-PERSPECTIVESThe River Continuum Concept’ROBIN L. VANNOTEStroud Water Research Center, Academy of Natural Sciences of Pf~i...
PERSPECTIVESparcours d’une riviere. Faisant appel a la theorie de l’equilibre Cnergetique desspecialistes de lagtomorpholo...
equilibrium. Therefore, over extended river reaches,biological communities should become establishedwhich approach equilib...
PERSPECTIVES 133t.II0Faa 0.1a2{‘-Relative Diversity of Soluble Organic Compounds-6t:-5 ;Index to Biotic Diversity>E4W>~‘2’...
-i134 CAN. J. FISH. AQUAT. SCI., VOL. 37, 1980highly stable physical systems, biotic contribution toecosystem stability ma...
PERSPECTIVES 135iedy.namic equilibrium, assume processing strategies~vOlvlngminimum energy loss (termed maximum+raling ” b...
136 CAN. J. FISH. AQUAT. XI.. VOL. 37. 1980‘44tion, is useful because it suggests that community struc- bLEOPOLD, L. B., A...
PERSPECTIVES 137,,@TE, R. L., AND B. W. SWEENEY. 1979. Geographic WEBSTER, J. R. 1975. Analysis of potassium and calciuman...
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  1. 1. -PERSPECTIVESThe River Continuum Concept’ROBIN L. VANNOTEStroud Water Research Center, Academy of Natural Sciences of Pf~iladelphiu, Avorrdale, PA 19311, USAG. WAYNE MINSHALLDepartment of Biology, Idaho State University, Pocatello, ID 83209, USAKENNETH W. CUMMINSDepartment of Fisheries and Wildlife, Oregon State University, Corvallis, OR 97331, USAJAMES R. SEDELLWeyerflauser Corporation, Forestry Researcft, 505 Nortfl Pearl Street, Centralia, WA 98531, USAAND COLBERT E. GUSHINGEcosystems Department, Battelle-Pacific Northwest Laboratories, Ricftlaild, WA 99352, USAVANNOTE, R.L.,G. W. MINSHALL, K. W. CUMMINS, J.R. SEDELL,AND~. E. GUSHING. 1980.The river continuum concept. Can. J. Fish. Aquat. Sci. 37: 130-137.From headwaters to mouth, the physical variables within a river system present a con-tinuous gradient of physical conditions. This gradient should elicit a series of responses withinthe constituent populations resulting in a continuum of biotic adjustments and consistentpatterns of loading, transport, utilization, and storage of organic matter along the length of ariver. Based on the energy equilibrium theory of fluvial geomorphologists, we hypothesize thatthe structural and functional characteristics of stream communities are adapted to conformto the most probable position or mean state of the physical system. We reason that producerand consumer communities characteristic of a given river reach become established in harinonywith the dynamic physical conditions of the channel. In natural stream systems, biologicalcommunities can be characterized as forming a temporal continuum of synchronized speciesreplacements. This continuous replacement functions to distribute the utilization of energyinputs over time. Thus, the biological system moves towards a balance between a tendency forefficient use of energy inputs through resource partitioning (food, substrate, etc.) and anopposing tendency for a uniform rate of energy processing throughout the year. We theorizethat biological communities developed in natural streams assume processing strategies involvingminimum energy loss. Downstream communities are fashioned to capitalize on upstreamprocessing inefficiencies. Both the upstream inefficiency (leakage) and the downstream adjust-ments seem predictable. We propose that this River Continuum Concept provides a frame-work for integrating predictable and observable biological features of lotic systems. Implica-tions of the concept in the areas of structure, function, and stability of riverine ecosystems arediscussed.Key words: river continuum; stream ecosystems; ecosystem structure, function; resourcepartitioning; ecosystem stability; community succession; river zonation; stream geomor-pholouVANNOTE, R. L.,G. W. MINSHALL, K. W. CUMMINS, J. R. SEDELL, AND C. E. CUSHING. 1980.The river continuum concept. Can. J. Fish. Aquat. Sci. 37: 130-137.De la tCte des eaux & l’embouchure, un r&eau fluvial offre un gradient continu de condi-tions physiques. Ce gradient devrait susciter, chez les populations habitant dans le rCseau, unesCrie de rCponses aboutissant B un continuum d’ajustements biotiques et & des schCmas uni-formes de charge, transport, utilisation et emmagasinage de la mat&e organique sur tout le‘Contribution No. 1 from the NSF River Continuum Project.Printed in Canada (J-5632)ImprimC au Canada (55632)i7n
