Northern Prairie Wildlife Research Center
Wigeongrass is a main source of primary production in some subtropical lagoons (Edwards 1978) and often totally dominates certain portions of estuaries where proper conditions of depth, salinity, and shelter exist (Reed 1979). Other submersed macrophytes often replace wigeongrass quickly when environmental conditions change. Wallentinus (1979) believed that the limited competitive abilty of wigeongrass, not its reaction to salinity, nutrient loadings, or other habitat characteristics, is one important reason the plant is restricted to certain shallow habitats in thalassic waters. Hammer and Heseltine (1988) thought that lack of competition allows wigeongrass to dominate more saline habitats. Husband and Hickman (1985) postulated that Ruppia may require saline water and not merely be at a competitive disadvantage in fresh water. However, recent research does not support the theory of salt enhancement of metabolic activity for submersed vascular plants (Jagels and Barnabas 1989).
In any case, wigeongrass commonly occurs in mixed stands at both ends of the salinity gradient and coexists well with some other submersed angiosperms. For example, Harrison (1982) found little effect of wigeongrass on growth of Zostera japonica, and Keddy (1987) observed that the presence or absence of wigeongrass did not affect the number of spathes produced by the annual form of Z. marina.
Wigeongrass also intersperses with small-to-medium-sized emergents, such as Cladium (sawgrass), Eleocharis (spikerush), or Juncus (rush; Joanen 1964; Joanen and Glasgow 1966; Baldwin 1968;) or taller forms such as Scirpus americanus (Smerican bulrush), S. maritimus (alkali bulrush), or Typha domingensis (southern cattail; Chapman 1960; Jefferson 1974; Zedler and Nordby 1986). Indeed, a more diverse flora occurs in coastal wigeongrass impoundments than in adjacent tidal wetlands (Kelley and Porcher 1986). Emergent, floating, and submersed plants found with Ruppia sp. in some New Zealand lakes are listed by Tanner et al. (1986). Britton and Podlejski (1981) list many emergents associated with wigeongrass in the French Camargue.
A great variety of submersed macrophytes is associated with wigeongrass around the world (Table 8). Several other submersed macrophytes not shown in this table (such as the Charophytes Nitella and Tolypella) sometimes associate with wigeongrass (Kornas et al. 1960; Verhoeven 1980a; Getsinger et al. 1982).
Although the information in Table 8 does not reflect a random sample of wigeongrass habitat, it is probable that the most important potential competitors of wigeongrass are Potamogeton pectinatus, Chara spp., and Zannichellia spp. (poolmats). These taxa also have wide global distribution. That these taxa are generally euryhaline perhaps supports the exclusion of wigeongrass from the true seagrasses, even though the plant commonly grows with at least two seagrasses, Zostera spp. and Halodule wrightii.
The distributions of the plants listed in Table 8 shows the wide environmental tolerance of wigeongrass. The plant occurs in fresh to saline coastal and interior waters on several continents. On the basis of species preference for fresh or saline wetlands, wigeongrass occurrences on the right side of table 8 probably are in saline and quite turbid waters, whereas those on the left are probably clear and fresh and neutral, or only slightly acidic, wetlands, where wigeongrass is likely a weak competitor among the specialist taxa (Pip 1984).
Verhoeven (1980a) believed that intra- and interspecific competition for space, light, and nutrients greatly alters the survival of all Ruppia taxa in thalassic waters. He stated that the ultimate success of these plants is determined primarily by the number of hibernating propagules that began spring growth, the pattern and rate of growth under prevailing conditions, and the ability to survive and adapt to temporarily unfavorable conditions.
