Fully 10 percent of all the present meadow area in the Rocky Mountains
is estimated to have been created by beaver, the only animal besides man
that engineers extensive changes in the environment to suit its own
needs.
When beavers dam a stream, they set in motion another form of
succession. If the resulting backwater floods a forest area, the trees
are soon killed, creating a broad opening in the forest canopy.
Water-associated plants and shrubs quickly invade the pond and
shoreline, creating favorable habitat for waterfowl, moose, blackbirds,
amphibians, wading birds, warblers, marsh hawks, and a score of other
animals.
After many years the water becomes shallow, filling in with silt and
plant debris. When the beavers abandon the site, the dam may rupture for
lack of maintenance and the pond will rapidly drain. Or it may continue
to hold, delaying for several more years its slow conversion to meadow.
Stimulated by the nutrient-rich mud, the water grasses, sedges, and
shrubs finally choke the water with their accumulating debris,
transforming the area into a bog.
Gradually the ground firms as more humus is created and more silt is
trapped. The area becomes meadow, supporting grasses, sedges, and other
flowering plants. Trees begin to reinvade the drier ground, and
eventually the meadow reverts to forest. Centuries may be required to
see this cycle through, from forest to pond, to bog, to meadow, to
forest again. At each stage many of the animal inhabitants change: the
song of the western robin and the chatter of a red squirrel in the
original, pre-beaver forest give way to the croak of a heron; the heron
is replaced by the insect-and-berry-eating cedar waxwing; the waxwing is
followed by the tree-dwelling western robin and red squirrel.