Applicability of Ecological Theory to Riverine Ecosystems

Verh. Internat. Verein. Limnol. 28 443–450 Stuttgart, February 2002

Applicability of ecological theory to riverine ecosystems

J. V. Ward, C. T. Robinson and K. Tockner

Introduction pristine rivers. Therefore, it is difficult to ascer- tain whether perceived differences between The conceptual foundations of lotic ecology were derived from diverse sources over a relatively long temperate and tropical running waters are in period (MINSHALL 1988). How do the most influen- fact real. tial of the concepts dealing with fluvial systems con- The conceptual foundations of running water form with current knowledge in ecological theory? ecology formed after most river systems in How effectively is new knowledge incorporated into developed (temperate zone) countries had the thinking of running water ecologists? These questions are important, not only for the further undergone substantial human-induced modifi- advancement of the discipline, but also to ensure cations. This was especially true in Europe that river conservation and management initiatives where river engineering began centuries ago are based on a sound theoretical foundation. (PETTS et al. 1989), but also applies to large riv- The objective of this study was to briefly examine ers in the United States (BENKE 1990). Flood- the extent to which current understanding of the plain reaches have been altered to the greatest structural and functional attributes of riverine eco- extent (WARD & STANFORD 1995a). Because the systems was consistent with contemporary ecological theoretical foundations of stream ecology were theory. Length restrictions precluded a detailed analysis of all the concepts that have had an influence on based mainly on geographical regions where running water ecology. Rather, after addressing rele- riverine reaches had been severely modified, vant historical constraints, concepts considered one might expect concepts of lotic ecology to herein to be particularly influential, especially those reflect a misconception of the natural condi- with a broad ecosystem perspective, were assessed tion. based on their concordance with ecological theory. Literature citations were used sparingly and selec- Until quite recently, ecology was typically tively. conducted according to the following unstated presuppositions: that nature is more or less Historical constraints deterministic, homeostatic and spatially homogeneous, that equilibrium conditions generally Historical constraints have markedly influenced prevail, and that scale is not a critical variable our perspectives of river ecosystems. These (WIENS 1999). Of course ecologists of this ear- include possible biases relating to (1) the types lier period were aware that these were simplify- and geographical locations of fluvial systems ing assumptions, but much research was from which concepts were derived, (2) a long designed and results interpreted accordingly. history of river engineering, and (3) a paradigm The ‘new paradigm in ecology’ (sensu TALBOT shift in ecology. 1996), in contrast, views natural systems as The conceptual foundations of running water open, spatially heterogeneous, non-determinis- ecology emanated largely from European and tic, non-equilibrial, and with patterns and pro- North American stream ecologists studying cesses that are highly scale dependent. Although small forested streams (a mesic, small forested this contemporary perspective has become stream, temperate zone bias). In stark contrast, firmly ingrained (sometimes to the point of studies in the tropics occurred later and initially dogma) in the thinking of most ecologists, focused on fish communities of large, relatively some of the most influential concepts in run-

0368-0770/02/0028-000443 $ 2.00 ©2002 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart 444 Verh. Internat. Verein. Limnol. 28 (2002) ning water ecology contain relicts from the ear- TAKER 1956). Given that unidirectional flow is lier period. However, a note of caution is the defining feature of rivers, it is natural that needed here: rather than an ‘either/or’ dichot- examining gradients from headwaters to the omy (deterministic versus stochastic), the disci- lower reaches has been a dominant theme in pline can profit from a balanced view recogniz- lotic ecology. ing that various ecological phenomena may The stream zonation concept (ILLIES & BOT- dominate under different conditions and at dif- OSANEANU 1963) envisions a series of distinct ferent times. communities along rivers, separated by major faunal transition zones (e.g. the rhithral–pota- Theoretical frameworks mal transition); the river continuum concept Herein, selected lotic ecology concepts are con- (VANNOTE et al. 1980) is a clinal (rather than sidered under the following theoretical frame- zonal) perspective of gradually changing works: gradient analysis, disturbance, hierarchy, resource gradients along which stream biota are ecotones, and connectivity (Table 1). Most of predictably structured; the hyporheic corridor the lotic ecology concepts listed in the table concept (STANFORD & WARD 1993) defines an relate to more than one of the theoretical alternating series of constrained reaches and frameworks. Figure 1 presents examples, in a alluvial flood plain reaches, analogous to beads lotic ecosystem context, for each theoretical on a string. The zonation and river continuum framework. concepts provide an essentially unidirectional (longitudinal) perspective, whereas the Gradient analysis hyporheic corridor concept also includes inter- Ecology has a long-standing interest in how active pathways in the lateral and vertical patterns and processes change along environ- dimensions within alluvial flood plains. It is mental gradients. The term gradient analysis proposed herein that the unidimensional per- was first applied to changes in mountain vege- spective of the zonation and continuum con- tation along an altitudinal continuum (WHIT- cepts reflects situations where the dynamic mul-

