Donalt

Imagine a European Serengeti full of the animals that used to live there:hippos, rhinos, elephants, hyenas and lions.What rewilding reintroduces,alongside the missing animals and plants,is that rare species called hope.It tells us that ecological change need not always proceed in the same direction. The silent spring could be followed by a wild summer.

Rewilding is about bringing back the species that drive dynamic processes and then letting nature take its course. But it's essential that rewilding must never be used as an excuse to push people off the land. It should happen only with the consent and enthusiasm of the people who work there. Imagine standing on a cliff in England,watching sperm whales attacking shoals of herring as they did within sight of the shore until the 18th century.By creating marine reserves in which no commerical fishing takes place, that can happen again.

They tell us that ecosystems that have lost just one or two species of large animals can behave in radically different ways from those that retain them.All over the world, new movements are trying to catalyze the restoration of nature in a process called rewilding. This means undoing some of the damage we've caused,reestablishing species which have been driven out,and then stepping back.There is no attempt to create an ideal ecosystem, to produce a heath, a rainforest or a coral reef.

which also ate the berries on the returning shrubs.Bison numbers rose as they browsed the revitalized forests.The wolves changed almost everything.This is an example of a trophic cascade,a change at the top of the food chain that tumbles all the way to the bottom,affecting every level.The discovery of widespread trophic cascades may be one of the most exciting scientific findings of the past half century.

Feedback processes might seem counterintuitive because many of us are used to more predictable linear scenarios of cause and effect. For instance, it seems simple enough that spraying pesticides would help plants grow by killing pest insects, but it may trigger a host of other unexpected reactions. For example, if spraying pushes down the insect population, its predators will have less food. As their population dips, the reduced predation would allow the insect population to rise, counteracting the effects of our pesticides.

The height of some trees quintupled in just six years.As forests returned, so did songbirds.Beavers, which eat trees, multiplied in the rivers, and their dams provided homes for otters, muskrats, ducks, frogs and fish.The wolves killed coyotes, allowing rabbits and mice to increase, providing more food for hawks, weasels,foxes and badgers.Bald eagles and ravens fed on the carrion that the wolves abandoned.So did bears,

So even if we can't use DNA to bring back ground sloths and giant armadillos,we can restore bears, wolves, pumas lynx, moose and bison to the places where they used to live.Some of these animals can reshape their surroundings,creating conditions that allow other species to thrive.When wolves were reintroduced to the Yellowstone National Park in 1995,they quickly transformed the ecosystem.Where they reduced the numbers of overpopulated deer, vegetation began to recover.

How? By making use of abandoned farms.As the market for food is globalized,infertile land becomes uncompetitive.Farmers in barren places can't compete with people growing crops on better land elsewhere.As a result, farming has started to retreat from many regions, and trees have started to return.One estimate claims that two-thirds of land in the US that was once forested but was cleared for farming has become forested again.Another estimate suggests that by 2030,an area in Europe the size of Poland will be vaccated by farmers.

The American pronghorn can run so fast because it evolved to escape the American cheetah.The surviving animals live in ghost ecosystems adapted to threats from species that no longer exist.Today, it may be possible to resurrect those ghosts,to bring back lost species using genetic material.For instance, there's been research in to cloning woolly mammoths from frozen remains. But even if it's not possible, we can still restore many of the ecosystems the world has lost.

We all know about the dinosaurs that once roamed the planet,but long after they went extinct,great beasts we call the megafauna lived on every continent.In the Americas, ground sloths the size of elephants pulled down trees with their claws.Saber-toothed cats the size of brown bears hunted in packs,but they were no match for short-faced bears,which stood thirteen feet on their hind legs,and are likely to have driven these cats away from their prey.

There were armadillos as big as small cars,an eight foot beaver,and a bird with a 26 foot wingspan.Almost everywhere, the world's megafauna were driven to extinction, often by human hunters.Some species still survive in parts of Africa and Asia.In other places, you can still see the legacy of these great beasts.Most trees are able to resprout where their trunk is broken to withstand the loss of much of their bark and to survive splitting, twisting and trampling, partly because they evolved to survive attacks by elephants.

Despite their stabilizing effects, many of these habitats and their ecosystems develop and change over time, as do the harmonies they create. Deforestation may turn lush tropics into a barren patch, like a successful ensemble breaking up after losing its star performers. But an abandoned patch of farmland may also become a forest over time, like a garage band growing into a magnificent orchestra.

Ocean environments dominated by predator-prey interactions, and strong negative and positive loops stabilized by self-damping feedback, are powerful and loud, with many oscillations. Desert ecosystems, where the turn over of biomass is slow, and the weak feedbacks loops through dead matter are more like a constant drone. And the tropical rainforest, with its great diversity of species, high nutrient turnover, and strong feedbacks among both living and dead matter, is like a lush panoply of sounds.

A food web containing twenty populations can generate thousands of loops of up to twenty links in length. But instead of forming a disordered cacophany, feedback loops in ecological systems play together, creating regular patterns just like multiple instruments, coming together to create a complex but harmonious piece of music. Wide-ranging negative feedbacks keep the positive feedbacks in check, like drums maintaining a rhythm. You can look at the way a particular ecosystem functions within its unique habitat as representing its trademark sound.

Note that each feedback is the product of the links in the loop. Add one negative link and it will reverse the feedback force entirely, and one weak link will reduce the effect of the entire feedback considerably. Lose a link, and the whole loop is broken. But this is only a simple example, since natural communities consist not of separate food chains, but networks of interactions. Feedback loops will often be indirect, occurring through longer chains.

This is an example of positive feedback, an essential force in the buildup of ecosystems. But it's not called positive feedback because it's beneficial. Rather, it is positive because it amplifies a particular effect or change from previous conditions. These positive, or amplifying, loops can also be harmful, like when removing a forest makes it vulnerable to erosion, which removes organic matter and nutrients from the earth, leaving less plants to anchor the soil, and leading to more erosion.

And the natural world is full of these mechanisms formed by the links between living and nonliving things that build resilience by governing the way populations and food webs respond to events. When plants die, the dead material enriches the soil with humus, a stable mass of organic matter, providing moisture and nutrients for other plants to grow. The more plants grow and die, the more humus is produced, allowing even more plants to grow, and so on.

In contrast, negative feedback diminishes or counteracts changes in an ecosystem to maintain a more stable balance. Consider predators and their prey. When lynx eat snowshoe hares, they reduce their population, but this drop in the lynx's food source will soon cause their own population to decline, reducing the predation rate and allowing the hare population to increase again. The ongoing cycle creates an up and down wavelike pattern, maintaining a long-term equilibrium and allowing a food chain to persist over time.

Testing, testing, one, two, three. When your band is trying to perform, feedback is an annoying obstacle, but in the grand orchestra of nature, feedback is not only beneficial, it's what makes everything work. What exactly is feedback? The key element, whether in sound, the environment or social science, is a phenomenon called mutual causal interaction, where x affects y, y affects x, and so on, creating an ongoing process called a feedback loop.