Habitat Heterogeneity and The Functional Signifiance of Fish in River Food Webs

Predation by fish (roach, Hesperoleucas symmetricus, and steelhead Oncorhynchus mykiss) produced strong cascading effects on biota associated with boulder—bedrock substrates in pools of a northern California river, but not on gravel—dwelling biota. Enclosure—exclosure experiments in the South Fork Eel River of northern California (39°44' N, 123°39'W) showed that fish, by suppressing densities of damselfly nymphs and other small predators, released algivorous chironomids (Pseudochironomus richardsoni) from predation. Chironomids in turn dramatically reduced algal standing crops. In contrast, fish had little effect on algae or invertebrates associated with gravel. Gravel—dwelling heptageniid mayflies were behaviorally inhibited from using tops of stones in fish enclosures, and stone surfaces had more chironomid tubes in fish enclosures than in fish exclosures. However, no effects on epilithic algae or densities of invertebrates comparable to those of biota on boulder—bedrock substrates were detected. These spatially varying predator effects in a river parallel results from marine benthic systems, where strong effects of large predators documented for rocky intertidal habitats and unvegetated soft bottoms are not conspicuous in seagrass beds.