72Fresh water
Fresh water covers less than one
percent of the Earth’s surface. Falling
rain flows back to the ocean in
streams and rivers, or gathers to
form ponds, lakes, and wetlands.
TAKE A LOOK: LIFE OF A RIVER
u SOURCE Rivers start
out as fast-flowing streams.■ The ecology of a river changes as it runs from the hills
to the sea. At its source, the water is too fast for plants
to take root, but invertebrates and fish thrive in the well-
oxygenated conditions. As it slows, a wider variety of
plants take root in mud brought down from the hills.
Animals make homes and hunt along its banks.u MOUTH When they reach
the sea, rivers are slow and wide.FISH HAVENS
Nearly 40 percent of fish
species live in fresh water.
Many fresh waters contain
unique species, such as this
African cichlid, because rivers
and lakes rarely connect for
species to colonize new areas.Freshwater habitats
The creatures that live in freshwater habitats face
many challenges. There might be flooding or the
water can shrink to almost nothing, silt up with
mud, or be choked by pollutants. Many animals
have adaptations that help them cope. Salmon
(right) spend part of their lives at sea and have a
special mechanism that allows them to change
from fresh to salt water and back again when they
return to their home river to breed.u POND LIFE
A sample of water
from a pond shows
how many species
live there, such as
insects, snails,
tadpoles, and
pond weed.Food chains
Food chains in rivers depend on
inputs from the land around. This
can be nutrients from farmland or
fallen leaves, which provide food
for algae and bacteria. These
are eaten by insect
larvae and snails,
which are
then eaten
by fish
and frogs.Water snailENVIRONMENTAND ECOLOGY
(c) 2012 Dorling Kindersley. All Rights Reserved.(c) 2012 Dorling Kindersley. All Rights Reserved.