Table 1. Selected concepts of river ecosystems, positioned within their general theoretical framework, and the extent to which each addresses the four-dimensional perspective of lotic ecosystems (sensu WARD 1989). Theoretical framework Lotic ecology concept Four-dimensional perspective Longitudinal Lateral Vertical Temporal Gradient analysis Stream zonation X –– – (WHITTAKER 1956) (ILLIES & BOTOSANEANU 1963) River continuum X––(X) (VANNOTE et al. 1980) Hyporheic corridor XXX(X) (STANFORD & WARD 1993) Disturbance Serial discontinuity XX–(X) (PICKETT & WHITE 1985) (WARD & STANFORD 1995b) Flood pulse –X–X (JUNK et al. 1989) Telescoping ecosystem –(X)XX (FISHER et al. 1998) Ecotones Aquatic–terrestrial ecotones –XX(X) (CLEMENTS 1905) (NAIMAN & DECAMPS 1990) Hierarchy Catchment hierarchy X––X (ALLEN & STARR 1982) (FRISSELL et al. 1986) Connectivity Hydrologic connectivity (X) X – X (MERRIAM 1984) (AMOROS & ROUX 1988) J. V. Ward et al., Current ecological theory and riverine ecosystems 445 et OCKNER et al. 2000; (B) expan- URNELL unpublished; (C) modified from T unpublished; (C) modified from OCKNER et al. 1999b. ARD unpublished; (E) modified from W from modified (E) unpublished; OCKNER al. 1998; (D) from T sion/contraction of the channel network of an island-braided reach of the Fiume Tagliamento, from T from Tagliamento, of the Fiume of an island-braided reach sion/contraction of the channel network Fig. 1. Examples from lotic systems of the five theoretical frameworks addressed herein. Sources: (A) modified from G from modified (A) Sources: herein. frameworks addressed theoretical five the systems of lotic from Examples 1. Fig. 446 Verh. Internat. Verein. Limnol. 28 (2002) tiple channel networks of alluvial flood plains with alternating wet and dry phases over annual have been engineered into constrained single- cycles in large floodplain rivers. The focus is, thread channels. These models may indeed therefore, on the lateral and temporal dimen- serve as suitable frameworks for investigating sions. The flood pulse concept emphasizes the patterns and processes occurring along man- importance of alternating dry and wet phases in aged rivers, but they should not be invoked to enhancing biodiversity and productivity, as well portray the natural condition. as the dynamic edge effect created by the ‘mov- ing littoral’. Perhaps the most important contri- Disturbance bution of the flood pulse concept (based largely The role of disturbance, a topic of long-stand- on pristine tropical rivers) was to make river ing interest in ecology, has undergone a para- ecologists of the temperate zone aware of the digm shift (PICKETT & WHITE 1985). Histori- extent of hydrological extremes possible in cally, disturbance was viewed as a deviation flood plains, and the tight coupling of the biota from the equilibrium conditions prevailing in with the flood regime. The views of WELCOMME nature, whereas disturbance is now generally (1995), who believed that temperate rivers in recognized as an agent responsible for sustain- the pristine state probably behaved in a similar ing the ecological integrity of ecosystems. For manner to tropical rivers, are generally con- example, the critical role of natural disturbances curred with herein (but see TOCKNER et al. such as fire, hurricanes and tidal action in 2000). maintaining high levels of biodiversity became The telescoping ecosystem model (TEM) of a central theme in ecology (CONNELL 1978). FISHER et al. (1998) specifically addressed the According to the ‘new paradigm in ecology’, differential recovery trajectories within four non-equilibrium conditions predominate in subsystems (surface stream, riparian, hyporheic, nature. It is, in fact, lack of disturbance that parafluvial) following disturbance (flood, suppresses biodiversity, and this is perhaps most drought). Disturbance/recovery are defined in a apparent in highly managed rivers. In reality, biogeochemical context as ‘processing length’ river ecosystems in the natural state are gener- (i.e. material cycling), with a short processing ally more dynamic than terrestrial or marine length reflecting high rates of cycling. Distur- systems, but in much of the world the natural bance increases processing length (extends the disturbance regime has been largely eliminated cylinders of the telescope that represent the four by a variety of river regulation measures. subsystems); recovery involves a return of pro- The serial discontinuity concept (SDC) is a cessing length (contraction of the cylinders) to theoretical model for rivers whose natural predisturbance rates. The TEM is an important dynamics have been suppressed by regulation. concept because (1) it recognizes subsystems The original model (WARD & STANFORD 1983) adjacent to surface waters as integral parts of had a unidimensional perspective that perceived the stream ecosystem, and (2) it emphasizes dams as disruptions of longitudinal resource that the subsystems may respond differently to gradients. The expanded SDC model (WARD & a disturbance and may exhibit quite different STANFORD 1995b) also included alluvial flood recovery trajectories. The concept focuses on plains, thereby encompassing dynamics along interactive pathways