Applicability of Ecological Theory to Riverine Ecosystems
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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 anal- ysis 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 homo- geneous, 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 ecosystemstheory riverine ecological and Current al., et Ward V. J. 445 Fig. 1. Examples from lotic systems of the five theoretical frameworks addressed herein. Sources: (A) modified from GURNELL et al. 2000; (B) expan- sion/contraction of the channel network of an island-braided reach of the Fiume Tagliamento, from TOCKNER unpublished; (C) modified from TOCKNER et al. 1998; (D) from TOCKNER unpublished; (E) modified from WARD et al. 1999b. 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,