomes: describe the global distribution of the major terrestrial biomes and explain how temperature and precipitation determine the type of biome found in a region.
A focused answer to APES Topic 1.2, covering the major terrestrial biomes, how temperature and precipitation define them, latitude and altitude patterns, and biome shifts under a changing climate, with a worked climograph question.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)