Developments in Stream Ecosystem ~heory' G. Wayne Minshall Department of Biology, Idaho State University, Pocatello, ID 83209, USA Kenneth W. 6umrnins2 Department sf Fisheries and Widlife, Oregon State University, CorvalEis, OW 9733 1, USA Robert C. Petersen Stream and Benthic Ecology Group, Institute of Limnology, University of bund, 5-22! 1 08 Lund, Sweden Colbert E. Cushing Environmental Sciences Department, Battelle Pacific-Northwest Laboratories, Richland, WA 99352, USA Dale A. Bruns EC & C Idaho, dnc., Environmental Sciences Section, ddaho Falls, ID 834 15, USA

Forestry Sciences Laboratory, United States Forest Service, Corvalbis, 88 9733 1, USA and Robin L. Vannote Stroud Water Research Center, Academy of Natural Sciences si Philadelphia, Avondale, PA 198 1 1, USA Minshall, G.We, K. W. Curnrnins, R. C. Petersen, C. E. Cushing, B. A. Bruns, J.R. SedeH, and W. %.Vannote. 1985. Developments in stream ecosystem theory. Can. J.Fish. Aquat. Sci. 42: 1045-1055. Four significant areas ~f thought, (I) the holistic approach, (2) the linkage between streams and their terrestrial setting, (3) material cycling in open systems, and (4) biotic interactions and integration of community ecology principles, have provided a basis for the further development of stream ecosystem theory. The River Continuum Concept (RCC) represents a synthesis of these ideas. Suggestions are made for clarifying, expanding, and refining the RCC to encompass broader spatial and temporal scales. Factors important in this regard include climate and geology, tributaries, location-specific lithology and geo- morphology, and long-term changes imposed by man. It appears that most riverine ecosystems can be accolr~modatedwithin this expanded conceptual framework and that the WCC continues to represent a useful paradigm for understanding and comparing the ecology of streams and rivers. Quatre importantes Iignes de pensee fournissent une base pour Irelaboration d'une thkorie detailfee sur les $cosyst&mes lotiques : (1) I'approche holistique; (2) le lien entre les cours d'eau et leur emplacement terrestre; (3) $ecycle de matieres dans les systemes ouverts; et (4) Bes interactions biotiques et 01int6gration des principes sus B'ecologie des comrnunaut6s Botiques. Le concept de continuum du milieu fiuvial repre- sente une synthese de ces idees. On presente des suggestions pour I'eclaircissement, B'expansion et le perfectionnernent de ce concept de facon a y inclure des echelles spatiale et temporelle plus larges. Sous ce rapport, les facteurs importants comprennent le climat, la geologie, les tributaires, la litholcsgie et Ba geomorphologie de chaque emplacement et les perturbations humaines long terme. II semble que %a plupart des $cosystemes fluviaux peuvent Gtre inclus dans cette structure conceptuelle elargie et que le concept de continuum represente toqjours un paradigme utile pour comprendre et comparer I'ecologie des ruisseaux et des rivieres. Received March 26, 1984 Re~ule 26 mars 1984 Accepted January 25, 1985 Accepte Be 2%janvier 198% (J77.35)

t the beginning of this century, limnslogists studying led several researchers to seek simplifying generalities in terns streams focused in great detail on the morphology, life of the observed fauna (e.g. Steinmann 1915; Thienemann 1925; cycles, behavior, and trophic relations of stream organ- Carpenter 1928; Needham and Lloyd 1930). These early efforts isms (e. g . Steinmann 1907; Wesenberg-Lund 191 1 ; stressed observation, and although a quantitative approach was Thienemann 1912). The observed diversity of the stream biota soon adopted, the approach and the investigations conducted through the 1950's were largely descriptive and autecological in 'Contribution No. 29 sf the River Continuum Project. nature. Since that time, the direction of stream research has 'Present address: Appalachian Environmental Laboratory, Univer- changed to incorporate a more synecological and holistic sity of Maryland, Frostburg, MB 21532, USA. approach. Inthe following we briefly discuss four major areas in

Can. 3. Fish. Aquat. Sci., Vol. 42, 6985 1045 TABLE1. Major areas in which developments in stream ecosystem cerning the magnitude and variation through time and space of theory have taken place in the past decade. the organic matter supply, the structure of the invertebrate community, and resource partitioning along the length of a 1. hogres~iionfrom an individualistic to a holistic viewpoint river. These views are summ~zedby Cuskiwg et al. (1983b) 2. Realization of the critical lidcage between a stream and its temes- who stated, ". .. streams are best viewed as gradients, or con- trial setting tinua, and that classification systems which separate discrete 3. Development of ideas about material cycling in open systems reaches are of little ecological value." The WCC is undergoing 4. Recognition of the importance of biotic interactions to the stream evolution, testing, and refinement - some aspects of which community will be addressed below. However, even if nothing else were to come from it, this paradigm has served to focus attention on which changes in perspective regarding stream ecosystems have rivers as integrated systems and to stimulate the fornulation and occurred (Table B) and which we believe form the foundation testing of systems-level hypotheses which have helped to move for further advances in stream ecology. With this foundation in lotic ecology from a descriptive to a predictive mode. mind, we then propose several adjustments to our earlier concept of riverime ecosystem dynamics (Vannote et al. 1986) Realization of the Critical Linkage between the Stream md Its Terrestrial Setting which have grown out of our continued interest in this area (Cummins et al. 1983; Cushing et d. 1983b; Minshall et al. Bllies (1961), Illies and Botosmeanu ( 1963), and Ross (1963) 1983, 1985; Petersen 1984; Sedell and Froggatt 