It remains to be seen if the destruction wrought by the Victorians continues to be the nadir for the fauna of Scotland. In addition to other difficulties the marine environment faces, climate change is a challenge facing all of Scotland's habitats. Among the birds, ptarmigan, dotterel and snow bunting in particular may be affected as they depend on high-altitude habitats, and populations are likely to decline if warmer weather brings competitors into their restricted ranges. Mammals and other vertebrates may fare better, although localised invertebrate populations are at risk. Marine life is already being affected. Planktonic species that prefer cold water are declining and are not able support the crucial food chains on which many seabirds depend. Further evidence of problems for marine species has been provided by the St Andrews University Sea Mammal Research Unit. An analysis suggests that common seal populations in Orkney and Shetland fell by 40% from 2001 to 2006, prompting the then Scottish Executive to announce the likelihood of a new protective conservation order.