A brief history of river ecosystem function, part 4: riverscapes to river ecosystem synthesis

The hunt was on for a single concept that synthesized the various ones that had been proposed so far, or at least reconcile the patchiness that is evident in rivers, with the continuum of flow from upstream to downstream. This was important for both theoretical and practical reasons. And indeed, building on the dynamic landscape model (and developments) of population ecology and life history of riverine fish (e.g. Schlosser 1991, 1995a, b; Schlosser and Kallemeyn 2000), Kurt Fausch and co-workers challenged river fish ecologists to take a broader, more holistic approach to rivers and the fish that inhabited them. In their Riverscapes Concept (RC), Fausch et al. (2002) identified three imperatives:

1. to make the research or management we do in streams spatially explicit, because of the loss of information, lack of interpretability and inability to find explanations and make predictions if this is not done (divorcing fish from their habitat template or context is nonsensical but commonplace in stream fish research);

2. we must marry scales at which research and conservation are carried out, both spatially and temporally (in other words, if fish, during a year or during its life, moves hundreds of kilometres, what good is it sampling and monitoring at the reach scale?); and

3. we need to reconcile the hierarchical nature of streams (viz. the HFSHC and the PDC) with the continuous downstream flow or materials and energy (viz. the RCC) and the upstream and downstream linkages that clearly occur via fish movement.

These were – and still are – really tough challenges.

One recent, ambitious attempt to synthesize concepts associated with river function, has been the riverine ecosystem synthesis (RES) of Jim Thorp, Martin Thoms and Mike Delong (2006; 2008). The RES is proposed as merging of eco-geomorphology with a landscape model of hierarchical patch dynamics (Thorp et al. 2006; Thorp et al. 2008). But it does not see the river as a continuum, despite considering the river ecosystem in its entirety. Instead the RES conceptualises rivers as “downstream arrays of large hydrogeomorphic patches (e.g. constricted, braided and floodplain channel areas) formed by catchment geomorphology and climate”. These patches are equivalent to the ‘functional process zones’ mentioned in the previous post and analogous to the ‘process domains’ of Montgomery (1999). The 14 tenets or hypotheses proposed by the RES relate to:

• the distribution of species and species diversity and the factors that influence these; • community (or assemblage) regulation;

• ecosystem and riverscape processes (such as autochthonous and allochthonous production, nutrient spiralling and life history), all largely governed by climate;

• flow and geomorphology.

It is, in effect, a merging of the RCC, FPC, RPM and the various concepts dealing with geomorphic process zone/domains within a nested hierarchical framework. The weakness and strength of the RES are related. As a synthesis, the RES is somewhat lacking for the very fact that it needs to be encapsulated in 14 tenets. The RES seems to involve a high degree of complexity and detail, rather than simplicity and elegance, which would be the ideal outcome. On the other hand, it proposes a one-stop-shop for hypotheses that can be tested and so provides wonderful opportunities for research for decades to come. As for fish in particular, the RES does not say much more than has already been discussed in the individual models above. Its attraction is that the RES brings together aspects of assemblage composition, diversity, feeding and life history of riverine fishes in one conceptual location.

Despite the comprehensiveness of the RES, there is still great potential to find a river ecosystem model that is both simple and elegant. Einstein was convinced, like many physicists and mathematicians, that simplicity, elegance and beauty should in many ways guide our search for fundamental concepts that describe the natural world, or perhaps should be our goal. The premise being that where these are found, truth is most likely to be. I tend to share this view in regard to river functioning. My intuition is that, despite the complexity of river ecosystems, there is a simple, elegant, beautiful truth out there. Perhaps you will find it.

References

Fausch KD, Torgersen CE, Baxter CV and Li HW (2002) Landscapes to riverscapes: Bridging the gap between research and conservation of stream fishes. BioScience 52, 483-498. Montgomery DR (1999) Process domains and the River Continuum. Journal of the American Water Resources Association 35, 397-410. Schlosser IJ (1991) Stream fish ecology: a landscape perspective. BioScience 41, 704-712. Schlosser IJ (1995a) Critical landscape attributes that influence fish population dynamics in headwater streams. Hydrobiologia 303, 71-81. Schlosser IJ (1995b) Dispersal, boundary processes, and trophic-level interactions in streams adjacent to beaver ponds. Ecology 76, 908-925. Schlosser IJ and Kallemeyn LW (2000) Spatial variation in fish assemblages across a beaver-influenced successional landscape. Ecology 81, 1371-1382. Thorp JH, Thoms MC and Delong MD (2006) The riverine ecosystem synthesis: Biocomplexity in river networks across space and time. River Research and Applications 22, 123-147. Thorp JH, Thoms MC and Delong MD (2008) ‘The Riverine Ecosystem Synthesis.’ Elsevier, London.

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