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Tracking Success in Managed Populations Over Time

By Alina Tugend
min read

An Investment in the Greater Good

Zoo and aquarium populations tend to be “closed,” meaning that, unlike in the wild, it can be difficult to bring in new genetic diversity. And that can lead to reproductive problems, health issues, and population declines.

For decades, the Association of Zoos and Aquariums has looked to address those issues with its cooperatively managed Species Survival Plan® programs. Started in the 1980s, there are now almost 500 such programs; each one develops a breeding and transfer plan that identifies population goals and recommendations and works toward a genetically diverse, demographically robust, and biologically sound population.

But the overall success of these programs has been difficult to measure—until now.

That’s because the first long-term assessment of cooperatively managed programs in zoos and aquariums was just published in June. It found that genetic and demographic health of many of these populations was more sustainable over the long-term—and its genetic decline slower—than many experts hypothesized.

The findings, which considered almost 20 years of data for about 400 ex situ vertebrate populations “are enlightening and show the resources being invested in cooperative management are having a positive impact,” said Megan Brown, AZA’s director of population management strategy.

Two cheetah cubs

The analysis, published¹ in the journal Frontiers in Ecology and the Environment, was conducted by the Alexander Center for Applied Population Biology at the Lincoln Park Zoo in Chicago, Ill., together with the AZA Population Management Center (PMC) also at Lincoln Park Zoo. The Alexander Center began collecting the data from SSPs in 2009.

“Starting in 2018, we really started building this specific research question: how are the SSPs doing over time?” said Judy Che-Castaldo, a research scientist at the Alexander Center and the lead author on the paper. The research focused primarily on mammals, birds, and reptiles.

The expectation was that “because zoo and aquarium populations are small, they might decline in genetic health in particular over time,” said Che-Castaldo. “But we found the opposite. We found that they’re actually able to maintain that gene diversity over time for most of these populations—which is unexpected.”

Therefore, based on the information now available, she said, the majority of zoo and aquarium populations “are stable and not declining.”

Part of the impetus for the paper was research that found that a large number of zoo populations are very small or have low genetic diversity and therefore may not be viable in the future.

A 2019 special issue of Zoo Biology devoted to this concern noted that, “over the last ten years, zoos and aquariums around the world have been coming to grips with the ‘sustainability crisis’—the realization that most of our collaboratively managed animal populations are not viable for the long-term.”

Adult female lion with a cub

But the research by Che-Castaldo and her colleagues stated that previous studies focused on “snapshots” of present status or future projections, “but did not examine whether and how population status has changed historically. Placing the current status of each population in the context of an earlier time point enables examination of whether populations have improved over time even though they may not be fully self-sustaining.”

Much of the success in this area, according to Kristine Schad Eebes, director of the AZA PMC and a co-author of the study, is the increasing emphasis on the importance of population management among zoos and aquariums—including the creation of the AZA PMC itself in 2000 and staffed with professional population biology advisers as well as adjunct biologists for the SSPs.

“Having trained program leaders, population biologists, and researchers is key to this,” said Joe Barkowski, vice president of animal conservation and science at Tulsa Zoo in Tulsa, Okla., and chair of AZA’s Animal Population Management (APM) Committee. “Without them, we wouldn’t have had the data and been as far as we are in this process.”

He noted that these findings show that zoos and aquariums need to invest resources not only into the animals in their care but ensure that people have the training and means to manage the animals.

“It wasn’t just going to happen on its own,” he said.

Each SSP keeps a studbook—an electronic database for a species or sub-species—and a studbook keeper, who maintains the database.

Adult female lion with three cubs

“The population biologists can go in and analyze the population to see what’s going on in terms of genetics and demography,” said Schad Eebes. “Are they breeding? Are they not? And at the same time, they can take that information and advise the SSPs on management decisions for the future.”

The successful population management of cheetahs is a good example, said John Andrews, senior population biologist at the AZA PMC.

“Moving animals, especially bigger ones, can be expensive,” he said. “We do it in the hope that animals will come together and breed, but sometimes that’s not successful.”

So, through its SSP, certain AZA-accredited zoos are designated as breeding centers for cheetahs, and other facilities are exhibit centers, Andrews said. “Any cheetah in an exhibit center, where they’re displayed for guests, can be pulled into a breeding center if we need to get their genetics more represented and then send additional animals for the exhibit.”

“Before 2013, this formalized management strategy didn’t exist for the cheetah program,” said Andrews. Currently there are 53 AZA zoos that have cheetahs and ten of those are breeding centers with many potential mates in one place. Other SSPs, such as the blue-gray tanager and red-capped cardinals, have implemented similar strategies.

Another example: lion breeding.

“Contraception in cats is difficult,” said Hollie Colahan, vice president of living collections at the Birmingham Zoo in Birmingham, Ala., and vice chair of AZA’s APM Committee. 

Sometimes the contraceptive did not reverse as soon as expected and “we were concerned that it could cause long term or even permanent effects,” she said. “Thanks to the work of the AZA Reproductive Management Center based at the Saint Louis Zoo in St. Louis, Mo., we were able to analyze data on lions across the SSP and adjust the methods and breeding strategies. A few years later, we had a record number of births and now the population is thriving.”

Cheetah cub

Cooperative management programs are “an investment in the greater good,” Colahan added.  “We know that we have to work cooperatively in order for these programs to be successful.”

“It is difficult to point to a single strategy that works for many species because of how much they vary biologically,” said Che-Castaldo.

The aim of the scientific paper was not just to look backwards, but forwards as well; the APM Committee is in the process of reimagining how the SSP program will look in the future, Barkowski said, adding that while zoos and aquariums are passionate about trying to manage and save as many species as possible, there will be some hard choices, given that resources are not unlimited. 

He anticipated that there will be an initial reduction of 20 to 30 percent in the number of SSPs as new criteria are developed.

“This research is showing us that it’s better than we thought, or it’s going to be better,” he said. “Before we were looking at just one piece of this giant puzzle and now that we’re looking at all these pieces, it’s really heartening. And if we want to keep this up as we gain more and more programs, we need to ensure this model stays on its trajectory.”


(1) Judy Che-Castaldo, Steven M Gray, Kathryn M Rodriguez-Clark, Kristine Schad Eebes, Lisa J Faust. Expected demographic and genetic declines not found in most zoo and aquarium populations. Frontiers in Ecology and the Environment.
First published: 03 June 2021 

Alina Tugend is a writer based in Larchmont, N.Y.

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