1984; Vannote were among the first to note similarities among stream commu- and Minshall 1982) and out of recent studies by others. nities over broad geographic areas. Ross in particular was struck by the association between stream macroinvertebrates and Major Contributions in Stream Ecosystem Theory terrestrial biomes or what he termed the landscape aspect. He postulated the importance to the stream of the combination of Progression from an hdividudistic ts a Holistic Viewpoint climatic conditions necessary for the existence sf such vegeta- A truly holistic view of streams as ecosystems did not begin tion zones plus the factors imposed by the physical nature of the until the late 19509s,being strongly influenced by Eugene and vegetation itself. The importance of the land-stream interaction Howard Odum (e.g. Odum 1957; Teal 1957) and Ramon also is implicit in the watershed approach to the study of Mugdef (e.g. Mzgalef 1960). The energy budget approach of terrestrial ecosystems (e.g. Likens et al. 1977). Odum for stream segments led to a flood of stream ecosystem Also in the early 19609s,Hynes (1963) published the first of budgets (Wmen et al. 1964; Mann et al. 1972; Hall 1972; a series of studies which chew attention to the importance of Fisher and Likens 1973; Westlake et al. 1972) for both large and terrestrially derived (allochthonous) detritus to stream dynam- small streams which has continued to the present (Cummins et ics. He summarized the importance of the stream-land linkage al. 1983). in his paper "The stream and its valley9'(Hynes 1975). Among The past BO yr also have seen a shift in perspective from that other things, he noted that the slope of the valley, the depth and of viewing streams, or different stretches within a stream, as permeability of the soil, and the patterns of precipitation greatly individual entities to that of a synthetic (integrated) view which affect the pattern of flow and dissolved organic matter content of seeks generalizations about streams as ecosystems (e.g. Cum- the stream. Hynes (1975) and others have stressed the impor- mins 1974; Webster 1975; Wallace et al. 1977; Newbold et al. tance of the type and density of the terrestrial vegetation and of 1982a, 1982b; Minshall 1978; Minshall et a%.1983b). The its chemical and biological dynamics once it reaches the stream. prevalent synthetic view of stream ecosystems is that of the The pioneering work in this area emphasized conditions in piver Continuum Concept (RCC) (Vannote et al. 1980), its streams draining forests and consequently, allochthonously mtecedmts (see Ide 1935; Margalef 1960; Hllies 1961; Hynes derived organic matter. But the broader implication of these 1963; Ross 1963), and correlaries (Wallace et al. 1977; studies is clear: the terrestrial setting for the stream - be it Newbold et al. 1981, 1982a, 1982b; Elwood et al. 1983; Ward forest, desert, taiga, or grassland - is crucial to the operation of and Stanford 1983) which emphasize the unifying aspects of stream ecosystem processes. Additional support for this idea Wowing water (pdcularly geomorphology and fluid hydraul- has come from investigations by Fisher and Likens (1973), ics) in structuring stream communities and providing a templet Minshall (I 978), Cushing et al. (1980), Fisher et al. 419821, for stream ecosystem function. Historically, a number of stream Molles (1982), Cummins et al. (1984), Minshall et al. (1983b), ecologists have recognized shifts in community structure along Naimm (1983), Strayer (1983), and others. Early research on the course of a river. Most focused sn macroinvertebrates and the importance of allochthonous detritus has in turn given rise to fish (see papers reviewed by Hawkes 1975) and many viewed additional major developments in the processing of this mater- the relationship as a series of distinct zones (Illies 1961) rather ial, particularly by microbes (see reviews by Anderson and than a gradually integrating continuum (but see Maitland 1966). Sedell 1979 and Bird and Kaushik 1981). Few attempted to treat streams and rivers holistically, and Rorto a surge sf interest in the role of allochthonous detritus particular1y to view them as ecological systems 4e.g. Rzoska in streams, considerable attention had been given to the primary 1978). Margalef (1960) and Mann (1979) are noteworthy producers (e.g. Butcher 1932, 1933, 1948, 1946; Blum 1957; exceptions. Douglas 11958; Gumtow 1955) and to their role in stream food The WCC (Vannote et al. 1980) conceptualizes the entire chains (e.g. Percival and Whitehead 1929; Jones 1950). With fluvial system as a continuously integrating series of physical the interest generated by almost continuous breakthroughs in the gradients and associated biotic adjustments. Streams are seen as area of alIochthonous detritus dynamics, the important support- longitudinally linked systems in which ecosystem-level pro- ive or primary role of autotrophy in stream ecosystems was cesses in downstream areas are linked to those in upstream virtually ignored. By the mid-1970qs, however, stream re- areas. This approach has provided useful generalizations con- searchers began to direct their attention outside of forested

