By Louisa Cheung

Meadow fescue (Festuca pratensis) has disappeared

Three generations of Charles Darwin's descendants have replicated their great grandfather's 150-year-old survey near his old home, Down House, in Kent.

Between then and now, the number of plant species in Great Pucklands meadow has declined by 15%.

Changes in farm practice are believed to be behind the decline.

Darwin's observations in the meadow contributed to the development of the natural selection theory in his seminal book On The Origin of Species.

Darwin recorded 142 plant species in the meadow, which is just behind Down House, in 1855.

"While scientists tend to focus on rare or unusual species, we have studied what is in essence a rather ordinary piece of grassland," said Johannes Vogel, keeper of botany at the Natural History Museum, one of the scientists involved in replicating the study.

"It is this ordinariness that makes it significant."

Wandering scientist

One day in June 1855, Charles Darwin set about wandering the meadow behind his home in the village of Downe.

I have just made out my first grass, hurrah! hurrah! Charles Darwin

He went through the meadow randomly and at regular intervals, recording the number and the features of different plants he found. In just a year, he built up his species list.

Though his method is very different from what botanists do today, modern botanists have followed Darwin's method in this study so they can identify what has changed during the last one and a half centuries.

"We also, as part of the activity there, put a line that went across the field and recorded what we found in particular along the line in 1m by 1m plots," Gill Stevens, head of biodiversity at the Natural History Museum in London, told the BBC News website.

'Good luck'

In 1855 there were 142 plant species in the meadow; but the modern scientists, who included several of Darwin's descendants, found the number had fallen to 119.

A new inhabitant in the field is the Beaked Hawk's-Beard

"That's not a huge decrease; it's quite remarkable really," observed Dr Stevens.

The 119 extant species include sweet vernal grass, which once moved Darwin to observe: "I have just made out my first grass, hurrah! hurrah!

"I must confess that fortune favours the bold, for, as good luck would have it, it was the easy Anthoxanthum odoratum - nevertheless it is a great discovery; I never expected to make out a grass in my life."

Many mosses and fungi are also present in Down House and the surrounding area. As a replica of Darwin's survey, scientists deliberately left them out.

"We didn't include for instance mosses," said Gill Stevens. "We actually followed Darwin's interests and it is just flowers, plants and grasses."

Remarkable task

When scientists began the study last year, they were keen to emphasise how remarkable a task it was.

Rarely are researchers able to revisit a site that has been documented in such detail a century and a half before.

Flowers and grasses were found in Great Pucklands

One message coming out of the replication is that air quality seems to have had little effect on biodiversity.

"There is now significant nitrogen deposition from the air that will involve enrichment of the ground," said Dr Stevens.

"This is one reason that we may have expected a lot bigger loss in species diversity."

One possible loss is meadow fescue (Festuca pratensis), a type of grass.

"Now that we have recorded the changes to Great Pucklands over the last 150 years, we will be working with English Heritage to recover the field's plant diversity," said Johannes Vogel.

In fact English Heritage is working to get World Heritage status for Darwin's home, gardens and meadow.

This would make it the first site to be recognised for its importance to the history of science.