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Report on adder and slow-worm status:

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Jim Foster
Joined: 24 Jul 2003
No. of posts: 19

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Posted: 25 Feb 2004

English Nature has just published a report summarising last year's survey of adder and slow-worm status in England, undertaken by Froglife. You can get free hard copies of this report from English Nature's enquiry service (tel 01733 455100/ 01 /02), and it can be dowloaded as a PDF from our website at:



The full reference is: Baker, J, Suckling, J & Carey, R (2004) Status of the adder Vipera berus and the slow-worm Anguis fragilis in England. English Nature Research Report no. 546.

NOTE: All those who contributed to the survey will be sent a hard copy in the post over the next few days.


For your information, the summary and conclusions of the report follow:


Concern about the status of two widespread reptiles, the adder Vipera berus and slow-worm Anguis fragilis, prompted an investigation of their status in England.  A questionnaire survey was widely circulated to gather data, ideally long-term, pertinent to sites known well to observers.  Responses were received for 249 sites.  More than half of the sites were nature reserves or other similarly designated sites.  Most of the population information reported (68 per cent of sites) was based on non-systematic surveys.  However, although non-systematically collected data tended to result in a greater proportion of populations not being scored for size (in the case of the adder) and status (for both the adder and slow-worm), where size and status were estimated, the data did not differ from those collected by systematic techniques, except in the case of the slow-worm for which non-systematic techniques yielded a smaller proportion of population decreases.  A great deal of information was based on long-term knowledge of sites; more than a quarter of the sites had been known to the reporters for more than 15 years and almost half of them had been visited on more than 50 occasions.


Although many populations were regarded as being stable, there is evidence of declines in status nationally in adders but not slow-worms.  The Midlands is a region of particular concern as the adder is in greater decline here than elsewhere and, to a lesser extent, slow-worm population declines are also evident.  Many of the populations reported on were relatively small.  A third of adder and almost a quarter of slow-worm populations were reported to consist of fewer than 10 adults.  There were more decreases and fewer stable adder populations among small (fewer than 10 adults) populations, while the converse was true for the largest populations (more than 50 adults).


Habitat management was the factor most frequently regarded as affecting adder and slow-worm populations.  In spite of reports of individual sites being harmed, habitat management or creation was regarded as a positive factor at more than 40 per cent of adder and more than 50 per cent of slow-worm sites.  The most frequently reported negative factor was public pressure (disturbance), affecting both species.  Persecution was also reported to negatively affect adder populations, whereas building development and predation adversely affected slow-worms.


Just over one third of all sites were isolated.  On isolated sites adders showed more population decreases and fewer stable populations.  There was no detectable effect of site isolation for slow-worms.  Site size also seemed to have some effects on population status.  In both the adder and slow-worm, population decreases were more frequent on small (up to 5ha) sites, and in the adder population stability was more frequent on large (more than 5ha) rather than small sites.


Although this report gathers and quantifies otherwise disparate information about adder and slow-worm populations and the sites that they inhabit, it does not provide a fully representative picture of national status.  The information in this report is biased towards sites with protected status or those that are managed for nature conservation.  The status of both adders and slow-worms on such sites was found to be more favourable than on non-designated sites.  Hence, the true national status of adders and slow-worms may be even less favourable than the reported information suggests. 


Relatively few data pertinent to brownfield sites a key habitat for slow-worms were received during the current study.  Hence, the questionnaire was inconclusive with regard to slow-worm status on these potentially threatened sites.


Further conservation measures for adders and greater research into slow-worm status, particularly in brownfield habitats, is recommended.




Jim Foster. Reptile & amphibian specialist, Natural England.

- Report on adder and slow-worm status

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