AThere was a time, not very long ago, when travelling to another country for pleasure was a privilege reserved for the wealthy. For most people throughout history, journeys were undertaken out of necessity rather than curiosity. All of that has changed. Cheaper flights, rising incomes and a flood of online information have turned international travel into an ordinary activity enjoyed by enormous numbers of people. In a typical recent year, well over a billion international trips were taken worldwide. Tourism has become one of the largest industries on the planet, and with its growth has come a set of difficult questions about its effects, both good and bad.

BThe economic benefits of tourism are considerable and easy to appreciate. The industry employs many millions of people around the world, in hotels, restaurants, transport and countless related businesses. For some countries, particularly smaller ones with few other resources, the money brought in by foreign visitors is among the most important sources of national income, paying for schools, roads and hospitals. In regions where traditional industries have declined, tourism can offer a lifeline, providing work that would otherwise not exist and encouraging investment in places that might otherwise be neglected.

CTourism is often praised for its cultural value as well. Those who travel are exposed to ways of life very different from their own, and such encounters, at their best, can foster tolerance and mutual understanding. The income that tourists provide can also help to preserve the very things they come to see: historic buildings are restored, traditional crafts and festivals are kept alive, and local languages and customs acquire a new economic value. Money spent by visitors can flow directly to the craftspeople, performers and guides who keep a tradition alive. Without the interest of outsiders, some argue, much of this heritage might quietly disappear.

DYet the sheer scale of modern tourism has created a problem that has come to be known as overtourism. When too many visitors descend on a single place, the effects can be overwhelming. Famous cities such as Venice and Barcelona, and celebrated sites such as Machu Picchu, now struggle with crowds so large that they damage the very attractions people come to admire. Streets become impassable, historic centres are given over entirely to souvenir shops, and the character that made a place special begins to erode. Perhaps most seriously, local residents can find themselves priced out of their own neighbourhoods, as housing is converted into short-term accommodation for tourists and rents rise beyond what ordinary people can pay.

EThe environmental costs of tourism are equally troubling. Air travel, on which much long-distance tourism depends, is a significant and growing source of the emissions that drive climate change. Popular natural attractions, from coral reefs to mountain trails, can be worn away or polluted by the very visitors who come to enjoy them. Fragile ecosystems are disturbed, water is consumed in vast quantities by hotels and swimming pools in regions where it is scarce, and mountains of waste are left behind. The irony is painful: tourism can destroy the natural beauty that draws people to a place in the first place.

FFaced with these pressures, destinations have begun to respond in a variety of ways. Some have introduced limits on the number of visitors allowed at any one time, or taxes designed both to raise money and to discourage excessive crowds. Others have tried to spread visitors more evenly, promoting lesser-known regions or encouraging travel outside the busiest seasons. Restrictions on large cruise ships, whose passengers can flood a small port in a matter of hours, have become common. Underlying many of these efforts is the idea of sustainable tourism: the principle that travel should be managed so that it can continue indefinitely without destroying the places and communities it depends upon.

GIt would be neither realistic nor desirable to call for an end to tourism. The benefits it brings, economic and cultural alike, are too valuable, and the desire to see the world is a deeply human one. What is increasingly clear, however, is that the way tourism has developed in recent decades cannot simply continue unchecked. The challenge for governments, businesses and travellers alike is to find a balance: to preserve the genuine rewards of travel while limiting the damage it can do. The goal, in other words, is not to stop people from exploring the world, but to ensure that there is still a world worth exploring when they arrive.

AWalk through a forest and you might imagine that each tree stands alone, a solitary individual competing with its neighbours for light, water and space. For much of the history of science, this was more or less how forests were understood. In recent decades, however, a very different picture has emerged. Beneath the forest floor, the roots of trees are linked together by an immense network of fungal threads, so extensive and so interconnected that some scientists have nicknamed it the wood wide web. Through this hidden network, it is claimed, trees may exchange resources and even information. The discovery has captured the public imagination, but it has also provoked a sharp debate among biologists about how far such claims can really be pushed.

BAt the heart of the story is an ancient partnership between plants and fungi known as mycorrhiza, a word that simply means fungus root. The fungi involved grow in and around the roots of a plant and send out extremely fine threads, called hyphae, that spread through the surrounding soil. These threads greatly extend the area from which the plant can draw water and nutrients such as phosphorus and nitrogen. In return, the plant supplies the fungus with sugars produced by photosynthesis, which the fungus, unable to make its own food, cannot obtain in any other way. This relationship is extraordinarily old and extraordinarily common: the great majority of land plants depend on such fungal partners, and the arrangement may date back to the time when plants first colonised the land.

CWhat transformed this familiar biology into a sensation was the realisation that a single fungal network might connect not one plant but many. Professor Diane Falk, whose experiments have done much to popularise the idea, used carbon labelled with a traceable form of the element to show that carbon absorbed by one tree could later be detected in a neighbouring tree of an entirely different species. To Falk, this was evidence that trees linked by fungi can share resources with one another. She has gone further, describing large, old hub trees, sometimes called mother trees, that are connected to many younger plants around them and that appear to help seedlings grow by passing carbon and nutrients to them through the fungal web.

DThe implications, as Falk and others have presented them, are striking. If trees can move resources to one another, then a forest begins to look less like a battlefield of competing individuals and more like a cooperative community. Dying trees, on this account, may unload their remaining resources into the network for others to use. Some researchers have even suggested that chemical warning signals about insect attacks can travel from tree to tree through the fungal connections, allowing plants to prepare their defences before the pests arrive. In popular books and talks, this vision of a nurturing, interconnected forest has proved enormously appealing.

ENot all scientists are persuaded. Dr Nils Eriksson, who has carried out detailed reviews of the published studies, argues that the popular image of the wood wide web has run far ahead of the actual evidence. The amounts of carbon shown to pass between trees, he points out, are often very small, and it is frequently unclear whether the carbon travels through the fungal network at all, rather than simply through the soil. Nor is it always obvious which tree benefits, or whether either tree benefits rather than the fungus itself. The idea of mother trees deliberately nurturing their offspring, Eriksson maintains, is a particularly bold claim for which convincing proof is still lacking.

FOthers question the very language of the debate. Professor Wei Chen cautions that words such as sharing, communication and cooperation carry meanings drawn from human life that may badly mislead us when applied to trees and fungi. A related concern is raised by Dr Amara Okoro, who stresses that the fungus is not a passive wire connecting one tree to another but a living organism with interests of its own. Far from being a neutral channel, she argues, the fungus may take more than it gives, and in some circumstances behaves almost as a parasite. To interpret every movement of carbon as an act of generosity between trees, on this view, is to overlook the organism that actually controls the network.

GDespite these disagreements, there is a good deal that both sides accept. Few now doubt that mycorrhizal networks are real, that they are vital to the health of forests, and that they deserve far more attention than they once received. What remains genuinely uncertain is how much cooperation, if any, they involve, and settling the question will require careful field experiments rather than appealing metaphors. Professor Louisa Bianchi argues that the practical lesson is in any case clear enough: whatever the outcome of the scientific debate, old trees and the fungal networks in the soil around them are precious and easily destroyed, and forestry and conservation should protect them.