How can the age at which organisms first reproduce affect population growth?

The age at which an organism first reproduces is a critical factor in determining the dynamics and growth rate of a population. Generally, populations in which individuals begin reproducing at an early age have the potential to grow much faster than those in which individuals delay reproduction.1

This relationship is central to understanding population growth because early reproduction shortens the mean generation time—the average interval between the birth of an individual and the birth of its offspring. A shorter generation time allows for a higher intrinsic rate of natural increase, meaning the population can expand more rapidly over time.2

Conversely, many species have evolved to delay reproduction to prioritize growth, defense, or the production of more robust offspring, which can increase the survival chances of the individual and its progeny. This trade-off is often categorized into two strategies:3
* r-selected species: These are opportunistic organisms that reproduce at an early age, often producing many offspring to maximize their growth rate in variable environments.34
* K-selected species: These organisms typically delay reproduction, producing fewer, higher-quality offspring in more stable environments where they remain near the carrying capacity of their habitat.34