Northern Prairie Wildlife Research Center
This phenomenon was evident in early studies and observations of adult breeding waterfowl (Pirnie 1935; Ward 1942; Mendall 1948; Bue et al. 1952; Dzubin 1955; Evans and Black 1956), and in later investigations (Munro 1963; Larsson 1969; Hopper 1972; March et al. 1973; Bjork 1976; Piest 1982). Preferences of dabbling ducks (Anatinae) for wetlands with openings in the marsh canopy or for flooded emergent vegetation of a shorter type are well documented (Marshall 1952; Glover 1956; Johnsgard 1956; Smith 1968; Drewien and Springer 1969; Poston 1969b; Hines 1975; Weller 1975a; McEnroe 1976; Bishop et al. 1979). Diving ducks (Aythyinae), of course, show strong relations with open water areas (Hochbaum 1944; Siegfried 1976; Stoudt 1982).
Detailed studies have related the daily activity patterns of breeding waterfowl to the increased attractiveness of wetlands that contain an interspersion of cover and open water. Such areas may provide better food resources according to Girard (1941), McDonald (1955), Sowls (1955), Williams and Imber (1970), Courcelles and Bedard (1978), Beule (1979), Kaminski and Prince (1981b), and Murkin et al. (1982). Multiple regression analyses indicate that an increase in the ratio of open water to emergent vegetation may manifest itself in dabbling duck populations through better isolation of conspecific pairs, and may provide a cue to quality feeding habitat (Kaminski and Prince 1984).
Nest densities or hatching success may also be greater in broken versus solid stands of emergent marsh vegetation (McDonald 1955; Steel et al. 1956; Nelson and Dietz 1966; Mihelsons 1968; Ward 1968; Krapu and Duebbert 1974; Mednis 1974; Murkin 1979). The importance of openings or bare areas along shorelines for preening, resting, or waiting sites for adult waterfowl is also evident (McDonald 1955; Smith 1955; Sowls 1955; Sugden and Benson 1970; Williams and Imber 1970; Seymour 1974; Fog 1976). Partial destruction of Typha spp. stands by herbicides resulted in a 300-400% increase in adult ducks per unit of shoreline (Keith 1961).
Waterfowl broods also prefer semi-open or open emergent vegetative cover, as shown by early observations and investigations (Bennett 1938; Wellein 1942; Stoudt 1944; Evans et al. 1952; Beard 1953; Berg 1956; Evans and Black 1956; Johnsgard 1956), and later studies by Keith (1961), Trauger (1967), Williams and Imber (1970), Bengtson (1971), Stoudt (1971), Sugden (1973), Whitman (1974, 1976), Mundinger (1975), Newton and Campbell (1975), Patterson (1976), and Wheeler and March (1979).
In the single instance where more broods were observed in closed stands of vegetation, Ignatoski (1966) postulated that nest success might have been higher there or that predation might have been greater in the more open areas. Studies showing that broods of dabbling ducks prefer semi-open marsh include those of Chura (1961), Perret (1962), Parnell and Quay (1965), Quame and Grewe (1970), Thompson (1974), Hines (1975), Courcelles and Bedard (1978), Mack and Flake (1980), Godin and Joyner (1981), Ringelman and Longcore (1982), Sjoberg and Danell (1982), and Talent et al. (1982). Similar results have been reported for diving ducks (Hochbaum 1944; Hilden 1964; Lokemoen 1966; Hilliard 1974; Stoudt 1982). Use of wetlands by broods increased as the number of vegetative types at the edge of the open water zone increased (Hopper 1972).
Other relations between breeding waterfowl and the physical features of their wetland habitat have been proposed. Openings in shoreline emergent vegetation may make nest sites on nearby uplands more easily accessible to hens (Mednis 1974; Mihelsons et al. 1974). Some studies indicate that waterfowl may be less susceptible to predation in more open situations (Furniss 1938; Beard 1953; Trauger 1967; Moller 1975) or that predator pressure may be buffered from waterfowl by the presence of other forms of prey in more open areas (Weller 1979). It has also been noted that a heavy buildup of marsh vegetation can make nesting islands accessible to predators (Mihelsons 1968). Rogers (1964) postulated that predation on lesser scaup (Aythya affinis) nests may have increased in situations where females were forced to walk, rather than swim, to their nests.
