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Just read an article about RHS wanting gardeners to monitor sightings of the green cellar slug which is endangering our native yellow cellar slug, but it didn't say why the green one is so bad, and when I did a search on the RHS website nothing came up! Does anyone know the full story? Is it just that it's better that the native creatures survive?

This site - http://www.slugwatch.co.uk - has a lot about slugs but has no details on why one is better than the other.