Environmental changes during the growing season are often mentioned as factors that allow wigeongrass to coexist with other submersed plants. Pulich (1985) suggested that, even when behaving as a perennial, Texas wigeongrass would be replaced by the seagrass H. wrightii under favorable growth conditions because of the latter plant's greater belowground biomass. He later showed how organic C and N gradients, combined with seasonal temperature cycles, could control competitive interaction between the two species by controlling sulfate reduction activity in the sediments (Pulich 1989). Wigeongrass prospered in the cool spring and fall months when sediments were low in free H2S, whereas H. wrightii grew during the warm summer months when sediments often contained free H2S and high levels of NH4. These two species also coexist in a Florida bay, with H. wrightii most prominent during winter and wigeongrass most prominent when salinities fall to 13.2-14.7 g/L during July (Tabb et al. 1962). When salinities decrease to 5-10 g/L, Chara nearly eliminates the wigeongrass. Although there is some overlap, wigeongrass and Zannichellia palustris (horned poolmat) separate temporally in some North Carolina creeks (Davis et al. 1985). There, the latter grows better in late winter and spring, while wigeongrass flourishes in summer and fall. Newly flooded ditches in Utah had an initial flush of Z. palustris and Chara sp. in June, but these were replaced by wigeongrass and Potamogeton pectinatus by September (Kadlec and Smith 1984).
In North Carolina impoundments, wigeongrass shows poor growth when mixed with P. pectinatus if salinities fall below 10 g/L (Heitzman 1978). Reed (1979) saw P. perfoliatus replace wigeongrass in a North Carolina estuary as water temperatures rise during midspring and summer. Jensen (1940) believed wigeongrass could not replace P. pectinatus in bottom substrates of insufficient clay and organic matter.
Replacement of Ruppia-dominated communities by emergent communities is uncommon in highly saline habitats - wigeongrass productivity is usually low there, and organic matter accumulations are insufficient to noticeably raise bottom elevations (Verhoeven 1980a). Nevertheless, Davis (1978) noted that wigeongrass trapped silt that aided colonization of other plants in silty-bottomed, hypersaline solar evaporation ponds. In brackish waters with sandy bottoms, Dahlbeck (1945) and Gillner (1960; both in Chapman 1974) noted that a community dominated by Zostera nana, Ruppia maritima, and R. spiralis later became dominated by Eleocharis parvula (dwarf spikerush). Baldwin (1968) recommends protecting the natural Cladium-Juncus community around wigeongrass impoundments that are managed for waterfowl in the southeastern United States in order to prevent invasion by Typha domingensis. Stands of wigeongrass along the Oregon coast can be replaced by Scirpus americanus where soils are sandy and S. maritumus where soils are silty (Jefferson 1974). Probably because of shading, Juncus roemerianus (needle rush) and other emergents can lower wigeongrass production more than 50% in coastal North Carolina impoundments. Prevost (1987) lists Juncus romerianus and Spartina alterniflora (smooth cordgrass) as major invaders of such impoundments in coastal South Carolina.
Lists of algae associated with wigeongrass in North America are available for British Columbia (Carl 1937), Florida (Gidden 1965), North Carolina (Davis et al. 1985), Saskatchewan (Tones 1976), and Texas (Conover 1964b). Agardhiella, Cladophora, Enteromorpha, Gracilaria, Rhizoclonium, and Ulva are important wigeongrass associates in thalassic waters of the eastern United States (Springer and Darsie 1956; Conover 1958; Grizzell and Neely 1962; Nixon and Oviatt 1973; Harlin and Thorne-Miller 1981; Thorne-Miller et al. 1983). Several of these genera, as well as Spirogyra and Oedogonium, are serious reducers of wigeongrass production in coastal salt marsh impoundments managed for wigeongrass (Heitzman 1978). Gilmore et al. (1982) showed how impoundment and flooding of Florida salt marshes replaces emergent vegetation with wigeongrass and various algae. In Europe, Cladophora, Enteromorpha, and Vaucheria are the most common noncharaceous macroalgae associated with wigeongrass (Verhoeven and Van Vierssen 1978b; Van Vierssen 1982a, 1982b). Millard and Scott (1953) noted that Enteromorpha, Cladophora, Ectocarpus, and Lyngbya form most luxuriant growths in a South African estuary after the wigeongrass community dies down. Enteromorpha is also a common wigeongrass associate in Iraq (Chapman 1960). Ectocarpus and Lamprothamnium (characeous genera) commonly occur with Australian wigeongrass (Wood 1959; Brock and Lane 1983).
