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 <strong>of</strong> other wetland species have been mixed. The historical destruction <strong>of</strong> the East Anglian Fens and other large marshlands caused the extinction <strong>of</strong> species such as the marsh fleawort 6 and large copper. Intensive draining since the 1940s has seen many local populations <strong>of</strong> wetland species disappear throughout the UK. But it’s not all bad news. While imported species are a cause for concern, the natural arrival <strong>of</strong> new colonists is more welcome. A surprising number <strong>of</strong> 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 <strong>of</strong> 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 <strong>of</strong> 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 <strong>of</strong> 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, <strong>of</strong>ten 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 <strong>of</strong> these new conditions. A few mosses and liverworts, which are <strong>of</strong>ten thought <strong>of</strong> 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 <strong>of</strong> the rate at which fungi are colonising the UK. 72 STATE OF NATURE 2013
Little egret Ernie Janes The little egret used to be a rare sight in the UK, but now there are over 700 breeding pairs.
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