Benthic Invertebrate Fauna, Small Streams J Bruce Wallace, University of Georgia, Athens, GA, USA S L Eggert, USDA Forest Service, Northern Research Station, Grand Rapids, MN, USA ã 2009 Elsevier Inc. All rights reserved. Introduction invertebrate taxa have been recorded in a mountain stream on Bougainville Island, Papua New Guinea. Small streams (first- through third-order streams) Incredibly, there are many headwater invertebrate spe- make up >98% of the total number of stream seg- cies that remain undescribed in both isolated and popu- ments and >86% of stream length in many drainage lated regions of the world. networks. Small streams occur over a wide array of With the great diversity of foods available for con- climates, geology, and biomes, which influence tem- sumption by invertebrates (i.e., deposited and retained perature, hydrologic regimes, water chemistry, light, on substrates, or suspended in the water column), it is substrate, stream permanence, a basin’s terrestrial not surprising that invertebrates have evolved diverse plant cover, and food base of a given stream. Small morphobehavioral mechanisms for exploiting food streams are generally most abundant in the upper resources. Their diverse feeding behaviors have been reaches of a basin, but they can also be found through- lumped into a broad functional classification scheme, out the basin and may enter directly into larger rivers. which is based on mechanisms used by invertebrates to They have maximum interface with the terrestrial acquire foods. These functional groups are as follows: environment, and in most temperate and tropical cli- scrapers, animals adapted to graze or scrape materials mates they may receive large inputs of terrestrial, or (periphyton, or attached algae, fine particulate organic allochthonous, organic matter (e.g., leaves, wood) matter, and its associated microbiota) from mineral from the surrounding plant communities. In locations and organic substrates; shredders, organisms that com- with open canopies such as grasslands and deserts, minute large pieces of decomposing vascular plant tis- autochthonous or primary production in the form of sue such as leaf detritus (>1 mm diameter) along with algae, or higher aquatic plants, may serve as the main its associated microflora and fauna, or feed directly on food base. Hence, headwater streams display a diverse living vascular hydrophytes, or gouge decomposing fauna, which is often adapted to physical, chemical, wood; gatherers, animals that feed primarily on depos- and biotic conditions of the region. ited fine particulate organic matter (FPOM 1mm diameter); filterers, animals that have specialized ana- tomical structures (e.g., setae, mouth brushes, or fans) Diversity of Benthic Invertebrates in or silk and silk-like secretions that act as sieves to remove particulate matter from suspension; and pre- Small Streams dators, those organisms that feed primarily on animal The benthic invertebrate fauna of small streams is com- tissue by either engulfing their prey, or piercing prey posed primarily of aquatic insects, crustaceans, mol- and sucking body contents. lusks, and various other invertebrate taxa. The insect Functional feeding groups refer primarily to modes fauna consists primarily of Odonata (dragonflies and of feeding and not type of food per se. For example, damselflies), Ephemeroptera (mayflies), Plecoptera many filter-feeding insects of high gradient streams (stoneflies), Megaloptera (alderflies and dobsonflies), are primarily carnivores, whereas scrapers consume Coleoptera (beetles), Trichoptera (caddisflies), occa- quantities of what must be characterized as epilithon, sional Lepidoptera (moths), and Diptera (true flies). a matrix of polysaccharide exudates, detritus, micro- Crustaceans (including amphipods, isopods, and cray- flora, and microfauna associated with stone surfaces, fish) can also be found in small streams as well as and not solely attached algae. Shredders may select microcrustaceans such as cladocerans, ostracods, and those leaves that have been ‘microbially conditioned’ copepods. Other common invertebrates found in small by colonizing fungi and bacteria. Shredders also ingest streams include nematodes, oligochaetes, turbellarians, attached algal cells, protozoans, and various other and mollusks such as snails, limpets, and finger-nail components of the fauna during feeding. Some ‘shred- clams. Total invertebrate diversity in small streams ders’ have been shown to grow by harvesting primar- can be quite high. The Breitenbach, a first-order stream ily the epixylic biofilm, the matrix of exudates, in Germany, contains at least 1004 described inverte- detritus, microflora, and microfauna found on wood brate taxa. At least 293 invertebrate taxa have been