To anyone with an interest in the fate of the world’s wildlife, it’s a familiar story: Bringing back predators like wolves and wildcats is crucial to re-wilding ecosystems. The most famous example might be the return of gray wolves to Yellowstone National Park in 1995, and its ripple effect on other species, including plants. The wolves’ presence after 70 years forced elk herds in certain areas to start moving again, which then gave the willow and aspen they had over-browsed a chance to regrow. That, in turn, provided beavers with enough sticks and logs to make a comeback of their own—from eight colonies in 1953 to 100 colonies today.
It’s a tidy story of nature’s resilience. But as biologists are now learning, reintroducing top predators to ecosystems where they’ve been absent for generations can affect other species in unexpected, and perhaps unwelcome ways.
Take the Iberian pear, a small, heat- and cold-resistant tree found only in Spain, Portugal, and Morocco—a hotspot of plant biodiversity. The species grows up to 32 feet high and sports round, marble-sized fruit that are so hard when unripe, shepherds used them as slingshot projectiles. The tree is considered stable but in decline with a “severely fragmented” population due to agricultural development, according to the IUCN’s Red List of Threatened and Endangered Species.
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Another species endemic to the Iberian Peninsula, the Iberian lynx, was reintroduced to several areas in Spain and Portugal in recent years. Inspired by the Yellowstone wolf findings, Tamara Burgos, a researcher in ecology at the Rey Juan Carlos University in Madrid, was curious to find out how the presence of lynx may be affecting the pear on the Spanish side of the border. From a 2009 study by colleagues, she knew that the trees depend on small mammals like foxes and badgers to spread their seeds. The animals consume the ripe fruit and later excrete the seeds, often some distance away, spreading the pear to new sites. Burgos and her co-investigators set up camera trapsin Sierra de Andújar Natural Park, one of the largest remaining refuges for Iberian lynx. They placed some cameras in the wildcat’s territory and some outside it to see if the lynx’s presence might be influencing where, when, and how red foxes, Eurasian badgers, and stone martens forage for Iberian pear fruit in the area.
The team placed Iberian pears beneath fruiting trees and, using the images from the cameras, kept an eye on the sites to see how often these important seed-spreaders visited them, how much fruit they ate, and how long they spent foraging. They found that within lynx territory, the red fox visited the sites less often, ate less fruit, and foraged less efficiently. The stone martens didn’t turn up at the sites at all, while badgers seemed unperturbed. The pear sites outside lynx territory, on the other hand, were far more popular: Across the entire study area, 70 percent of fox visits and 100 percent of stone marten visits to pear trees occurred beyond the lynx’s range.
Burgos and her collaborators concluded that the lynx has kept these frugivorous carnivores on the move and is likely affecting how they disperse pear seeds.
“We got really interesting results,” Burgos says. “We don’t think carnivores have any impact on plants because they prey on animals. But in many ecosystems, like this one, they are super important for plants.” The next step is to figure out exactly how the presence of lynx is affecting pear distribution—a dynamic that could have implications for Iberian pear recovery efforts.
While the study sites were in a lynx stronghold where the predator didn’t need to be reintroduced, Burgos says the seed-spreaders’ behaviors suggest that there could be a similar effect in areas where the wildcats have returned.
Lynx habitat on the Iberian peninsula mainly overlaps with private lands, so Burgos had to use some unusual tactics to gain access to her study sites. “We had to reach a deal with them, because they had to allow us to work inside their properties,” she says of the farmers and other landowners in the area. “That was the hardest part for this project. There were many dinners, many meetings in bars. It took one year of work, building these relationships.”
Bill Ripple, an ecologist at Oregon State University, calls the work enabled by those deals an “innovative” study and encourages more experts in the field to explore these kinds of effects. Still, as the co-author of a 2012 study on the “passive restoration” of Yellowstone ecosystems after the reintroduction of gray wolves, he cautions that other factors may have also influenced foraging behavior in lynx territory, such as human activity or the presence of wolves, which also live in the park.
“This has really piqued my interest,” he says. “But at the same time, I want to emphasize that nature can be complex. We know so little; we have to be humble.”
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In the past, ecologists and biologists have primarily focused on how the return of predators affects prey numbers. Now they’re beginning to realize that their influence on behavior merits just as much attention, Ripple adds. “The behavioral impact needs to be considered as much or even more so than the mortality effects [on prey]. And this study is definitely looking at the ‘landscape of fear’ and those effects.”
Burgos says she plans to continue studying how the comeback of the lynx is affecting the pear, stone martens, and other species. And she hopes the fruit of her research will inspire other ecologists to explore the complicated and sometimes unexpected effects of returning long-absent predators to their native habitats. Understanding how bringing back one species can influence others can also help wildlife managers improve reintroduction programs, she explains.
“The lynx is a super charismatic species,” she says, “but we need to think about the rest of the species in the ecosystem.”
