The Cascading Consequences of the Worst Disease Ever

Nature is clearly in crisis—but what do researchers do when they only have imperfect data on the extent of the losses?

Cope's vinesnake
Cope's vinesnake (Kevin Enge)

Karen Lips could hear that the frogs had gone. Since 1997, she had been working in the national park near El Copé, Panama—an area whose forests were rich in amphibians, and whose air resounded with their croaks and ribbits. But since 2004, when a deadly fungus called Bd swept through the region, that chorus has all but disappeared. “It’s pretty obvious,” says Lips, who is based at the University of Maryland. “You don’t hear them. You go out at night, and you know numbers are down.”

Bd, or Batrachochytrium dendrobatidis, is a pathogen like no other. While most diseases affect a few host species, Bd indiscriminately kills amphibians—an entire class of animals. In El Copé alone, it wiped 30 species from the region, and reduced overall amphibian numbers by more than 75 percent.

This sudden froglessness has consequences, some of which are apparent to human senses. The streams, for example, are now slippery and hard to walk through, because the rocks are overgrown by algae that was once eaten by frog tadpoles. Other changes are harder to pinpoint. Many frogs are food for snakes, and if the prey vanish, the predators should suffer. But tropical snakes are notoriously hard to find: They’re not vocal like frogs, can remain motionless for hours, and often blend into their backgrounds. Besides, many are naturally rare. “I could go to a site, and if you asked me how the snakes are doing, I wouldn’t know,” Lips says.

Still, she tried to find out. She and her team, including snake expert Julie Ray of Panama’s La MICA Biological Station, conducted more than 1,000 surveys of the El Copé region. They began in 1997, anticipating Bd’s onslaught, and for 13 years found, identified, and counted every reptile and amphibian they could. Before the epidemic, the team saw 30 snake species. Afterward, they saw just 21.

That seems like clear-cut evidence of a decline, but it’s not. Some of the snakes are so rare that during the team’s years of work, they saw 12 of the species just once. And they only saw a few of these after Bd had passed through. It’s not as if those snakes had suddenly migrated into the area; they were probably there before, and just hadn’t been spotted. But if that’s the case, how could the team possibly know if any of the trends in their data were real? If a snake was found in the pre-Bd era but not the post-Bd years, was it actually missing, or did the team just miss it?

“I don’t think I appreciated how difficult it was going to be,” Lips says. “It took a long time to find someone to help us analyze the data.”

That person was Elise Zipkin. A number-crunching ecologist at Michigan State University, Zipkin specializes in estimating how animals react to changing environments, especially when the underlying data are imperfect. (“My job is making lemonade out of lemons,” she told me.) Working with Lips, she built a mathematical model that represents the snake community in El Copé. The model assumes that different species will vary in abundance—some extremely common, some extremely rare, and many in the middle. It then predicts how likely these species are to be in a given stretch of forest at any one time, and how likely a herpetologist walking through that stretch is to actually spot them. By looking at the model’s predictions and working backwards, Zipkin could use the number of snakes that Lips saw to estimate how many snakes there actually were at the time.

Zipkin calculated that there’s an 85 percent chance that the number of snake species went down after Bd slaughtered El Copé’s frogs, and a 99 percent chance that the community became more homogenous—that is, what’s left is much the same everywhere. “That’s horrible,” says Patricia Burrowes, a herpetologist at the University of Puerto Rico. “When we homogenize a place like El Copé that’s so diverse, you end up losing amazing species—and those that are the rarest go first.”

There are other worrying signs. Of the more commonly observed species, Zipkin estimated that around half declined in number after the epidemic, a quarter increased, and a quarter were unaffected. For the six species that were seen most often, four were smaller after the epidemic, and two were bigger. “Some things do better, but most things do badly, and everything looks more similar on a regional scale,” Zipkin says. The same trends befall most ecosystems under stress.

“The picture that emerges is very concerning,” adds Ariadne Angulo, who co-chairs the amphibian specialist group at the International Union for Conservation of Nature. Humans inadvertently spread Bd around the world, and can easily do the same for other wildlife diseases. “In a complex and globalized world where disease is easily moved across different ecosystems, the potential to wreak havoc is considerable,” Angulo adds.

But the world is changing so quickly that scientists and conservationists are struggling to make sense of it. They’re having to rush species that once seemed stable into captive arks before they go extinct in the wild. They’re watching as years of work are undone by natural disasters, such as Australia’s recent bushfires, that are occurring on once-unimaginable scales. And more often than not, they have to deal with these problems using incomplete information. The natural world is in bad shape, but how bad? Which bits are most in need of help? The urgency of the world’s biodiversity crisis is growing, but the data about that crisis are as imperfect as ever.

Many disagreements have sprung from that tension between knowing that there’s a problem and not knowing its extent. Other researchers have chastised the authors of the recent study that estimated how many amphibian species have been affected by Bd for using weak evidence and poor data-handling practices that have made it hard to replicate the results. (The critique, and the authors’ response, is set to be published soon.) Another prominent study which claimed that 55 percent of the ocean is fished was slammed for looking at the seas at too low a resolution; a different team, using the same data, calculated a figure of just 4 percent. A third study which looked at changes in North American bird populations was criticized for overplaying a splashy stat—3 billion fewer birds since 1970—over subtler details, some positive and some negative. A much-hyped narrative about a looming insect apocalypse has been questioned because there’s only long-term data for a vanishingly small proportion of insect species.

Scientists, on average, tend to be cautious types. They’re less likely to cry wolf, and more likely to say that the evidence suggests that a wolf is around but we’d ideally like to see more data before coming to firm conclusions. But in many cases, the problem is not that they haven’t done the work to get data. It’s that they have no option for collecting more.

Consider El Copé. Here was a team of experts who had funding for many years of surveys, and who knew that Bd might hit their region and so could start doing censuses before that event. And yet, they could barely collect enough information on the local snakes to analyze because, well, fieldwork is hard. It is difficult enough to assess obvious animals like elephants and giraffes, let alone smaller, rarer, well-camouflaged species, like snakes. Zipkin’s solution was to offer probabilities instead of hard, media-friendly numbers. “It’s hard for us to pinpoint how many species there were before and after, and there’s a wide range of possible numbers,” Zipkin says. Instead “we can talk about the probability of decline. That’s the best we’ll ever be doing, because there’s no scenario where we could just collect more data. We now have probably the strongest evidence that we’ll ever have that there are cascading effects.”

Is that enough? Will an 85 percent chance that snakes have been hit resonate as strongly as, say, the alleged disappearance of 3 billion birds? “This is a very common problem for conservation,” adds Lips. “You know that something is wrong and you have a gut feeling that things are much reduced, but to have the robust scientific data that you need to support your claim and get policy change … that can be very difficult.”