Life of a River - Biology
drift (pg 3 of 3)
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An interesting aspect of stream ecosystems is
the flow of organisms known as invertebrate
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.