1846 Can. J. Fish. Aguat. Sci., Vol. 42, 1985 headwater strems. It became clear in the broader river basin Recognition of the Importance of Biotic Interactions within the and interbiome contexts, as well as in moderately shaded Stream Community headwaters everywhere, that autotrophs contribute significantly One of the underlying biases sf research in lotic ecology has to ecosystem dynamics in both the mass and quality of material been that streams are highly variable and hence unpredictable. they provide (Minshall 1978). Now it is generally recognized Consequently, stream communities often are regarded to be that both terrestrial plant debris and aquatic primary production dominated by physical factors and highly individualistic (e.g are important sources of simple carbon compounds and that they . Winterbourn et al. 1981; Grossman et al. 1982). However, we commonly complement one another, bath seasonally and along suggest that while stream communities are not strictly detemin- a river, to provide a more reliable and varied food base for lotic istic, neither are they a hodge podge of organisms resulting consumers than either would do alone (Hornick et al. 1981; solely from stochastic events (see e.g. Bmes and Minshall Cushing and Wolf 1982; Gregory 1983). 1983). The probability theory of fluvial geomo~hologists (Leopold and Langbein 1962), and the thenmad equilibrium Development of Ideas on Material Cycling in Open Systems theory (Vannote 1978; Sweeney and Vannote 1978; Vmnote Webster (1975) pointed out that nutrients in a stream do not and Sweeney 1980), clearly suggest that there is a high degree cycle in place, but are displaced downstream as they complete a of predictability (and hence opportunity for determinism) em- cycle. He described this coupling of transport and cycling as bedded in the apparent stochasticity of streams. Also, as pointed "spiraling'bnd suggested that the ability of a stream to utilize out by Turelli (1981) and Schoener (1982), relatively small nutrients is associated with the tightness and magnitude of the amounts of environmental fluctuation have small biological spirals. Webster (1975) and Webster and Patten (1979) pro- effects so that the simpler deterministic approach often is posed nutrient spiraling as a mechanism to account for the adequate. In addition, not only does the opportunity for apparent ability of stream ecosystems to withstand and recover density-dependent (deterministic) phenomena exist in streams from disturbance. Later, Wallace et al. (1977) applied the idea (Howitz 1978; Shiozawa 1983; Minshall et al. 198%)but, in in describing the role of filter feeders in strems. They suggested fact, biotic determinants are operable (e. g . Hildebrand 1974; that filter feeders, though their capture of seston, impede the Hildrew and Townsend 1976; Bohle 1978; Peckasky 8979, downstream transport of organic matter and sene to reduce the 1986, 1981; Peckarsky and Ddson 1986a, 1980b; Hart 1981, distance between spirals. The present state of our knowledge 1983; Wiley and Kohler 1981; Allan 1982; Fisher et al. 1982; in this area is largely a result of the mathematical models Bruns and Minshall 1983; McAuliffe 1983, 1984). developed by Newbold and his colleagues at Oak Ridge The RCC is primarily a detemimistic mdel to some National Laboratories (Newbold et al. 1981, 1982a, 1982b; (Winterbourn et al. 1981; Winterbourn 1982; Grossman et a%. Elwood et al. 1983). These collective studies are termed the 1982); others view it as largely stochastic (Bamuta and Lake nutrient spiraling concept. 1982; Fisher 1983). Actually it lies somewhere in between (see Hm four stream systems studied in an interbiome test sf the e.g. Levins and Lewontin B 986). For example, the WCC focuses RCC (Minshall et al. 1983b), all first-through third-order on physical features of the environment as providing much of stations had "tight" spirals associated with a high retention the habitat templet (sensu Southwood 1977) for stream com- capacity; the larger stream locations (fifth- to seventh-order) all munities, but it also is founded on trophic responses that are had long distances between spirals. Rate of recycling was generally considered to be largely deterministic (Richardson considered "fast" for about half of both the small- and large- 1986; Fisher 1983; Gorman and Karr 1978; Matthews 1982). sized streams, indicating differences in degree of biological In addition to the relative spatial (longitudinal) and temporal influence (Minshall et al. 1983b). But in all streams studied, predictability now believed to exist in stream environments, spiraling distance was determined primarily by current velocity certain characteristics of the biota allow them to recover quickly and the presence and effectiveness of physical retention devices; from environmental fluctuations and thereby minimize the biological processes played a lesser quantitative role. Thus, in effects of what commonly are viewed as significant perturba- many cases, the retentive capacity of the stream exerts a tions. In particular, the mobility, short life cycles, high repro- significant influence on ecosystem dynamics. In another study, ductive rates, and ability to encyst or to burrow into the sub- Wallace et al. (1982) have shown that experimental reduction of stratum allow stream organisms to avoid or adjust quickly to the lotic insect fauna reduces the breakdown, utilization, and regular variations in the stream environment (e.g. flood resets) subsequent downstream transport of organic matter, indicating and to treat these variations essentially as time constants (see that consumers are important in regulating energy flow and Hutchinson 1953) (e.g. Lehrrakuhl and Anderson 1972; Min- nutrient cycling in stream ecosystems (see also Brock 1967; shall et al. 1985; Wiwget 1984). Studies of invertebrate coloni- EBwd et al. 1983). Thus, physical retention and macroinver- zation on introduced substrata (Minshdl 1984) or following tebrate processing are important mechanisms, along with the "catastrophic" flood and dewatering (Minshall et al. 1983a) microbial action mentioned earlier, for closing or tightening the also support the idea that rapid recovery from physical distur- recycling process in streams and preventing the rapid through- bance in streams is commonplace. Thus, these reset events are put of materials. not really "stress," at the system level at least, and community Together, the WCC and nutrient spiraling concepts point to a structure is dependent on them in order to persist. Further, shift from the view of streams as discrete segments to one of a within the periods of physical variation (e.g. between spates) continuous, interacting set of biological and physical processes there commonly are sufficient time spans to permit the establish- along the stream gradient (Newbold et al. 1982b; Cushing et al. ment and maintenance of equilibrium conditions. There seem 1983b). This shift should lead to a reevaluation sf the idea that to be certain periods during the year when equilibrium (hence strems are conduits that simply transport materials from the deterministic or density dependent) conditions may prevail. land to the sea, and hence to a greater appreciation of the Most lotic consumers are trophic generalists or selective metabolic and retention roles streams play. omnivores (Hutchinson 1981) that feed in proportion to the

