Why does increasing survival and fecundity at earlier ages impact fitness more than at later ages?

Increasing survival and fecundity at earlier ages impacts fitness more significantly than at later ages primarily because of the higher probability of individuals facing risks such as predation and accidents as they age. In early life stages, individuals have the potential to produce more offspring before encountering these risks, thus enhancing their overall reproductive success.

For organisms that are subject to a variety of environmental pressures, such as predation, disease, and competition, earlier reproduction allows them to pass on their genes to the next generation more efficiently. If an individual can survive to reproduce at a young age, any additional offspring produced during that time contributes directly to its fitness. Once individuals reach older ages, the likelihood of facing fatal risks increases, and the opportunity for further reproduction diminishes. Therefore, increasing survival and reproductive success at younger ages yields a greater cumulative effect on an individual's fitness compared to later reproductive events, which may not contribute as meaningfully to the gene pool due to decreased survival probabilities.