M10 Ecosystem Pop Quiz · L04

The Brakes

HS-LS2-2HS-LS2-4

Technical Specification

Learning Objectives

Distinguish density-dependent from density-independent limiting factors.
Interpret predator–prey cycles and explain why predator peaks lag prey peaks.
Predict how removing a brake (predator, disease, competition) alters population growth.

Data Protocol

Integrity Guidelines

Classify a factor by its mechanism, not its severity.
Cite the cycle source (here: MacLulich 1937, representative Hudson Bay fur returns).
Carry units — fur returns in ×1000 pelts, never bare integers.

The Hook

Track lynx and snowshoe hare in the Canadian boreal forest for a century and neither levels off at a flat line. You see two jagged waves rising and falling in a repeating rhythm — the predator's wave always trailing the prey's by a couple of years. That pattern is one of the most famous datasets in ecology, and it reveals exactly how populations brake.

Density-dependent vs independent

A limiting factor is anything that restricts population growth. The key split is whether its effect scales with how crowded the population is:

Density-dependent — intensity rises as crowding rises. Competition, Disease, Predation, Parasitism, food depletion, nest-space shortage.
Density-independent — strikes regardless of crowding. Hurricane, Frost, Drought, Toxic waste, extreme temperature.

Dependent factors are the built-in brakes. Independent factors are weather.

Case — Channel Island fox decline

On San Miguel, Santa Rosa, and Santa Cruz islands, island fox populations fell ~95% between 1994 and 2000. Two brakes hit at once: golden eagles colonized the islands (drawn by non-native feral piglets) and began predating foxes, while a canine distemper outbreak tore through the isolated population. After 2001, captive breeding + eagle relocation reversed the collapse — demonstrating that removing the right brake can rescue a free-falling population.

Interactive Ledger · 04

Limiting Factors

Read the cycle, watch it move, then classify the brakes.

Dual-axis dataset

Hudson Bay Co. lynx–hare fur returns, 1900–1920

Representative fur-return counts — lynx peak lags hare peak ~1–2 yr (after MacLulich, 1937)
Year	Hare (×1000)	Lynx (×1000)
1900	24	4
1902	48	8
1904	90	20
1906	60	35
1908	20	50
1910	8	40
1912	12	18
1914	40	10
1916	95	24
1918	70	45
1920	30	60

Hare column tinted green; counts in ×1000 pelts.

Live cycle

Predator–prey oscillation

Hare leads; lynx trails ~1–2 yr behind — classic delayed-brake signature.

Factor Sort

Classify each factor — click to cycle

Tap a card to cycle: classify → Density-dependent → Density-independent.

Defend your sort

Justify one dependent and one independent classification with mechanism.