Environment, Ecology & Earth's biodiversity

[Sully]

A great article about trophic cascades and landscapes of fear, as well as the competing theories surrounding these. Below I'll post some parts I found most interesting as it's a long article. By all means read the whole thing though, these types of things are why ecology fascinates me.

Can Rewilding Large Predators Regenerate Ecosystems?

"Estes’s observations were quickly followed by a series of observations from other researchers that pointed to the existence of trophic cascades in other aquatic ecosystems, such as those in lakes and rivers. In the 1990s, Os Schmitz, an ecologist at Yale School of the Environment, discovered that a terrestrial predator, the nursery web spider Pisaurina mira, could similarly create a trophic cascade, although in this case he uncovered an entirely different mechanism. It turns out that the spiders didn’t have to kill their prey to affect the ecosystem; they just had to scare them into skipping a meal. Schmitz placed leaf-chewing grasshoppers, specifically Melanoplus femurrubrum, in a small, grass- and herb-filled enclosure, and observed that the vegetation flourished after he added nursery web hunting spiders, even without any immediate change in grasshopper numbers. Then, in a series of experiments in which Schmitz glued the spiders’ mouthparts shut so they could still instill fear but not consume their prey, he demonstrated that their mere presence was enough to allow grasses to flourish, as grasshoppers would forgo a meal to avoid becoming one. Biologists now call such fear-driven effects behaviorally mediated trophic cascades, distinguishing them from density-mediated ones that involve the predators’ consumption of their prey."

"Utah State University ecologist Dan MacNulty questions whether adult elk, which are large and thus difficult for wolves to kill, would be so afraid of wolves as to miss out on a good meal, he says. Indeed, he and his colleagues have tracked wolves and elk with radio collars and found that elk often don’t avoid areas frequented by the predators, and that the ungulates seem to be more concerned with avoiding cougars (another name for pumas, Felis concolor). To Schmitz, this makes sense, given pumas’ sit-and-wait hunting strategy. As he’s learned from comparing spiders with different hunting styles, predators that tend to ambush their prey are more likely to create behaviorally mediated cascades. The wolves at Yellowstone typically hunt by chasing elk across the landscape. Because the elk can often see them coming from a distance, Schmitz explains, there’s no point avoiding certain areas. "

"Even without reintroducing predators into the wild, researchers elsewhere are using experimental approaches to detect trophic cascades already in action. In 2008, in the rugged Andean terrain of San Guillermo National Park in Argentina, Donadio wanted to understand how pumas influenced their prey, llama relatives known as vicuñas (Vicugna vicugna). He noticed that, in open grasslands where they’d easily see a predator approaching, the vicuñas’ heads were usually buried in the grass eating, only occasionally popping up to look around. In meadows with taller grasses and canyon areas where pumas could lurk behind rocky outcroppings, on the other hand, the vicuñas spent less time eating and more time on watch. To test the effects of these behavioral differences on vegetation, Donadio constructed a number of 20-meter-by-20-meter exclosures—fenced areas intended to keep the vicuñas out, though the herbivores (as well as the pumas) could still frequent the general area. Sure enough, he observed that the growth of grass inside the exclosures in the grasslands shot up compared with grass in surrounding control plots, while grass growth in the canyon and meadow exclosures did not, suggesting that vicuñas were indeed sacrificing grazing opportunities there to avoid an ambush. This behaviorally mediated cascade is created by the complexity of the animals’ habitat, Donadio says, and in turn, it helps shape the environment. If the pumas weren’t there, “the vegetation in the canyons [and meadows] would look exactly like the vegetation in the plains.” By enhancing the diversity of habitats in San Guillermo, pumas may be creating new niches for other species, he explains, and in doing so, enhancing biodiversity."

"In several locations within Iberá’s 1.3 million–hectare protected area, biologists with specialties in entomology, ornithology, predator ecology, and animal behavior are busy characterizing various facets of the ecosystem that they suspect the jaguars might influence. Populations of oversized rodents called capybaras (Hydrochoerus hydrochaeris) might plummet after the predators’ arrival, and their behavior may radically change, PhD student Belén Avila of Argentina’s Institute of Subtropical Biology hypothesizes. Right now, the capybaras are acting fearlessly, Donadio says, even dozing on the paths cutting through the area. But once they realize there are killers lurking about, individuals are likely to become more cautious and vigilant, which means they’ll spend less time eating, possibly affecting grass abundance. The jaguars could also reduce the number of smaller predators such as pampas and crab-eating foxes, which are abundant at the moment, and in doing so protect the endangered birds that the foxes sometimes eat. As the researchers track these and other outcomes over the coming years, Donadio says, “it’s going to provide really, really good information when it comes to the importance of large predators on landscapes and biodiversity.” 

"Pringle did, however, find evidence of cascading interactions between other species in the same ecosystem. In clearings where impala (Aepyceros melampus) gather to avoid being ambushed by leopards (Panthera pardus), the researchers noticed an abundance of acacia trees (Acacia etbaica), which carry thorns to defend against intense herbivory. In other areas where the bush was thicker—areas less frequented by the impala, probably due to fear of predation—“there were a bunch of plants that actually tend to be much more palatable,” Pringle says."

"Similar studies have reported that underwater playbacks of mammal-eating killer whales (Orcinus orca), but not of local fish-eating killer whales, trigger harbor seals (Phoca vitulina) to dive to safer depths." Reply