Course: BIS 105 Environmental Biology

Unit 5 Learning Module

Historical Population Growth

Historical Population Growth

The growth of the world’s human population (using estimates by scholars in the field for the time before census data) shows a classic J shape on this 12,000-year scale. Can you distinguish the decline due to “black death” in the early middle ages?

Let’s begin by looking at the data. Worldwide human population from 10,000 BCE through today is graphed above. The theoretical J (exponential) and S (logistic) growth curves are reviewed in the previous unit. Overall, our growth resembles exponential growth (the J curve), increasing very slowly at first, but later growing at accelerating rates which show no sign of nearing carrying capacity. We appear to be r-selected for rapid growth; indeed, some have described humans as the most successful “weed species” Earth has ever seen as we are fast growing, rapidly dispersing, and colonize habitats from pole to pole. If Earth has a carrying capacity for humans, it is not yet visible in our growth curve – at least on this scale.

However, closer study of human population dynamics reveals more complexity. Different countries show different patterns of population growth today, and history shows varying patterns of growth across time. The history of human population growth can be divided into four stages. Today’s countries show snapshot views of these stages. In this section, we will look at early human population growth.

As scientists currently understand human history, Homo sapiens arose about 200,000 years ago in Africa. Living as nomadic hunter-gatherers, we migrated to Eurasia and Australia about 40,000 years ago and into the Americas 30,000 years later. Throughout this period, both birth rates and death rates were probably high – as much as 5%. Our human population grew slowly as we spread throughout the world, out-competing other hominid species with our apparently superior reproductive and competitive adaptations. Ice ages, warming periods, and volcanic eruptions were density-independent factors which severely limited our population growth. For example, a “supervolcanic” eruption at Toba in Sumatra 74,000 years ago covered India and Pakistan with more than 5 feet of ash, causing 6 years of nuclear winter, a thousand-year ice age, and the death of up to 99% of the humans living at the time!

With the invention of agriculture 10,000 years ago, we began to develop settled civilizations and trade. Disease associated with animal domestication and city living increased death rates, but reliable food supplies, shared childcare, and division of labor increased birth rates. These effects may have offset each other; slow and uneven growth probably continued. However, the development of agriculture, like many advances in technology, almost certainly raised carrying capacity.

Beginning about 6000 years ago, political states evolved, cooperated or competed, and sometimes waged war. Empires formed, connecting previously independent populations. In the Middle Ages, technology advanced, and the 17^th century brought the Scientific Revolution. Throughout this long period of human history, death rates and birth rates continued to be high. Density-independent factors such as drought and the “little ice age” combined with density-dependent factors such as disease to keep death rates high and variable. The “black death” of the mid-fourteenth century killed as many as 75 million people worldwide and the disease is one of the very few events whose effects are visible in any graph of human population growth. Birth rates continued at a high level throughout early human history. Carrying capacity rose with major advances in technology, as humans modified the environment by irrigating land, building cities, and transporting animals, plants, and products. The overall result was slow growth and a young population. By 1804 CE, the world’s human population had reached 1 billion.

As we move to examine more recent population growth, we will use a few new vocabulary terms. Total fertility rate (often abbreviated TFR) represents the average number of children born to a female in her lifetime. Replacement fertility is number of children per female that are needed to maintain a consistent population size. In most developed nations this is approximately 2.1, 2 children to replace the parents and a fraction of a child extra to make up for early mortality of some individuals.