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What lies beneath? Rivers: diverse habitats with broadly varying niches. Communities reflect and influence local, upstream, downstream, and broader landscape conditions.
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Introduction
Water current pervades every facet of existence for life in lotic (flowing water) habitats. Maintaining position in the face of flow can be energetically costly, but provides access to a conveyer belt-like food-delivery system. Stream and river organisms reflect their localized niche, and surrounding landscape both upstream and downstream. River organisms have evolved in diverse and fascinating ways in the varied environments between river source and mouth.
Streams and Rivers: Habitats Partitioned at Different Spatial Scales
Large-scale differences: Source to mouth.
The blue line of a river on a map conveniently represents rivers as two-dimensional habitats beginning (usually) in a mountainous region, and ending in a far-off sea (or inland basin). But the physical changes in three dimensions along a river’s length have important implications for river inhabitants.
River sources are usually small, and in the case of mountain streams, steep and erosional (Montgomery and Buffington 1997). In temperate environments, small streams tend to be shaded by an interlocking, overhead tree canopy. Such conditions result in cool, well oxygenated streams that are abundantly supplied with a food base of leaves. Fine particles of organic matter are released as the leaves are broken down by biological communities in the streams (River Continuum Concept; Figure 1; Vannote et al. 1980).
The river continuum concept.
Figure 1: The river continuum concept.
The river continuum concept is a conceptual model that predicts biological community responses to physical changes along the lengths of rivers.
© 2011 Nature Education Modified from Vannote et al. 1980. All rights reserved. View Terms of Use
At some point along their path to the sea, rivers have typically gained enough water and width to preclude interlocking tree canopies. This open-canopy state frequently coincides with somewhat lower gradient landscapes. Streams at this point are warmer, and less abundantly supplied with leaves than was the case upstream. These larger streams remain well oxygenated because air is entrained by turbulent flow in riffles. Open canopy, and fairly shallow water, means that light can reach the river benthos, increasing in-stream primary productivity.
Very large rivers are usually low gradient and very wide, resulting in negligible influence of riparian canopy in terms of shading and leaf-litter input. Water currents keep fine solids in suspension, reducing light penetration to the benthos. Organic matter in suspension is by far the largest food base in these very large rivers.
Changes in physical habitat and food base from river source to mouth profoundly influence biological communities. Aquatic ecologists classify benthic macroinvertebrates into functional feeding groups: shredders that eat leaves, collectors consuming fine particulates, grazers that scrape periphyton from substrates, and predators of animal prey (Cummins & Klug 1979). Smaller temperate streams tend to be co-dominated by shredders primarily consuming leaf litter, and collectors consuming particles (Figure 1). As canopies open in larger streams, grazers become common with increased periphyton production. With less canopy cover in wider streams, shredder abundance is reduced. Collectors utilize particles in streams of all sizes, but they dominate benthic communities in larger streams where suspended organic matter is common. Predators represent a small but important fraction of benthic communities in rivers of all sizes.
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