Many ecologists and aquaculturists view crayfish as either detritivores or herbivores. The nutritional importance of detritus and plant material, seemingly confirmed by examination of stomach contents is exaggerated. Examination of stomach contents emphasizes indigestible residues, for example, plant fragments and detritus, while neglecting the ingestion of easily digestible soft‐bodied metazoans. The few comparative studies that have measured the assimilation and metabolic use of ingested items reveal animal protein to be the chief substance promoting rapid growth. Crayfish are not indiscriminate omnivores; in fact, they demonstrate a predilection for animal protein. Crayfish rank among the chief carnivores found in lakes and streams. To completely understand their role, crayfish should be viewed from two perspectives: that of an individual attempting to complete its life cycle, and the collective functioning of a population in the ecosystem, which can produce a serious ecological impact. While ...

Redefining the role of crayfish in aquatic ecosystems

We conducted five 4- to 12-wk in situ enclosure–exclosure experiments in four macrophyte assemblages in three northern Wisconsin lakes to test the impact of grazing by the crayfish Orconectes rusti...

Reductions in Submersed Macrophyte Biomass and Species Richness by the Crayfish Orconectes rusticus

Nous avons quantifie les sources d'energie des ecrevisses Orconectes luteus et O. punctimanus dans la riviere Jacks Fork (Missouri) a l'aide d'isotopes stables (δ 13 C et δ 15 N) et de l'analyse du contenu des intestins. Un modele mixte combinant les deux isotopes a fait ressortir que les deux tiers environ de la production des ecrevisses provenaient de sources allochtones de carbone, et que 30-50 % de cette production provenait de la consommation directe de matieres animales. Les jeunes de l'annee et les adultes d'O. luteus etaient plus carnivores que les adultes plus gros d'O. punctimanus. Les contributions des sources d'energie calculees a l'aide du modele combinant les isotopes stables etaient similaires aux resultats de l'analyse du contenu des intestins, apres correction pour l'assimilation differentielle des composantes alimentaires. Les proportions de la production des ecrevisses attribuees a la matiere animale selon ces deux methodes etaient quatre a cinq fois plus elevees que les estimations obtenues sans correction par l'analyse du contenu des intestins. Cette analyse sans correction surestimait le pourcentage de la production provenant de l'absorption directe des detritus. Les calculs des taux de consommation de la nourriture par les ecrevisses a partir de la production indiquaient que ces crustaces etaient les consommateurs dominants des invertebres benthiques, des detritus et des algues, et pourraient avoir une influence importante sur les niveaux trophiques inferieurs, la transformation de la matiere organique et le flux de l'energie dans ce systeme.

Energy sources and ecological role of crayfishes in an Ozark stream: insights from stable isotopes and gut analysis

By comparing distributions of functional group production among different habitats in an Appalachian mountain stream, the influence of site-specific geomorphology upon the overall functional group composition of the animal community was demonstrated. By replicated monthly sampling, substrate particle size distributions, current velocity, standing crops of benthic organic matter, and production of macrofauna were measured in each of three principal habitats: bedrock-outcrop, riffle, and pool. Samples were taken at randomly assigned locations and the relative number of samples taken from each habitat was assumed to be proportional to the area of the habitat within the stream. These proportions were used to weight production measured in each habitat and the resulting values were summed to obtain production per unit area of average stream bed. The bedrock-outcrop habitat was characterized by high material entertainment and export as indicated by significantly higher current velocities and lower standing crops of detritus compared to the riffle and pool habitats. Pools were sites of low entertainment and high retention of organic matter as demonstrated by significantly lower current velocities and higher accumulations of detritus than other habitats. The riffle habitat was intermediate to the bedrock-outcrop and pool habitats in all parameters measured. Annual production of collector-filterers was highest in the bedrock-outcrop (ash-free dry mass 1920 mg/m2 ), followed by riffle (278 mg/m2 ) and pool (32 mg/m2 ). Although constituting only 19% of the stream area, the bedrock-outcrop habitat contributed 68% of the habitat-weighted collector-filterer production. Annual production of shredders was highest in pools (2616 mg/m2 ), followed by riffles (1657 mg/m2 ) and bedrock-outcrop (579 mg/m2 ). The pool habitat, constituting 23% of stream area, contributed 36% of shredder production. Annual production of scrapers was highest in the riffle habitat (905 mg/m2 ), followed by bedrock-outcrop (517-mg/m2 ) and pool (238 mg/m2 ). Riffles constituted 58% of total stream area and were the source of 77% of the habitat-weighted scraper production. Annual production of engulfing predators was greatest in the pool habitat (2313 mg/m2 ), followed by riffles (1765 mg/m2 ) and bedrock-outcrop (687 mg/m2 ). The relatively lower production of engulfing predators in the bedrock-outcrop habitat reflects a functional shift in mode of resource acquisition by predators, with predaceous collector-filterers (Arcto-psychinae: Trichoptera) predominating in the bedrock-outcrop. Collector-gatherer production was more evenly distributed, with the bedrock-outcrop, riffle, and pool habitats each contributing 14, 54, and 33% to the habitat-weighted production, respectively. Unlike all other functional groups, this distribution was not significantly different from the distribution of stream area among habitats and reflected lack of dependence on specific physical attributes of the local environment for access to food by members of this functional group. Local geomorphology determined the diversity and spatial distribution of bedrock-outcrops, riffles, and pools in the study stream. In turn, the functional structure of the macrofauna, when viewed holistically, was the result of the integration of the relative contributions of each habitat type of total stream area. Total habitat-weighted annual production in the study stream was estimated at 5093 and 1921 mg/m2 for primary and secondary consumers, respectively. The distribution of habitat-weighted production among functional groups was: collector-gatherers (39%), followed by shredders (225), engulfing predators (22%), scrapers (13%), and collector-filterers (8%). This functional structure agrees favorably with current conceptual models of head water streams draining forested catchments.

