# Human Populations

These factors that impact the growth of wild populations apply to humans as well. We are influenced by the dynamics of population ecology, experience our own population dynamics, and have our own limiting factors.

## Age Structure Diagrams

**Age structure diagrams** display the relative numbers of individuals in a population and their respective ages. Ecologists use this data, along with information about reproductive potential, to predict the future growth rates of a population. These same diagrams can be used for human populations as well, as is pictured to the right with an age structure diagram of the United States. It is important to note that males and females are separated in this diagram - there are not 11 million people ages 0-4, but 11 million *males* ages 0-4.

Growth rates can be inferred based on the shape of the structure that forms from the diagram.

When looking at age structure diagrams of humans, they can be split up by age into roughly the following categories:

**Pre-reproductive: **ages 0-14

**Reproductive:** ages 15-44

**Post-reproductive: **ages 45+

The relative size of these categories is what is used to determine the growth rate. Rapidly growing populations have high reproductive rates, which means there will be a larger proportion of younger people. If the pre-reproductive group is the largest, the population is currently growing and there will most likely be future growth. If the pre-reproductive and reproductive groups are roughly the same size, the population is fairly stable and may be experiencing small amounts of growth. If the reproductive group is larger than the pre-reproductive group, the population is in decline and has a negative growth rate.

There can be variation within these patterns. For example, two diagrams can both have growing populations. The one with the most extreme pyramid shape, where the pre-reproductive is the widest in comparison to the reproductive population, will have an even larger growth rate.

# Factors Affecting Human Population Growth

There are a variety of factors that can influence human populations and their growth rates. These can be social, cultural, and economic factors.

## Fertility Rates

A common measurement used in discussions of human population growth is the **total fertility rate** (**TFR**). This is the average number of children that a woman in a particular population will have. Typically, the higher the TFR, the more rapid the population growth.

There are a variety of factors that can influence the TFR of a population, with development of a country being one of the largest. Richer, more developed countries have lower TFRs and therefore lower population growth rates. As there are more educational and economic opportunities for women, as well as increased access to contraceptives, women tend to have kids later in life, resulting in fewer births overall.

There are government policies that can influence the TFR as well, such as China's two child policy, tax incentives for having fewer children, and even countries that engage in forced sterilization. Some countries are beginning to reverse course and offer tax incentives to have children due to the TFR being below the replacement level fertility.

If the TFR is equal to the **replacement level fertility**, births are offsetting deaths and the population is relatively stable.

## Infant Mortality

The **infant mortality rate** (**IMR**) is the number of deaths of children under the age of one per 1,000 people in a population. It is higher in less developed countries due to lack of access to adequate health care, stable food supplies, and clean water. A higher IMR results in a higher replacement level fertility, which typically results in a higher TFR.

## Human Carrying Capacity

Just like other species, we have limiting factors that impart a carrying capacity on us. **Malthusian theory** dictates that the human population is growing faster than our food production capabilities are increasing and warns of a catastrophe once we pass this point. If we exceed our carrying capacity, there will be famines, as well as likely wars over resources.

While technological advancements, such as the invention of synthetic fertilizers or the production of high-yield GMO crops, have increased our carrying capacity by singificantly increasing food production, we still are influenced by a variety of limiting factors. Major storms, heat waves and droughts are density-independent factors that are witnessed throughout the world, and are becoming increasingly more common as climate change worsens. Access to clean water and air, food availability, available land, and disease transmission are all density-dependent factors that continue to be stretched thin or threatened as our population grows and we live unsustainably.

## Standard of Living

The **standard of living** is the overall measure of the quality of life for people in a population. It consists of two major parts: GDP and life expectancy.

GDP (the Gross Domestic Product) is a key economic indicator as it relates to the total value of foods and services provided. The life expectancy is a key health indicator as it relates to the average age a person in the population will live to and increases with better healthcare, cleaner water, and more stable food sources.

A higher GDP and higher life expectancy are both associated with development of a country and lower population growth.

# Calculating Growth Rate

## Growth Rate

The growth rate (**r**) of a population is the percent increase in the population per unit time (usually per year). It can be calculated using the **crude birth rate** (births/1000 people) and **crude death rate** (deaths/1000 people) of a population.

As the answer is a *percent* (with is out of 100) and the crude rates are out of 1000, you divide by ten as part of the equation to convert to a percent. If you get a positive percent, the population is growing; a negative percent growth rate means the population is shrinking.

Let's do a practice one!

In a population of ring-tailed lemurs, the crude birth rate is 47 and the crude death rate is 38. What is the growth rate of this population?

### Click here for the solution.

r = (CBR - CDR)/10

r = (47-38)/10

r = 9/10

r = .9%

## The Rule of 70

Once you have calculated the growth rate, you can use that to determine how many years it will take the population to double in size. You simply need to divide 70 by the growth rate.

A population of warthogs has a crude birth rate of 176 and a crude death rate of 115. How many years will it take this population to double?

### Click here for the solution.

r = (CBR - CDR)/10

r = (176-115)/10

r = 61/10

r = 6.1%

doubling time = 70/r

doubling time = 70/6.1

doubling time = 11.5 years