This essay is taken from his new book Demons in Eden: The paradox of plant diversity, published this month by the University of Chicago Press (ÂŁ13.97/$25, ISBN 0226757714)

A LIGHT drizzle of honeydew moistens the air. Aphids in the lime trees overhanging our path puncture the leaves with a billion stylets and the sap surges through each miniature thief, spilling sugared water to the ground like ale from a tiny spigot. Even billions of wastrel insects cannot diminish the grandeur of this avenue of tall, arching boughs that meet to form a glowing green canopy far above our heads. Its breadth and stature suggest that the avenue leads to something grander than an ordinary farmhouse, and sure enough, at the end of the single-track road are crested iron gates, set obligingly open to receive us. Inside is Rothamsted Manor, its facade of old red bricks and mullioned windows radiating warmth in the June sunshine; the tower clock perched on the tiled roof shows it is nearly noon. This is good haymaking weather, something that cannot always be relied on in England.

The drone of a tractor can be heard nearby, a sound you might normally shun, but this particular machine is harvesting something rather special – in fact, something unique. It is the 150th annual harvest of the Park Grass experiment – an event we will be privileged to witness. We follow the gravel drive that crosses in front of the Hertfordshire manor house and head through a small spinney toward the noise of the tractor. In front of us is a large meadow divided across its width into strips, each about 20 metres wide. This is our first glimpse of one of the world’s longest-running ecological experiments .

Let’s take a closer look. Here is plot 3, a now scarce fragment of species-rich hay meadow vegetation. Back in 1856, when the experiment was started, the whole of Park Grass meadow was like this and every farm in the neighbourhood had meadows just like it. The plot is a tapestry of different textures and colours, with the warp of the fabric formed by red fescue – a stiff, wiry-looking grass that features long, graceful flower stems. Another half-dozen grasses lend subtle variations of colour and texture to the background, but the highlights are broadleaved plants like bird’sfoot trefoil, ladies’ bedstraw and fairy flax – species also found in chalk grassland.

Take a step back from plot 3 and it is easy to see, even at a casual glance, that other plots in the meadow are quite different. A few plots along to our left, for example, is an area that receives an annual dose of fertiliser containing nitrogen and other minerals. In contrast to plot 3, which has had no fertiliser for 150 years, there are very few colourful flowers, the grasses are taller, and the vegetation seems more bulky.

At the beginning of the experiment in 1856, this plot had as many species growing on it as plot 3, but the fertiliser applied to it increased the abundance of grasses at the expense of other species. The result was the now familiar story of a few demon species supplanting the rest when extra nutrients gave them the opportunity to do so. The Park Grass experiment is a microcosm of the larger world of plant communities, where we can see, played out in less than three subdivided hectares, how soil nutrients hold the ring in the battle between plants, the outcome of which determines species diversity.

These results showed that the fertiliser treatments at Park Grass had two separate kinds of negative effect on diversity. First, any fertiliser treatment that increased the total yield of hay would quite quickly lead to an exactly corresponding reduction in the number of species present in the plant community. This is because adding nutrients favours the few species best able to exploit them, and the growth made by these demons increases total yield at the expense of other species.

If the fertiliser contains nitrogen, it is a few competitive grasses that benefit, but if nitrogen is absent and only phosphorus and potassium are added, plants of the pea family take over. Either way, it is demons versus diversity. The second mechanism operates in an entirely different way and has less to do with competition than with soil chemistry. Simply put, most plants accustomed to living in neutral soils have a hard time dealing with acidity, and fewer and fewer species are able to cope as acidity increases.

Fertilisers are not the only source of nitrogen in the experiments at Rothamsted – there are inputs from the atmosphere too. It is estimated that back in 1850, each hectare of ground received about 10 kilograms of nitrogen per year, mostly from compounds dissolved in rain. This was a negligible amount, unable to compensate for the nitrogen removed in plant tissues with each annual harvest and having little effect on soil acidity. In consequence, it probably had no ecological effect on plant diversity. Over time, atmospheric nitrogen inputs have risen until today, about 45 kilograms per hectare is deposited annually – a very significant amount.

Most, if not all of the increase is from the burning of fossil fuels, which produces various oxides of nitrogen. Car exhaust gases are one source and fuel burnt to heat homes another. In summer, when people in Harpenden and other nearby towns turn off their gas-fired central heating, the nitrogen dioxide readings at Rothamsted halve, climbing steadily again as winter approaches. Then, at Christmas and New Year, there is a brief but detectable downward blip in the data as people leave their cars in the garage and give the atmosphere a festive break – a salutary reminder that our everyday activities have a measurable impact on the environment.

Atmospheric nitrogen inputs to ecosystems are now a global phenomenon, found everywhere that fossil fuels are burnt or intensive agriculture is practised. Plants now get significant amounts of usable nitrogen from the atmosphere, though not through any natural process, and these inputs are threatening the diversity of plant communities. The threat is so insidious and all-pervasive that ecologists are only just beginning to realise quite how serious it is.

