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The Sustainability Index

By Alina Tugend
min read

When Tim Snyder, curator of birds at the Chicago Zoological Society—Brookfield Zoo in Chicago, Ill., agreed to participate in research to develop a new tool called the S-Index to help manage animal collections and improve population sustainability, he didn’t know what to expect.

And some findings turned out to be a surprise, especially about the Zoo’s perching birds. Although a lot of chicks were being hatched, many weren’t surviving long enough to meet the Zoo’s goals.

“Those general trends were apparent, but the tool helped to quantify them and direct further research,” said Snyder.

And that’s exactly why Colleen Lynch, curator of birds at Riverbanks Zoo and Garden in Columbia, S.C., decided to create the S-Index (the S stands for sustainability). Lynch is also a consulting population biologist with the Association of Zoos and Aquariums’ Population Management Center at Lincoln Park Zoo in Chicago, Ill.

As part of the research, Lynch asked 27 zoos to give their bird collection inventory data from January 2006 through December 2010 and January 2011 through December 2015. Lynch said she asked for information on birds, because she is most familiar with them, but the S-Index can be used on any taxa for any collection. Of the 27 facilities, 25 participated in both sets of samples, representing 1,000 reported taxa and 10,000 individual animals.

The S-Index uses five data points (the number of individuals in a collection, births, deaths, acquisitions, and dispositions) to calculate five metrics, or outcomes, that describe the collection.

These five metrics are three lambdas (which are a commonly used metric of growth in populations) that indicate the extent of growth or decline in a population and tell if the growth or decline is driven within the collection by births and deaths, outside of the collection by transfers in or out, or both.

Two other metrics are the Producer/Consumer Index–the net balance of animals provided to and accepted from other zoos–and the Community Impact Index, which is the net change in the collection plus the producer/consumer index representing the impact on the regional association collection due to changes in the facility’s collection.

“Zoos are very good at keeping records,” said Lynch, “but now we’re not just keeping track of animal inventories but looking at collections analytically to help us boil down what the trends are.”

She likened it to balancing a checkbook; just filling in the numbers and making sure it balances out doesn’t tell you where your money is going. You must also plan and evaluate your budget.

The goal is to use all of the S-Index metrics together to develop a deeper understanding of the dynamics of an animal collection, and to use this insight to better manage zoo animals; which species does a zoo need to acquire from other zoos and which can they provide for themselves or even supply to other facilities, Lynch said.

“Through this understanding we can also identify which species are true success stories within the collection, which might be better managed with informed allocation of resources, and which might not be best suited for continued management in the facility,” Lynch said. “It’s important that we all contribute to the system fairly.”

Scarlet ibis

That doesn’t necessarily mean consuming is bad or producing is good, she added. 

“We all do some producing and some consuming—it is the balance that is key. This helps show where my strengths and weaknesses are and how I can use that to build relationships with other facilities that complement those strengths and weaknesses.”

Dr. Candice Dorsey, AZA’s senior vice president for conservation, management, and welfare sciences, said the S-Index should prove helpful in carrying out AZA’s Animal Population Management Committee’s new strategic plan, which promotes assisting institutional collection planning.

The strategic plan addresses the “urgency of the animal population management crisis throughout the AZA community,” said Dorsey. “Without broad sweeping change to AZA animal programs, practices, and resource allocation, many species will no longer exist in zoos and aquariums. Facilities need to realize the critical role that they play in population management. 

“We want to engage with facilities more, promote their expertise and resources, and help them take ownership of their part in enhancing sustainability within the community,” said Dorsey. “Performing the S-Index exercise is a solid step. It’s something we would love more facilities to do.”

AZA has more than 500 Species Survival Plan® programs and most are not genetically and demographically sustainable over 100 years, which is a common definition of sustainability.

“Oftentimes, when discussing sustainability, we’ve focused on the population managers and not the zoos themselves,” said Dr. Megan Brown, AZA’s director of population management strategy.