  2. 2. PERSPECTIVESparcours d’une riviere. Faisant appel a la theorie de l’equilibre Cnergetique desspecialistes de lagtomorphologie fluviale, nous avancons l’hypothese que les caracteristiques structurales etfonctionnelles des communautes fluviatiles sont adaptees de facon a seconformer a la positionou condition moyenne la plus probable du systeme physique. Nous crayons que les commu-nautes de producteurs et de consommateurs caracteristiques d’un segment donne de la rivieresemettent en harmonie avec les conditions physiques dynamiques du chenal. Dans des reseauxfluviaux naturels, on peut dire que les communautes biologiques forment un continuum tem-pore1 de remplacements synchronises d’especes. Grace a ce remplacement continu, il y arepartition dans le temps de I’utilisation desapports Cnergetiques. Ainsi, le systeme biologiquevise a un Cquilibre entre une tendance vers l’utilisation efficace des apports d’energie en par-tageant les ressources (nourriture, substrat, etc.), d’une part, et une tendance opposee vers untaux uniforme de transformation de l’energie durant l’annee, d’autre part. A notre avis, lescommunautes biologiques habitant dans des tours d’eau naturels adoptent des strategies detransformation comportant une perte minimale d’energie. Les communautes d’aval sontorganisees de facon a tirer profit de l’inefficacite de transformation descommunautes d’amont.On semble pouvoir p&dire a la fois l’inefficacite (fuite) d’amont et les ajustements d’aval. Noussuggerons ce concept d’un continuum fluvial comme cadre dans lequel integrer les caracteresbiologiques previsibles et observables des systemes lotiques. Nous analysons les implicationsdu concept quant B la structure, fonction et stabilite des Ccosystemes fluviaux.Received May 14, 1979 Recu le 14 mai 1979Accepted September 19, 1979 Accept6 le 19 septembre 1979131Statement of the ConceptMany communities can be thought of as continuaconsisting of mosaics of integrading population aggre-~(ptes (McIntosh 1967; Mills 1969). Such a con-coptualization is particularly appropriate to streams.&Vera1 workers have visualized streams as possessingassemblages of species which respond by their occur-wnces and relative abundances to the physical gradientsprcscnt (Shelford 1911; Thompson and Hunt 1930;sicker 1934; Ide 1935; Burton and Odum 1945; Van&trscn 1954; Huet 1954, 1959; Slack 1955; Minshall“1968; Ziemer 1973; Swanston et al. 1977; Platts 1979).Expansion of this idea to include functional relation-hips has allowed development of a framework, the“River Continuum Concept,” describing the structurernd function of communities along a river system.,; Basically, the concept proposes that understanding of.thc biological strategies and dynamics of river systemsEquircs consideration of the gradient of physical fac-IOrJ formed by the drainage network. Thus energy~IQNit, and organic matter transport, stora.ge, and useby macroinvertebrate functional feeding groups may!. bo regulated largely by fluvial geomorphic processes./ fie patterns of organic matter use may be analogousb those of physical energy expenditure proposed byPomorphologists (Leopold and Maddock 1953; Leo-i Wld and Langbein 1962; Langbein and Leopold 1966;1 Curry 1972). Further, the physical structure coupledj1with the hydrologic cycle form a templet (Southwood1977) for biological responses and result in consistentPatterns of community structure and function and or-@nit matter loading, transport, utilization, and stor-‘ge along the length of a river.Derivation of the ConceptAs the cyclic theory for explaining the evolution ofland forms and streams (young, mature, ancient)proved unsatisfactory, the concepts gradually were re-placed by a principle of dynamic equilibrium (Curry1972). The concept of the physical stream networksystem and the distribution of watersheds as open sys-tems in dynamic (“quasi”) equilibrium was first pro-posed by Leopold and Maddock ( 1953) to describeconsistent patterns, or adjustments, in the relationshipsof stream width, depth, velocity, and sediment load.These “steady state” systems are only rarely character-ized by exact equilibria and generally the river and itschannel tend toward a mean form, definable only interms of statistical means and extremes (Chorley1962) ; hence, the idea of a “dynamic” equilibrium.The equilibrium concept was later expanded to includeat least nine physical variables