along vertical, temporal the lateral dimension. Both of these models and, to a lesser extent, lateral dimensions. were based on hypothetical reference river eco- Although developed for relatively small, single- systems, in the first case derived from the river thread, canyon-constrained streams without continuum concept and in the second case also extensive flood plains, it may be possible to incorporating the flood pulse. expand the model to encompass floodplain riv- The flood pulse concept (JUNK et al. 1989) is ers. a theoretical framework for examining the Recent studies of relatively pristine alluvial adaptive strategies employed by aquatic and ter- flood plains in the Alps have greatly heightened restrial biota to exploit the dynamics associated awareness of the remarkable levels of habitat J. V. Ward et al., Current ecological theory and riverine ecosystems 447 heterogeneity and fluvial dynamics possible in investigations into the role of ecotones in mate- natural river systems (e.g. TOCKNER et al. 1997, rial and energy flux are required, in order to WARD et al. 1999a). Based on these studies, it is provide insight into the major determinants of essential to include the shifting mosaic of lotic, biodiversity and productivity in river ecosys- lentic and riparian habitats of fringing flood tems. Clearly a holistic perspective would plains, as well as contiguous alluvial aquifers, as include various interactive pathways between integral parts of the total river ecosystem. land and water, surface water and ground water, and instream transition zones. Preliminary evi- Ecotones dence suggests that management of ecotones CLEMENTS (1905) regarded ecotones as tension should be an integral part of river protection zones between adjacent communities. Eco- and restoration programs. tones are now viewed as semi-permeable boundaries between relatively homogeneous Hierarchy patches, transition zones where the rates of Recognition that ecological phenomena mani- change in ecological patterns or processes are fest across a diverse array of scales led to the increased relative to the surroundings (WIENS development of the nested hierarchical model 1992). The book on aquatic–terrestrial eco- (ALLEN & STARR 1982). Perhaps its most impor- tones edited by NAIMAN & DECAMPS (1990) tant aspect is recognizing that phenomena firmly established the ecotone concept in structuring one hierarchical level may or may aquatic ecology, yet very little empirical work not be operative at another level. This was well on ecotones per se has been conducted in river exemplified by the investigation of ARSCOTT et ecosystems. Nonetheless, it is apparent that riv- al. (2000) of the spatio–temporal heterogeneity erine ecotones, operating across a broad range at three hierarchical levels (corridor, floodplain of spatio–temporal scales, play important roles and habitat scales) in six geomorphic reaches relating to speciation, evolutionary invasion of along a dynamic alluvial river system. This fresh waters, biodiversity, bioproduction and work clearly demonstrated that the patterns of nutrient transformation (WARD & WIENS variation were scale- and variable-dependent. 2001). Ecotones are perhaps most apparent in FRISSELL et al. (1986) presented a rather ele- intact alluvial flood plains, which themselves gant hierarchical framework for stream net- constitute a large-scale ecotone between the works, in which they identified the spatial river and the upland. Within flood plains, eco- extent of subsystems (from microhabitat to tones are manifest at a variety of scales, includ- catchment) and their corresponding temporal ing the boundaries between different riparian persistence. This catchment hierarchy concept, communities, surface water–groundwater tran- despite its utility and influence, has severe limi- sitions (e.g. upwelling/downwelling zones, tations. The model was based on small headwa- springs), lotic–lentic transitions and oxic– ter streams and does not address interactions anoxic boundary zones in the soil. with flood plains or alluvial aquifers. MINSHALL Natural disturbances, by forming a variety of & ROBINSON (1998) demonstrated that mea- patch types and successional stages, play an sures of habitat heterogeneity influenced biota important role in maintaining a diversity of differentially among streams of different size. ecotonal habitats in riverine flood plains. The WARD et al. (1999b) formulated a framework suppression of disturbance (fluvial dynamics) in for examining alpha, beta and gamma diversity managed rivers undoubtedly reduces ecotone at hierarchical levels ranging from physio- diversity and affects their functional properties graphic region (e.g. Alps) to habitat patches. (e.g. permeability), although few data are avail- The views of TOWNSEND (1996), who proposed able. It is likely that the importance of ecotones that various concepts in river ecology “be in sustaining ecological integrity has been meshed together into the broad spatio–tempo- underestimated because of their reduced diver- ral context of the catchment hierarchy of an sity/function in managed rivers. Rigorous entire river”, are concurred with herein. 448 Verh. Internat. Verein. Limnol. 28 (2002)