Zoo Knoxville in Knoxville, Tenn., decided to use the S-Index last summer for its entire collection over a ten-year period.

“We know that our zoo populations are not sustainable, and most facilities exhibit and hold more species of wildlife than they breed for sustainability purposes,” said Lisa New, the Zoo’s president and chief executive officer. “It seemed like a really objective, interesting tool to be able to see where our Zoo landed.”

“We’re always saying we need to save animals from extinction. A tool like this gives us more than lip service,” said Phil Colclough, director of animal care, education, and conservation at the Zoo.

Collecting the information was an extensive paperwork exercise and could create misinterpretations; without digging into the data “it would appear on the surface that the Zoo has a bleak track record with some species,” said Colclough.

That’s because the overall analysis doesn’t always reflect special circumstances; for example, Zoo Knoxville took in a large number of snakes confiscated by the local wildlife agency from a religious snake-handling minister.

“By the time we got the snakes they were in really, really bad shape and we had to euthanize them,” he said. “I think it’s an awesome tool,” but added that there needs to be a way to account for those anomalies.

Red capped cardinal

Michael Ogle, curator in the Zoo’s Department of Herpetology, said looking at the wide expanse of data also brought some good news—he found out that over a nine-year period he had delivered 335 turtles to other zoos.

“I was really astounded we had sent out that many,” he said. “It’s a good big-picture look at collections and how they change and who does well with what and how we can each better build moving forward. This information exists, but in a more fragmented way.”

The S-Index may uncover some uncomfortable information, “but I think we have to be really honest about our problems and what we’re going to do to change if we’re going to have a sustainable population of zoo animals in the future,” said New.

Christopher Kuhar, executive director of Cleveland Metroparks Zoo in Cleveland, Ohio, which completed the S-Index, also agreed that the numbers don’t always tell the whole story about who is a consumer or a producer or why. Cleveland Metroparks Zoo, for example, has been a holder of bachelor gorillas for a long time, which means the Zoo is a consumer of those animals, but is also providing a needed service.

“It’s a conscious decision,” said Kuhar. Like Zoo Knoxville, there were also some pleasant surprises. Kuhar said the Zoo was producing and sending more macropods—kangaroos and wallabies—to other zoos than he expected.

“It was a good exercise for the team to go through,” said Kuhar. “We all have species we know we’re investing resources in. The question is are you producing at the level you think you are? I don’t think an absolute number has a lot of value because there are so many complicating factors, but it’s good to do a gut check—‘are you doing what you want to be doing?’”

Kuhar thinks the S-Index could be particularly helpful for new zoo directors, especially if that person is coming in from outside the animal world.

Paul Schutz, animal operations manager for Disney’s Animal Kingdom in Orlando, Fla., participated in Lynch’s initial study and said he found the information valuable because it goes deeper than simply, say, how many chicks have hatched, “but did that contribute to the overall population in AZA?”

A number of those who have completed the S-index also say that it is useful in highlighting how a curator’s own interests are affecting a collection.

That’s not necessarily bad, New said, but “we know that if someone at a facility has a strong interest in a species and the expertise to make it happen, you’ll see an upsurge of that species at a national level. If that person leaves or dies, the expertise is gone. We have to normalize and standardize against individual interests and preferences.”

Unfortunately, with many zoos reeling from the pandemic, most work with the S-Index has been put on hold, but some, like Kuhar, say they hope to revisit it in the next year or so.

And what about Snyder’s perching birds? Why were they dying off? After a fair amount of research, he focused his efforts on nutrition, and as he dug deeper, it turned out other programs at other zoos were dealing with the same problem.

For Snyder, the lesson is clear. “I think it’s negligent if we don’t use the S-Index,” he said. “If we don’t use it as one of the tools for managing our collections, we may be missing some key information to help us improve how we take care of our animals.”

Zoos and aquariums interested in analyzing their collection with the S-Index can contact Megan Brown at for further information.

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

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