Can. J. Fish. Aqmt. Sci., Vol. 42, 1985 18347

':-:~~I.::~.:.m~~~~~:~~.:.-.:.~.:.m:".:~.:.:.~.m.:.~.-.:.~.:.m<.~.~.~.~.~:.:.~;.:.~.:.:.~x.~:.:.:.~,I.~.:? .m~<.~.>m.:.~,~-.,~7.~ . -. ---.. -r ...-. .-,m>.7.w.v.m.,.-,.,.,T, .. .. _ ...... m!.;.~.I..x'_.'_..'_..'_..'_..'_., .'_..-..------broad kinds and amounts of foods available. Thus, the belief tions from the genera1 pattern are a predictable result of specific that competition for food is not a major factor in structuring variation in climate and geology, tributary influence, and loca- stream comunities is related to the widely held view that tion-specific lithology and geomorphology, as well as of long- streams are highly variable and hence stochastic in nature. The tern changes imposed by man. These factors are discussed RCC (and the data subsequently collected to test it: Hawkins and more fully below. Sedell 1981; Minshall et al. 1982, B983b; Cushing et al. 1983a) Climate and Geology suggests that fwd relationships are important in structuring stream communities (see also Mackay and Wiggins 19778; Many geologists have accepted for sometime the idea that Molles 1982). Lotic consumers also show morphological climate and geology are the ultimate determinants of river speciallization with respect to food-gathering structures (Cum- morphology through their effect on discharge and sediment load mins and Klug 1979; Hde 1981 ; Merritt and Curnmins 1978) (e.g. Leopold and Wolman 1957; Gregory and Walling 1973; resulting in considerable partitioning of food resources based on Lotspeich 1980). Together, climate and geology provide many particle size, palatability, particle type, and constituents (Cum- factors influencing the characteristics of a river basin or mins 1973; Wallace et al. 1977; Anderson and Cummins 1979; watershed ecosys~m.Tnis has been accentuated by (1) the Cummins and Klug 1979; Sheldon 1980; McAuliffe 1983). realization that river morphology and discharge can provide Ongoing competition for food has been demonstrated by Hart the templet for stream ecosystem structure and function (Cuq (1983) and McAuliffe (1983, 1984) whereas in other cases, 1972, 1976; Homitz 1978; Vannote et al. 19801, (2) exam- resource partitioning (resulting in the 'kontrslled" avoidance ination of stream ecosystem dynamics over different North of competition) is evident (Minshall 1968; Mackay and Kalff American biornes (Minshall et al. 1983b), (3) efforts to classify 1969; Vannote 1978; Bans and Minshall 1983). watersheds as ecosystems (Warren 1979; Lotspeich 19801, and Another important step in the development of stream ecosys- (4) attempts to evaluate streams outside the north-temperate tem theory is the integration of certain principles of community region in a broader context (Winterbourn et al. 1981). Climate, ecology within the broader geomorphic/ecosystekncontext. The of course, affects the type and density of vegetation (Kuchler RCC, in conjunction with recent developments in population 1964; Bailey 1978; Garrison et al. 1977) and the effect sf and community ecology (e.g. Cody and Diamond 1975; climate on the stream is in turn intimately bound up with the Scsuthwood 1977; Anderson et al. 1979; Brown 1981; May pattern of vegetation (Ross 1963; Gregory and Walling 1973; 198I), suggests that the organization of lotic macroinvertebrate Hynes 1975). For example, precipitation and vegetation interact communities can be explained by the mean state of environmen- to affect runoff and erosion and hence sediment yield and tal variables and their degree of temporal variability and spatial organic matter loading. In oher words, the action of flowing heterogeneity. The gradient from headwaters to high-order water, and hence such things as bed fom and stability and reaches may be viewed as a spectrum of differentially variable organic matter storage and transport, varies with climate (e.g . habitats, and the patterns of species diversity for the total Peltier 1958; Gregory and Walling 1973; Minshall et al. 1983b). community and for specific functional groups appear to be Geology affects the relative erosiveness of the parent material interpretable within the context of habitat templets (see South- in a drainage basin and, consequently, watershed topography, wood 1977). For example, we have found that components of chemical load, bed composition, and the like. Geology gives community organization of lotic macroinvertebrates (e.g . spe- each physiographic region its distinctive appearance. Within cies richness, niche breadth and overlap, and community each region, geologic structure, acted on by processes con- complexity and stability) can be explained by the degree of trolled by climate, gives shape to each land form (Lotspeich temporal variability adspatial heterogeneity (D.A. Bruns, A. 1980). Recently, Strayer (1983) has shown that stream size B . Male, and G. W. Minshall, unpubl. data; Minshalil et al. and surface geology are the major factors controlling mussel 198%). This structural approach complements previous RCC distribution in the streams of southeastern Michigan. Others studies which emphasized