largely determined by the terrain and vegetation of the area through which they
flow and is also determined by adjacent and overhanging vegetation, the
weathering of rocks in the area, and soil erosion. The downward flow of surface
water and groundwater from higher elevations to the sea can be separated into
three zones:
SOURCE ZONE—contains headwaters or headwater streams and often begins as
springs or snowmelt of cold, clear water with little sediment and relatively few
nutrients. The rocky channels are usually narrow, creating swift currents. The
water has relatively high oxygen levels and may include freshwater species such
as trout.
TRANSITION ZONE—contains slower, warmer, wider, and lower elevation moving
streams, which eventually join to form tributaries. The water is less clear as it
contains more sediment and nutrients with the substrate beginning to accumulate
silt. Species diversity is usually greater than in the source zone.
FLOODPLAIN ZONE—due to large amounts of sediments and nutrients, the water
is murky and warmer. Tributaries join to form rivers, which empty into oceans at
estuaries.
Terrestrial Biomes
The major terrestrial biomes that you should know for the APES test are deserts,
forests, taiga, grasslands, and tundra. Some general information on terrestrial
biomes follows below.
■ Many places on Earth share similar climatic conditions despite being
located in different areas. As a result of natural selection, comparable
ecosystems have developed in these separated areas.
■ The geographical distribution (and productivity) of the various terrestrial
biomes is controlled primarily by the climatic variables of precipitation
and temperature.
■ Most terrestrial biomes are identified by the plant life primarily found
within them.