Nitrogen pollution can change an ecosystem from a pristine state where nitrogen limits growth to one where phosphorus or some other nutrient becomes the limiting factor. This change can favour invaders and may be what has happened to the native grasslands of the western US and Canada where, among other alien plants, no fewer than five non-native species of knapweed belonging to the genus Centaurea dominate pastures.

The artificial fertilisation of natural habitats all over the industrialised world with atmospheric nitrogen pollution is like a massive, unplanned, and reckless experiment in plant nutrition. It has revealed that many natural plant communities are nitrogen limited. In habitats like British grasslands, being freed from nitrogen limitation has unleashed the demonic potential of native grasses, and competition from these has eroded plant diversity.

Elsewhere, particularly in the New World, nitrogen pollution has opened natural communities to invasion by non-native plants. Many plants have a habit of running amok when they are introduced to new environments. This is when some plants really earn the title of “demon”, invading and changing whole plant communities.

OUR knowledge of the natural history of China has scarcely increased
in the West since the days when 19th-century naturalists gave some of China’s
rarest creatures alien epithets, such as Lady Amherst’s pheasant, which
was called Chrysolophus amherstiae, Pere David’s deer Elaphurus davidianus
and Przewalski’s horse Equus przewalskii. Although there have been books
about particular regions of China, or particular groups of animals and plants,
this appears to be the first to attempt to deal with the big picture.

China is huge, stretching across a sixth of the world’s circumference
and spanning 50 degrees of latitude. The Natural History of China deals
with the full range of ecosystems, from the tropical rainforest of Hainan
Island and Taiwan, the alpine vegetation of Tibet to the Gobi Desert. The
authors manage remarkably well to pack all this into a book of modest length
that is profusely illustrated with colour photographs, yet remains affordable.

The four authors are from the Beijing Normal University, so this is
not merely a picturebook. It begins with two chapters on the geography of
China, which include a relief map and a map of the important vegetation
zones. The rest of the book is organised into chapters on forests, rivers,
lakes and sea coasts, mountains, grasslands, and deserts. Because not all
these categories fall easily into discrete geographical regions, I found
myself constantly referring to the map. The chapter on forests deals with
the full range from the cool-temperate forests in the mountains of the far
northeast to the tropical rainforest in the extreme south-west. Millennia
of human settlement and agriculture have removed forests from virtually
all but the mountain areas. China’s timber reserves now consist of a relatively
small region in the northeast and the forests of Tibet. The authors treat
Tibet as an integral part of China and no mention is made of the environmental
destruction that reports say timber extraction has caused in the region
since China first colonised this area.

Ninety-four per cent of China’s huge population of more than 1.1 billion
live in an area that is under half of the total land area, and which lies
east of a diagonal line from Yunan province in the southeast to Heilongjiang
in the northwest. This is also the area most tourists to China will see
on the ‘milk run’ around the cities and classic sites. The land here is
intensively cultivated and little wildlife habitat exists. Most of China’s
remaining wildlife is to be found outside the area of heavy settlement,
particularly in the mountains.

The mountain flora of China has provided European gardeners with a wealth
of plants, which receive surprisingly little mention in this book. However,
the plant statistics are impressive: Yunan is the home of 60 monotypic genera,
about half of which are found nowhere else. Representatives of more than
half the vascular plant families of the world are found in China and there
are 24 500 species of flowering plants and conifers. These include more
than 2800 species of tree, more than half of which are found on the tropical
island of Hainan.

As well as garden plants, fruit and vegetable crops and tea, China’s
natural wealth has given us some familiar animals: the goldfish, the chicken,
the feral pigeon or rock dove of Trafalgar Square and the Mandarin duck,
which lost most of its habitat in China and is now perhaps better represented
in British parks than in Chinese forests from which it came. The most famous
of endangered animals in China is, of course, the giant panda. Efforts to
preserve its few remaining populations are well known, but China also harbours
many other animals on the brink of extinction: Przewalski’s horse, the Bactrian
camel, the Chinese river dolphin and the Chinese alligator. Fewer than 40
South Chinese tigers remain. China is rich in birds, especially pheasants
and other game birds, and in cranes. Sixteen endangered pheasants and nine
of the world’s 15 species of crane are found in China.

Breeding centres for cranes have been established in Heilongjiang Province,
while other nature reserves protect endangered plants. Some of these reserves,
as well as captive-breeding programmes in zoos and botanical gardens, are
described in the final chapter of the book. One of the largest res erves
is the monsoon rainforest nature reserve of Xishuang banna in Yunan. Within
its 2000 square kilometres are found half of the plants on China’s national
list of protected species, and one-third of China’s species of birds. In
the 1970s, botanists discovered a fast-growing species of dipterocarp tree
new to science, the Chinese parashorea. It is now protected legally.

China faces national problems of soil erosion, air and water pollution,
and desertification. This book describes comprehensively the natural history
that is under threat and candidly states that, despite measures already
taken, the outstanding problems of nature conservation ‘are huge’.

Jonathan Silvertown is in the Department of Biology at the Open University,
Milton Keynes.