Northern Prairie Wildlife Research Center
The heterotrophic primary consumers in prairie wetlands consist of herbivores and detritivores. Invertebrates, muskrats, and waterfowl are important herbivores in prairie wetlands. A large group of invertebrates are known to ingest living phytoplankton and attached algae, but little is known of the role of algae in the support of these primary consumers in prairie wetlands. Some of the most important of these herbivores in prairie wetland. include macro- and microcrustaceans, dipteran and ephemeropteran insects, and freshwater mollusks. Domestic animals also consume much of the primary production from prairie wetlands, but are unimportant in many other of the world's wetland ecosystems. At least one emergent hydrophyte, Scolochloa festucacea, is recognized as a highly valuable livestock forage grass and is actively managed for and harvested in prairie wetlands (Neckles et al. 1985).
Secondary consumers are extremely important and provide foods for most of the higher organisms found in prairie wetlands. Murkin stated that knowledge of the basic ecology and life-history of consumers in prairie wetlands is so limited as to make work on secondary production impossible at this time.
A third, nontrophic user of primary production is the large and diverse group of organisms that depend on living and dead plants for thermal cover, nesting material, sites for egg attachment, etc. These organisms will not be discussed here, but some information on nontrophic use is presented in the preceding chapters on various animal groups.
Net primary production, both above and below ground level, is particularly high in prairie wetlands (van der Valk and Davis 1978b), but shows great temporal variation, mainly because of the presence or absence of surface water; water levels are also important in this regard. In semipermanent wetlands, highest primary productivity likely occurs as emergent hydrophytes regenerate when surface water is replenished after prolonged drought, and lowest productivity probably occurs after several years of high water levels when emergent hydrophytes are replaced by submersed species (van der Valk and Davis 1978a). In temporary and seasonal wetland basins, productivity likely is highest when water is present, as judged by increases in height, biomass, and density of emergent macrophytes (Neckles 1984: H.A. Kantrud, pers. obs.).
Examples of production rates and above- and below-ground biomass for various vegetation zones and some common species in prairie wetlands are shown in Table 10. In many instances, algae do not contribute greatly to standing-crop biomass, and thus are not usually considered an important part of primary production. However, recent investigations by Hooper and Robinson (1976) and Shamess et al. (1985), and unpublished data of van der Valk (Murkin 1989), show that algal production and biomass in prairie wetlands can be quite high (Table 10). Algal groups especially important as primary producers in prairie wetlands are epiphytes, which grow on surfaces of submersed plant material: epipelics, which grow on bottom substrates: and metaphytes, which usually form floating mats. Prairie wetlands can produce high midsummer populations of planktonic algae of sufficient density to cause fish kills (Kling 1975).
Virtually nothing is known about the most primitive producer group in prairie wetlands, the chemosynthetic bacteria.
There is no doubt that huge herds of wild ungulates once removed considerable amounts of primary production from prairie wetlands. This phenomenon has been continued on many wetlands through man's livestock and hay-cutting operations. Birds (primarily waterfowl) are also known to consume up to 86.8 gm/m2 vegetative biomass annually from portions of certain prairie wetlands (Anderson and Low 1976). The emergence of aquatic flying insects may also remove significant primary production, although not enough is known about the foods of larval insects to quantify this relationship. However, as many as 19,713 adult chironomids/m2 have been trapped in a single month as they emerged from a semipermanent North Dakota wetland (Nelson and Butler 1987). Nevertheless, the generally high productivity of most prairie wetlands ensures that a considerable amount of material enters the system as detritus.
An important part of the detritus pool in prairie wetlands is standing dead emergent hydrophytes. Much of this litter enters the pool with the onset of killing frosts at the end of the growing season, but in less permanent wetlands, many emergent hydrophytes die from drought during late summer and early fall. Leaf and shoot death of various emergent species can occur at any time during the growing season (van der Valk and Davis 1978b).
The fallen litter compartment is usually considered to consist of material in the water column, including fallen or bent-over stems and leaves of emergents, submersed macrophytes, and algae. Some living emergent material can enter this compartment directly through muskrat activities (van der Valk and Davis 1978b) or those of man and his livestock. The third component of the detritus pool is the dissolved organic substances that leach into the water column from both standing and fallen litter.
De la Cruz (1979) recognized three simultaneous processes whereby detritus is decomposed: (1) rapid leaching of soluble substances from newly dead plant tissue, (2) weathering and mechanical fragmentation, and (3) biological decay from bacteria and fungi. Particle sizes rapidly decrease, and the dead tissue and soluble organic substances become food for a host of invertebrates and other life forms whose excretory products can again be colonized by microbes.
There is no doubt that detritivory is an important aspect of food chain support in prairie wetlands, even though there are no estimates of detrital consumption and utilization for any wetlands. Coarse particulate matter likely enters prairie wetlands as a pulse associated with increased spring water temperatures. Bacteria, fungi, and other microorganisms form the first level of consumption of dead plant material and are considered the base of the detrital food chain, with secondary production assumed to occur at higher trophic levels. Many of the invertebrate herbivores common to prairie wetlands are also suspected to be detritivores, but some authors argue that such organisms may simply be assimilating microorganisms associated with litter of small particle size. Thus Murkin (1989) concluded that the primary role of detritus in prairie wetlands may be to provide habitat necessary for algae and invertebrate production in these systems.