The Society for Ecological Restoration was founded in California, in 1989, by a few dozen Americans and Canadians. They believed that damaged landscapes could be restored and that conservation didn’t mean just fencing off supposedly pristine sites.

But it was probably beyond even their ambitions to imagine that, a quarter of a century later, the society would attract 1,200 delegates from 54 countries, including Brazil and China, to its latest conference, in Wisconsin. They would certainly have been very happy to know that, by 2013, the UN Convention on Biological Diversity would have urged – indeed, officially “required” – the world’s government’s to restore 15 per cent of degraded ecosystems by 2020. And that the European Union would have set even more ambitious targets.

But what does restoration mean? Does it just mean “greening” clapped-out sites with any kind of plant, especially trees, as many international and local policymakers still seem to think? Or does it mean the painstaking task of restoring entire ecosystems, with their vastly complex interactions, from soil to atmosphere, and the enormous range of indigenous animal and plant communities in between, as most people at the conference would say?

The society’s members agreed a comprehensive set of definitions a decade ago, stressing the importance of at least attempting to restore each degraded site to its “historical reference” – the native biodiversity that had previously flourished there.

No sooner was the ink dry on those definitions, however, than the evidence for the acceleration of climate change and other human effects on the environment made some restoration ecologists change tack. They argue today that this target was hopelessly optimistic, even dangerously misleading.

A more realistic target, they say, is to try to manage “novel” ecosystems as they emerge under the probably unprecedented conditions of the 21st century. Some of them accept that in many cases key restoration battles – against alien invasive species, for example – will be lost.

The counterargument is that restoration techniques are improving rapidly and that more biodiverse historically-based systems are being restored.

There were debates around these issues at the conference, often leading to the conclusion that the greatest obstacle to restoration is not climate change, formidable though it is, but our continuing failure to allocate sufficient funds.

Rory Harrington, whose restoration of the Annestown catchment area, in Co Waterford, featured recently on these pages, threw down the challenge facing his colleagues in Wisconsin baldly, from a scientist’s point of view: “To understand how something works, you must build it, or rebuild it.”

Harrington has built more wetlands, more successfully, than most people. In a fascinating workshop he traded ideas with experts from around the world. But his success raises another question, which was echoed in other sessions in the conference.

Luc Gnacadja, former executive secretary of the United Nations Convention to Combat Desertification, put it like this: “Degraded landscapes are underperforming assets. Restoration should be very attractive to investors. And for governments, the cost of continuing inaction is much higher than the cost of restoring those landscapes. So why isn’t more restoration being done?”

Nobody came up with a clear answer. There was much talk of the need for “paradigm shifts” in economic thinking but little clarity about how such a shift could be achieved. Perhaps there was a clue in the adaptation one speaker made to a familiar aphorism. “Politics should not be just the art of the possible. It should be the art of changing what we think of as possible.”

Perhaps this is already happening, and more than we realise, given the international and national mandates for restoration. But if restoration is not to become just another UN buzzword, more observed on paper than in practice, like “development” and “sustainability”, a great deal needs to be done.

Costa Rica is often, with some justification, held up as an example of what a poor small country can achieve through putting conservation, and restoration, close to the top of its agenda. One of the founders of its national-park system, Alvaro Ugalde, who has worked with the biologists Daniel Janzen and Winnie Hallwachs to restore and conserve entire landscapes in his country’s Guanacaste province, spoke about some of the principles he lives and works by.

“There can be no conservation without justice,” he said. “You must be patient, you must listen to the people, you must speak their language. You must never take territory from indigenous people. But you can work with them for common aims. If you fulfill your promises they will fulfill theirs.”

Listening to people has not always been a strong point of restoration movements, especially in the US. Volunteer movements have sometimes run far ahead of public opinion, especially when working on public land.

A vision of restoring an almost extinct system of prairie savannah to the forest preserves of greater Chicago, for example, produced some suberb results in terms of restored native plants and biodiversity in the 1980s and 1990s. But it also sparked a political backlash from people who felt aggrieved that their local parks were being transformed without consultation. Restoration was banned for 10 years in many parts of Cook County as a result.

Restoration has come a long way in its first 25 years as a formal movement, but some very simple lessons seem to need to be learned again and again.

Paddy Woodworth's new book on ecological restoration projects around the world, Our Once and Future Planet: Restoring the World in the Climate Change Century, is published by University of Chicago Press