CHAPTER 1I: APPROACH AND METHODS 45
and fewer high-quality wetlands are available to filter impurities, reduce runoff and recharge ground
water supplies. Marsh and mud beach communities were once common along the coastline and
submergent vegetation was a prominent feature of the Mississippi Sound.
Aquatic habitats have been greatly impacted by modern land use changes and channel modifications.
Land clearing has resulted in increased rates of runoff, erosion, sedimentation and elevated water
temperatures. Many stream channels have been dammed, dredged, desnagged or flanked with levees, all
of which greatly alter stream habitats and reduce the frequency and duration of floodplain interaction
with the stream. Channel modifications have caused headcutting (upstream migration of bed and bank
erosion) resulting in wider and shallower streams. A significant portion of Mississippi’s coastline has
been converted to artificial sand beach and portions of the Mississippi Sound, river mouths and bays
have been dredged for navigation channels.
Detailed descriptions of conditions are included in the description of habitat type and subtypes in
Chapter IV, Part 3.
EFFECTS OF HABITAT CONDITION ON WILDLIFE
Habitat changes generally benefit some species while harming others. Habitat generalists are better able
to tolerate changes than habitat specialists. Land use practices, such as conversion of mature forests to
row crops or plantations, fire suppression, use of pesticides and herbicides, livestock grazing and stream
channel modification have significant effects on terrestrial communities. Following disturbances,
vegetation goes through phases of succession as plant species respond differently to changes in light
exposure. Opportunistic, shade intolerant plants and plants with small or wind distributed seeds are
more likely to colonize cleared areas. Shade tolerant species are more likely to reproduce and dominate
more established stands. Birds and mammals aid in the distribution of seeds also. Animal species
composition will not only be influenced by the disturbance event, but also by the phases of plant
community succession. Effective management of disturbed and successional habitats is necessary to
minimize effects on less tolerate species while providing more benefits to those that may take advantage
of these conditions.
Many factors impact freshwater and marine communities. Changes in water chemistry, nutrient levels,
sediment load, channel shape or bank vegetation all contribute to species diversity and composition.
Changes and their effects on species may be direct, localized and apparent. However any alteration
within a watershed may have far-reaching impacts downstream, or even upstream in the case of
headcutting. Duration and extent of alterations are also important factors in determining effects.
Impacts of indirect or short-term events can be significant but sometimes may be difficult to link to the
sources.