Other algae, including other chlorophytes as well as cyanophytes, rhodophytes and phaeophytes, coexist with various Ruppia taxa (Grontved 1958; Kornas et al. 1960; Ravanko 1972; Hammer et. al. 1975; Nilssen 1975; Lindner 1978; Zimmerman and Livingston 1979; Congdon and McComb 1981). Carpelan (1957) and Davis (1978) listed algae associated with wigeongrass in hypersaline solar evaporation ponds.
Algae cause significant reductions in wigeongrass growth by midsummer (Mahaffy 1987; Prevost et al. 1978; Whitman and Cole 1987). Mahaffy (1987) recorded six algal genera that form mats and reduce wigeongrass production in Delaware wetlands. Many algae differ from wigeongrass in dates of peak abundance. Because of their early growth, all European Ruppia taxa are able to out-compete benthic macroalgae; however, summer growth of floating macroalgae not only shades out wigeongrass, but weakens stems, increasing susceptibility to damage from wave action (Verhoeven 1980a). Algal mats on the surface of the water column also cause thermal stratificaton that slows flowering and drupelet production (Richardson 1980). Dense mats of floating filamentous algae that shade out wigeongrass and reduce its biomass and drupelet production are a serious problem for managers of coastal wetlands in the southern and eastern United States (Grizzell and Neely 1962; Joanen 1964; Joanen and Glasgow 1965; Harlin and Thorne-Miller 1981).
Some have suggested that algal mats provide some benefits to wigeongrass. Richardson (1980) noted that mats of filamentous algae that remain moist on the bottom temporarily protect wigeongrass plants and drupelets in areas subject to dessication. He also observed that algal mats help diminish the effects of wind in roiling sediments, and that the shading effects of surface algae limits the growth of both epiphytes and phytoplankton, thereby stimulating wigeongrass growth and fruit production.
In Ruppia-dominated systems in Baltic waters, angiosperms and benthic algae account for nearly all of the primary production - contributions by phytoplankton are minor (Ankar and Elmgren 1977). Similarly, Gallup (1978) noted that phytoplankton productivity is relatively low in a saline Alberta wetland where wigeongrass was the dominant macrophyte and productivity of benthic algae was extremely high.
Nevertheless, in some situations phytoplankton can greatly lower wigeongrass production and limit distribution of the plant to very shallow (< 40 cm) waters (Verhoeven 1980a). Alternating dominance by wigeongrass and phytoplankton was recorded by Flores-Verdugo et al. (1988) in a shallow, river-fed Mexican lagoon having an ephemeral outlet to the ocean. They hypothesized that the cycle is controlled by the occurrence of rainfall and subsequent river flow that brings nutrients into the lagoon, opens the inlet to the ocean, and flushes out existing wigeongrass beds. They also believed that nutrients promote light-limiting blooms of phytoplankton, but when river flows cease, phytoplankton growth diminishes, and wigeongrass exploits nutrients to grow in sediments not readily available to the phytoplankton. Nevertheless, there is poor understanding of the cycles between dominance by phytoplankton, macroalgae, and submersed angiosperms in wetlands (Gibbs 1973).
Because wigeongrass frequently assimilates essential gases and nutrients from the water column, epiphytes can seriously reduce wigeongrass biomass and propagule formation by inhibiting nutrient uptake and photosynthesis (Conover and Gough 1966; Richardson 1980). Peak epiphyte populations coincided with the period of rapid decay of wigeongrass in a Massachusetts estuary (Conover 1958). In a Maryland river, Anderson (1966) and Anderson et al. (1968) found fall densities of the epiphytic diatom Melosira arenaria great enough to visually obscure the presence of wigeongrass; Cladophora and Merismopedia (a blue green) are also epiphytic there. Blades of wigeongrass in a North Carolina estuary develop a rich epiphytic and animal biota during the growing season (Copeland et al. 1974). In a South African estuary, Ectocarpus sp., Polysiphonia sp., Rhodochorton sp., Cladophora sp., and Rhizoclonium sp. heavily coat wigeongrass (Scott et al. 1952). Sullivan (1977) listed 57 epiphytic diatom taxa found on wigeongrass in thalassic New Jersey wetlands. These algae formed a golden brown felt completely covering the leaves; Navicula pavillardi the most abundant taxon. However, in other thalassic habitats (Grontved 1958; Wood 1959; Kornas et al. 1960; Zimmerman and Livingston 1979; Congdon and McComb 1981) and in rivers (Conover and Gough 1966; Richardson 1980) wigeongrass is relatively free of epiphytes, perhaps because of grazing invertebrates or current flow.