and was progressivelydeveloped in terms of energy inputs, efficiency inutilization, and rate of entropy gain (Leopold andLangbein 1962; Leopold et al. 1964; Langbein andLeopold 1966). In this view, equilibration of rivermorphology and hydraulics is achieved by adjustmentsbetween the tendency of the river to maximize theefficiency of energy utilization and the opposing tend-ency toward a uniform rate of energy use.Based upon these geomorpholsgical considerations,Vannote initially formulated the hypothesis that struc-tural and functional characteristics of stream com-munities distributed along river gradients are selectedto conform to the most probable position or mean stateof the physical system. From our collective experiencewith a number of streams, we felt it was possible totranslate the energy equilibrium theory from the phys-ical system of geomorphologists into a biologicalanalog. In this analysis, producer and consumer com-munities characteristic of a given reach of the rivercontinuum conform to the manner in which the riversystem utilizes its kinetic energy in achieving a dynamic
  3. 3. equilibrium. Therefore, over extended river reaches,biological communities should become establishedwhich approach equilibrium with the dynamic physicalconditions of the channel.Implications of the ConceptIt is only possible at present to trace the broa.d out-lines of the ways the concept should apply to streamecosystems and to illustrate these with a few examplesfor which reasonably good information is available.From headwaters to downstream extent, the physicalvariables within a stream system present a continuousgradient of conditions including width, depth, velocity,flow volume, temperature, and entropy gain. In de-veloping a biological analog to the physical system,we hypothesize that the biological organization in riversconforms structurally and functionally to kinetic energydissipation patterns of the physical system. Biotic com-munities rapidly adjust to any changes in the redistribu-tion of use of kinetic energy by the physcial system.STREAMSIZEANDECOSYSTEMSTRUCTUREANDFUNCTIONBased on considerations of stream size, we proposesome broad characteristics of lotic communities whichcan be roughly grouped into headwaters (orders l-3),medium-sized streams (4-6), and large rivers (>6) (Fig.1). Many headwater streams are influenced strongly bythe riparian vegetation which reduces autotrophic pro-duction by shading and contributes large amounts ofallochthonous detritus. As stream size increases, the re-duced importance of terrestrial organic input coincideswith enhanced significance of autochthonous primaryproduction and organic transport from upstream. Thistransition from headwaters, dependent on terrestrialinputs, to medium-sized rivers, relying on algal orrooted vascular plant production, is thought to be gen-erally reflected by a change in the ratio of gross primaryproductivity to community respiration (P/R) (Fig. 2).The zone through which the stream shifts fromheterotrophic to autotrophic is primarily dependentupon the degree of shading (Minshall 1978). Indeciduous forests and some coniferous forests, thetransition probably is approximately at order 3 (Fig. 1) .At higher elevations and latitudes, and in xeric regionswhere riparian vegetation is restricted, the transitionto autotrophy may be in order 1. Deeply incisedstreams, even with sparse riparian vegetation, may beheterotrophic due to side slope (“canyon”) shading.Large rivers receive quantities of fine particulateorganic matter from upstream processing of dead leavesand woody debris. The effect of riparian vegetation isinsignificant, but primary production may often be lim-ited by depth and turbidity. Such light attenuated sys-tems would be characterized by P/R < 1. Streams oflower order entering midsized or larger rivers (e.g.the 3rd order system shown entering the 6th order* 51 PERlPHYTedN 11132 CAN. J. FISH. AQUAT. SCI. VOL. 37, 1980ICOLLECTORSPHYTOPLANKiON9-lU-I2 RELATIVE CHANNEL WIDTH -FIG. 1. A proposed relationship between stream sizethe progressive shift in structural and functional attribof lotic communities. See text for fuller explanation.river in Fig. 1) have localized effects of varyingtude depending upon the volume and natureinputs..The morphological-behavioral adaptations ofning water invertebrates reflect shifts in types andtions of food resources with stream size (Fig. 