became source populations during interglacial periods. When the glaciers receded, plants and animals spread rapidly into the newly opened areas. Different taxa responded to these rapidly changing conditions in different ways. Tree species spread outward from refugia during interglacial periods, but in varied patterns, with different trees dominating in different periods. Insects, on the other hand, shifted their ranges with the climate, maintaining consistency in species for the most part throughout the period. Their high degree of mobility allowed them to move as the glaciers advanced or retreated, maintaining a constant habitat despite the climatic oscillations. Despite their apparent lack of mobility, plants managed to colonize new areas rapidly as well. Studies of fossil pollen indicate that trees recolonized these lands at an exponential rate. Mammals recolonized at varying rates. Brown bears, for instance, moved quickly from refugia with the receding glaciers, becoming one of the first large mammals to recolonize the land. The Last Glacial Period ended about 10,000 years ago, resulting in the present distribution of ecoregions. Another factor contributing to the continuity of Holarctic ecosystems is the movement between continents allowed by the Bering land bridge, which was exposed by the lowering of sea level due to the expansion of the ice caps. The communities found in the Palearctic and the Nearctic are different, but have many species in common. This is the result of several faunal interchanges that took place across the Bering land bridge. However, these migrations were mostly limited to large, cold-tolerant species. Today it is mainly these species which are found throughout the realm. Threats As the Holarctic is an enormous area, it is subject to environmental problems of international scale. The primary threats throughout the region result from global warming and habitat fragmentation. The former is of particular concern in the north, as these ecosystems are adapted to cold. The latter is more of a concern in the south, where development is prevalent. Global warming is a threat to all the Earth's ecosystems, but it is a more immediate threat to those found in cold climates. The communities of species found at these latitudes are adapted to the cold, so any significant warming can upset the balance. For instance, insects struggle to survive the cold winters typical of the boreal forest. Many do not make it, especially in harsh winters. However, recently the winters have grown milder, which has had a drastic