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Life in Freshwater

Invertebrate Drift

This is a phenomenon that has had ecologists baffled for a long time. The process of drift is straightforward. In lotic systems the current may cause animals that live on the bottom to be dragged from where they are attached and pushed down stream. It has be shown that the distance maybe just a few metres or substantially more especially when a river is in spate.

River in spate
A river in spate. Animals living on the bottom will have problems hanging on with this high volume of water and the increased turbulence caused.

The species most at risk will be those that have to come out to feed on algae and diatoms attached to the rock, like mayfly larvae. The larger species of stonefly larvae as well as flatworms and other species are predators that feed on the grazers and scrapers. All will be at risk of becoming detached.

Until you actually study drift it is difficult to appreciate the tremendous quantity of invertebrates involved. A net needs to be anchored in the stream (not in spate and a small stream at that!) and studied over 24 hours by examining and emptying the net every 2 or 3 hours. There is a summary of some data plotted as a graph over a 24 hours period below.

Graph showing how many invertibrates were caught over a 24 hr period

This data was summarised from a small stream approximately one metre across and 40cms deep. The net was half a metre wide and the experiment performed in late April. During the day there was minimal drift: invertebrates remained under stones, sheltering from predators. As the sun set so the herbivores emerged to gather food. During this move to the top of the rocks they were most prone to be knocked off the rocks. Whilst feeding they were not moving so much and there is a slight dip between 10pm and 4am. However, the data includes all species and the predators began to follow the prey up to the top of the rock. A second peak occured at 4am as the animals began to move back down again to shelter beneath the stones before it became light. Repeating this each month will produce slightly differing results and Spring is a good time to do this as the larval invertebrates are at their highest density.

So, why are ecologists baffled? With this volume of invert traffic going down river how come the upper reaches do not become depopulated? The obvious suggestion is that either in their larval form or adult stage they are always moving against the current and going up river. However, there is conflicting evidence. For larvae they do have a tendency to go against the current if possible. Some like Black Fly larvae have adaptations to try to secure themselves if they do enter the drift. This means they do not move too far down river. Ecologists have used special suction traps located on the river bank to see if there is a movement of adults back up stream to breed. But there is no clear evidence of this. It may be that the density is so high up river the population can just cope with the loss and this is made up with sufficient offspring in the next breeding season.

Is this all bad? New ideas suggest that drift may not be all about risk. There are advantages. In many cases the organisms drifting could be the surplus at times of high density. Competition will be reduced and dispersal will occur to new and possibly species depleted areas, e.g. at times of drought in lowland areas. This could mean that drift may be a pro-active phenomenon such that organisms deliberately release themselves into the current to move them on to new habitats. When drift has been studied experimentally there does seem to be correlation with density. In some cases for some species there is little correlation between release and time of day or night.


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