APES Unit 2 Study Guide

Key concepts

Tap a module to open it. Each ends with the trap students fall into most.

M8Introduction to Biodiversity▸

Big idea: More diversity at every level makes an ecosystem better able to absorb disturbances and recover.

Three levels of biodiversity

Genetic diversity: variety of genes within a population. More variety means some individuals are likely to survive a new disease or a changing climate.
Species diversity: the number and relative abundance of species in an area.
Ecosystem (habitat) diversity: the variety of ecosystems in a region. More habitats support more species.

Measuring species diversity

Species richness = how many different species are present.
Species evenness = how equal the population sizes are across those species.
Two communities can have the same richness but very different evenness. The more even community is considered more diverse.

Why low genetic diversity is dangerous

A bottleneck event (disaster, overhunting, habitat loss) shrinks a population sharply. Survivors carry only a fraction of the original genes.
Small populations face inbreeding depression: more harmful traits show up, and fitness drops. Classic examples: cheetahs, Florida panthers, northern elephant seals.
Habitat loss hurts specialists more than generalists.

Trap: Richness is not the same as diversity. A forest with 10 species where one species is 95% of the trees has low evenness and lower overall diversity than a forest with 10 evenly mixed species.

M9Ecosystem Services▸

Big idea: Ecosystems do work for humans for free. When we damage them, we pay economically as well as ecologically.

The four categories

Category	What it means	Examples
Provisioning	Goods we take directly	Food, timber, fresh water, fiber, medicine
Regulating	Natural processes kept in balance	Climate regulation (carbon storage), flood control, water and air purification
Cultural	Non-material benefits	Recreation, tourism, spiritual value, education, research
Supporting	Processes that make the others possible	Nutrient cycling, soil formation, pollination, habitat

Human disruption has costs

Cutting a forest gives timber (provisioning) but removes carbon storage, flood control, and erosion control (regulating).
Losing pollinators raises the cost of growing crops that depend on them.
Draining wetlands increases flooding and water treatment costs downstream.
On FRQs, name the specific service, its category, and the economic or ecological consequence.

Trap: Pollination sometimes appears as "regulating" in other sources. The AP course framework lists it as a supporting service, so go with supporting on the exam unless her teacher says otherwise.

M11Ecological Tolerance▸

Big idea: Every organism survives only within a range of conditions. Push past it and it stresses, then dies.

The tolerance curve (from center outward)

Optimal range: organisms thrive, grow, and reproduce best.
Zone of physiological stress: organisms survive but grow and reproduce poorly.
Zone of intolerance: organisms die or leave.
The full span from lower to upper survival limit is the range of tolerance.

Things to know

Tolerance applies to individuals and species. Individuals vary, which is where genetic diversity matters again.
Common factors: temperature, salinity, pH, dissolved oxygen, sunlight, water availability.
Example: corals have a narrow temperature tolerance. Water just 1–2 °C above normal causes coral bleaching (they expel their algae).
Example: freshwater fish in water that gets saltier from road salt runoff enter physiological stress.

Trap: "Stress" zone does not mean the organism dies. It survives but with reduced growth and reproduction. Death happens in the zone of intolerance.

M12Natural Disruptions to Ecosystems▸

Big idea: Ecosystems change naturally on every time scale, and some natural disruptions affect ecosystems more than human ones do.

Three patterns of disruption

Type	Pattern	Examples
Periodic	Regular, predictable intervals	Day/night, seasons, tides, annual floods
Episodic	Occasional, somewhat irregular	Hurricanes, droughts, El Niño, wildfires
Random	No pattern at all	Volcanic eruptions, earthquakes, asteroid impacts

Type

Pattern

Examples

Periodic

Regular, predictable intervals

Day/night, seasons, tides, annual floods

Episodic

Occasional, somewhat irregular

Hurricanes, droughts, El Niño, wildfires

Random

No pattern at all

Volcanic eruptions, earthquakes, asteroid impacts

Long-term natural change

Earth's climate has swung between glacial (ice age) and interglacial periods. Scientists reconstruct this from ice cores, tree rings, and sediment.
When glaciers grow, they lock up water and sea level falls. When they melt, sea level rises.
Organisms respond by migrating, adapting, or going extinct. Wildlife migrate to follow food, water, or suitable temperatures.
Disruptions also reset succession (see M14), which can increase diversity at the landscape level.

Trap: Students mix up periodic and episodic. Ask: "Could I put it on a calendar?" Yes = periodic. Happens now and then, roughly = episodic. Totally unpredictable = random.

M14Ecological Succession▸

Big idea: After a disturbance, communities change in a fairly predictable sequence. Whether soil survived decides where it starts.

	Primary succession	Secondary succession
Starts on	Bare rock, no soil	Existing soil
Triggered by	Glacier retreat, lava flow, new island	Fire, logging, abandoned farm, hurricane
First arrivals	Lichens, mosses (pioneer species)	Grasses, weeds, wildflowers
Speed	Very slow (centuries+)	Faster (decades)

Primary succession

Secondary succession

Starts on

Bare rock, no soil

Existing soil

Triggered by

Glacier retreat, lava flow, new island

Fire, logging, abandoned farm, hurricane

First arrivals

Lichens, mosses (pioneer species)

Grasses, weeds, wildflowers

Speed

Very slow (centuries+)

Faster (decades)

How it unfolds

Pioneer species break down rock (lichens release acids) and add organic matter, slowly forming soil.
Stages run roughly: lichens/mosses → grasses → shrubs → fast-growing sun-loving trees → shade-tolerant trees.
Early species tend to be r-selected (grow fast, disperse well). Later species tend to be K-selected (larger, longer-lived, better competitors).
Over succession: soil depth, nutrients, biomass, and species richness generally increase. Productivity is high in middle stages.
A stable, late-stage community is often called the climax community.
Aquatic succession: a lake or pond fills with sediment and becomes a marsh, then a meadow or forest.