Local Geomorphology as a Determinant of Macrofaunal Production in a Mountain Stream.

Three pairs of cobble riffle study sites were established in a second-order stream in North Carolina and logs added to the downstream riffle at each site. At log addition transects, stream depth increased, current velocity decreased, cobble substratum was covered by sand and silt, and both coarse and fine paniculate organic matter increased dramatically. Log additions had less dramatic effects on uptake lengths of ammonium, nitrate, and phosphate, but they had immediate and significant impacts on invertebrate community structure: abundances and biomass of scrapers and filterers decreased; collectors and predators increased; overall shredder biomass did not change, but biomass of trichopteran and dipteran shredders increased, while that of most plecopteran shredders decreased; and plecopteran predators also decreased despite greater abundances of potential prey. These observations suggest that physiological and morpho- behavioral constraints preclude many animals from tracking resources among patches when patches display very different abiotic conditions. Secondary production of scrapers and filterers decreased, whereas that of collectors and predators increased. The shifts in functional group abundances, biomass, and production between reference and debris-dam transects, which differed considerably from those previously reported for low-gradient, sandy-bottom streams, accentuate the importance of localized abiotic factors in structuring invertebrate communities within patches.

physical and biotic characteristics of a mountain stream

An understanding of the population dynamics of crayfish and their trophic position is critical to understanding energy transformation in lakes. In many lakes crayfish dominate the annual production of the benthic biomass, at times reaching mean biomass values of 1000 1400 kg/ha and numerical densities of 15/m2. Density determines the rate of growth of various populations and determines reproductive capacity, age at maturity and life span. For Orconectes virilis, within the lakes we studied, population stability was mostly due to the short lag between a change in population density and an adjustment in fecundity. As a result, these populations appear to be self-regulated through density-dependent control of brood stock size. In Orconectes virilis, annual production was a function of mean annual biomass. Examination of available data on the ratio between other species showed them to fall within the same range as O. virilis. It seems that the evolutionary strategy governing crayfish biomass replacement and energy turnover has remained essentially unchanged. As regards their role in the community, crayfish do not fit the trophic level concept. Attempts to quantify their food habits show them to be polytrophic and a key energy transformer between various trophic links through utilization of all the trophic levels in lakes. Because of the multiple trophic roles of the crayfish, a comparison with the bioenergetics of various freshwater crust?cea is difficult. Crayfish may be most important in lakes with low phytoplankton production. In such lakes detritus and benthic algae, both of which are ingested by the crayfish, are important sources of overall energy flow. Since crayfish also function as major predators on the benthos, they most likely contribute to the stability of lake communities by interacting with a whole subweb of species ranging from large benthic invertebrates and submerged higher aquatics to the periphytic microcommunity. Hence, crayfish act as an important mechanism contributing to the regulation of most production processes and most specifically to the regulation of benthic production available to fish. In low nutrient lakes, because of their biomass dominance and functional role, crayfish greatly surpass the usual role assigned to them from their occurrence in fish diets. Manipulation of the benthic food chain would seem to be the most appropriate strategy to increase the fish production of such lakes. However, a greater appreciation of the complexity of the trophic organization of such food webs is necessary before such modifications can be attempted. 1 A contribution under Dingell-Johnson Project F-30-R, Michigan. 2 Present address: Department of Biology, Lakehead University, Thunder Bay, Ontario, Canada.

Patricia D. Jones

Papers

The dynamics of crayfish and their role in ecosystems.