Unit 2.6 — Environmental Consequences of Connectivity

Topic 2.5 showed connectivity's cultural upside — ideas, technology, and travelers moving freely. Topic 2.6 is the other side of the same coin: the same roads that carried Marco Polo and gunpowder also carried a disease that killed a larger share of humanity than almost any event in world history.

The Black Death rides the Mongol network

The bubonic plague originated in Central Asia and spread west along the very trade routes that the Pax Mongolica (Topic 2.2) had made faster and safer than ever before. Fleas carried by rodents traveling with merchant caravans and ships moved the disease from Central Asia into the Middle East, North Africa, and Europe during the 1340s–1350s. This is the clearest possible example of a general rule the exam wants you to internalize: connectivity is not automatically good — the same infrastructure that spreads trade goods and ideas spreads pathogens just as efficiently, and often faster.

A death toll that reshaped continents

Estimates vary, but the plague is thought to have killed roughly 75 to 200 million people across Eurasia, including an estimated 30–60% of Europe's population — almost certainly the deadliest single demographic event covered anywhere in this course. No other topic in Units 1–2 comes close to this scale of population loss.

Labor shortages changed European society

The demographic collapse had a direct economic consequence: with so many workers dead, the surviving peasants and laborers were suddenly scarce and could demand higher wages and better terms from landowners. This shortage of labor is a major factor historians point to in the gradual weakening of serfdom in Western Europe — peasants who once had little bargaining power now had leverage precisely because there were so few of them left.

Beyond disease: crops and animals moved too

Trade networks didn't only transmit goods, ideas, and disease — they also carried crops and animals that reshaped agriculture and diets across connected regions, feeding into the same networks discussed in Topics 2.1–2.4 (Silk Roads, Indian Ocean, trans-Saharan routes). The exam frames this broadly: connectivity moved everything a network could physically carry, for better and for worse.

Why this matters for the exam

Topic 2.6 pairs directly with Topic 2.5 as a cause-and-effect unit: 2.5 is the upside of connectivity (culture, technology, travelers), 2.6 is the downside (disease, demographic collapse). Expect comparison or causation essay prompts that ask you to weigh both consequences of the same network — the Pax Mongolica that let Marco Polo travel safely is the identical infrastructure that let the Black Death reach Europe. Know the specific death-toll range and the labor-shortage/serfdom connection — both are common short-answer and multiple-choice targets.

Sources: Albert.io AP World History Review, OER Project.