True nature: revising ideas on what is pristine and wild

Are there any pristine ecosystems out there? The evidence is growing that our ideas about virgin nature are often faulty. In fact, the lush rainforest or wind-blown moorland we think is natural may be a human creation, with alien creatures from distant lands living beside native species. Realizing this will change our ideas about how ecosystems work and how we should do conservation.

We like to think that most nature was pristine and largely untouched until recent times. But two major studies in recent weeks say we are deluded. In one, Erle Ellis, a geographer at the University of Maryland Baltimore County, and colleagues have calculated that at least a fifth of the land across most of the world had been transformed by humans as early as 5,000 years ago - a proportion that past studies of historical land use had assumed was only reached in the past 100 years or so.

The human footprint was huge from the day, perhaps 60,000 years ago, when we began burning grasslands and forests for hunting, according to the Ellis study. It extended further with swidden "slash-and-burn" agriculture, and became more intense when farmers began to domesticate animals and plow the land.

This seems odd given how few we were back then - tens of millions at most - and how primitive our technology was. But, says co-author Steve Vavrus of the University of Wisconsin, "early farmers didn't need to be as efficient as modern farmers and therefore, counterintuitively, they used much more land per capita." In other words, they spread out.

In fact, they farmed large areas that today look like virgin forests. But we now know that as much as a tenth of the trees in the Amazon rainforest grow on man-made "dark earths," orterra preta, which archaeologists believe were created by pre-Columbian farmers who added organic wastes and charcoal to improve nutrient supply and boost yields.

Much of the Amazon, Ellis concludes, is actually forest regrowth. Or - judging by the profusion of fruit trees and other valuable species still growing in terra preta areas – perhaps overgrown gardens.

Other tropical rainforests also seem to have been farmed. In the past couple of years, James Fraser of Lancaster University in England has found dark earths in until-recently forested West Africa. And last year Doug Sheil and colleagues reported similar findings from Borneo. Other studies have found oil-palm nuts over wide areas of the central African jungle, suggesting the place was covered in palm-oil plantations 2,000 years ago.

Nor is this just about rainforests. The bison-grazed plains of North America were remade by Native Americans long before Europeans showed up. Many of the mist-shrouded treeless grasslands of the tropical Andes, known as the paramos, are the result of burning and grazing after locals cut down the natural forests centuries ago. In colder climes, the Scottish highlands tell a similar story.

Just as geographers and archaeologists are hard-pressed to find untouched landscapes, so biologists are having similar trouble locating pristine ecosystems.

A new book, Novel Ecosystems, edited by Richard Hobbs of the University of Western Australia and others, shows how many superficially natural ecosystems are heavily influenced by the introduction of alien species. Whether intentional or accidental, most introductions seem to have human origins.

This is disconcerting. "Over large parts of the globe, the 'wilderness' that people refer back to never existed," says one of the book's authors, Michael Perring, also of the University of Western Australia.

Nature has always had open borders for alien species on the move. Those itinerants may have been a driving force of evolution. But human activity has dramatically increased their travel options. We move many deliberately, as commercial crops or domesticated animals, for instance. Today, others can hitch a ride on ship hulls or in ballast tanks, aboard planes or on the wheels of trucks or the backs of domesticated animals. This phenomenon seems to have been going on for much longer than we sometimes imagine.