ecosystem function. It also serves to (e.g . Minshall and Minshall1978; Magdych 1984) have demon- introduce a conceptual basis for understanding lotic communi- strated how differences in water quality mitigated or caused by ties which in the past have been studied predominately from a the underlying geology can profoundly affect major segments descriptive aspect. And finally, it points the way for experimen- of the benthic invertebrate community. In fact, as noted by tal investigations whereby mechanisms of community organiza- Magdych (1984), these findings suggest the need to modify the tion can be addressed directly. RCC to dlow for multiple gradients if it is to be useful in the generation of ecological models in regions where abnormal (nongradient) chemical inputs are impowant. Expansion of the River Continuum Concept &o a The riparian system (Swanson et al. 1982) can override the Broader Geographic and Historical Model effects of climate and geology to some extent, especially in small (first to fourth) order streams below the tree line. The The RCC defines a standard for natural, unperturbed lotic riparian vegetation is seen as a "ribbon of continuity" that makes systems against which existing streams can be compared. The many headwater streams look very similar worldwide (Cum- utility of the RCC lies in its identification of a set of general mins et ale 1984). Within rather broad climatic limits undis- conditions and relationships that can be used to study and turbed headwater streams have temporally stable accumulations compare stream systems. In this context, the RCC provides a of large woody debris. Also, these seem to be biochemical framework for understanding the ecology of streams and rivers equivalents along these streams in terns of riparian plant inputs and is not intended as a description of biological components sf that show fast, intermediate, and slow rates of decomposition all rivers in the individualistic context (i.e. it is an abstraction in (K. W. Cummins, pers. obs.). Thus, shading, geomorphic the sense of Levins and Lewontin 1980). The WCC defines a structuring by woody debris, and $iochemical%yanalogous general condition and in so doing explains the interrelationships inputs from the riparian system result in similar functional group between sets of unique conditions. Regional and local devia- assemblages in spite of differences in geology and climate.

Can.J. Fish. Aquaf. Sci., Vol. 42, 1985 Further, it is evident that even in larger stretches of a river el SYAX (greater than fourth-order) the floodplain or riparian zone cannot MAXIMUM be uncoupled from the stream channel itself. The RCC (Vannote >% I SPECIES et ale% 980) stressed that the influence of the terrestrial system on I RICHNESS a stream diminishes as the stream becomes larger and that autochthonous carbon inputs to higher order streams (seventh- to tenth-order) should increase while allochthonous inputs of carbon decrease. However, Sedell and Froggatt (1984) have shown that many rivers in pre- and early settlement times also were heavily subsidized by their local flood plains. This subsidy took several forms including local litter fall from extensive riparian forests and carbon inputs from sloughs and side chan- nels. In addition, Triska (1984), in his review of the Red River, reported that between 1828 and 1876, woody debris from the riparian zone of this eighth-order stream was enough to cause log jams dong a 400- to 508-km stretch of the river. Besides being a direct carbon input to the river, the logs acted as a physical retention device that caused flooding of the riparian zone, drowning of trees, and a further addition of material to the river. FIG 1 The effect of a tributary on stream ecosystem parameters depends on where the tributary enters the larger stream. If a tributary Tributary and Related Influences enters above the point of system maximum it will drive the strearn to that maximum. Hf it enters below it will move it back to reset that Tributary additions to master strems have a significant parameter. In this example, species richness reaches a maximum in influence on the continuum pattern. Where tributary inputs third- to fourth-order streams. Tributaries entering above this cause deviate from their usual summative influence on river water the community to reach its maximrnm sooner. Tributaries entering quality, expected (sensu Vannote et al. 1980) trophic and below the peak increase richness by driving the system back. community patterns may be altered. The magnitude of change high-gradient , turbulent, rocky headwaters to low-gradient, depends on tributary size (Fig. I), regional drainage density, sluggish, mudbottomed rivers, and with a channel pattern vegetation cover, land use, and lotic versus lentic source of progressing from straight, through braided, to meandering. inputs. A variety of effects may be envisioned. Concentrations Further reflection indicates that the ideal rarely is so clearly of nutrients and/or food items in the tributaries may be higher achieved. For example, in the Salmon River the steep-gradient than in the mainstream and serve to increase the mainstream headwaters descend immediately to a low-gradient glacial levels. This could lead to increases in algal production or filter valley, and channel pattern varies from straight to meandering feeder densities if limiting concentrations were exceeded. to braided and back to meandering, thew straight, then braided. Alternatively, a tributary may serve to dilute the mainstream In addition to differences in hydraulic dynamics associated with concentrations or to provide a sustained input of coarse materials these changes, one may expect major shifts in such factors as to a mid- or high-order master channel which could result in a biotic structure and function similar to upstream reaches. These riparian inputs, channel storage of organic