The only instance I found where epiphytes were said to possibly benefit wigeongrass was the account by Flores-Verdugo et al. (1988), reporting that a second, smaller crop of wigeongrass had a heavy cover of epiphytes, but may have benefited somewhat by their nitrogen-fixing properties. Howard-Williams and Allanson (1981) suggested that epiphytic growth helps another submersed angiosperm (P. pectinatus) absorb P.
Wigeongrass probably is less troubled with diseases than several other submerged angiosperms. Hisinger (1887) stated that "tubercles" on Ruppia are a pathological response to the fungus Tetramyxa parasitica. Vegetative reproduction usually allows wigeongrass to survive Rhizoctonia infestations (Bourn and Jenkins 1928). Motta (1978) collected 24 fungal isolates from Chesapeake Bay wigeongrass; although he determined no specific host-parasite relations, the evidence suggested that some pathogenic activity existed.
Wigeongrass provides cover for many estuarine and marine invertebrates (Bourn and Cottam 1939; Day 1952; Kerwin et al. 1975 in Stevenson and Confer 1978) and wigeongrass detritus is an important food source for invertebrates (Tenore 1972; Nixon and Oviatt 1973; Edwards 1978; Verhoeven 1978). Lists of invertebrates found with wigeongrass or in impoundments managed for the plant are available for Africa (Scott et al. 1952; Millard and Scott 1953), Australia (Geddes et al. 1981), California (Carpelan 1957), France (Hoffman 1958), Maine (Hyer 1963), Mexico (Edwards 1978), North Carolina (Heitzman 1978); Saskatchewan (Hammer et al. 1975; Tones 1976), South Carolina (Taniguchi 1986; Wenner and Beatty 1988), Sweden (Ankar and Elmgren 1977), and Texas (Hellier 1962; Johnson 1974).
Invertebrates associated with Ruppia-dominated communities in western Europe number up to 43,800/m2 with biomasses of up to 22.9 g/m^2 ash-free dry weight (Verhoeven 1980a). Verhoeven (1980a) found that only 15 of 75 species intimately associate with wigeongrass plants, that only one or two species strongly dominate, and that poor correlationsn exist between numbers of species and plant biomass or water salinity. Van Vierssen (1982a) listed many invertebrates found in European waters inhabited by wigeongrass and noted that faunal diversity decreases from north to south as salinity fluctuations increase. Hoffman (1958) also found relatively low invertebrate diversity in European wigeongrass communities.
Many mollusks, polychaete worms, crustaceans, and an echinoderm inhabit a Florida bay dominated by Halodule wrightii and lesser amounts of wigeongrass when salinities are 18-35 g/L; the echinoderm disappears when salinities fall to 5-18 g/L, and Chara becomes co-dominant with wigeongrass (Tabb et al. 1962). Carl (1937) listed invertebrates of a wigeongrass-dominated lagoon in British Columbia where salinity varies from nearly 0 g/L in winter to 17.7 g/L in summer. Rotifers, polychaetes, nematodes, gammarid amphipods, and grass shrimp (Paleomonetes spp.) associate with wigeongrass in a North Carolina estuary where salinity usually ranges from 3-10 g/L (Copeland et al. 1974).
