How can age structure predict future population trends of animals?
Look at an age pyramid. A large cohort of immature to barely mature individuals signals a rise in population is coming. Conversely, few young individuals signals a decrease in population. You do have to interpret the pyramid in light of the survivership cirve. Some animals have high mortality in the early stages and low later. Others, more even mortality. Still others die mostly after reproduction.
As for animals that are in captivity until they expire, their ages and health are monitored. Using that as a barometer, biologists can determine whether or not they are healthy enough to reproduce. If any animal in its family dies, the day and year is recorded and subsequently every other animal is too. From that they would know whether or not the animal lived to its longevity and if not, possible tests will be taken (autopsy) to see if the animal had a disease.
