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Riparian Ecosystem Creation and Restoration:
A Literature Summary

Functions

Fish and Wildlife Habitat


Wise management of remaining riparian ecosystems or replacement of these communities is extremely important because of their high value as fish and wildlife habitat. Riparian ecosystems generally are characterized by increased structural diversity of vegetation compared to surrounding plant communities and an increased edge effect for area occupied. In the arid Southwest, riparian habitats support higher species richness and densities of wildlife than any other desert habitat (Anderson and Ohmart 1984). Riparian areas of Western rangelands provide food, water, shade, and cover for fish and wildlife, and forage for both wild and domestic grazing animals, as well as provide recreational areas (U.S. General Accounting Office 1988).

Fish and Aquatic Invertebrates

The riparian zone influences several elements of fish habitat, including temperature, cover, and food (Reeves and Roelofs 1982). Loss of vegetative cover and undercut banks can decrease the amount of suitable habitat, thereby reducing stream productivity and fish carrying capacity. Streambank vegetation also can be an important source of fish food. Small fish use slower water along margins of larger streams and depend on terrestrial organisms from streamside vegetation for food because most aquatic drift organisms escape them.

There are only a few reports of attempts to create, manage, or enhance manmade backwaters or to evaluate the effectiveness of specific backwater habitat manipulations (Matter and Mannan 1988). Longevity, productivity, and habitat quality of man-made backwaters are greatly affected by the amount of protection from main river channel flooding and sedimentation, number and type of connections to the river, flushing rate, and degree of water-level fluctuation. Studies of Colorado River and Mississippi River backwaters indicate that some interconnections of backwaters and river channel are important. Direct openings to the river permit water exchange that can prevent stagnation and oxygen depletion, renew organic material and nutrients, and allow export of materials such as detritus, plankton, and aquatic invertebrates to the river. Fish are known to readily enter backwaters, especially for spawning, and the free movement of fish into and out of these areas in response to changing conditions is important for maintaining healthy populations. However, if there are numerous uncontrolled connections to the main channel, then high rates of water movement throughout the backwater will flush out nutrients and preclude development of slow-water habitat features. Numerous openings also contribute to increased water-level fluctuations, which can be detrimental to aquatic plants and animals.

Water velocity, water depth, and cover are important factors regulating stream trout populations (Burgess 1985). In general, cover increases habitat complexity, which can lead to a richer species complex. Cover provides hiding places for both adults and fry to escape predation. Its slowing effect on water velocity provides a metabolic resting place and, under some circumstances, cover provides increased substrate for food items and for egg attachment.

Severe dewatering caused by water storage or diversions for extended periods of time can be detrimental or even disastrous to biological systems of streams, particularly fisheries. Trout have been shown to regularly use cover areas, either natural or artificial, particularly during these periods (Cooper and Wesche 1976). In areas where minimum instream flow criteria cannot be met, development of riparian ecosystems may serve to enhance trout habitat, thus easing the impact of low flows, which reduce trout habitat.

Observations of the physical development of several reclaimed streams in surface-mined areas of the Midwest indicated that erosion control, which is linked with proper stream channel design and rapid revegetation of side slopes, was as critical to stream fauna recovery as stream restoration (Thompson 1984). Habitat inundated by heavy silt loads becomes useless as fish and invertebrate habitat. Spoil materials that segregate out in these reclaimed streams form many microhabitats and thus add significantly to invertebrate diversity. Lack of riparian woodlands surrounding relocations had a detrimental effect on both fish and invertebrate recovery. Many species associated with habitats made available by riparian trees were absent from the relocations.

Robertson (1988) found that the density of macroinvertebrates inhabiting a relocated channel of Sink Branch in a mined area of central Florida was identical to that of an unmined control area 2 km upstream. The experimental site was a narrow, 1-ha corridor planted with a variety of native hydric and mesic tree species. After 8 months, species richness (total number of species collected) was 20% greater in the mined channel, indicating the development of new niches. Disparity in richness value was expected to persist until planted trees on the streambank formed a canopy over the channel and shaded out the abundant aquatic vegetation that had colonized the stream. Robertson et al. (1987) found that aquatic invertebrate richness of central Florida's phosphate mined stream systems reclaimed by creation of marshy areas exceeded that of undisturbed streams, but richness of reclaimed lotic sections matched that of similar undisturbed streams after 2 years.

Birds and Mammals

Presence of riparian vegetation can substantially increase the wildlife value of created wetlands. For example, borrow pits surrounded by bottomland hardwoods along the lower Mississippi River were generally associated with a greater frequency of both bird and mammal observations compared to pits containing few hardwoods (Landin 1985; U.S. Army Engineer Waterways Experiment Station 1986). During a 3-year study of 26 pits, analysis of variance tests revealed numerous differences among pit characteristics, year, season, and mammal and bird use. Other factors contributing to greater frequency of bird and mammal use were reclaimed pits with moderately grazed or occasionally mowed understory, flooded less than 1 month yearly, at least 6 feet at tankful, at least 30 acres in size, and more than 1 mile from the river.

High foliage density and diversity in the vertical and horizontal dimension were among the variables most frequently associated with high avian densities and diversities in riparian zones along the lower Colorado River (Anderson and Ohmart 1985). Cottonwood and willow (Salix spp.) attracted, or were correlated with vegetational factors that attract, the greatest density and diversity of insectivorous birds. Doves and quail were associated with riparian shrubs. Riparian revegetation schemes should be directed toward achieving high diversity and density of woody vegetation if goals are to increase bird use of the habitat.

Riparian ecosystems not only supply breeding and foraging habitats for resident birds, but also provide productive habitats for migrants. A greater percentage of a bird's total energy may be channeled into reproduction if habitats used by migrants have provided adequate resources for physiological needs and for increasing endogenous reserves (Fredrickson and Reid 1986). Thus, revegetation schemes should incorporate habitat requirements for migrating birds as well as residents.

The study of mammalian populations in created or restored riparian ecosystems is generally lacking in the literature, and usually only occasionally referred to by noting occurrence of deer or small mammal sightings or use during routine surveys of other parameters. Anderson and Ohmart (1985) recommended creation of a riparian ecosystem that is horizontally diverse to benefit most rodent species. Habitat preferences varied considerably among rodent populations found on their study site along the lower Colorado River.


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