Fig. 2. A modular framework for developing an integrated model of dynamic river ecosystems.

Connectivity important determinant of functional processes The concept of connectivity originally referred in aquatic and riparian systems (BRUNKE & to gene flow between subpopulations of a GONSER 1997, WARD et al. 1998). metapopulation (MERRIAM 1984). Connectivity However, as stressed by AMOROS & BORNETTE is a relatively new concept in ecology and has (1999), connectivity is a complex phenomenon only recently caught the attention of lotic ecol- that “cannot be reduced to a simple gradient”. ogists. Hydrological connectivity (sensu AMO- This is well exemplified by the biphasic rela- ROS & ROUX 1988) refers to the exchange of tionship between hydrological connectivity matter (including organisms) and energy via (defined as length of channels with an upstream the aqueous medium between different units of surface connection with the main channel/total the riverine landscape. Floodplain water bodies channel length) and channel length in a braided differing in connectivity with surface waters of glacial flood plain (WARD et al. 2002). The ini- the main channel exhibit different successional tial period of expansion in early summer, in trajectories and contain different biotic com- which channel length more than doubled, was munities. In a Danubian flood plain, each of not accompanied by a substantial increase in the faunal and floral groups examined exhibited connectivity because snow melt recharged aqui- maximum species richness at a different point fers via subsurface pathways. Initial channel along a connectivity gradient (TOCKNER et al. expansion mainly involved groundwater-fed 1998). The degree of connectivity between channels that did not form upstream connec- ground waters and surface waters also is an tions with the main channel. However, once a J. V. Ward et al., Current ecological theory and riverine ecosystems 449 certain threshold was exceeded (a channel Connectivity in Landscape Ecology: 125–130. – Muen- length of about 15 km), there was a strong pos- sterische geographische Arbeit, Muenster, Germany. itive relationship between connectivity and ARSCOTT, D. B., TOCKNER K. & WARD. J. V., 2000: Aquatic habitat diversity along the corridor of an Alpine floodplain total channel length. Exactly how the epigean river (Fiume Tagliamento, Italy). – Arch. Hydrobiol. 149: and hypogean biota are influenced by connec- 679–704. tivity patterns in this glacial flood plain remains BENKE, A. C., 1990: A perspective on America’s vanishing to be seen. Rigorous analyses of the role of con- streams. – J. N. Am. Benthol. Soc. 9: 77–88. nectivity in river ecosystems hold considerable BRUNKE, M. & GONSER, T., 1997: The ecological significance promise for furthering the understanding of of exchange processes between rivers and groundwater. – functional processes and biodiversity patterns. Freshwater Biol. 37: 1–33. CLEMENTS, F. E., 1905: Research Methods in Ecology. – Univer- sity Publ. Co., Lincoln, Nebraska, USA. Conclusions CONNELL, J., 1978: Diversity in tropical rainforests and coral In general, concepts of riverine ecosystems are only reefs. – Science 199: 1302–1310. partially concordant with current ecological theory. FISHER, S. G., GRIMM, N. B., MARTI, E., HOLMES, R. M. & This reflects (1) misconceptions of what constitutes JONES, J. B., 1998: Material spiraling in stream corridors: a the natural state of river ecosystems, (2) failure to telescoping ecosystem model. – Ecosystems 1: 19–34. fully recognize the major interactive role between FRISSELL, C. A., LISS, W. L., WARREN, C. E. & HURLEY, M. 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