As wigeongrass beds are fragmented by wave action from fall winds, the floating masses are eaten and turned into smaller particles by gammarids and isopods; this stimulates a large detrital food chain (Verhoeven and Van Vierssen 1978b). In a New England bay, Nixon and Oviatt (1973) found that amphipods are abundant in wigeongrass detritus. The soft, highly organic sediments where the plants grow were suitable for small worms, nematodes, ciliates, ostracods, and copepods; however these substrates were poor for large infaunal invertebrates. Poff (1973) reported that the annelid worm Peloscolex gabriellae disappears when wigeongrass does in a Texas bay. Heck and Orth (1980) listed temporal and diel variation in use of mixed Ruppia-Zostera meadows by decapod crustaceans in the Chesapeake Bay. Conover (1961) found abundant zooplankton in Rhode Island waters that supported better stands of wigeongrass. Jones (1975) correlated a decrease in macrozooplankton with a decline in wigeongrass in a Texas bay, and noted that some invertebrates use the plant as an attachment site for eggs.
Invertebrates can benefit wigeongrass. Grazing on wigeongrass epiphytes by snails (Richardson 1980) and amphipods (Greze 1968; Zimmerman et al. 1979; Van Montfrans et al. 1984) increases fruit production.
I found little information on the direct consumption of living wigeongrass by invertebrates. Edwards (1978) noted that wigeongrass is the main food of the gastropod Cerithidea mazatlanica in a Mexican lagoon. Among the seven most common invertebrates in a Netherlands pond, only Gammarus zaddachi directly consumes Ruppia cirrhosa (Verhoeven 1978). Nevertheless, the animal may reduce fall biomass of this plant by nearly 40%. Copeland et al. (1974) show wigeongrass as a major food item of marine crabs (Callinectes spp.) in a North Carolina estuary. Zieman (1982) reported that the blue crab (Callinectes sapidus) consumes Ruppia in south Florida.
Data on concentrations of various insecticides found in irrigation drainwater evaporation ponds supporting wigeongrass is available (Schroeder et al. 1988).
In an African estuary, Millard and Scott (1953) found Rana and Xenopus tadpoles common where wigeongrass was abundant. Water snakes (Natrix sipedon) and American alligators (Alligator mississippiensis are regularly observed in wigeongrass impoundments in the southeastern United States (Heitzman 1978; Epstein and Joyner 1986). The plant can be an important food of some sea turtles (Felger et al. 1979).
Fish extensively use wetlands dominated by wigeongrass (Carl 1940; Chapman 1960; Scott et al. 1952; Millard and Scott 1953; Hellier 1962; Jeffries 1972; Nixon and Oviatt 1973; Edwards 1978). Verhoeven and Van Vierssen (1978b) and Verhoeven (1980a) found fish in all except the smallest Ruppia-dominated habitats in western Europe. Copeland et al. (1974) listed permanent resident, seasonal (absent in winter), and migrant fish in a wigeongrass-dominated North Carolina estuary, and Heitzman (1978) lists the fresh- and saltwater fish that live in impoundments in that state, many of which are managed for wigeongrass. Species compositions of fish in South Carolina wigeongrass impoundments and adjacent tidal wetlands are compared by Wenner et al. (1986). A few species of fish eat wigeongrass and its detritus, but probably more often use stands as a nursery (Hildebrand and Cable 1938; Sculthorpe 1967; Austin and Austin 1971; Congdon and McComb 1981).
Of the many fish species that use wigeongrass beds in the lower Chesapeake Bay, only one group, consisting of two combtooth blennys (Hypsoblennius hentzi and Chasmodes bosquianus), a toadfish (Opsanus tau), and a sea bass (Centropristis striata), likely prefer these beds to adjacent beds of Zostera (Weinstein and Brooks 1983). Of 22 fish species that use the wigeongrass-dominated saline lagoons of the Carmargue, France, seven - including the carp - are of freshwater origin (Hoffman 1958). Greatest fish use of Ruppia-dominated coastal wetlands occurs in spring and fall (Nixon and Oviatt 1973). In a Florida bay, Tabb et al. (1962) found more fish, but less wigeongrass, at salinities of 18-35 g/L than at 5-18 g/L. Davis (1978) listed marine fish occurring in hypersaline (50-73 g/L) solar evaporators dominated by wigeongrass. Changes in fish populations occurred, along with increases in wigeongrass, when an additional ship canal was opened from the lower Laguna Madre, Texas, to the Gulf of Mexico (Breuer 1962).
