AP Environmental Science Unit 2 — The Range of Tolerance: Limits, Acclimation, and a Sea That Moved (2026)

Unit 2 keeps asking the same question in different costumes: when conditions change, who lives and who doesn't? The concept behind almost every answer is ecological tolerance — the range of conditions a species can actually take.

This episode builds that idea from one weekend in June 2021, when the Pacific Northwest heat dome landed on the year's lowest tides, and then follows it through four more cases: the heat threshold NOAA uses to predict coral bleaching, a three-generation experiment with a small reef fish, a coral that changes how its genes are read, and a shellfish hatchery where the water itself had moved past what oyster larvae can build a shell in. It closes with five new practice questions worked through out loud, answers and reasoning included.

The short version

Tolerance looks like a hill, not a wall, and the Unit 2 study guide draws it the same way: an optimal range, zones of physiological stress on either side of it, and a zone of intolerance where survival stops. Performance fades before survival does, so a population can look healthy while it is already past its best conditions.

Acclimation, epigenetics and adaptation run on three different clocks. Acclimation is one organism adjusting inside its own lifetime. Epigenetic tags can be inherited, changing what the next generation handles without changing the DNA sequence. Adaptation is a shift in allele frequencies across generations, and it can only work with the variation a population already holds.

The real margin is narrower than a one-factor graph suggests. Warm water holds less oxygen, acid water makes shells harder to build, and whichever factor runs out first is the one that sets the limit.

The cases behind the numbers

Case	What was measured
Salish Sea intertidal beaches, 26–28 June 2021	Seattle hit 108 °F, an all-time record, on a day when the lowest tide of the year fell near noon. A survey led by the University of Washington collected 203 reports from 108 beaches covering 24 species: acorn barnacles, highest on the shore and driest longest, were hit hardest, while clams and oysters lower down mostly survived, and California mussels on the exposed outer coast mostly survived where bay mussels in sheltered water died 1 2
Coral bleaching threshold	Water 1 °C above the hottest month's average is enough to stress corals, so Coral Reef Watch accumulates the excess into degree heating weeks: more than 4 °C-weeks brings significant bleaching, more than 8 °C-weeks severe bleaching and mortality 3
Spiny damselfish, three generations	Reared at control temperature, +1.5 °C and +3 °C. At +3 °C, juveniles from unexposed parents lost most of their aerobic scope; juveniles whose parents had also been reared warm kept normal aerobic scope 4
Corals under low pH	A heat-sensitive coral kept at low pH changed its DNA methylation and calcified less, while a more robust species held its calcification steady with no change in methylation 5
Pteropods, California Current, 2011	53% of near-shore pteropods carried severe shell dissolution damage against 24% offshore; severe dissolution there had doubled since pre-industrial times and was projected to triple by 2050 6
West Coast oyster hatcheries, 2007–2010	Hatcheries supplying oyster seed failed repeatedly, and Whiskey Creek Shellfish Hatchery in Oregon fell to production its owners called non-economically viable. The failures tracked the carbon dioxide in the water where larvae build their first shell in the first 24 hours 7

Ocean pH has fallen about 0.1 units since the industrial revolution, which is roughly a 30% increase in hydrogen ion concentration, because pH is logarithmic 8.

Vocabulary you should be able to use in a sentence

Optimal range — the conditions in which an organism performs best, at the top of its tolerance curve.
Zones of physiological stress — conditions a species can survive but not thrive in; energy goes into staying alive rather than growth or reproduction.
Zone of intolerance — conditions beyond a species' tolerance, where survival stops.
Aragonite saturation state — the chemistry threshold for shell building: above 1, calcium carbonate shell material forms; below 1, it dissolves.
Acclimation — a short-term adjustment by one organism inside its lifetime. It is reversible and passes nothing on through genes.
Epigenetics — chemical tags such as DNA methylation that switch genes on or off without changing the DNA sequence, and which can be inherited.
Transgenerational plasticity — when the environment a parent experienced changes the performance of its offspring.
Adaptation — a shift in allele frequencies across a population over generations, under natural selection.
Degree heating week — one week of water 1 °C above the hottest-month average; the unit NOAA accumulates to predict coral bleaching.
Specialist versus generalist — a narrow tolerance curve belongs to a specialist tied to particular conditions; a wide curve belongs to a generalist that tolerates change.

The five practice questions

1. Two species, one warming stream. Species A peaks at 22 °C and runs from 6 °C to 34 °C; species B peaks at 14 °C and runs from 8 °C to 20 °C. The stream warms from 16 °C to 24 °C. What happens to each species, and what can't the graph tell you?

Species A is just past its optimum at 24 °C — stressed, but alive. Species B is beyond its range entirely, so expect it to disappear from that stretch of stream. The graph cannot tell you how many fish there are, because a tolerance curve's vertical axis is performance or relative abundance against that one factor, not population size.

2. Does three weeks of heat bleach the reef? A reef's hottest summer month averages 29 °C; a marine heatwave holds the water at 31 °C for three weeks.

Two degrees above the summer maximum for three weeks is six degree-weeks. That is past the four degree-week threshold for significant bleaching and short of the eight degree-week level where severe bleaching and mortality are expected — so expect significant bleaching and real losses if the heat continues. Bleaching is not the coral dying immediately: it is the coral expelling the algae that live in its tissue and feed it, which leaves the coral white and starving. If the water cools in time, it takes the algae back.

3. Name the process. A fish moved from 18 °C water into 24 °C water grows larger gill surfaces and performs normally within weeks; juvenile reef fish raised 3 °C above normal have normal aerobic scope only if their parents were raised warm too; over thirty generations, the share of fish carrying a heat-tolerant gene version climbs from 10% to 60%.

Acclimation, transgenerational plasticity, and adaptation, in that order. Only the third hands new alleles to the next generation — the first two change how the same genes are used.

4. Which water dissolves a pteropod's shell? pH is 8.2 before the industrial revolution and 8.1 now; aragonite saturation is 1.2 in one water mass and 0.8 in the other.

The water at 0.8, because below a saturation state of 1 shell material dissolves faster than it forms. A classmate's claim that "the ocean is turning acidic, so fish will die" fails twice: at pH 8.1 the ocean is still basic, and acidification lands hardest on shell builders, on animals that read water chemistry to navigate, and on the life stage with the least margin — oyster larvae in their first day, not adult fish.

5. Which species is most vulnerable? Species W: a temperature range 2 degrees wide, one small lake, low genetic diversity. Species X: a range 30 degrees wide, twenty lakes, high genetic diversity. Species Y: a narrow temperature range, twenty lakes, high diversity. Species Z: a wide temperature range, one small lake.

W, on three counts at once: the narrowest tolerance, a single place to live, and the least variation for selection to work with. Y has the same narrow tolerance but twenty separate populations. Z is the trap — a wide tolerance still fails if there is nowhere to go.

Four ways this part of the unit gets missed

Acclimation and adaptation are different events. One organism adjusting is acclimation; alleles shifting across a population over generations is adaptation. Read who is changing, and over how long.

Check the vertical axis before reading a tolerance curve. These graphs plot performance or abundance against one factor, so a curve sliding sideways is not a population growing.

pH is logarithmic, and acidification does not mean acidic. A tenth of a unit is about 30% more hydrogen ions, and the ocean is still basic.

Thresholds are about time as much as intensity. One degree above the summer maximum, held for weeks, bleaches a reef; the same warmth for two days does not. On those Salish Sea beaches, the heat and the year's lowest tides landed together.