Life of a River - Biology
Consequences (pg 1 of
3)
Threats to the diversity of life are due to
alterations of the flow, shape, connections, and
water quality of river systems, which result in
habitat loss or degradation. As examples, dams
block migration or completely alter natural flow
regimes; excessive sedimentation degrades habitat
quality; and straightened streams simplify
essential habitat diversity.
Plant diversity declines because of unnatural
water flows, sedimentation, and high turbidity from
human activities. Backwater habitats that no longer
are refreshed with natural periodic flooding have
declined while seasonal communities are being
damaged by artificially large flood events.
Natural succession of plant communities has also
been disrupted. Swamp white oak and river birch
were more common in the Mississippi River valley
before European settlement, but with river
alterations have seriously declined. In floodplain
forests of the Minnesota River Valley, natural
succession typically unfolds when thickets of
sandbar willow first establish on sandbars,
cottonwoods then come in, and silver maple follows
(Noble,
1979). Many of these communities have been
logged and converted to agricultural crops or
agroforestry crops, leaving only remnants of huge
silver maple canopies as evidence of their
potential.
Invasive non-native plants add insult to already
injured communities. For example, attempts to
reforest floodplain forests along the Vermillion
Bottoms have been unsuccessful due to dense stands
of reed canary grass. Reed canary grass either
directly interferes by occupying space or
indirectly prevents establishment because it
shelters mice that eat the planted seedlings. Even
native invasive plants such as prickly-ash (a shrub
that establishes on disturbed, usually overgrazed
land) can interfere with successful seedling
survival.
Pictured plants:
Floodplain forest, silver maple samaras, silver
maple female flower, silver maple male flower,
black willows, black willow (46 cm diameter),
cottonwood