Because the flora and fauna of fluvial ecosystems are so intimately associated with the substrate, a great deal of research has been directed toward unraveling how substrate acting as habitat influences biological assemblages. In gravel-bed rivers, a diverse macroinvertebrate fauna exhibits a patchy spatial distribution that surely is determined at least in part by the heterogeneity of the substrate. In fact, abundance and taxa richness typically are low in fine substrates and increase with substrate size at least up to gravel and cobble (Minshall 1984, Mackay 1992). The density and richness of invertebrates have been shown to correlate with amount of detritus, algal biomass, substrate stability and complexity, depth, and velocity (Rabeni and Minshall 1977, Barmuta 1989, Quinn and Hickey 1994); and strength of correlation has been found to depend upon spatial scale at which substrate-related variables are measured (Downes et al. 1995, Beisel et al. 2000). Substrate size tends to decline downstream, for reasons discussed in Chapter 3, but tributaries can interrupt the longitudinal fining of substrate with inputs of coarse material. In gravel-bed rivers of western Canada, Rice et al. (2001) found an increased abundance of taxa that prefer coarse substrate at these points of sediment recruitment, as well as an overall increase in diversity.
A series of studies of substrate-induced habitat complexity conducted in Steavenson River, a stony upland stream in southeastern Australia, found that stone surface area accounted for some 70-80% of variation in species richness, small stones had fewer species because they had less surface roughness, and the filamentous red alga Audouinella hermannii enhanced roughness and the presence of macroinverte-brates (Douglas and Lake 1994, Downes et al. 1995). Using clay bricks as experimental substrate, Downes et al. (1998) manipulated three aspects of habitat structure: large surface pits and cracks, surface texture (small pits), and abundance of macroalgae. Sampled after 14 and 28 days of macroinvertebrate colonization, the majority of common species reached higher abundances on rough substrates, there was a disproportionate accumulation of small individuals, and each of the three manipulated elements of habitat structure had separate, additive effects on taxa richness (Figure 5.9). By employing a statistical procedure known as rarefaction to compare number of species (S) standardized for overall abundance (n), Downes et al. concluded that S increased disproportionately to n, indicating that species richness was augmented by habitat complexity. Although this study provided strong evidence for the importance of substrate roughness, the mechanisms by which crevices and surface roughness affected the biota were unclear. In particular, because A. hermannii responded strongly to surface texture, it was not possible to separate the effects on the fauna of increased algal cover alone from that of increased algae in combination with a rough surface.
Crevices and surface roughness evidently protect both microorganisms and invertebrates from disturbance associated with high flows, sediment scour, and tumbling of substrates. Using a range of substrates from beer bottles to smooth and rough stone types, Bergey (2005) showed that algal biomass remaining after an artificial disturbance (scrubbing with a toothbrush) was enhanced by crevice quantity and surface roughness. On rough stones, some 60-80% of the algae remained, indicating the effects of increased surface area and greater protection. In laboratory flumes, nymphs of Baetis rhodani were able to maintain position in fast velocities that are typical of their preferred microhabitat only on the roughest substrates (Lancaster and Mole 1999). The presence of certain organisms may add a biological dimension to substrate texture and stability. Cardinale et al. (2002a) showed that the presence of net-spinning caddis larvae of the family Hydropsychi-dae increased the velocity at which particle movement was initiated in a laboratory flume, presumably due to their silk secretions, and sta bilized smaller particles to a greater extent than larger particles. Thus the abundance of caddis larvae may influence the ability of other taxa to retain position when current increases.
Streambed regions of saltating bed load likely provide unstable habitat and fewer food resources for benthic invertebrates, and thus may explain why invertebrate abundance generally increases with increasing substrate size, because larger stones are more stable than smaller ones (Death 2000). By monitoring individual marked stones that experienced high flows in a gravel-bed stream on the South Island of New Zealand, Matthaei et al. (2000) observed strong invertebrate declines on unstable stones but no change on stable stones. This finding raises the possibility that the influence of various measures of current and hydraulic forces on organisms discussed earlier in this chapter may be useful because they are predictors of bed stability, rather than as measurements of forces acting directly on individuals.
To conclude this topic it is apparent that the interaction between current and substrate provides considerable insight into the local-scale distribution and abundance of the stream biota. They remain inextricably interwoven, however, and this is unsurprising when one considers that the flow variables used by stream ecologists have their origins in the efforts of fluvial geomor-phologists to understand particle transport, and bed instability is a key element of habitat unsuit-ability.
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