Other vertebrates

Amphibians, reptiles, birds, and mammals are all represented in lotic food webs. The taxa most likely to have a significant impact on riverine food webs probably are the salamanders of headwater streams, and fish-eating snakes, birds, and crocodiles in larger rivers, but perhaps this reflects lack of knowledge concerning other groups. Salamanders can attain large size, including Megalobatrachus of the Orient, Cryptobranchus (the hellbender), and Necturus (the mud puppy) of Eastern North America, and Dicamptodon ensatus of the Pacific Northwest of North America (Hynes 1970, Nickerson and Mays 1973). They are carnivores of invertebrates, other amphibians, and fish. Small salamanders may be the principal vertebrate predators in headwater streams. Petranka (1984) concluded that the larval two-lined salamander Eurycea bislineata was an opportunistic generalist, consuming a variety of insect larvae and crustaceans. In cave streams, however, Eurycea may be a de-tritivore rather than a predator based on stable isotope analyses (Simon et al. 2003).

The larvae of some frogs and toads feed on algae in small streams, and a few possess powerful suckers that provide attachment and allow leech-like maneuverability (Hynes 1970). Tadpoles of the web-footed frog, Rana palmipes, widely distributed in the Neotropics, are epi-benthic consumers that feed on algae and sediments and can grow on sediments alone (Flecker et al. 1999).

Reptiles that feed in rivers include the Croco-dylia, many families of snakes but especially the Colubridae (water snakes), and the Chelonia (turtles). The former two groups are predators of fish and invertebrates; the latter are omnivores of sluggish streams and rivers and consume substantial amounts of invertebrates and fish. Size of prey relative to size of predator is a common constraint, and many predators increase the size and breadth of their diet as they grow. An aquatic population of the Oregon garter snake Thamnophis atratus fed on small prey along the stream margin as juveniles, but as adults they consumed a wider variety of prey types and sizes, especially concentrating on larvae of the Pacific giant salamander in midstream substrates (Lind and Welsh 1994). Fish are the dietary mainstay of the alligator snapping turtle Macrochelys temminckii, the largest of North American turtles, which possesses a unique lingual lure to attract its prey (Harrel and Stringer 1997). Nonetheless, they consume a wide mix of plant and animal matter. Even young crocodiles feed primarily on invertebrates until they reach a length of about 2 m, when they become preda tory on a wide range of aquatic and terrestrial vertebrates (Corbet 1959, 1960), as do American alligators, which consume fish, turtles, muskrats, and rabbits, among other prey. Hynes (1970) remarks that, considering the abundance of crocodiles along tropical rivers where exploitation has not reduced their numbers, they are likely to have a major impact on lower trophic levels. In the Big Cypress Swamp of Florida, the American alligator Alligator mississippiensis is unusually abundant, and based on a model of indirect trophic effects, it appears to benefit invertebrates, frogs, mice, and rats through its predation on snakes and turtles (Bondavalli and Ulanowitz 1999).

At least 11 orders of birds make use of rivers and streams as feeding habitat (Hynes 1970). Many are fish predators but some feed directly on invertebrates (e.g., the Cinclidae or dippers, Ormerod 1985). Diving ducks also consume significant amounts of invertebrates, especially mollusks, although submerged aquatic vegetation is their primary food (Perry and Uhler 1982), and ducks can be important consumers of macrophytes (Lodge 1991). There are many piscivorous birds, but at present the weight of evidence suggests that they do not have a major impact on fish populations except when fish are easily captured, such as during low water conditions (Draulans 1988). However, some studies of bird predation report a substantial effect on population size or on the foraging behavior of the prey species. Steinmetz et al. (2003) altered the abundance of Great Blue Herons (Ardea herodias) and Belted Kingfishers (Ceryle alcyon) along an Illinois prairie stream by suspending plastic bird netting along an exclusion reach and adding kingfisher perches along an augmentation reach. The mean sizes of two abundant prey, striped shiners, and central stone rollers, decreased under normal and elevated predation but increased in the reduced predation reach, in accord with preferred prey sizes of the two predators, apparently due to a combination of direct mortality and prey emigration. Armored catfish in Panamanian streams experience significant predation risk from fishing birds (Power 1984a, b), and this causes larger individuals to avoid shallow waters. Because these fish are effective herbivores, the depth distribution of periphyton inversely mirrors the distribution of fish.

A diversity of mammals feed within running water. Taxa ranging from shrews to racoons to bears occasionally or frequently consume invertebrates and fish. Others such as the river otter Lutra canadensis are fully aquatic and feed almost entirely on aquatic resources. The duckbill platypus Ornithorhynchus anatinus, a nocturnal hunter in Australian rivers, possesses electro-receptors capable of detecting the muscle activity of invertebrate prey (Scheich et al. 1986). Very large river-dwelling mammals include the plant-eating manatees of South America and West Africa (Campbell and Irvine 1977), and dolphins, which feed on invertebrates and fish. River dolphins are top predators and those from the Amazon have been found to eat at least 50 fish species from 19 families, including individuals up to 0.8 m in length (Best and Da Silva 1989); in addition, they occasionally consume mollusks, crustaceans, and turtles.

Seasonal fluxes of anadromous fishes into rivers provide nourishment for a great many mammal species (Willson and Halupka 1995). Spawning salmon or their fry have been shown to provide critical sources of food for martens during years of low rodent abundance (BenDavid et al. 1997), influence the body mass and litter size of North American brown bears (Hil-derbrand et al. 1999), and even be significant components of the diet of wolves (Szepanski et al. 1999).

Although different vertebrate predators are capable of a variety of hunting tactics, most are morphologically constrained to hunt primarily by wading, diving, or swimming. Wading birds typically fish in water no deeper than 20-30 cm. Leg length and striking distance must limit their success at greater depths. Diving and skimming predators such as kingfishers and bats usually fish very close to the surface, although kingfisher dives to a depth of 40 cm are not unknown (Power 1984b) and mergansers fish at depths of one meter or more. Swimming predators typically fish at greater depth, either to minimize their own risk of predation or, especially if they are of large body size, to have more room to maneuver. The need to capture and swallow prey generally results in a rough correspondence between prey size and predator size, even in species able to extend their gapes or rend prey into pieces. The combination of a predator's depth range and size range may significantly affect the size and depth distribution of fishes in streams, and perhaps affect other members of the biota as well (Power 1984b). Indeed, many vertebrate predators may have their impact on riverine communities by influencing the foraging location of their prey. As we shall see in subsequent chapters, the consequences can ramify widely through the food web.

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