Wigeongrass can also provide excellent food and cover for fish in some inland waters (Terrell 1923). Certain saline (about 19-31 g/L) lakes contain wigeongrass and fish (e.g. Cyprinodon , Coregonus, Pungitus), but most saline interior wetlands are generally inhospitable to fish (Navarre 1959 in Cole 1963; Hammer et al. 1975; Tones 1976).
Fish seldom consume large amounts of wigeongrass. Carr and Adams (1973) found low consumption of wigeongrass among 10 dietary groups of Florida fish; of 21 species, only three had a herbivorous stage. In Louisiana, only the gulf sheepshead (Archosargus oviceps) eats significant amounts (Darnell 1958). Nevertheless, when usual sources of essential fatty acids for fish and invertebrates are exhausted in tidal marshes, wigeongrass sometimes provides these nutrients (Jeffries 1972).
Fish can negatively affect their association with wigeongrass by raising turbidity and thus limiting wigeongrass growth in wetlands with easily resuspendible bottom sediments; young plants are especially vulnerable to such light limitation (Joanen 1964; Joanen and Glasgow 1965). Conversely, some fish feeding likely aids the dispersal of wigeongrass drupelets, and the germination rate of drupelets passing through the digestive systems of some fish can greatly increase (Agami and Waisel 1988). Grizzell and Neely (1962) believed that fish consumption of algal scums benefits wigeongrass.
Many water birds eat wigeongrass vegetation and drupelets. The invertebrates birds find in living and decomposing wigeongrasss are also important foods (Nixon and Oviatt 1973; Verhoeven and Van Vierssen 1978b). Unfortunately, agricultural land development and the construction of irrigation reservoirs have destroyed or seriously degraded many of the vast natural beds of wigeongrass in coastal and interior Mexico (Saunders and Saunders 1981). These beds helped support huge numbers of wintering water birds from all over North America.
In subtropical climates, wintering waterfowl quickly consume entire stands of wigeongrass (Heit 1948), but, with proper water-level manipulations in managed impoundments, stands reestablish in only a few weeks (Jemison and Chabreck 1962; Joanen 1964; Joanen and Glasgow 1965). Stieglitz (1966) believed that waterfowl can consume at least 50% of the standing crop without damaging stands. Australian black swans (Cygnus atratus) can eat 20% of the standing crop (Congdon and McComb 1981). A major problem for managers of coastal impoundments in the southeastern United States is high summer temperatures - these can prevent the fall growth of wigeongrass on which wintering waterfowl largely depend (Kelley and Porcher 1986).
Hurricanes along the Gulf Coast may spread wigeongrass into interior wetlands and where it then receives increased use by waterfowl (Kimble and Ensminger 1959). Similarly, cyclic changes in the vegetation of climatically unstable prairie wetlands cause changes in species composition of waterfowl that eat the vegetation or its associated invertebrate fauna (Swanson et al. 1988). Cycles of dominance by Potamogeton pectinatus and Ruppia maritima are fairly common in saline wetlands in this region as dissolved salts are alternately diluted and concentrated (H.A. Kantrud, personal observation).
Studies throughout the world confirm the attractiveness of wigeongrass or Ruppia-dominated wetlands to waterfowl and show that all parts of the plant are eaten. Swiderek (1982) showed that some waterfowl species feed mainly on wigeongrass drupelets, whereas others select the vegetative portions of the plants. Over 5,000 wigeongrass drupelets can be found in one duck (McAtee 1915; Kubichek 1933). Table 9 suggests that wigeongrass is primarily a food of dabbling ducks (Anatini) and pochards or diving ducks (Aythyini). The plant also rates as good food for geese (Anserini) (McAtee 1939; Quay and Critcher 1962) and swans (Cygnini) (McAtee 1939; Sincock 1962; Gaevskaya 1966; Congdon and McComb 1979, 1980, 1981). Saunders and Saunders (1981) reported use by whistling-ducks (Dendrocygnini).
