For many dog owners, watching your pet race around after a crow or leap joyfully into a stream is a source of great pleasure...but these natural behaviours all have an impact on the environment. Estimates of the UK dog population vary from 10 million all the way up to 13 million and the number has been rising in recent years, so their environmental paw print is growing. In this programme Tom Heap visits a nature reserve where dogs have been banned from some areas after being blamed for frightening wildlife, damaging rare habitats and adding excess nutrients to the soil via their excrement. He meets a farmer and dung beetle expert, who shows him how the drugs found in flea treatments and worming pills can leach out into nature. And what to do with all that poo - especially when it's wrapped in plastic bags? Meanwhile, across the world free-roaming dogs are having wide reaching effects. Dr Abi Vanak from the Ashoka Trust for Research in Ecology and the Environment says the 60-80 million free-roaming dogs in India are putting some native species at risk of extinction. Producer: Heather Simons