# Age Structure

Another characteristic of populations that ecologists measure is population age structure. This characteristic is as simple as it sounds: it’s a summary of the number of individuals of each age in the population. Age structure is useful in understanding and predicting population growth.

For instance, if you know the age of first reproduction and reproductive strategy of a population, from the life history we mentioned earlier, then you can predict future population growth patterns based on age structure data.

If most of the individuals in the population are below the age of first reproduction, then you can predict that in the near future, the population is likely to grow. However, if most of the individuals are beyond reproductive age, then you can expect the population to shrink. An understanding of population age structure is critically important to industries that harvest living organisms. Even if they can calculate Maximum Sustainable Yield, if they don’t know the age structure of their population of interest, they might over- or under-harvest.

Ecologists use nifty graphs called age pyramids to depict the age structure of populations. Age pyramids show age groups like 0–4, 5–9, or 10–14, along the vertical axis (y-axis) and population size along the horizontal axis (x-axis). Each age group is broken into males and females, with a bar graph for each running horizontally to the left and to the right. Human age pyramids from different countries are especially interesting to examine. In industrialized countries, the age pyramids are often not pyramid-like at all, with the largest numbers of people near the middle or top of the graph. In developing countries, however, the vast majority of people are in the youngest age categories, with few surviving to older ages.

The question you should ask about human age pyramids is, "Why do different populations of humans have different age structures?"

We will not answer that question here (hey, we can’t do everything), but the answer’s impact on foreign policy and the world’s future is not insignificant.

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### Population Distribution

The last population characteristic we will discuss here is population distribution. This aspect deals with how the individuals in a population are located relative to one another across the environment.
• Some populations have clumped distributions, with multiple, similarly sized groupings of individuals spread fairly evenly across the landscape.

• Other populations have a random distribution, with some clumping and some more even spreading of individuals.

• Still other populations have an even distribution, with no clumping whatsoever.

Populations with different distributions respond differently to both density-dependent and density-independent factors. What’s more, sometimes it can be difficult to determine whether a population is clumped or if, in fact, there are multiple distinct populations. Often, the distinction is a matter of understanding the degree to which the clumps, or subpopulations, interact with one another. At any rate, it is obvious that a population with a clumped distribution is likely to have different characteristics than a population with an even or a random distribution.

One dot = 7500 people.

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Brain Snack

The oldest human ever recorded was 122 years and 164 days old, but there is a group of people in Okinawa, a small area of Japan, who are on average the most long lived.