Geoscience Reference
In-Depth Information
Changing British and Irish flora
NEW DEVELOPMENT
A significant database recording changes in flora is the New Atlas of British and Irish Flora (2002) which maps the
distribution of 2,412 species in each 10 km square of the British Isles. The maps can be compared with the first atlas
of 1962, and even with some earlier records of 1930. The New Atlashighlights several key factors in causing changes
to the flora and the distribution of species:
•
Widespread adoption of herbicides has led to decline in the category of 'arable weeds', which include broad-fruited
cornsalad, corn buttercup, cornflower, corn marigold and shepherd's needle.
•
Conversion of sheep grassland to arable crops on chalk downland has led to decline in grassland species, making
rarities of carline thistle, field gentian, orchids, purple milk vetch, and slender bedstraw.
•
Increasing nitrogen pollution from cars and lorries has led to declines in plants of nutrient-poor habitats such as
bogs, heaths and limestone grasslands such as grass of Parnassus, orchids, sheep's bit scabious and sundews.
There are equivalent increases in species of nutrient-rich habitats such as cow parsley and nettles along roadside
verges.
•
Changing built environments have led to expansions in the ranges of some plants. For example, Danish scurvygrass
(Cochlearia danica) and round-leaved cranesbill (Geranium rotundifolium) were coastal species in the 1960s but
have since spread rapidly inland via salt-treated roads and motorways, and in railway ballast. The Welsh poppy
(Meconopsis cambrica) illustrates another dispersal mechanism, as a garden escape which now colonizes
roadsides and waste land.
•
Climate warming is responsible for the increase in Mediterranean species, such as Abraham-Isaac-Jacob
(Trachystemon orientalis) in southern England.
•
Alien species now account for an increasing percentage of the flora, with some, like the butterfly bush (Buddleja
davidii), being very invasive.
Overall it is surprising that only ten species have become extinct in Britain and Ireland since 1930, although the New
Atlaspoints out that threatened species are now much more dependent on protection within national and local nature
reserves, Sites of Special Scientific Interest (SSSIs) and protected agricultural areas such as Environmentally Sensitive
Areas (ESAs) and Countryside Stewardship. Orchids are especially vulnerable because they suffer from many
environmental impacts like habitat change, nutrient pollution and competition from taller plants (
Plate 28.10
).
Studies of British woodlands during 1971-2001 by English Nature (the predecessor of Natural England) and the Centre
for Ecology and Hydrology (CEH) provide a valuable database of the effects of recent environmental changes on their
biodiversity and structure (Kirby et al. 2005). Such effects are often gradual, relying on careful vegetation inventories
to be identified, but the conclusion is that the species diversity of woodland ground flora has declined significantly
over thirty years. Although some woodland flowers such as wood anemone, tutsan, herb Robert, common cow wheat
and black bryony have increased in abundance, others such as wood horsetail have declined.
In response to the question 'Why have some species declined?' there are four potential causes, namely (1) changing
woodland management, e.g. coppicing or grazing intensity, (2) air pollution, (3) successional changes over time, and
(4) climate change. It is likely that all four factors will have an influence. Regarding (3), woods become shadier as
the dominant trees increase their canopies during the succession, and heliophytes are less favoured under the lower
light conditions. This illustrates the importance of knowing the ecological status of the vegetation community in order
to make meaningful interpretations (see
Chapter 20).
Under (4), increased temperatures from January to March are
the trend, with different species responding differently. The increasing abundance of holly may, in part, be related
to milder winters, whereas increased summer drought over the next fifty years may reduce the competitiveness of
beech and primrose. It is an extremely complex calculation to predict how plants in any community will respond to
climate change, but in general larger populations are more likely to be able to adapt and survive than small ones.
This is an argument for protecting rare communities from extinction by overgrazing and excessive shade, and
encouraging them to spread throughout the countryside by providing corridors such as field margins.
