Consider Australia. When managing our national parks it's widely assumed their ideal state would resemble what it was at the coming of the Europeans. But this is a purely arbitrary baseline. Indigenous people who lived in the landscape at that time had changed it hugely through hunting and the use of fire, and by hunting to extinction many species of large herbivores that themselves had shaped the landscape with their eating habits. So why fix the baseline at 223 years ago? Why not go back to the arrival of the people now called indigenous? Or even further, to the rise of the giant herbivores? Within that broad picture there are smaller problems, such as the dingo. It has been here a mere 4000 years, so does it really deserve a place in any effort to recreate pristine wilderness? And if we are to allow dingoes into national parks, why not indigenous hunters and gatherers too? (Or, if none can be found, other people performing the same functions?) Then there's the problem of climate. It has changed a lot over the millennia, thereby altering ecosystems. Should that be taken into account when choosing the baseline? And what do we do when climate changes in the future: do we try to preserve current - or 1788 - dispersals of flora and fauna? Or do we accept change? And if so, do we separate natural change from that induced by humans, and allow for one but not the other? A growing number of scientists around the world are thinking about such questions. Their motive is not reduced concern for the environment but a desire to engage with it perhaps more helpfully. This is controversial. Perhaps the most contested area of discussion involves introduced species. An entire body of knowledge, called invasion ecology or biology, has grown up to study introduced species, on the assumption their impact is generally malevolent. In many cases, of course, it is: consider the rabbit in Australia, or avian malaria, which wiped out half of Hawaii's bird species. Invasion biologists use the terms "alien" and "native" and sometimes resort to military language to describe the fight against foreign flora and fauna. Large amounts of money and effort are spent each year around the world on this battle.

In Nature in June, the American Mark Davis and 18 other ecologists published an article calling for a rethink. One of their examples was the major 20-year effort to eradicate the Mexican devil's claw plant from the Gregory National Park in the Northern Territory. "Is the effort worth it?" they asked. Today the plant is still found in the park, and "There is little evidence that the species ever warranted such intensive management - it does not substantially change the fundamental character of its environment by, say, reducing biodiversity or altering nutrient recycling''. More generally the authors observed, "many of the claims driving people's perception that introduced species pose an apocalyptic threat to biodiversity are not backed by data … recent analyses suggest that invaders do not represent a major extinction threat to most species in most environments - predators and pathogens on islands and in lakes being the main exception. In fact, the introduction of non-native species has almost always increased the number of species in a region." They urged conservationists and land managers to stop worrying about whether a species was alien and instead focus on whether it produces "benefits or harm to biodiversity, human health, ecological services and economies". These fighting words were criticised by many letters in a subsequent issue of Nature, one signed by 141 scientists and arguing that Davis and his fellow authors had exaggerated the scientific opposition to introduced species. Whether this is true or not, there's no doubt that in the popular media, heavily influenced as it is by the environmental movement, introduced species are nearly always portrayed as potent and dangerous.

But some writers have compared attitudes to human and non-human immigrants. The New School anthropologist Hugh Raffles wrote a widely discussed opinion piece in The New York Times in April, where after referring to hostility towards immigrants (such as himself) he went on: "[N]ativism runs deep in the United States. Just ask our non-native animals and plants: they too are commonly labelled aliens, even though they also provide significant benefits to their new home." He claimed that many biologists, like members of the Tea Party hostile to immigration, were motivated "by the fear of being swamped by aliens" and urged readers to "embrace the impurity of our cosmopolitan world". This overlooks the point that all humans are members of the same species - although Raffles might say that is being too literal. Still, the immigration theme has been picked up by others, including Davis and his co-authors: "Most human and natural communities now consist both of long-term residents and of new arrivals, and ecosystems are emerging that never existed before. It is impractical to try to restore ecosystems to some 'rightful' historical state." Another area of reconsideration is the alleged harmony of natural ecosystems. Emma Marris and others say this is not always realistic, and that ecosystems are often a result of the ancient accidents of natural history. From certain viewpoints, such as the system's health and resilience, some of the species in them may be less than ideal, and some desirable species might even be missing. I suspect that's a difficult view for many of us, implying a right to judge nature and find it wanting. New terms have been devised to describe the new ways of looking at nature. Perhaps the most important is "novel ecosystem", used for a heavy mix of introduced and native species that most environmentalists and ecologists would describe in far less flattering terms, perhaps as debased or degraded, or even as trash. Calling it a novel ecosystem makes it sound more positive, and also reminds us that most ecosystems were novel once - species move around in nature too, even if not nearly so often as they do under human influence. If novel ecosystems can be created accidentally by human activity, why not create them deliberately? One of the proponents of this is Professor Richard Hobbs, of the University of Western Australia, one of Mark Davis's co-authors in the article described above. He has been a leader in putting the argument that the desire to return ecosystems to a state of "naturalness" can sometimes, depending on how changed they are, be futile. Speaking to the Herald, Hobbs commended many restoration efforts but added, "Climate and land use change, invasive species and the like, are ongoing forces driving our ecosystems in new directions. Holding back that change will be a huge and ongoing task in many instances, and we may have to think of new ways of doing things and new goals to aim for."

He has written that such goals might include enhancing a system's health and resilience. Interventions could involve the introduction of new species to, for example, provide shade or assist in pollination. "These ideas are challenging," he said. "The key challenge is working out where standard [restoration] approaches are still likely to work, and where we need to switch to new ways of doing things." One of the most interesting developments of the reformation in environmental thinking occurs when people start to mix up some of the above ideas. The strangest example of this is known as re-wilding, as seen in a place in the Netherlands called Oostvaardersplassen. This is a 56-square kilometre nature reserve on land reclaimed from the sea, where an attempt is being made to reproduce the ecosystem of 10,000 years ago. This involves introducing contemporary species to fill ecological niches once occupied by animals now extinct. For example, the small Konik horse has been brought from Poland to stand in for the tarpan, a wild horse that lived in the area long ago. From Germany come Heck cattle, developed a century ago to resemble the auroch, the ancestor of modern cattle that became extinct in the 17th century. It's perhaps too early to say what we can make of Oostvaardersplassen, except that it's probably the strangest nature reserve in the world and very interesting. As Emma Marris asks, "Is the result a wilderness? A garden about wilderness?" Or is it something else? When the ecologist Josh Donlan proposed a somewhat similar "Pleistocene re-wilding" in America, many people told him it was preposterous. "A big criticism of this is 'You are playing god'," Donlan told Marris. "Well, I don't buy that. We are already playing god [in how we look after nature]." What is needed, he said, "is admitting to ourselves that we live in an intensely managed world."

Some critics of the new environmental thinking see it as a surrender, even as anti-environmental. But this is too easily dismissive, like those Catholic leaders of the Counter Reformation who claimed Martin Luther was working for the devil. Emma Marris quotes the ecologist and novel ecosystem student Joe Mascaro on this: "People come up to me and say, 'It sounds like you've given up.' I want to say, 'I never took up arms, my man.' This isn't about conceding defeat; it is about a new approach." Follow Environment on Twitter