ABOVE: The female jaguar Isis is part of a breeding project in the Iberá wetlands in Argentina. Her offspring will be reintroduced into the wild. MATÍAS REBAK
Around the world, predators have powerful, yet diverse and nuanced effects on the living world around them. These effects often manifest as trophic cascades, whereby predators indirectly influence the abundance of plants or other organisms at the bottom of the food web, either by killing their herbivore prey or changing prey behavior.
So far, much knowledge about the role of apex predators is based on correlational evidence, often based on the effects of their population declines. But researchers around the world are using a number of experimental approaches to produce stronger evidence, and in some cases, are actively tracking the effects of predator reintroductions to secure a better understanding of the animals’ ecological roles.
How wolves changed Yellowstone
The reintroduction of more than 40 gray wolves (Canis lupus) to Yellowstone National Park in the 1990s has become the best-known predator reintroduction, yet researchers still debate the wild canid’s precise ecological role.
Early studies from Yellowstone spearheaded by Oregon State University ecologists William Ripple (pictured) and Robert Beschta show that aspen trees are taller in streamside areas frequented by wolves, suggesting that the predators are indirectly boosting tree growth by deterring vegetation-munching elk from visiting those areas.
More-recent research by Colorado State University and others involving measurements and experimentally constructed dams suggests that some tree species in Yellowstone, such as willows that grow on streambanks, haven’t regenerated following wolf reintroduction.
DAVID COOPER, COLORADO STATE UNIVERSITY TOM HOBBS, COLORADO STATE UNIVERSITY
Returning jaguars to the world’s second-largest wetland
In the next couple of months, conservationists at Argentina’s Iberá wetlands hope to release five jaguars, including the two-year old Mbareté (pictured).
Scientists are poised to monitor changes both in abundance and behavior of large rodents known as capybaras (Hydrochoerus hydrochaeris) after the jaguars return. If capybaras spend more time on the lookout for their predators, they could spend less time eating, affecting the growth of local grasses.
RAFAEL ABUíN CONSTANZA PASIAN
Tracking wild dogs in Gorongosa
In Mozambique, ecologists are monitoring the ecological impact of reintroducing wild dogs (Lycaon pictus) into Gorongosa National Park in an extensive project involving GPS collars and scat analyses for predator and prey, as well as satellite imagery.
In studies involving GPS-collared antelopes such as bushbuck (Tragelaphus sylvaticus), ecologists have found that the loss of wild dogs and other apex carnivores during Mozambique’s civil war have altered the herbivores’ behavior, making them less vigilant of their predators.
In Argentina’s San Guillermo National Park in the Andean mountains, ecologists have found that vicuña (Vicugna vicugna), a relative of the llama, are more vigilant in areas where pumas (Felis concolor) are a greater threat, affecting the local vegetation.
MARCO ESCUDERO JOE RIIS
Ecologists were able to demonstrate the trophic cascade by constructing exclosures to experimentally exclude vicuñas from particular patches of ground. In the relatively flat, sparsely vegetated areas of the park where pumas are easily spotted by vicuñas and less likely to hunt there, grass flourished inside the exclosures, demonstrating that those areas were subject to heavy grazing. This was not the case in meadows with tall grass and rocky canyons where pumas could hunt more efficiently.
Fishing for trophic cascades in the ocean
In Shark Bay, Australia, researchers used exclosures to help demonstrate that dugongs (Dugong dugon) and sea turtles (Chelonia mydas) graze heavily in areas that aren’t patrolled by tiger sharks (Galeocerdo cuvier).
SHARK BAY ECOSYSTEM RESEARCH PROJECT
Research is also underway to characterize a long-held hypothesis that “grazing halos,” white barren rings such as these spots visible in the Red Sea, result from the eating activity of reef fish, which don’t venture far from their rocky refuge due to the risk of predation.
Using sound to scare prey
Some ecologists study the behavioral influence of predators on their prey by playing the carnivores’ vocalizations. Automated behavioral response systems such as the one pictured couple playback devices with a camera trap to document the prey animal’s response to hearing a predator.
LIANA ZANETTE, WESTERN UNIVERSITY
One study demonstrated that when racoons heard the sounds of barking dogs, they foraged fewer crabs and other seafood, showing that trophic cascades can be triggered through fear of predators alone.
LIANA ZANETTE AND JUSTIN SURACI
Not only do large apex predators, such as mountain lions (another name for pumas, (Felis concolor) instill fear in others, they themselves are afraid of humans, suggesting that Homo sapiens is the ultimate predator.