This week we will focus on population – the movement of humans through time. Our emphasis will be on population geography which focuses on the number and distribution of humans over specific areas in space, over specific regions, at specific locations. Like all other aspects of geography, population geography looks at how the environment impacts population trends. Today, we will introduce some basic measurements and metrics used to describe populations. And we will introduce the population pyramid, which is a powerful way to view population distribution by age cohorts.

In recent years, world population has been growing at about 77 million people per year or almost 211,000 per day. World population is over 7 billion, almost double what it was fifty years ago Population increases at an exponential rate whereas resources increase at a linear rate. This discrepancy means at some point population will outstrip the available resources. Understanding the trends that drive population growth is a fundamental and important scientific enquiry.

In the example, we are looking at per capita Olympic medals. Although large nations like China and the USA take home the most medals, looking at medals in a per-capita way (controlling for size) allows us to see which nations get the most medal “bang” for their population buck.

Our first metric is that of the crude rates – crude birth rate (CBR) and crude death rate (CDR). “Crude” in this sense just means gross or total or unadjusted or raw. CBR and CDR are usually expressed not per capita but per thousand. So a CBR of 6 means that for every 1,000 people, there will be 6 newborns in that group this year. We can generate a new measurement, the Natural Increase Rate, by using the equation NIR = CBR CDR. A positive NIR means your population is growing.

Once you have your NIR in percentage form, you can use the Rule of 72 to quickly calculate doubling times. 72 divided by the NIR gives you the number of years needed to double your population.

Very different answers based on different NIR’s. In the 70’s, with an NIR of 2.2 – our population was expected to double in just 30 years. Today, with a much lower NIR, we have more than fifty years before another doubling will occur.

Africa, Central America and Western Asia have the highest NIR. Note how many countries (including the USA) have NIR’s less than 1 – these are the low growth nations. Note that some countries (Russia, parts of Europe) have NIR’s less than zero, meaning their populations are shrinking. Quick review and check for comprehension: is the projection on the NIR map a conformal one or an equal-area one?

Fertility is a measure of how many children a woman will have in her lifetime. Despite the term “Total”, TFR is a per-capita term. The number given for TFR refers to the number of children, on average, each woman in a population will have. Any TFR less than 2.0 will result in a population decline as not even the mother and father are fully replaced by their children. Given accidental deaths or disease, most TFR’s will need to be at least 2.2 or higher to simply maintain a population at its current levels.

Notice that the high Total Fertility Rates are almost always associated with high Natural Increase Rates. Remember that both environmental and cultural factors can impact an individual woman’s decisions regarding children.

The Infant Mortality Rate or IMR measures the number of children who are born and die within the same year. It is a “per-thousand” result. An Infant Mortality Rate of 20 means that for every 20,000 babies born, twenty will die with a year. Convert to a percentage by dividing by ten. In this example, 2% of all infants born will die within a year.

There are many, many environmental factors behind the IMR. Access to medical care, the number of available doctors and the quality of medical facilities plays a large part. All of these, by the way, show up in straight economic metrics like GDP per capita (more money, lower IMR). Famine and war obviously impact IMR. Birth control access plays a role. Women who find themselves in substandard conditions and who have access to birth control will postpone pregnancy until more favorable conditions arrive.

You can see the impact of war on IMR by looking at the deep-red, high IMR of Afghanistan and Angola. Otherwise, IMR seems to match up pretty well with GDP (high GDP, low IMR).

The “population pyramid” is a convenient visual data representation population geographers use to view many aspects of a population by displaying the data in a cohort-based, gender-symmetric fashion. The broader the horizontal column, the more people it contains. The higher the column, the older the population it contains. And males and females are put on different sides of a zero center line. Asymmetries become easy to spot. Top-heavy pyramids have high dependency ratios – the older generation is more populous than the younger cohorts tasked with their care. If the pyramid tilts to the right or left, sex ratios are not 50-50 (example: China has an abnormal surplus of young men after two decades of their “One Child” policy).

High growth population pyramids have a broad base of young people and children (who will soon grow up to be fertile young people). Slow and zero-growth pyramids are top-heavy with fewer young people and more older folks. Note the asymmetry for older (80+) females. Longer female lifespans is one example of an inclusive fitness trait.

These pyramids are from the United States. The Hispanic pyramid is in a high growth shape, the white-only is designed for slower growth (or perhaps no growth).

Here, we see that foreign born Hispanics in the US typically don’t bring infant or toddlers with them when they migrate. Native born Hispanics are poised for high growth over the next decades.