1).relative dominance (as biomass) of the generaltional groups - shredders, collectors, scrapers (grand predators are depicted in Fig. 1. Shredderscoarse particulate organic matter (CPOM, > 1such as leaf litter, with a significant dependenceassociated microbial biomass. Collectors filtertransport, or gather from the sediments, fine andfine particulate organic matter (FPOM, 50 pm-1UPOM 0.5-50 pm). Like shredders, collectorson the microbial biomass associated with the p(primarily on the surface) and products of mimetabolism for their nutrition. Scrapers areprimarily for shearing attached algae fromjThe proposed dominance of scrapers followsprimary production, being maximized in midsi/
  4. 4. PERSPECTIVES 133t.II0Faa 0.1a2{‘-Relative Diversity of Soluble Organic Compounds-6t:-5 ;Index to Biotic Diversity>E4W>~‘2’3’4’5’6’7’8’9’~o’t~ 12STREAM ORDERFIG. 2. Hypothetical distribution of selected parameters through the river continuum fromheadwater seeps to a twelfth order river. Parameters include heterogeneity of soluble organicmatter, maximum die1 temperature pulse, total biotic diversity within the river channel,coarse to fine particulate organic matter ratio, and the gross photosynthesis/resp@ation ratio.with p/R > 1. Shredders are hypothesized to becdominant with collectors in the headwaters, re-flecting the importance of riparian zone CPOM andFP()M-UPOM derived from it. With increasing Streamb and a general reduction in detrital particle Size,wllcctors should increase in importance and dominate%a macroinvertebrate assemblages of large rivers(Fig. 1).* The predatory invertebrate component changes littlein relative dominance with stream order. Fish popula-:jions (Fig. 1) show a shift from cool water species low$I diversity to more diverse warm water communitiestag. Huet 1954).jnVcrtivores.Most headwater species are largelyPiscivorous and invertivorous specieskharacterize the midsized rivers and in large riversmC planktivorous species are found - reflecting the#mi-lentic nature of such waters.The expected diversity of soluble organic compoundsthrough the continuum is shown in Fig. 2 (dashedline). Headwater streams represent the maximum inter-&e with the landscape and therefore are predominantlyecumulators, processors, and transporters of materialsrrom the terrestrial system. Among these inputs are%rogeneous assembla.ges of labile and refractory dis-mlved compounds, comprised of short- and long-chain‘*@nits. Heterotrophic use and physical absorption of,Jb’l’ e organic compounds is rapid, leaving the moreFprractory and relatively high molecular weight com-j:;,punds for export downstream. The relative importancefof large particle detritus to energy flow in the systemis expected to follow a curve similar to that of thediversity of soluble organic compounds; however, itsimportance may extend further downstream.Thus the river system, from headwaters to moutg,can be considered as a gradient of conditions frog’ astrongly heterotrophic headwater regime to a seasoiial,and in many cases, an annual regime of autotrophy inmidreaches, and then a gradual return to heterotrophicprocesses in downstream waters (Fisher 1977). Majorbioenergetic influences along the stream continuum arelocal inputs (allochthonous litter and light) and trans-port from upstream reaches and tributaries (Fig. 1).As a consequence of physical and biological processes,the particle size of organic material in transport shouldbecome progressively’smaller down the continuum (re-flected by CPOM:FPOM ratio in Fig. 2, except forlocalized input of lower order tributaries) and thestream community response reflect progressively moreefficient processing of smaller particles.RIVERECOSYSTEMSTABILITYStability of the river ecosystem may be viewed as atendency for reduced fluctuations in energy flow, whilecommunity structure and function are maintained, inthe face of environmental variations. This implicitlycouples commcinity stability (sensu Ricklefs 1979) tothe instability (“noise”) of the physical system. In
