"Save the aliens!" is the cry – and an unusual one too. Safeguarding Britain's flora and fauna from the ravages of mankind and "non-native invader" species has become the largely unquestioned cause célèbre of a generation.

In a new book, however, a leading historian argues this "culturally-determined" idea of native and non-native species is fundamentally flawed, and calls attempts to preserve the genetic identity of British wildlife "quasi-racist". Professor Christopher Smout, Scotland's Historiographer Royal and the founder of the Institute for Environmental History at St Andrews University, said species needing conservation should receive it regardless of "ethnicity". Those which cause problems, such as native bracken or non-native giant hogweed, should be dealt with in the same way and classed as "pests".

"The preoccupation with alien species is comparatively recent and not something which worried scientists and ecologists 50 years ago," said Professor Smout, whose book, Exploring Environmental History, is published in May. "They were concerned with pests. In recent times, the emphasis has been on the fact these pests are aliens and it has tended to a blanket condemnation to all species not classed as natives."

Download the new Independent Premium app Sharing the full story, not just the headlines

One such unfortunate is the ruddy duck, an American species accidentally released from the Slimbridge Wetland Centre in Gloucestershire – described as the "birthplace of modern conservation" – in the 1950s. The population expanded to such an extent that the bird migrated to Europe and, in Spain, started breeding with the white-headed duck, threatening the latter's status as a distinct species. The RSPB persuaded the British Government to carry out a decade-long cull of ruddy ducks.

"Conservationists are up in arms because they fear the ducks will all get turned into some kind of mish-mash," said Professor Smout. "The conservationists would say 'We're doing this because it is endangering the genetic integrity of the white-headed duck'.

"I don't think that's a scientifically valid point of view. The concern with genetic integrity seems almost quasi-racist. Our attitude towards alien species is culturally determined and sometimes you end up with rather bizarre actions by scientists."

Another case in point is the sika deer from Asia. Scientists have warned breeding with native red deer in Scotland threatened the famous "Monarch of the Glen". Professor Smout dismisses it as "no big deal", adding: "If one species can successfully interbreed with another, it might assist its survival in evolutionary terms. If it is a failure, the hybrid will die out."

Andre Farrar, of the Royal Society for the Protection of Birds, was outraged by the claim of "quasi-racism", saying the more extreme wing of the animal rights movement had suggested that conservationists who talked about alien species "are one step from Goebbels". "These are dedicated people who have given their lives to give beleaguered native fauna a chance."

Not welcome here: 'Alien' species

*Ruddy ducks were introduced from North America to Britain in the 1950s and their population grew rapidly. They have been repeatedly culled down to about 4,100 over-wintering birds to stop their migration to Spain, where interbreeding with white-headed ducks is threatening the latter's existence as a separate species.

*Reeves's pheasant is endangered, with just 2,000 left worldwide. It is native to China but there are a few wild escapees in Britain. According to the RSPB, they have never formed a sustainable population. Professor Smout argues that they should be considered for conservation projects here, despite their alien status.

*Sika deer were brought to the UK in 1860 from Asia. Scientists recently voiced concern that they are interbreeding with red deer, threatening the red deer's genetic identity.