matter, and inter- postulates have been substantiated by the studies of Bavns et al. change with the flood plain (Fig. 2). (1984) in the Middle Fork of the Salmon River. Not only were Studies on the distribution of Gonidea angu&ataand Mar- tributary influences demonstrated but the differing effects of garitiferafalcata in relation to substrate and periodic temporal small and large tributaries were contrasted. variations in runoff amply demonstrate the influence of local lithologic and gmmoi-phic processes on the relative abundance A corollary to the RCC has recently been developed which emphasizes the impact of man-made dams on stream dynamics and population size structure of these organisms (Vanwote and (Hauer and Stanford 1982; Ward and Stanford 1983). Ward and Minshall 1982). @onidpa angukata was predominant and Stanford (1983) visualized the effect of relatively high dams as possessed high population densities on stabilized sand and gravel bars; M.fakcata was predominant in cobble and boulder producing a shift in stream ecosystem structure and function in areas. On cobble areas unprotected by block boulders, most either an upstream or downstream direction depending on the size of the stream on which the dam is located and the position of Margariti$era populations were relatively young (20-48 yr) and had an approximately normal size distribution. On boulder outflow relative to the thermwline. The expected effects are controlled substrates, the size frequency was skewed towards similar to those postulated for tributaries. Extensive pools, the larger, old ( 108 yr) age-classes Periodic floods, perhaps whether generated by dams or associated with low gradient . approaching a 58- to 108-yr return period, apparently cause (Brown and Brown 1984), appear to have predictable impacts high mussel mortality by bed scour in most canyon habitats. on stream conditions which represent local modifications to the trend expected for rivers in general (Vannote et al. 1980; This periodic scour keeps populations relatively young with an approximately normal size distribution. In contrast, where Magdych 1984). mussel beds are protected by a field of large block-boulders, the dissipation of kinetic energy during floods is primarily through Local Lithology and Geomorphology turbulence within the water column rather than bed shear. Hn Here the focus is on location-specific variations in local these rare but highly stable habitats, M.fakcata attains maximal geology and in the morphology of the river channel (perhaps density, old age, and a population structure that is skewed most evident in flow patterns and substratum composition). The towards large individuals. Thus, local (site specific) conditions classic view of a river system is one of progressive change from may dter both community and population structure and con-

Can. /. Fish. Aquat. Sci., Vol. 42, 1985 PARAMETER

Stream Surface

FIG.2. SMts in important ecosystem parmeters in response to changes in geomorphic features re- sulting from differences in hydraulic dynamics at different points in a stream reach.

found efforts to obtain an accurate holistic view unless they are domestic animals; and fire suppression. These changes, many taken into account. of them so subtle as to go unnoticed, have made it difficult for modem stream ecologists to place their findings in perspective Long-Term Effects of Man with reference to natural, pristine conditions or to obtain mean- ingful measurements of certain aspects such as carbon budgets Tswnsend (1980) contended that the spatial aspect of streams (Cumins et a%. 1983). Nevertheless, incopration of a broad can be viewed without regard to the temporal. However, recent research (Molles 1982; Cummins et al. 1983, 1984) suggests temporal perspective has begun to aid substantially in the understanding of stream ecosystem structure and function both that both a spatial and a temporal scale must be employed to past and present (Molles 1982; Romme and Knight 1982; Sedell obtain m understanding of stream dynamics. This is particularly and Luchessa 1982; Sedell et al. 1982; Cummins et al. 1983; important, since our concept of aboriginal conditions presently F~dingand Claflin 1984). is so poor (at least 188 yr of nowendemic, man-made effects in yr Side channels, braided channels, and of-channel backwaters South America and Africa, up to 4400 in North America, and (seasonal) may have an effect similar to that of tributaries. 1000 yr or more in Europe). However, the massive snag removal that took place on essen- The impact of man on the structure of ecological communities tially all North American rivers in the late 1800's and early is a common theme running though modern environmental 1900's, together with large-scale channelization efforts, has science and pollution biology. However, mam's greater responsi- converted the majority of streams greater than fourth- to bility for wholesale alteration of cornunities and extinction sf sixth-order from heavily braided or meandering systems to species over the entire historical record has only recently been single, relatively straight channels (see e.g. Cummins et al. suggested (Goudie 1981 ; Lewin 1984) and may be a much larger effect than thought. Hn stream research the intimate connection 1984; Fremling adClaflin 1984; Sedell and Froggatt 1984; Triska 1984). Thus, instead of a single fifth-order channel there between the stream md the adjacent terrestrial environment has might have been five third-order channels. Among other things, focused attention on how man-caused disturbances such as fire this would mean that the riparian effect was much different prior (Minshall et al. 19811, silviculture (Molles 19821, riparian control (Curnmins et al. 1984; Sedell and Froggatt 19841, and to such alterations. Wood, removed by man, probably produced tree debris removal (Tkska 1884) have potential for long-term a