Environmental Engineering Reference
In-Depth Information
ala , the hoverfly Eristalis aeneus , white-tailed bumblebee Bombus lucorum , red-shanked bumblebee B.
ruderarius and early mining bee Andrena haemorrhoa.
Most of the plants grow in deep crevices in the limestone cliffs, forming cushions of compact leaf
rosettes, although young specimens are often found in turf on thin soil around and below the primary cliff
habitat. Their density is largely determined by the availability of suitable crevice sites and is commonly 1
to5plantspersquaremetre.Thespeciescannotregeneratefromdetachedrosettes,becauseitcannotform
adventitious roots and has no dormant buds. It also appears to be difficult for a seedling to survive for
more than a few months near a well-established plant, but when an older plant dies it may be replaced by
thegrowthofnearbyseedlings survivingfromthepreviousautumn.Thelightseedsareonlyspreadafew
metres from the parent plant and wider dispersal appears difficult. Yellow whitlowgrass is, for example,
usuallyabsentfromapparentlysuitablehabitatsinformerlimestonequarrieswhichhavenotbeenworked
since the late nineteenth century, and the only walls it grows on are those of Pennard Castle.
Since 1987 whitlowgrass on the coast between Rhossili and Mewslade has been monitored on a reg-
ular basis. Its overall distribution has remained stable, but the counts have shown that there has been a
slight decline, mainly ofplants within easyreach ofsheep,orofenthusiastic gardeners andcollectors. On
Pennard Castle there has been a 25 per cent decline since the early 1980s, with plants disappearing from
the lower section of the walls. But there is no problem in growing it from seed, as it produces copious
amounts and is self-fertile. Indeed it is better to grow native whitlowgrass than the imported continental
material available from suppliers of alpine plants.
Yellow whitlowgrass is usually associated with a restricted plant community that includes three
grasses, red fescue, sheep's-fescue Festuca ovina and cock's-foot Dactylis glomerata , together with
greater knapweed Centaurea scabiosa , salad burnet, Portland spurge Euphorbia portlandica , wild carrot
Daucus carota , small scabious Scabiosa columbaria and wild thyme. This restriction to a narrow habitat
and plant community is striking in view of the wide range of habitats that occur adjacent to the current
populations.
VEGETATION MANAGEMENT
Despite the rich list of plants the first sight of the cliffs can be extremely disappointing. Grazing pressure
fromsheepalongmostofthesouthcoast,especiallythewesternend,isveryhighandasaconsequencein
placestheturfisextremelyshortandmanyoftheflowerseatenbeforetheysetseed(Fig.48).Infactatthe
western end of the coast there is almost a total lack of flowers, despite the very high number of plant spe-
cies present in the close-cropped grassland. The absence of flowers means that there are no pollen or nec-
tar sources for the invertebrates. A reduction in grazing pressure would, in theory, help the conservation
ofthe fauna enormously.Allowing higher vegetation todevelop andmore plants toflower would connect
currently isolated sites, as more mobile species, such as bees and wasps, would forage more widely and
havetheopportunitytocolonisenewnestingareas.Thereisacleargradientalongthecoast,rangingfrom
cliffs dominated by long ungrazed scrub and bracken at the eastern end to severely overgrazed cliffs at
the western end. Current sheep numbers are about three times higher than in 1866 if the data from parish
returns are to be believed. Information drawn from the Rhossili Agricultural Returns suggests that sheep
stocking levels dropped during the Second World War, but then increased rapidly to almost five times the
number of grazing livestock units by 1984. This change reflects the improvement of grassland due to ap-
plication of inorganic fertiliser.
In some areas the species composition resembles semi-improved grassland due to recreational pres-
sureandproximitytoimprovedpastures.ThisisparticularlynoticeablealongthecoastnearWormsHead,
where the effect is intensified by sheep lying up at night, which leads to localised heavy dunging in
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