Temperate woodlands and grasslands are characteristic of middle latitude areas, where warm subtropical and cool subpolar air masses meet. This boundary is not fixed but moves north and south with the seasons and varies a great deal according to the geography and relief of the region. In the lower temperate latitudes, the western edges of the continents tend to have hot, dry summers and mild, damp winters, while the eastern edges are warm and humid all the year round with frequent summer thunderstorms. In higher latitudes the cool subpolar air masses are the more dominant influence and the general eastward movement of the air brings rain to the western margins, giving damp, humid conditions in both summer and winter.
The typical vegetation in humid areas is deciduous forest, but, in places where the rainfall is high and there is little difference between summer and winter temperatures, evergreen forests of both coniferous and broadleaved trees are found. Most of the tree species present are influenced by soil type and local relief. Pines are found on gravelly soils and rock outcrops, and alders and willows on waterlogged soil by rivers and streams - but the main types of tree are oak, ash, maple and beech. The characteristic feature of deciduous woodland is the difference between its summer and winter aspects. In the summer the leaves form an almost continuous canopy and little direct sunlight reaches the ground. After the annual shedding of leaves the trees stand stark and naked against the wintry skies and the inhabitants are faced with new conditions of lighting and cover as well as of temperature and precipitation.
They react to this in many ways, including hibernation and migration. The discarded leaf matter forms a thick, rich soil and contains three sources of plant nutrients - rotting plant material, humus and clay minerals. The humus slowly releases nutrients into the soil and also traps essential minerals such as nitrates and phosphates. The clay minerals store potassium, sodium and calcium (important raw materials necessary for photosynthesis).
In areas of seasonal rainfall where the total precipitation is between 25 and 75 centimeters, grass forms the dominant vegetation. Although all grassland areas have an annual period of drought lasting several weeks or months when the surface soil dries out completely, their fundamental characteristic is the total absence of moisture at depth in the soil. The lack of water at this level does not impede the growth of grass, which is shallow rooted, but prevents trees, which have deep roots, from establishing themselves.
The temperate woodlands and grasslands probably represent the habitats that suffered most during the reign of humanity some 50 million years ago. Humans cut down the forests to supply fuel and to provide space for agriculture and settlement. They ploughed large tracts of grassland to plant cereals and created wide expanses of pasture land for grazing domesticated nonhuman animals. These disturbed areas did not revert to their natural state until a long time after humankind's disappearance. This interference caused the extinction of a great number of animal genera native to the original habitats. However, some creatures did survive, and it was from these that the animals of today's temperate woodland areas are descended.
The undergrowth of a temperate wood, thick with humus and leaf litter and added to annually by the autumnal shedding of deciduous leaves, provides a rich source of nourishment and shelter for all sorts of animals. The primary consumers of this material are bacteria and invertebrates, such as earthworms and slugs, which in turn provide food for many mammals and birds. The insectivores are therefore well represented in this habitat, not only in their primitive role of small insect-eater but also in a number of varieties that have adopted a more advanced predatory, carnivorous mode of life.
Plant-eating mammals abound in the trees of the deciduous forests, eating shoots and leaf buds in the spring and fruits and nuts in the autumn.
It is really the birds that are the masters of the trees. After the great non-avian dinosaurs became extinct, over a hundred million years ago, the birds expanded into an enormous number of species. Being primarily designed for flying, birds had access to the treetops in a way that few other animals had, and finding that they were safer there than on the ground they soon became perfectly adapted to this new habitat. As a result many woodland birds have developed feet with curved opposable toes that are ideal for gripping branches.
As night falls in the temperate woodland, the sleeping animals of the day are replaced by a completely new set of creatures. Nocturnal birds, bats and insects - a whole array of creatures is found that are as diverse and numerous as those of the daytime. As dusk falls and the moths and night-active flies take to the air the insectivorous bats appear to feed on them. Bats have proved so successful in their shape and life style that in most parts of the world they have remained remarkably stable in shape and form ever since they first appeared over a hundred million years ago. Save for the development of a more sophisticated echolocation system, positioned at the front of the face and the absence of eyes in many species, little else has changed.
In temperate latitudes wetland areas are comparatively isolated pockets of land found scattered widely across the Northern Continent, As well as strictly water habitats such as ponds, lakes and rivers, they also include stretches of saltmarsh and fenland found near the sea, mires and peat bogs found in poorly drained inland regions and areas of regular inundation.
The conditions found throughout this range of habitats is so diverse in terms of salinity, oxygenation, light penetration and water currents that very nearly every individual location has its own little ecosystem and associated fauna, and almost every animal group is represented.