Special roles

Keystone species: has a much larger effect on its ecosystem than its numbers suggest. Remove it and the community changes dramatically. Examples: sea otters (control urchins, protect kelp), beavers (create wetlands), gray wolves in Yellowstone.
Indicator species: sensitive to environmental change, so their presence or absence signals ecosystem health. Examples: lichens (air quality), mayfly and stonefly larvae (clean water), amphibians.

Trap: A forest fire leads to secondary succession, even though it looks devastating, because the soil remains. Only "no soil" means primary.

APES math

No calculator is allowed on the AP exam for most years' FRQs, so practice by hand. Always show the setup, the work, and units.

1. Simpson's Diversity Index

Measures diversity using both richness and evenness. Ranges from 0 (no diversity) to close to 1 (very diverse).

D = 1 − Σ (n / N)²

n = number of individuals of one species · N = total individuals of all species

Worked example. Two meadows each have 4 species and 40 plants total.

Meadow A: 10, 10, 10, 10 each n/N = 10/40 = 0.25 → 0.25² = 0.0625 Σ = 4 × 0.0625 = 0.25 D = 1 − 0.25 = 0.75 Meadow B: 37, 1, 1, 1 (37/40)² = 0.925² ≈ 0.856 (1/40)² = 0.000625, × 3 ≈ 0.002 Σ ≈ 0.858 D = 1 − 0.858 ≈ 0.14

Same richness, but Meadow A is far more diverse because it is more even.

Your turn: 3 species with 20, 10, and 10 individuals

N = 40 (20/40)² = 0.25 (10/40)² = 0.0625, twice = 0.125 Σ = 0.375 D = 1 − 0.375 = 0.625

M14

2. Percent change

The most common APES calculation. Use it for any "how much did it increase or decrease" question.

% change = (new − old) / old × 100

Worked example. An abandoned farm field had 12 plant species. Forty years into secondary succession it has 30.

(30 − 12) / 12 × 100 = 18 / 12 × 100 = 1.5 × 100 = 150% increase

Your turn: a bird population drops from 2,000 to 1,400 after a hurricane

(1,400 − 2,000) / 2,000 × 100 = −600 / 2,000 × 100 = −0.3 × 100 = 30% decrease

3. Valuing an ecosystem service

Multiply area by value per unit area, then by time if asked.

Worked example. A 200-hectare wetland provides flood control worth $12,000 per hectare per year. What is it worth over 10 years?

200 ha × $12,000/ha/yr = $2,400,000 per year $2,400,000/yr × 10 yr = $24,000,000 (= $2.4 × 10⁷)

Your turn: a developer will drain 75 ha of it. How much flood-control value is lost per year?

75 ha × $12,000/ha/yr = $900,000 per year That's 75/200 = 37.5% of the wetland's value.

M12

4. Rates and unit conversion

Divide the change by the time. Convert units first so they match what the question asks for.

Worked example. Since the last glacial maximum about 20,000 years ago, sea level rose about 120 m. What was the average rate in mm per year?

120 m × 1,000 mm/m = 120,000 mm 120,000 mm / 20,000 yr = 6 mm/yr

Your turn (M11 tie-in): reef water is 28 °C and warming 0.02 °C/yr. Bleaching starts at 29 °C. How many years until bleaching?

Change needed: 29 − 28 = 1 °C 1 °C / 0.02 °C per yr = 50 years

Math habits that earn points

Write the formula first, then plug in numbers. Graders give credit for a correct setup.
Keep units on every line and in the final answer.
Use scientific notation for big numbers: 24,000,000 = 2.4 × 10⁷.
Say whether a percent change is an increase or decrease.
Sanity-check: diversity index between 0 and 1? A population can't drop more than 100%.

Practice questions

AP-style multiple choice. Pick an answer to see whether it's right and why. Two free-response questions follow.

Free response

M14 M11

FRQ 1. A wildfire burns a 500-hectare section of temperate deciduous forest in Michigan. The soil remains intact.

Identify the type of succession that will occur and justify your answer.
Describe one characteristic of the species likely to colonize the area first.
Explain how species richness is expected to change over the next 100 years.
Ten years after the fire, ecologists find mayfly larvae returning to a stream in the burned area. Explain what this indicates.
Fire is an episodic disruption. Identify one other episodic disruption and one random disruption.

Show scoring guide

(a) Secondary succession, because soil is still present after the fire.
(b) Any one: grow and reproduce quickly, small seeds that disperse widely, tolerate full sun, r-selected, short-lived (grasses, weeds, wildflowers).
(c) Richness increases as soil develops and more habitats/niches form (grasses → shrubs → trees), then may level off as the community matures.
(d) Mayflies are an indicator species sensitive to pollution and low oxygen; their return indicates improving water quality.
(e) Episodic: hurricane, drought, El Niño, flood. Random: volcanic eruption, earthquake, asteroid impact.

M9 M8

FRQ 2. A city plans to drain a coastal wetland to build a shopping center.

Identify one regulating service and one cultural service the wetland provides.
Describe one economic consequence to the city of losing the wetland.
The wetland contains 8 bird species. A survey shows one species makes up 90% of all birds. Explain why the wetland's species diversity is lower than its richness suggests.
Calculate the annual loss if the wetland's 150 hectares provide water purification worth $8,000 per hectare per year. Show your work.