You have probably read that wild horse herds grow at a rate of about 20% per year.
If you start with 100 horses, 20 foals will have to be born over the next year to bring the herd to 120, a 20% increase over the original 100. That’s if none of the adults die. A census would reveal that foals accounted for roughly 17% of the total.
If adults die at a rate of 5% per year, 25 foals would have to be born over the next year to bring the herd to 120. A roundup would yield 95 adults and 25 youngsters, with foals accounting for approximately 21% of the total.
When you look at gather stats and see foals representing 13% of the total, that herd may not be growing at a rate of 20% per year. Similarly, if you see foals accounting for 26% of the horses captured, that herd probably doesn’t conform to the 20% rate.
By how much can the death rate vary and not have an appreciable effect on birth rates?