  5. 5. -i134 CAN. J. FISH. AQUAT. SCI., VOL. 37, 1980highly stable physical systems, biotic contribution toecosystem stability may be less critical. However, inwidely fluctuating environments (e.g. stream reacheswith lage fluctuations in temperature), the biots mayassume critical importance in stabilizing the entire sys-tem. In this interpretation, ecosystem stability isachieved by a dynamic balance between forces con-tributing to stabilization (e.g. debris dams, filter feed-ers, and other retention devices: nutrient cycling) andthose contributing to its instability (e.g. floods, tem-perature fluctuations, microbial epidemics). in sys-tems with a highly stable physical structure, bioticdiversity may be low and yet total stability of thestream ecosystem still be maintained. In contrast, sys-tems with a high degree of physical variation may havehigh species diversity or at least high complexity inspecies function which acts to maintain stability.For example, in stream zones experiencing wide die1temperature changes, organisms may be exposed tosuboptimum temperatures for significant portions ofthe day, but over some range in the die1 cycle eachorganism encounters a favorable or optimum tempera-ture range. Under these conditions an optimum tem-perature will occur for a larger number of species thanif the thermal regime displayed minimum variance.Also, in the thermally fluctuating system, many popu-lations have an opportunity to process energy, and astemperatures oscillate around a mean position, variouspopulations may increase or decrease their processingrates. Thus, an important aspect of the predictablyfluctuating physical system is that it encompasses op-timum conditions for a large number of species. Thisinterplay between physical and biological componentscan be seen in terms of ecosystem stability by con-sidering the response of total biotic diversity in theriver channel as balanced against the maximum die1temperature range (AT max) (Fig. 2). Headwaterstreams in proximity to groundwater supply or infiltra-tion source areas exhibit little variation in AT max.With increased distance from subsurface sources andseparation of the forest canopy, AT max will attain itswidest variance because of increased solar input. TheAT max amplitude is greatly diminished in high orderstreams due to the buffering effect of the large volumeof water in the channel (Ross 1963). In headwatersprings and brooks, diversity may be low because bio-logical communities are assembled from those specieswhich can function within a narrow temperature rangeon a restricted nutritional base; the stability of thesystem may be maintained by the low amplitude ofdie1 and annual temperature regimes. Total communitydiversity is greatest in medium-sized (3rd to 5th orderin Fig. 2) streams where temperature variations tendto be maximized. The tendency to stabilize energy flowin midsized streams may be aided by high biotic diver-sity which mitigates the influence of high variance inthe physical system as characterized by AT max; i.e.variation due to fluctuating thermal regimes should beoffset by a high diversity of biota. In large rivers,stability of the system should be correlated with ,.duction in variance of die1 temperature. We wisb’temphasize that temperature is not the only factor rrsponsible for the change in community structure; it isimply one of the easiest to visualize. Other factorsuch as riparian influence, substrate, flow, and fqalso are important and change in predictable fas&downstream both absolutely and in terms of the relati,,heterogeneity of each.TEMPORAL ADJUSTMENTS IN MAINTAININGAN EQUILIBRIUM OF ENERGY FLOWNatural stream ecosystems should tend towards uaiformity of energy flow on an annual basis. Althaugithe processing rates and efficiencies of energy utilb,tion by consumer organisms are believed to approachequilibrium for the year, the major organic substrate,shift seasonally. In natural stream systems, both livin,and detrital food bases are processed continuou+,but there is a seasonal shift in the relative importannof autotrophic production vs. detritus loading and pracessing. Several studies (Minshall 1967; Coffman et rl197 1; Kaushik and Hynes 1971; MacKay and Kalfi1973; Cummins 1974; Sedell et al. 1974) have shownthe importance of detritus in supporting autumfiwinter food chains and providing a fine pa,rticle bagfor consumer organisms during other seasons of thyear. Autotrophic communities often form the majOtfood base, especially in spring and summer montht(Minshall 1978).Studies on headwater (order l-3) streams h&tshown that biological communities in most habitat1can be characterized as forming a temporal sequenccof synchronized species replacement. As a species COGpletes its growth in a particular microhabitat, it is I@placed by other species performing essentially the satifunction, differing principally by the season of gro@(Minshall 1968; Sweeney and Vannote 1978; VannOM1978; Vannote and Sweeney 1979). It is this contii$*ous species replacement that functions to distributetbutilization of energy inputs over time (e.g. Wall!