The feedback effect of shrubs on the tundra's permafrost is unclear. In the winter they trap more snow which insulates the permafrost from extreme cold spells, but in the summer they shade the ground from direct sunlight. The warming is likely to cause changes in the plant communities. Except for an increase in shrubs, warming may also cause a decline in cushion plants such as moss campion. Since cushion plants act as facilitator species across trophic level and fill important roles in severe environments this could cause cascading effects in the ecosystems. Rising summer temperature melts on Canada's Baffin Island have revealed moss previously covered which has not seen daylight in 44,000 years. The reduction of sea ice has boosted the productivity of phytoplankton by about twenty percent over the past thirty years. However, the effect on marine ecosystems is unclear, since the larger types of phytoplankton, which are the preferred food source of most zooplankton, do not appear to have increased as much as the smaller types. So far, Arctic phytoplankton have not had a significant impact on the global carbon cycle. In summer, the melt ponds on young and thin ice have allowed sunlight to penetrate the ice, in turn allowing phytoplankton to bloom in unexpected concentrations, although it is unknown just how long this phenomenon has been occurring. The northward shift of the subarctic climate zone is allowing animals that are adapted to that climate to move into the far north, where they are replacing species that are more adapted to a pure Arctic climate. Where the Arctic species are not being replaced outright, they are often interbreeding with their southern relations. Among slow-breeding vertebrate species, this often has the effect of reducing the genetic diversity of the genus. Another concern is the spread of infectious diseases, such as brucellosis or phocine distemper virus, to previously untouched populations. This is a particular danger among marine mammals who were previously segregated by sea ice.  3 April 2007, the National Wildlife Federation urged the United States Congress to place polar bears under the Endangered Species Act. Four months later, the United States Geological Survey completed a year-long study which concluded in part that the floating Arctic sea ice will continue its rapid shrinkage over the next 50 years, consequently wiping out much of the polar bear habitat. The bears would disappear from Alaska, but would continue to exist in the Canadian Arctic Archipelago and areas off the northern Greenland coast.