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How do age structure and sex ratio influence a population's fertility rate?

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Common wildebeest — population ecology A herd of common wildebeest (*Connochaetes taurinus*) migrating across a dusty savanna in Africa. The animal is a keystone species (i.e., a species with a disproportionately large effect on its biological community) in plains and acacia savanna ecosystems from southeastern Africa to central Kenya.

In population ecology, age structure and sex ratio are critical determinants of a population's fertility rate because they constrain the number of individuals capable of reproducing.1

Age Structure

Age structure refers to the proportion of individuals in various age categories. It influences fertility in the following ways:2
* Reproductive Capacity: A population with a large proportion of individuals in their reproductive years will generally have a higher fertility rate. Conversely, if a population has a high percentage of elderly individuals—as seen in many industrialized nations—there are proportionately fewer people of reproductive age, which contributes to lower fertility.12
* Life History Strategies: Species that begin reproducing at an earlier age have the potential for faster population growth compared to those that delay reproduction.3
* Feedback Loops: While fertility rates shape the age structure by adding new individuals to the youngest age group, the resulting age distribution subsequently dictates the future reproductive potential of the population.42

Sex Ratio

The sex ratio—the relative number of males and females—directly impacts fertility by influencing mating opportunities:5
* Marriage Patterns: In many populations, a scarcity of males of a specific age can depress marriage rates for females in that same or younger age groups, thereby reducing overall fertility.5
* Marriage Squeeze: Dramatic shifts in fertility can create a "marriage squeeze," where the number of available partners of the socially expected age is insufficient, leading to changes in marriage timing or age differentials, which in turn affects birth rates.5