@et al. 1977). Individuals within a species will tend tQexploit their environment as efficiently as possible. ‘@results in the biological system (composite species,@’semblage) tending to maximize energy consum$OfiBecause some species persist through time and bec?!tgnew species become dominant, and these too are$pploiting their environment as efficiently as posslblaprocessing of energy by the changing biological SYstemtends to result in uniform energy processing over tim&Thus, the biological system moves towards equilibmJnby a trade-off between a tendency to make most C@cient use of energy inputs through resource partiti?’ing of food, substrate, temperature, etc. and tend$$toward a uniform rate of energy processing thro@’out the year. From strategies observed on small-’$Jlmedium-sized streams (orders l-5>, we propose ,abiological communities, developed in natural strefl
  6. 6. PERSPECTIVES 135iedy.namic equilibrium, assume processing strategies~vOlvlngminimum energy loss (termed maximum+raling ” by Webster 1975).ECosys~~~P~~~~~~~~~A~~~~~~~C~~~~~~~~The dynamic equilibrium resulting from maximiza-tion of energy utilization and minimization of variationin its use over the year determines storage or leakageof mru Storage includes production of new tissue,nd physical retention of organic material for futureptoccssing. In stream ecosystems, unused or partiallyproc.essedmaterials will tend to be transported down-,trcam. This energy loss, however, is the energy income,Mg,,rher with local inputs, for communities in down-,tre.,nl reaches. We postulate that downstream com-,,,unities are structured to capitalize on these ineffi-ciencies of upstream processing. In every reach somenuneri;rl is processed, some stored, and some released.me amount released in this fashion has been used incrlcul;Lting system efficiency (Fisher 1977). Both theupstrc;im inefficiency (leakage) and the downstreamrdjtistments seem predictable. Communities distributedalong the river are structured to process materials(specificdetrital sizes, algae, and vascular hydrophytes)thereby minimizing the variance in system structuremnd function. For example, materials prone to wash-out, such as flocculant fine-particle detritus, might bemost clliciently processed either in transport or afterdeposition in downstream areas. The resistivity of finepurticle detritus to periodic washout is increased bytcdimcntation in depositional zones or by combinationIn a matrix with the more cohesive silt and clay sedi-ments.Thus, enhanced retention results in the forma-IiOn of a distinct community adapted to utilize thismcltcrial.The minimization of the variance of energynOW is t,he outcome of seasonal variations of energyinput rates (detritus and autotrophic production),Nuplcd with adjustments in species diversity, spe-:ialization for food processing, tempora1 expression ofhnctional groups, and the erosional-depositionalransport and storage characteristics of flowing waters.rJME INVARIANCE AND THE ABSENCE OF SUCCESSIONNSTREAMCOMMUNITIESA corollary to the continuum hypothesis, also arisingIron1 the geomorphological literature (Langbein andlcoPold 1966), is that studies of biological systemsMablished in ,a dynamically balanced physical setting:an be viewed in a time independent fashion. In the““text of viewing adaptive strategies and processes” continua along a river system, temporal change be-‘Omes the slow process of evolutionary drift (physical“d genetic). Incorporation of new functional com-‘Orients into the community over evolutionary timelecessitates an efficiency adjustment towards reducedcakage. In natural river systems, community structurelains and loses species in response to low probabilitycataclysmic events and in response to slow processesof channel development.The concept of time invariance allows integration ofcommunity structure and function along the river with-out the illusion that successional stages are being ob-served at a given location in a time-dependent series.The concept of biological succession (Margalef 1960)is of little use for river continua., because the com-munities in each