Two discarded pet turtles spotted basking in the spring sunshine near the University of East Anglia have prompted a discussion about the management of non-native species in the wild.

The red-eared sliders, small aquatic turtles closely related to terrapins, were photographed by a member of our iwitness community at the UEA Broad – part of a network of public nature trails which was opened in February.

The species is native to the southern states of America, but it has developed a population in the UK after creatures imported for the pet trade were dumped in lakes and rivers when no longer wanted by their owners.

Dr Iain Barr, of the UEA's School of Biological Sciences, published a biodiversity audit of the campus last year which recorded a staggering variety of birds, moths, mammals, amphibians and reptiles – including the two non-native invaders.

He said the reptiles had been at the lake, alongside the River Yare, for at least five years, and although they were categorised as a non-native species on the audit, there are no current plans to remove them.

'They have been flagged as a non-native species and they shouldn't be there, but you could say the same for Egyptian geese, rabbits and Spanish bluebells,' said Dr Barr. 'All sorts of life is there which shouldn't be. It is very easy to trap them and they could be removed, but they are not perceived to be a problem.

'They grow extremely slowly and they have a very slow metabolism. They are omnivores, so they eat everything from plants to slugs, snails, worms and maybe frogs.

'Some people will say that one newt eaten by a non-native species is one too many but I cannot see them having a major effect, otherwise I would have recommended we remove them. We do have a policy on non-native species, but where do you draw the line?'

Dr Barr said the two red-eared sliders, one 30cm long with a smaller 10cm companion, were 'almost certainly' not able to breed in the UK climate.

He said there were an estimated 20,000 of them in the UK, although further imports into Europe have been banned by the EU.

Their popularity has been attributed to the comic book craze of the Teenage Mutant Ninja Turtles in the 1990s, but they become more difficult to care for as they grow.

'You can buy them for maybe ten or 15 quid, they eat crickets and meal worms and they are very cute,' said Dr Barr. 'But when they get older they can take the edge of your finger off as they have very strong jaws.

'They are closely related to snapping turtles, and I think it shows. They make good pets when they are tiny but they tend to get angry when they are older.'

Author and naturalist Mark Cocker, who helped launch the UEA's nature trails, said: 'Non-native, alien species can sometimes become deeply problematic; Japanese knotweed and Himalayan balsam being classic examples.

'But this is such a hugely altered landscape that very few non-natives can cause such problems. There is often a knee-jerk reaction to them which does not always reflect the levels of threat from non-native species.

'This is really part of a much larger issue of the irresponsibility of pet owners and keepers. These reptiles are incredibly long-lived and people can just get fed up with keeping them. They think the kindest thing is not to kill them or repatriate them, so they just dump them in a lake.'