![]() Herbivore numbers were determined by the abundance of vegetation, and predator numbers by herbivore abundance. Modern predator ecology began in principle with a simple question: “Why is the world green?” In the late 1950s, when ecologists Nelson Hairston, Frederick Smith, and Lawrence Slobodkin were pondering this question, the prevailing notion was that the abundances of animals that inhabit ecosystems depend solely on the amount of plants and nutrients at the bottom of the food web. “It’s just that we don’t really have all the data yet to understand exactly when, where, and why.” A green new world “There’s still good reason to believe that trophic cascades will. What they’re finding is that predators have powerful, yet nuanced and complex effects that ripple through food webs in what are known as trophic cascades-effects that depend not only on the nature of the hunter itself, but also on characteristics of its prey and the habitat the animals share. In search of answers, scientists are employing novel approaches to study the ecological roles of large carnivores, from the African savannah and the Andean plateau to the ocean, and to understand how ecosystems change as they are lost or reintroduced. In Colorado, for instance, conservation organizations have been using such findings to push for the approval of a bill on the November ballot that would effectively mandate wolf reintroduction in the state to restore the ecosystem’s “natural balance.” But some ecologists caution that the ecological outcomes of such projects are unclear. In the absence of firm evidence, conservationists have been eager to interpret early predator reintroduction studies-largely based on the purported regenerative ecological effects of returning gray wolves ( Canis lupus) to Yellowstone National Park in the mid-1990s-as a rationale for bringing predators back to many parts of the globe. “We only have so many natural experiments,” notes wildlife ecologist Justine Smith of the University of California, Davis. ![]() In addition, predator reintroductions such as the ongoing project in Iberá are extremely rare and have lagged greatly behind herbivore rewilding projects, in large part because releasing animals capable of killing people and livestock is so controversial. Part of the problem is, while it’s well accepted that large carnivores play vital ecological roles, just how they shape ecosystems through cascading effects in different environments isn’t well understood. ![]() MATÍAS REBAK REWILDING ARGENTINA FOUNDATION ![]() “We think that an ecosystem devoid of predators will be in better shape when the predators come back,” Donadio says, but that supposition remains largely untested. The loss of jaguars ( Panthera onca), pumas (cervu concolor), and other predators from fragments of the Venezuelan rainforest after the construction of a hydro-electric dam, for instance, is thought to be a key factor in triggering an explosive proliferation of herbivores including monkeys, which ravaged the vegetation and caused what researchers described as an “ecological meltdown.” But seldom do ecologists get to investigate whether those negative effects can be reversed by restoring predators. Scientists know that losing large predators can have far-reaching, disruptive effects on ecosystems through cascading forces that reverberate from predators at high trophic levels-the top of the food web-to their prey and beyond, even sculpting the abundance and structure of plant life. To Emiliano Donadio, the foundation’s scientific director, the release not only is crucial to rebuilding Iberá’s ancient ecosystem, but is a scientific experiment that will provide a rare glimpse of how the return of one of the world’s largest carnivores could transform an ecosystem. ![]()
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