In 1986 when an enormous hydroelectric dam flooded the Caroni Valley in Venezuela, leaving only the man-made Lago Guri Islands – hills surrounded by water - large, wide-ranging predators, such as jaguar, puma and harpy eagle were quickly rendered homeless, leaving a host of herbivores, big and small, to feast on the foliage and multiply to extraordinary densities. The forest never stood a chance. The islands became hostile worlds of bottom-up regulation, overrun with leaf-cutter ants, iguanas, howler monkeys and bird egg-eating capuchins. What used to be prime tropical forest was stripped of its understorey. It fought back with toxins and thickets of thorns and vines, which did their best to smother the trees and block out the sun – preventing regrowth. The pollinating, seed-dispersing bird life vanished along with many other creatures, and the islands became largely treeless. Look up ‘ecological meltdown’ and you often find your way to Lago Guri.
So, could it be that plants rely on the regulatory role of top predators? Could a similar unravelling occur elsewhere? When you start digging, similar cases pop up all over the place. The decades, sometimes centuries-long absence of large carnivores across the United States, led to unchecked browsing of plants by various deer species on a massive scale. In Utah’s Zion National Park, increasing human presence beginning in the mid-1930s, drove out healthy populations of cougar, causing an explosion in mule deer numbers. Riverside woodland regeneration dropped off a cliff. In 2006, a study comparing biodiversity levels in the Park found that where cougars were common, wildflowers were far more abundant. Cougar areas held over 100 times as many amphibians, three times as many lizards and five times as many butterflies as areas where the big cats were rare or absent altogether.
Between 1880 and 1940, a similar story played out in the Yosemite, Olympic, Yellowstone and Wind Cave National Parks, where wolves and cougars were either hunted or trapped to extinction, or displaced by human disturbance. With the carnivores gone, more herbivores came out to play, and as the ancient landscape of fear between prey and predator faded, browsing pressure grew. Soon the vegetation structure changed, and new growth couldn’t get going in the presence of so many hungry mouths. Woodlands began to age and wither as new generations of trees struggled to establish. Fewer nutrients made their way into rivers, which were more susceptible to erosion in the absence of tree roots binding the soil. The pyramid crumbled and species by species, almost imperceptibly, the once common, became rare.
Where cougars were common, wildflowers were far more abundant.