Learn to use the Release Hub to restore endangered and extinct species in Let's Build a Zoo!

This guide is based on my own experimenting with releasing animals and trying to restore species and may not be entirely accurate.

The Release Hub is unlocked in the Research Hub and built from the Facilities menu. The Release Hub is located 2 tiles tiles to the left of "Animal Enrichment - Trampoline" on the research screen, and requires +40 good morality points to unlock and use.

Outside of your zoo there is a whole world full of animals, many of which are unfortunately already extinct. (Don't worry, you can still get those animals by trading with other zoos)

A species is:

• "Extinct in Wild" if it has less than 100 members.
  
• "Critically Endangered" from around 100 to 1,500
  
• "Endangered" from around 1,500 to 5,000
  
• "Vulnerable" from around 5,000 to 10,000
  
• "Near Threatened" from around 10,000 to 50,000
  
• "Conservation Dependent" from around 50,000 to 99,999
  
• Species with over 100k in the wild are of "Least Concern" and stop displaying detailed population counts.

Wild animal populations change over time in semi-predictable ways. When left alone, a population will either be stable or increase/decrease by a mostly consistent number of animals each day. This daily rate of growth/decay does occasionally change, so keep an eye out for previously stable populations beginning to decline.

When you release animals, you can affect the rate of the background daily population change. For example: my wild sheep population was declining at a constant rate of -27 sheep per day. Once I started releasing sheep into the wild, the background population slowly stopped declining and then started increasing. After about two weeks of releasing 1-4 sheep per day, the background population change stabilized to about +32 sheep per day.

Unfortunately, species which have gone completely extinct seem to have a very high background rate of decline that is unaffected by the animals you release into the wild. I've seen a previously extinct species lose upwards of 1200 animals from the wild population per day. This constant decline appears to continue even after the species is restored to "Least Concern" status with over 100k in the wild.

Note that the "increasing" and "decreasing" green and red arrows include your impact on the population. If you have released any animals of that species, the arrow may not indicate the actual daily rate of change.

Each animal you release adds to the wild population. The more total animals you release, the larger the impact of each animal.

More specifically, as long as the population is larger than zero, each animal you release adds (TotalReintroducedToWild + 2) to the population. That means that each animal you release is worth one more population than the animal before it. This also works for releasing animals in batches. Starting from 0 released, if you released 15 animals, the population increases by 3+4+5+6+...+17 = 150. And, the more animals you release, the greater your impact will be on the wild population.

However! Here's where it gets a little complicated: If the wild population ever drops completely to zero and is extinct in the wild, the release counter effectively resets. So, the counter might say you've released 30 platypuses, but if you let the population fall to zero, the 31st platypus you release will only be worth 3 population again.

These numbers appear to be independent of any biological information about the animal released. In other words: variant, age, sex, and pregnancy status do not matter for the purpose of wild population increase.

As best I can tell the "increasing" and "decreasing" tags reflect your impact on the population more than the wild population's daily change. When I started restoring the platypus population, the population trend changed to "increasing" even though the population's background rate of change was a loss of 501 per day. This leads to some conflicted messaging when a species is both population zero "Extinct in Wild" and nominally "Increasing".

The relationship between the number of animals released and the wild population increase is consistent for early population restoration (I tracked the platypus population for 24 game days up to about 10,500 animals and it stayed in this pattern). However, some higher population Near Threatened and Conservation Dependent species have a greater than predicted population increase per release. I'm not sure when or why this starts to happen.

In short: release enough animals each day to counteract the background wild population loss, and don't ever let the population hit zero.

You will likely need to heavily front-load your releases to avoid the population falling to zero and losing your progress. The more animals you release, the easier it will get to increase the population. When I started releasing platypuses, it took me 45 animals to get a wild population of 1125. Now that I have a streak of 200+ platypuses released, I can add that number to the population by releasing just 5 or 6 more.

To restore a completely extinct species, you will need to fill up a couple pens of animals before starting to release them. Depending on the species' rate of decay, you may need more animals for some species than others. If a wild population is losing 500 animals per day, that means you need to release enough animals the first day to bring the wild population above 500 (about 30 animals). Unfortunately, you often don't know the rate of decay before you start restoring the population. It's better to over-prepare than under-prepare, since if the population drops to zero you lose your release progress.

For folks who want to do math, you can calculate batches of future releases with an arithmetic series.
S = n[2a+(n-1)]/2
S is the sum of the series (the amount the wild population will increase)
a is the first term of the series (streak of animals previously introduced to the wild + 3)
n is the number of terms in the series (number of animals you will be releasing)

I recommend relying primarily on nurseries to produce animals for release, since the endangered species frequently have a low pregnancy probability. This also means that you won't accidentally release your breeder animals. If you are far enough into the game to have researched infinite CRISPR splicers, CRISPR is also a good way to generate more animals.

You can use pregnancy time, nursing time, and breeding success rate to calculate how many breeding pairs you will need to maintain a desired number of released per day. Note that animals appear to always have a one day gap between the end of nursing and start of the next pregnancy.

Breeding Pairs Needed = Desired Number to Release * Expected Days to Get One Baby

Expected Days to get One Baby = Pregnancy Time + Nursing Time + (1 Day / Breeding Success Rate)

If you've researched Reduce Pregnancy Time, you can just use the following table:
Expected days to get one baby (simplified)

Depending on space and nursery costs, it may be easier to stockpile animals over a longer period before starting your reintroduction than to obtain a high production of animals per day. If you want to release 6 animals per day at 2-day nursing, you will need at least 24 breeding pairs, which is 12 (or 6 upgraded) nurseries.

If the animal has a short gestation period, it can help to keep a breeding population in your display pen as well. Don't release pregnant females unless they're your only option to avoid the population dropping to zero.

Where to find the some very helpful research upgrades:

• Nursery is 1 down, 3 to the right of trampoline
  
• Reduce Pregnancy Time is 4 down from trampoline
  
• Breeding Facility - Add Rooms is 6 left, 3 up from trampoline
  
• Infinite CRISPR Splicers is 5 down, 9 right from trampoline

Updated 9/3/23 with more information on daily population changes, expanded animal production math.