People out on Therfield Heath in the sunshine at the weekend. Picture: Clare Swarbrick - Credit: Archant The rats migrated to the area as a result of the food waste left nearby - so Conservators have reissued a plea to those using the heath to take their rubbish home to help prevent the issue in future. They have also urged heath users to keep young children and dogs close and within eyesight to ensure that they do not come into contact with rats/rat remains in the area around the car park while the extermination takes place. Concerns over the impact on wildlife have been put to the conservators, with members of the community questioning the use of the poison on a nature reserve.