River Ecosystems and Animals

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River ecosystems present a complex web of interrelationships among vegetation, invertebrates, fish, mammals and birds. They can be thrown out of balance by things such as excessive nutrients causing algal blooms or the effects of invasive species. Because of food chain relationships, something affecting one link can cause ripple effects up the chain.

Food Chains Are More Like Webs

  • In a river, benthic (bottom-dwelling) insect larvae feed small fish, which feed larger fish, but there's more complexity to the food chain than that. Those larvae and nymphs, which mature into adult insects, emerge from the water to feed amphibians and birds. The amphibians might feed reptiles such as snakes, and the snakes in turn might feed mammals and larger birds. The fish might also feed birds like kingfishers or herons. At the same time, detritus from these animals, such as decaying bodies or waste matter, replenishes the nutrients feeding the plankton.

Starting from the Bottom

  • The condition of the river bottom greatly affects the river's ecosystem. "Cobble embeddedness" describes how much sediment fills and covers a riverbed's cobblestones, as explained by the West Virginia Department of Environmental Protection. A muddy bottom leaves little or no cover for benthic creatures like insect larvae, small fish and crayfish, who hide and hunt among the rocks. When their populations are low, there's less food for higher predators all the way up the food web, so all of their populations suffer, and there is less life in and around the river.

Apex Predators Vary

  • Apex, or top, predators are those which, once adults, are not preyed upon by other species in their ecosystem. Some are top predators in their part of the ecosystem -- such as dragonfly nymphs in the benthic environment -- but they can fall prey to small fish. That same small fish could fall prey to a largemouth bass, or a pike, who could be the top predator in the water. They in turn could fall prey to predators like otters and snapping turtles from the land.

Some Effects of Invasive Species

  • When a foreign animal species enters a river ecosystem, it can devastate the balance of animal populations. The so-called Asian carp, which actually comprises several carp species, thrives by robbing the lower levels of the food web, filter-feeding plankton or eating the benthic invertebrates that support the other species of fish in the river ecosystem. In Scotland, American crayfish threaten native animals by dominating the benthic environment and robbing the natives of food, as described by the Society of Biology.

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