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Life of a River - Biology
Adaptations: Invertebrate drift (pg 3 of 3)
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An interesting aspect of stream ecosystems is the flow of organisms known as invertebrate drift. (Waters, 2000). As the sun sets on the stream, some invertebrate species-stonefly and mayfly nymphs, caddisfly larvae, and crustaceans such as amphipods-wriggle out from the streambed and set loose in the current to drift downstream.

The drifting animals drop slowly from the water column to take shelter once again in the streambed. Drift is probably greatest at night to avoid sight-feeding fish and predaceous insects. Drift is also higher in summer than in winter.

Since some types of invertebrates drift downstream, are the headwaters eventually depleted of them? That question has puzzled stream scientists ever since the discovery that many organisms purposefully drift with the current. Many species of insects, such as mayflies and caddisflies, do emerge from the stream as winged adults and fly back upstream.

But what of the species that aren't capable of flight? They would have to crawl or swim. Other researchers have hypothesized that drift represents the dispersal of surplus organisms that otherwise would overrun their limited habitat.


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