surfaces. Although it appears valid to separate benthic found in headwater streams in the southern Appala- invertebrates according to these mechanisms used chian mountains of the United States. Over 182 known to obtain foods, many questions remain concerning 173 Encyclopedia of Inland Waters (2009), vol. 2, pp. 173-190 174 Invertebrates _ Benthic Invertebrate Fauna, Small Streams the ultimate sources of protein, carbohydrates, fats, 70 000 Abundance and assimilated energy to each of these functional Mixed substrate 60 000 groups. Rock face 50 000 Quantitative Measurements of Benthic 40 000 + 1 s.e. 2 30 000 Invertebrates in Headwater Streams #/m 20 000 Invertebrates are often enumerated by abundances or average numbers per unit area of stream bottom. Other 10 000 measures include average biomass, or weight, per unit 0 (a) Sh Sc Cg Filt Pred area ofstream, or more rarely, secondary production per unit area of stream bottom. Each of these will provide a different picture of the invertebrate commu- 800 Biomass nity. For example, Figure 1(a) shows abundances per unit area of moss-covered bedrock outcrop and mixed 600 substrates in three headwater streams (n ¼ 20 total stream years) at the Coweeta Hydrologic Laboratory + 1 s.e. 400 2 in western North Carolina, USA. Note that abun- dances are dominated by members of the collector- mg/m gatherer (Cg), functional group. In contrast, three 200 groups, predators, shredders, and collectors, represent the majority of the biomass in these small streams 0 (Figure 1(b) ). Secondary production, which represents (b) Sh Sc Cg Filt Pred the living organic matter or biomass produced by each functional group over a year regardless of its fate, i.e., 3000 Production losses to predation, or other sources of mortality, is fairly evenly distributed between the predator, collec- 2500 tor, and shredder functional groups (Figure 1(c)). The 2000 /y + 1 s.e. integration of production, feeding habits, and bioener- 2 getic data can yield a much better understanding of the 1500 role of animal populations in ecosystem function than 1000 either abundance or biomass. Distributional patterns for functional feeding group mg AFDM/m 500 abundance, biomass, and production in small streams 0 may differ among substrate types (Figure 1(a)–1(c)). (c) Sh Sc Cg Filt Pred Collectors and predators dominate abundances on Figure 1 both substrates. For biomass, predators > shredders > Total abundance, biomass, and production of invertebrates by functional feeding group for mixed substrate collectors dominate the mixed substrates, whereas and rockface habitats in headwater streams at Coweeta filterers > collectors > predators contribute most to Hydrologic Laboratory in western North Carolina, USA. These biomass on bedrock outcrop substrates. Most produc- data represent 20 total years of abundance, biomass, and > > production estimates. tion is attributed to predators shredders collectors in mixed substrates compared to filterers > collectors > predators on bedrock out crop substrates. Thus, habitats as these small streams are heavily shaded year distinct differences exist in functional feeding group round by dense riparian rhododendron. These data production among different substrates within a stream, emphasize the influence of local geomorphic processes whichcorrespondtodifferentavailablefoodresources. and riparian linkages on invertebrate productivity in Filterer production predominates in the bedrock forested headwater streams. habitats with high current velocities that transport FPOM. Collector production is also enhanced by Comparison of Secondary Productivity FPOM trapped in the moss on the bedrock outcrops. Measurements from Small Streams Conversely, predator, shredder, and collector pro- duction are similar in the retentive mixed substrate Secondary productivity measures for benthic inverte- habitats, which also have the greatest biomass, abun- brates from small streams from various temperate dances, and organic matter retention. Scraper abun- areas around the world are given in Table 1. Total an- dance, biomass, and production are low for all nual productivity is quite variable ranging from Encyclopedia of Inland Waters (2009), vol. 2, pp. 173-190 À À Table 1 Estimates of secondary production (g m 2 year 1) for various functional feeding groups, or primary and secondary consumers, for small streams from various regions of the world Country Biome Stream Scrapers Shredders Collectors Filterers Predators Total production Source USA, ID Cool desert Douglas Creek* 2.65 0.64 15.28 4.20 0.45 23.22 2 USA, ID Cool desert Snively 0.00 1.32 9.33 3.18 0.33 14.15 2 Springs* USA, ID Cool desert Rattlesnake 0.00 0.17 3.62 11.80 0.77 16.36 2 Springs* Denmark Deciduous Rold Kilde 0.14 5.93 2.58 0.01 0.88 9.54 6 Invertebrates