reach have a continuous heritagerather than an isolated temporal composition within asequence of discrete successional stages. In fact, thebiological subsystems for each reach are in equilibriumwith the physical system at that point in the continuum.The concept of heritage implies that in natural riversystems total absence of a population is rare, andbiological subsystems are simply shifting spatially(visualize a series of overla.pping normal species-abun-dance curves in which all species are present at anypoint on the spatial axis but their abundance differsfrom one point to the next) and not in the temporalsense typical of plant succession.On an evolutionary time scale, the spatial shift hastwo vectors: a donwstream one involving most of theaquatic insects and an upstream one involving molluscsand crustaceans. The insects are believed to haveevolved terrestrially and to be secondarily aquatic. Sincethe maximum terrestrial-aquatic interface occurs in theheadwaters, it is likely that the transition from landto water first occurred here with the aquatic forms thenmoving progressively downstream. The molluscs andcrayfish are thought to have developed in a marine en-vironment and to have moved through estuaries intorivers and thence upstream. The convergence of thetwo vectors may explain why maximum species diver-sity occurs in the midreaches.ConclusionWe propose that the River Continuum Concept pro-vides a framework for integrating predictable and ob-servable biological features of flowing water systemswith the physical-geomorphic environment. The modelhas been developed specifically in reference to natural,unperturbed strea,m ecosystems as they operate in thecontext of evolutionary and population time scales.However, the concept should accommodate many un-natural disturbances as well, particularly those whichalter the relative degree of autotrophy: heterotrophy(e.g. nutrient enrichment, organic pohution, alterationof riparian vegetation through grazing, clear-cutting,etc.) or affect the quality and quantity of transport(e.g. impoundment, high sediment load). In manycases, these altera’tions can be thought of as resetmechanisms which cause the overall continuum re-sponse to be shifted toward the headwaters or seawarddepending on the type of perturbation and its locationon the river system.A concept of dynamic equilibrium for biologicalcommunities, despite some difficulties in absolute defini-
  7. 7. 136 CAN. J. FISH. AQUAT. XI.. VOL. 37. 1980‘44tion, is useful because it suggests that community struc- bLEOPOLD, L. B., AND W. B. LANGBEIN. 1962. The conceatLture and function adjust to changes in certain geo-morphic, physical, and biotic variables such as stream’ flow, channel morphology, detritus loading, size ofparticulate organic material, characteristics of auto-trophic production, and thermal responses. In develop-ing a theory of biological strategies along the rivercontinuum, it also should be possible to observe anumber of patterns that describe various processingrates, growth strategies, metabolic strategies, and com-munity structures and functions. Collection of ex-tensive data sets over the long profile of rivers areneeded to further test and refine these ideas,AcknowledgmentsThe ideas presented here have been refined through dis-cussions with our associates in the River Continuum Project,especially T. L. Bott, J. D. Hall, R. C. Petersen, and F. J.Swanson; and other colleagues including S. A. Fisher, C. A.S. Hall, L. B. Leopold, F. J. Triska, and J. B. Webster. Weare grateful to J. T. Brock, A. B. Hale, C. P. Hawkins,J. L. Meyers, and J. R. Moeller for their constructivecriticism of a draft version of the manuscript. This workwas supported by the U.S. National Science Foundation,Ecosystems Studies program under grant numbers BMS-75-07333 and DEB-7811671.BURTON, G. W., AND E. P. ODUM. 1945. The distribution ofstream fish in the vicinity of Mountain Lake, Virginia.Ecology 26: 182-194.CHORLEY, R. J. 1962. Geomorphology and general systemstheory. U.S. Geol. Surv. Prof. Pap. 500-B: 10 p.COFFMAN, W. P., K. W. CUMMINS, AND J. C. WUYCHECK. 1971.Energy flow in a woodland stream ecosystem. I. Tissuesupport trophic structure of the autumnal community.Arch. Hydrobiol. 68 : 232-276.CUMMINS, K. W. 1974. Structure and function of streamecosystems. Bioscience 24: 631-641.CURRY, R. R. 1972. 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