True nature: revising ideas on what is pristine and wild

Conventionally, we regard these unwanted interlopers as a curse, destabilizing ecosystems and devouring indigenous species. Sometimes this is true, as Hobbs and his co-authors acknowledge. But they point out that, in the 21st century, aliens make up a substantial fraction of the planet's biodiversity, and many are actively useful, even essential parts of ecosystems.

Extinctions caused by new arrivals happen and can sometimes be devastating. The brown tree snake from New Guinea is eating its way through the wildlife of Guam, after arriving on a military plane. The zebra mussel, which came from the Black Sea region in the ballast water of ships, is notorious in the U.S., which returned the favor by inadvertently sending the Black Sea a jellyfish that devastated that ecosystem. But actually, such events are rare. Mostly, invaders swiftly settle down and become model eco-citizens, pollinating crops, spreading seeds, controlling predators, and providing food and habitat for native species. After a while we forget about them, or learn to love them. Where would North American be without the European honeybee?

Usually, invaded ecosystems end up with more species than they had before. Places like New Zealand, Hawaii, even the Galapagos islands - all notorious for species invasions due to human activities - are actually all more biodiverse than before. Ellis calls them "anthropogenic melting pots."

Scientists who research the invaders and their hosts are discovering much that is intriguing. British researchers recently reported finding two species of native tits that have learned to eat the larvae of a wasp that was introduced to the country from the Middle East 180 years ago and that lays its eggs on the Turkey oak, another introduced species. The tits are spending more and more time in the trees, eating the larvae, especially in spring because climate change means their young now hatch before their previous food source, leaf-eating moth caterpillars appear.

Novel ecosystems are different, but not necessarily worse. San Francisco Bay, for instance, is widely regarded as the most invaded estuary on the planet. But that didn't stop the U.S. government submitting it in January to the Ramsar Convention as a wetland of international importance, because of is a "key habitat for a broad suite of flora and fauna and a range of ecological services." Much of its rich biodioversity - and some of its ecological services - is due to its alien species.

Aliens may even contribute to rewilding those parts of the planet we no longer need. In Puerto Rico, abandoned sugarcane fields across half the island have sprouted new forest ecosystems, largely thanks to the invasive power of non-native species such as the African tulip tree, says Ariel Lugo of the International Institute of tropical Forestry. The tulip tree proved attractive to native birds and insects and now, after a few decades, native trees species have started to recover too.

The case that we have to accommodate the alien and novel when trying to conserve nature and restore ecosystems was made by Emma Marris in her 2011 book Rambunctious Garden. But the new analysis goes beyond that simple pragmatism, because it suggests we need to rethink many ideas about how nature works, too.

For instance, it calls into question the conventional view that ecosystems such as rainforests are complex machines, or super-organisms, that have emerged through a long process of co-evolution of species to fill ecological niches. But, if that is so, asks ecologist James Rosindell of Imperial College London, how come alien species are so good at invading other ecosystems, frequently becoming fully integrated neighbors?

Ecosystems begin to look a lot more accidental, random, and transient than niche theory would suggest. They are constantly being remade by fire and flood, disease, and the arrival of new species. They are a hodgepodge of native and alien species. This fits a rival model for how ecosystems work called "ecological fitting," first articulated by the legendary U.S. ecologist Daniel Janzen of the University of Pennsylvania. He said that co-evolution is a bit-part player in ecosystems; most of the time, species muddle along and fit in as best they can.

Far from reaching some equilibrium state with niches filled, ecosystems have always been in a constant state of flux, says Stephen Jackson, of the Southwest Climate Science Center in Arizona, in Novel Ecosystems. "Change, including rapid and disruptive change, is a natural feature of the world." Humans may have dramatically speeded that up, but novelty is the norm.

In that light, we need to look afresh at conservation priorities. Novel ecosystems cannot be dismissed as degraded versions of proper ecosystems, nor can alien species be demonized simply for not belonging. If novelty and change is the norm, Hobbs and colleagues ask, does it make sense for the growing business of ecosystem restoration to try and recreate static historic ecosystems? By doing that, you are not creating a functioning ecosystem; you are creating a museum exhibit that will require constant attention if it is to survive.

Conservationists, others argue, also need to be more positive about the ecological benefits of traditional farming. A widely held view is that we need more intensive industrial farming in order to provide the food the world needs while leaving land for nature. But that assumes farms can have no ecological value. Yet traditional farming methods are often better seen as novel ecosystems rich in biodiversity - havens for wildlife that is worth protecting.

Christian Kull of Monash University in Australia recently called for the protection of novel farming systems that operate within forests, such as the rubber gardens of Indonesia and the cacao farms of Cameroon, which "blur boundaries between human and natural, native and non-native, production and conservation."

The good news from all this is that nature emerges as resilient and adaptable, able to bounce back from the worst we can throw at it. And that raises a final heretical question. In an era of coming rapid climate change, if any species are going to thrive surely it will be the desperadoes, stowaways, and vagabonds that have been hitching a ride around the world with humans - species that, in some respects, closely resemble us. So if novel is the new normal, should we be encouraging their travels, rather than stopping them at the border?