Rivers are the Product of their Landscapes

In a prescient essay published in 1975, Noel Hynes wrote that ''in every respect the valley rules the stream.'' Geology determines the availability of ions and the supply of sediments, topography determines slope and degree of containment, climate and soils determine vegetation and hence the availability of organic matter and extent of shade, and so on. Decades of research support this view. The river continuum concept describes how basal resources and thus consumer assemblages and stream metabolism change along a river's length owing to changes in river size and terrestrial influences. The flood pulse model reminds us that rivers can interact extensively with surrounding land during regular cycles of inundation. In addition, our perspective on rivers within landscapes has expanded to encompass more explicit consideration of the physical template and spatial hierarchy provided by the river network (Section 14.1), and been enriched by concepts borrowed from the emerging field of landscape ecology. Intriguing-ly, Hynes (1975) also opined that every stream "is likely to be individual and thus not really very easily classifiable.'' The many efforts to classify streams and their limitations discussed in Section 3.4.3 do not make a convincing rebuttal to Hynes' statement. But three decades of effort to place the individuality of streams within the frameworks of scale and landscape has significantly

Product Life Cycle Nike

FIGURE 14.6 The riverine productivity model for large rivers (>fourth order) proposes that secondary production by macroinvertebrates and fishes depends on autochthonous organic matter produced in the river channel and in the riparian zone, which are more labile but less abundant than organic matter of allochthonous origin transported from upstream reaches. The latter dominates the total amount of organic matter transported by rivers and contributes to high rates of microbial respiration but contributes little to the higher food web. (Reproduced from Thorp and Delong 2002.)

FIGURE 14.6 The riverine productivity model for large rivers (>fourth order) proposes that secondary production by macroinvertebrates and fishes depends on autochthonous organic matter produced in the river channel and in the riparian zone, which are more labile but less abundant than organic matter of allochthonous origin transported from upstream reaches. The latter dominates the total amount of organic matter transported by rivers and contributes to high rates of microbial respiration but contributes little to the higher food web. (Reproduced from Thorp and Delong 2002.)

advanced our understanding of the causes of that individuality.

Rivers are shaped by environmental factors that control essentially all aspects of the river's physical appearance, vary from place to place, and can be organized hierarchically according to spatial scale (Snelder and Biggs 2002). Climate, topography, geology, and vegetation cover are fixed environmental variables that the river cannot influence, and because climate tends to be expressed at a larger spatial scale than topography, followed by geology and vegetation, their influence is approximately hierarchical

(Figure 14.7). These four controlling factors operate at the macroscale and mesoscale of the river basin and catchment. At smaller scales of network position and valley landform, local processes are of greater importance.

The emerging view of rivers within a landscape perspective builds from recent ideas concerning the dynamical nature of local patches of the environment, the hierarchical nature of environmental controls, and the interconnections (or lack thereof) of spatial elements of lotic ecosystems in longitudinal, lateral, and vertical dimensions. The study of patch

Valley Rules The Stream
FIGURE 14.7 The hierarchy of controlling environmental factors and the spatial scales at which various physical characteristics of the riverine environment are expressed. (Reproduced from Snelder and Biggs 2002.)

dynamics in streams (Pringle et al. 1988, Town-send 1989) initially emphasized how episodic disturbance could create patches within the environment, the biological assemblages of which were in varying stages of successional recovery, interconnected at larger spatial and longer temporal scales by dispersal and recolonization. Although the language is somewhat different, nutrient dynamics similarly depend on processes that occur in spatially distinct locations associated with the availability of organic matter and the supply or lack of oxygen. For complete N cycling to occur, processes that occur in different areas have to be linked by the waterborne movement of different forms of dissolved N from one location to another.

Landscape ecology studies the interactions between spatial pattern and ecological processes in heterogeneous systems across a range of scales, emphasizing the importance of discrete patches, ecotones (the boundaries between patches), and the connectivity among patches (Ward et al. 2002, Weins 2002). In general, ecological processes are scale dependent so that factors operating at larger scales influence smaller scale systems but not the converse, in accord with the hierarchical directionality of influence described by Frissell et al (1986), Snelder and Biggs (2002), and others. Borrowing some central themes from landscape ecology (Weins 2002), one can assert that patches vary in their physical, chemical, and biological conditions (habitat quality, nutrient processing, and productivity); patches can be relatively persistent and discrete; patch context has an important influence on patch processes (local flowpaths, supplies of nutrients, and organic matter); connectivity is important (e.g., to overall assemblage dynamics and nutrient cycling); and events at the patch scale integrate to produce patterns observed at large scales while simultaneously being influenced by larger-scale patterns and processes.

The landscape perspective is much like the view of a river as seen from the window of an airplane, or perhaps from several airplanes flying at different altitudes. There is a patchiness of physical environments, formed by the interaction of hydrologic and geomorphological processes, which gives rise to repeatable patterns within a region but different patterns in different regions because of the hierarchical arrangement of controlling environmental variables described in Figure 14.7. Because river systems are both heterogeneous and hierarchical in structure, it follows that species assemblages, inputs of nutrients and organic matter, ecosystem processes, and hydrologic exchanges differ among patches but are linked at larger scales (Wu and Louck 1995, Poole 2002) (Figure 14.2). The location and arrangement of these patches is due in part to the arrangement of tributaries in a river network and in part to local and chance processes (Benda et al. 2004). Assemblages of species, ecosystem pro cesses, and the relative magnitude of longitudinal, lateral, and vertical interactions differ among patches, but are interconnected at the larger scale of the river segment (Thorp et al. 2006).

Although it may be most intuitive to think of patches as small-scale elements, in the nested hierarchy of lotic ecosystems, patchiness is expressed at multiple scales, and dynamics within a patch are governed by both internal dynamics and by supplies of materials or organisms that are the outcome of processes in other patches and possibly at other scales. Patch structure and function is highly subject to the temporal pulsing of fluctuating hydrologic conditions, so that their state is highly dynamic and possibly ephemeral. This underlies the perspective that the river itself is a complex, shifting mosaic of channel features, habitat units, surface and subsurface zones, floodplains, and riparian corridors in which the diversity of the physical template provides the setting in which biologically diverse communities flourish and ecological processes interact across a range of scales (Poole 2002, Ward et al. 2002).

The Power Of Charisma

The Power Of Charisma

You knowthere's something about you I like. I can't put my finger on it and it's not just the fact that you will download this ebook but there's something about you that makes you attractive.

Get My Free Ebook


Post a comment