State of Nature report - RSPB

EXTINCTIONS AND COLONISATIONS
Case study
Wetland colonists
Until the 1990s, the little egret was a rare vagrant to Britain,
but breeding began in 1996 and now there are more than 700
breeding pairs 5 . However, the fortunes of other wetland species
have been mixed.
The historical destruction of the East Anglian Fens and other
large marshlands caused the extinction of species such as the
marsh fleawort 6 and large copper. Intensive draining since the
1940s has seen many local populations of wetland species
disappear throughout the UK. But it’s not all bad news.
While imported species are a cause for concern, the natural
arrival of new colonists is more welcome. A surprising number
of animals that have been expanding their range in Europe
have crossed the Channel or the North Sea and established
themselves in Britain. Just as it is hard to tell how many
species have been lost from the UK, it is also difficult to
know how many have colonised. Is a species a new arrival,
or just previously overlooked? Did it arrive naturally, or was
it introduced? With species such as the tree bumblebee and
French wasp, it is hard to tell, but for species from further
afield with poor dispersal abilities, it is more obvious that they
must have hitched a ride. Only those species for which we are
able to make these distinctions appear in the figure below.
There has been a rise in the number of new wetlands,
including reservoirs, flooded quarries and sites created
especially with conservation in mind. Many wetland species are
good dispersers and have quickly taken advantage of these
new habitats. Others, such as egrets and dragonflies, seem
to be spreading as the climate changes 5,7 . Large, connected
wetlands are likely to be an important resource for new
colonists to the UK.
Case study
The pool frog: neglected native
or undesirable alien?
Number since 1970
60
50
40
30
20
10
0
Mosses
Extinctions
Re-colonists
Colonists
Flowering plants
Stoneflies
Dragonflies
Figure 26
The number of species in various groups colonising naturally,
re-colonising or becoming extinct since 1970 9–13 .
Grasshoppers & allies
Bugs & hoppers
Moths & butterflies
Bumblebees
Amphibians
Terrestrial mammals
Birds
The pool frog was found at a single site in East Anglia until
the mid-1990s, when its population declined and then went
extinct. Just at that time, researchers started questioning the
long accepted wisdom that it was an introduced species, and
realised – too late – that it had been a native species with a
common origin to pool frogs found in Scandinavia 8 .
In 2006, the species was reintroduced to a single site and there
are plans to bring the species back to other sites in its former
East Anglian range.
Some newly-arrived species seem to be responding to
changes in the climate, moving north from continental
Europe, often in combination with another factor like a
food plant being more widely grown. The more mobile
species, such as moths, dragonflies, birds and bugs,
have been able to take advantage of these new conditions.
A few mosses and liverworts, which are often thought of
as immobile, are also rapidly colonising Britain, probably
dispersed by spores on the wind.
Less mobile organisms have not been able to cross the sea
without human assistance, although many are managing
to exploit our commercial activities and transport systems.
No snails, slugs, reptiles or amphibians have colonised the UK
naturally in the last 40 years. Only one mammal, Nathusius’s
pipistrelle, has become established 14 , and perhaps only one
plant, the small-flowered tongue-orchid, has arrived naturally
(and even that is debated) 6 .
Tony Gent (ARC)
Pool frog
Fungi, through their airborne spores, are likely to spread
further than flowering plants, but we do not yet have full
knowledge of the rate at which fungi are colonising the UK.
72 STATE OF NATURE 2013