very different channel fom in midsized rivers than that found changes in stream systems. This awareness has been accentu- today. Braids, off-channel backwaters, and side-channel streams probably caused many midorder rivers to behave more like ated by the need for resource managers to know the natural, headwaters. However, intensive beaver activity prior to the presettlement conditions of areas under their supervision mid-1800's may have had a countereffect in that the extensive (Romme 1982; Romeand Knight 1982; Sedell and Luchessa impoundments the beaver created would have made headwater 1982). Various postsettlement alterations likely to have had streams wider, slower, and more rnidorder in character. major impacts on the nature of stream ecosystems include the foliowing: urbanization; deforestation; tillage; irrigation; dam implications to Seeam Ecology construction; channel alteration for navigation, logging, min- ing, and flood control; virtual exterraaimation of beaver, bison, Deviations in regional and local conditions (associated with and other large mmmds including aboriginal man; grazing by such things as climate and geology, tributaries and related

1050 Can. J. Fish. Aqua. Sci., VuF. $9, 1985 FOREST I I 1 2 3 4 5 6 7 8 9 DESERT STREAM ORDER FIG. 3. Expected changes in particulate organic matter inputs and functional feeding group relation- ships dong a river system (after Vannote et al. 1988). The abscissa is shown as a "sliding scale" to emphasize the fact that different streams enter the continuum at different points. In the two eases illus- trated, forest streams begin with a strong terrestrial influence (reflected by a predominance of dlmh- thonous organic matter and detrital processors) whereas desert streams, due to the lack of shading and reduced influx sf aIIochthonous detritus, enter the sequence at a point dispjaced to the right and equiva- lent to a more ddownstrem position of the forest stream. influences, local lithology and geomorphology, and long-term streams below the springs are dominated by primary production effects of man) suggest that adjustments need to be made in the (macrophytes ad periphytic algae). The grazer functional RCC if it is to be applied across biomes, especially when group, consisting of snails, dominates the macrobenthos, and environmental extremes or unusual situations are involved. For due to a lack of particulate organics, filter feeders are rare but example, in some wooded pats of Africa (e.g. Zimbabwe), collector-gatherers feeding on decomposing macrophytes are large amunts of come particulate leaf detritus do not reach the abundant. In short, the system structure is similar to a midorder stream. Most leaf material is processed by terrestrial detritivores woodland river but without the filter feeders that normally feed during the dry season, and by the time the rains come, the coarse on the upstream losses of particles. The need to accommodate material has already been reduced to fine particles (W. C. these sorts of changes are explicit in the original fornulation of Petersen, pers. obs .) . Consequently, an important functional the WCC (Cumrnins 1975; Minshall 1978; Vannote et a$. 1980) feeding group (shredders) is reduced or absent from such but the point has been missed by some investigators (e.g. streams, and a sequence of adjustments in invertebrate trsphic Winterbourn et a$. 6981; Rounick and Winterburn I983a). structure results (Fig. 3). The absence of large-particle shed- Because streams are ecosystems with numerous linkages and ders also appears to be typical of unstable, poorly retentive feedback loops, care must be taken in developing and testing headwater streams in disturbed watersheds (Wounick and stream ecosystem theory that critical interrelations are not Winterhum %983a, 1983b). As another example, there are excluded or unccsnscisusly left uncoupled. For example, some many areas of the world in which, for one reason or another, investigations have taken the absence sf macroinvertebrate headwater streams (first-order woodland streams) simply do not shredders from certain headwater streams as failing to support occur. In areas dominated by porous calcareous geology such as the RCC (e.g. Winterbourn et al. 1981). But the various facets in Florida (Odum 1957) or the western mountains of Jamaica of organic matter processing cannot be viewed in isolation. (Petersen 1984; pers. obs.), the rivers begin as larger sized For one thing, it is unreasonable to expect macroinvertebrate systems. In these givers it still is expected that the basic shredders to be present if the coarse particulate organic matter sequential model will kx followed but the river will skip the supply is inadequate or unreliable. Also, various trade-offs may upper small-order categories and the effect that those small occur in which the macroinvertebrate shredders may be replaced systems have on the downstream reaches will be removed. In or supplemented by other size-reducing mechanisms (microbes, Jamaican springs, which begin with a width of 38-48m, the beaver, physical fragmentation). The important point in terms