Certain ducks seem especially fond of wigeongrass. In South Carolina, Gordon et al. (1987) and Gray et al. (1987) reported that communities where Ruppia maritima and Eleocharis parvula codominante are intensively used by wintering green-winged teals (Anas crecca), northern pintails (A. acuta), and American wigeons (A. americana). A coastal Massachusetts impoundment supporting wigeongrass was especially attractive to American black ducks (A. rubripes) (Portnoy et al. 1987). Euliss (1989) noted that, in irrigation wastewater evaporation ponds in California, American wigeons and redheads (Aythya americana) eat and uproot wigeongrass vegetation in deeper open water areas; northern pintails then feed mostly on drupelets from the plants that wash ashore. Wigeongrass-dominated wetlands in North Dakota were especially attractive to fall migrant gadwalls (Anas strepera), American wigeons, and redheads (H.A. Kantrud, personal observation).
Intensive feeding on wigeongrass by swans may significantly disturb anaerobic bottom sediments and affect turnover rates of organic materials and increase nutrient release (Congdon and McComb 1980).
Wigeongrass is often a food of coots (Fulica spp.) (Quay and Critcher 1962; Gaevskaya 1966; Holmes 1972; Prevost et al. 1978; Verhoeven and Van Vierssen 1978b; Swiderek 1982) and other aquatic birds (Sculthorpe 1967). Verhoeven (1978) calculated the consumption of Ruppia cirrhosa by individual coots as 70 g/d dry weight and estimated that about 20% of the fall decrease in biomass of this plant is from bird grazing. Waterfowl and coots exploit the Ruppia beds of western Europe mostly from the end of summer to winter (Verhoeven and Van Vierssen 1978b) and the birds help disperse drupelets (Verhoeven 1979).
Wigeongrass is sometimes an important food of red knots (Calidris canutus); dowitchers (Limnodromus spp.) and common snipes (Gallinago gallinago) also eat the plant Sperry (1940). Martin et al. (1951) also reported use of wigeongrass by knots and dowitchers as well as by other Calidris spp., and by purple gallinules (Porphyrula martinica), black-necked stilts (Himantopus mexicanus), and king rails (Rallus elegans). Bourn and Cottam (1950) indicated that wigeongrass was a minor food of various rails (Rallus spp.), yellowlegs (Tringa spp.), and willets (Catoptrophorus semipalmatus). Allen (1956) considered wigeongrass an important food of flamingoes (Phoenicopterus spp.).
The animal community associated with wigeongrass is an important food source for many breeding and wintering birds (Hoffman 1958; Verhoeven and Van Vierssen 1978b). A bewildering array of wintering and migrating wading birds, shorebirds, and waterfowl use South African estuaries ("vleis") where R. maritima and Zannichellia aschersoniana are often dominant and large numbers of invertebrates occur (Millard and Scott 1953; Scott 1954). A wide variety of birds also use Ruppia-dominated impoundments in the southeastern United States (Epstein and Joyner 1986). In these areas, the plant beds probably also provide foods for insect-hawking birds such as swifts (Apodidae), swallows (Hirundinidae), and martins (Hirundinidae). Morgan (1954) commented on the large numbers of invertebrate waterfowl foods found in an Australian wetland dominated by wigeongrass and Potamogeton pectinatus.
The only wild mammals known to consume living wigeongrass are West Indian manatees (Trichechus manatus latirostris) (Hartman 1971), muskrats (Ondatra zibethicus) (McCabe 1982), and nutria (Myocastor coypu) (R.H. Chabreck, personal communication in Garner 1962). Deer and cattle sometimes eagerly eat detached plants windrowed along shorelines (Campbell 1946).
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