I was thinking you found a hole here, but I realize it’s just another case of choosing the right tool for the right job. A species of only 1000 organisms is rare. It’s special, not mediocre. It’s probably fragile, not durable, and fragility carries more information. Maybe it just suffered a catastrophe, maybe it’s over-fitted to one niche. Whatever it is, you’ll likely know the special reasons why they’re so fragile. What are our vulnerabilities, how often do we face them, how lethal are they? If this is an information-rich scenario like that, then Bayes isn’t the right tool, because it can’t consider all of that information. You should pick the method that can.
It seems like birthrate stability is a dimension that would need considering here. If they’re at 1000 but they’ve been at 1000 for a million years with an annual birthrate of 10, they’re not fragile or special, they’re durable and mediocre. Clearly they face no special calamity or vulnerability, so there’s not much to know about risks. It’s a low-information situation where only durability is known, so this method becomes the best forecasting tool under those assumptions.