CQ~.I. Fish. Aquat. Sci., Vol. 42, 1985 105 B of the RCC is that when coarse particulate organic matter enters References a stream it will be reduced to fine particulate organic matter and that this phenomenon generally is quantitatively most signifi- ALLAN,9. D. 1982. Feeding habits and prey consumption of thee setipalpian stoneflies (Plecoptera) in a mountain stream. Ecology 63: 26-34. cant in the headwaters of a river system. It is only when these ANDERSON,N. H.,AND K. W. CUMMINS.1979. Influences of diet on the life features are shown to be unimportant or absent that this aspect of histories sf aquatic insects. J. Fish. Wes. Board Can. 36: 335-342. the RCC can be shown not to hold. Furthermore, abnormal ANDERSON,N. H., AND 9. R. SEDELL.1979. Detritus processing by (often anthropogenic) factors may obscure or obliterate the macroinvertebrates in stream ecosystems. Amu. Rev. Entomol. 24: 351-377. expected response such as when the entrance of geothermal ANDERSON,R. M., B. D. TURNER,AND L. R. TAYLOR[ED.]. 1979. Population waters or other adverse environmental conditions selectively dynamics. Blackwell Scientific. Oxford. 434p. eliminates certain components of the community. Sometimes BAILEY,R. G. 1978. Description of the ecoregions of the United States. U.S. such influences may occur on a large scale (e.g. acid rain), Forest Service, Inkmountain Region, Bgden, UT. 77 p. further obscuring the relationship and giving the impression that BA~UTA,L.A., AND P. S. LAKE.1982. On the value of the River Continuum Concept. N.Z.J. Mar. Freshwater Res. 16: 227-231. the general presettlement pattern is being examined. This is BARNES,J. W., AND G. W. MINSHALL[ED.]. 1983. Stream ecoIogy: application especially true if the event is a temporally distant one where the sand testing of general ecological theory. Plenum Press, Mew York, NY. cause is separated from its present-day effect by time as well as 399 p. space. Examples include acid rain in Europe which began with BIRD,G. A., AND N. K. KAUSHIK.1981. Coarse particulate organic matter in streams, p. 41-68. In M. A. Lock and D. D. Williams [ed.] Perspectives in the Industrial Revolution and deforestation sf !age tracts of running water ecology. Plenum Press, New York, NY. 430p. land which in parts of England may date back 3000 yr to the BLUM,J. L. 1957. An ecological study of the algae of the Saline River, Bronze Age (Dr. W. Bennington, Botany Department, Univer- Michigan. Hydrobiologia 9: 361-408. sity of Leicester, U.K.,pers. comm.). BOPELE,H. W. 1978. Relation between food supply, drift, and microdistribution Since stream ecosystem dynamics also are coupled closely to of larvae of Baeris rhodani. Investigations in a stream model. Arch. Hydrobisl. 84: 500-525. the fluvial geomorphic conditions, these conditions must be BRQCK,T. D. 1967. Relationship between standing crop and primary taken into account when testing the RCC or devising alternative productivity dong a hot spring themal gradient. Ecology 48: 566-571. explanations. This suggests that, in examining conditions along BROWN,A. V., AND K. B. BROWN.1984. Distribution of insects within riffles a river system, samples should be taken so that the mean or most sf streams. Freshwater Invehtebr. Biol. 3(1): 2- 11. BROWN,J. H. 1981. Two decades of homage to Santa Rosakia: toward a general characteristic gecsmoqhic conditions in each stretch we repre- theory sf diversity. Am. Zsol. 21: 1877-888. sented. For example, collections of organisms taken only from BRUNS,D. A., AND G. W. MINSHALL.1983. Macroscopic models of riffles along the course of a river may provide an erroneous view community organization: analyses of diversity, dominance, and stability in of general patterns of distribution, since the characterizing guilds of predaceous stream insects, p. 231-264. Pn J. R. Bmes and 6. W. Minshall [ed.] Stream ecology: application and testing of general geomorphic properties of the stream reach may be being over- ecological theory. Plenum Press, New York, NY. 399p. ridden by the sampling procedure and a depositional rather BRUNS,D. A., 4;. W. MINSMACL,C. E. CUSHHNG,K. W. CUMMINS,J. T. than an erosional biota may actually be the appropriate BROCK,AND R. L. &TANNOTE. 1984. Tributaries as modifiers of the River representative. Here, too, the long-term temporal perspective is Continuum Concept: analysis by polar ordination and regression models. rn important one. Due to flow regulation, many stream Arch. Hydrobiol. 9: 208-220. BUTCHER,R. W. 1932. Studies on the ecology of hvers. II. The microflora of channels, particularly in lowland areas, are over- (e.g. due rivers with special reference to the algae of the river bed. Ann. Bot. 46: to channelization) or under-fit (e.g. due to sedimentation) or 813-861. otherwise managed in ways that alter the natural conditions 2933. Studies on the ecology of rivers. I. On the distribution of (e.g . weed cutting). While these represent deviations, the extent macrophytic vegetation in the rivers of Britain. J. EcoB. 21: 58-91. of which can be measured against the RCG, they do not provide 1948. Studies on the ecology of rivers. IV. Observations on the growth and distribution of the sessile algae in the River MuBi, Yorkshire. J. evidence of conditions that invalidate the concept. Ecol. 28: 210-223. 8946. Studies ow the ecology of rivers. VI. The algal growth in certain highly calcareous streams. J. Ecol. 33: 268-283. Conclusion CARPENTER,K. E. 1928. Life in inland waters. Sidgwick md Jackson, London. 267 p. CODY,hf. L.,AND 9. M. DIAMOND[ED.]. 1975. Ecology mand evoiution of Stream ecosystem theory has seen a number of important communities. Hmard University Press, Cambridge, MA. advances, especially in the past decade. The RCC is consistent CUMMINS,K. W. 1973. Trophic relations of quatic insects. Annu. Rev. with these views and has served to synthesize most of them into Entomol. 18: 183-206. a workable general hypothesis. Although the worldwide gener- 1974. Structure and function of stream ecosystenas. BioScience 24: 631-642. ality of the model remains to be further evaluated, it appears that 1975. The ecology of mnning waters; theory md practice, p. 277-293. most riverime ecosystems generally can be accomodated within Proceedings Sandusky River Basin Symposium. Iwtemationai Joint Com- the current conceptual framework. In this regard, it appears that mission on Great Lakes. even systems identified as "exceptions" (e.g . Winterbourn et al. CUMMINS,K. W., AND M. J. KLUG. 1979. Feeding ecology of stream 1981) represent no more than variations on a central theme (e.g. invertebrates. Annu. Rev. Ecol. Syst. 10: 147- 272. 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