effect on the forest. Winter mortality of some insect species drastically decreased, allowing the population to build on itself in subsequent years. In some areas the effects have been severe. Spruce beetle outbreaks have wiped out up to ninety percent of the Kenai Peninsula's spruce trees; this is blamed primarily on a series of unusually warm years since 1987. In this case a native species has caused massive disturbance of habitat as a result of climate change. Warming temperatures may also allow pest species to enlarge their range, moving into habitats that were previously unsuitable. Studies of potential areas for outbreaks of bark beetles indicate that as the climate shifts, these beetles will expand to the north and to higher elevations than they have previously affected. With warmer temperatures, insect infestation will become a greater problem throughout the northern parts of the Holarctic. Another potential effect of global warming to northern ecosystems is the melting of permafrost. This can have significant effects on the plant communities that are adapted to the frozen soil, and may also have implications for further climate change. As permafrost melts, any trees growing above it may die, and the land shifts from forest to peatland. In the far north, shrubs may later take over what was formerly tundra. The precise effect depends on whether the water that was locked up is able to drain off. In either case, the habitat will undergo a shift. Melting permafrost may also accelerate climate change in the future. Within the permafrost, vast quantities of carbon are locked up. If this soil melts, the carbon may be released into the air as either carbon dioxide or methane. Both of these are greenhouse gases. Habitat fragmentation threatens a wide variety of habitats throughout the world, and the Holarctic is no exception. Fragmentation has a variety of negative effects on populations. As populations become cut off, their genetic diversity suffers and they become susceptible to sudden disasters and extinction. While the northern parts of the Holarctic represent some of the largest areas of wilderness left on Earth, the southern parts are in some places extensively developed. This realm contains most of the world's developed countries, including the United States and the nations of Western Europe. Temperate forests were the primary ecosystem in many of the most developed areas today. These lands are now used for intensive agriculture or have become urbanized. As