Other marsh-dwelling birds and mammals may benefit greatly from the presence of openings in marsh vegetation (Beard 1953; Seabloom 1958; Weller and Spatcher 1965; Willson 1966; Orians 1972; Vogl 1973; Gorenzel et al. 1982; Nudds 1982; Stenzel 1982). Such conditions may also result in avian communities of greater species diversity or richness (Weller and Spatcher 1965; Weller and Fredrickson 1973; Weller 1975a, 1978; Harris et al. 1981).
Biologists have often attributed decreased wetland use by aquatic birds to decreases habitat heterogeneity caused by disruption (usually a reduction) of natural ecological processes, resulting in domination by tall, robust hydrophytes in such genera as Scirpus, Carex, Typha, Salix, and Phragmites (Fig. 1). In the absence of these processes, autogenic successional processes tend to build dense stands of such hydrophytes in most wetlands (Walker 1959; Jahn and Moyle 1964; Whitman 1976). Prairie wetlands are particularly susceptible to the establishment of monotypes because of low gradient shorelines, small differences in soils or organic matter content within basins, and the ability of many species to survive under a wide range of water conditions (Hammond 1961; Walker and Coupland 1968).
|Fig. 1. A prairie wetland unburned for more than 45 years; dense stands of Phragmites australis (foreground) and Typha angustfolia (background) lie offshore from the wet-meadow zone, which is dominated by a mature stand of Salix amygdaloides. The area has seldom been grazed by livestock. (Roberts Cty., South Dakota, 6 miles southwest of Rosholt; photo by H. A. Kantrud.)|
Typha spp. has spread rapidly across a major portion of the prairie pothole region. For example, Metcalf (1931) and F. M. Uhler (Patuxent Wildlife Research Center, Laurel, Maryland, personal communication, 1984) saw few Typha-dominated wetlands in North Dakota during 1917-25. Metcalf found only common cattail (T. latifolia) in North Dakota, and the species was listed only for "springy places and in the vicinity of freshwater lakes." Since then, T. angustifolia and the extremely robust T. "glauca" (a presumed T. latifolia × T. angustifolia hybrid) have become dominant in thousands of prairie wetlands whose salinity ranges from fresh through slightly brackish (Stewart and Kantrud 1971).
Typha spp. is well-adapted to form monotypes (Linde et al. 1976). Typha seeds germinate under a wide range of water depths (Weller 1975b) and tolerate a wide range of soil types (Dean 1933). Older plants prevent competition from younger plants by autotoxicity (McNaughton 1968). Because shoot death in Typha spp. occurs late in the growing season, this plant's competitive advantage over other species is probably enhanced (Davis and van der Valk 1978). A process of self-thinning allows individual, Typha plants to grow large; decomposition of these large plants may take as long as 2 years (Mason and Bryant 1975). Mechanical control of Typha spp. is difficult and expensive (Nelson and Dietz 1966; Weller 1975b).
When tall, robust emergents such as Typha spp. dominate a wetland, drastic environmental changes occur. Less insolation of marsh soils and the water column caused by tall emergents and their litter may reduce or eliminate other species of plants in the understory (Bennett 1938; Buttery and Lambert 1965; Spence and Chrystal 1970; Vogl 1973) or lower productivity (Willson 1966). Submerged plants, in particular, require water of sufficient depth to reproduce (Anderson 1978; Courcelles and Bedard 1978), and the buildup of litter and organic material from emergent species may reduce water depth or eliminate shallow water areas (Ward 1942; Walker 1959; Hammond 1961; Ward 1968; Beule 1979). Buildup of litter and the shading effect also may result in lower soil or water temperature and slower rates of plant decomposition (Willson 1966; Godshalk and Wetzel 1978). Various emergent species may decompose at different rates as the result of differences in species composition of macroinvertebrate populations (Danell and Sjoberg 1979). Thus the development of monotypic stands of emergents may effectively remove some of the variation in decomposer organisms that could act to maintain or increase vegetative heterogeneity.