Stream velocity affects predation risk

Posted on June 4, 2011

Small-bodied animals typically show escape responses to larger predators, but the escape movements of prey individuals may displace them from preferred locations, such as patches of food.  Therefore, it might be expected that the behaviours of prey species will allow them to strike an optimal balance between the risks of predation and the costs of escape.  Flowing streams offer interesting opportunities to examine this idea because predation risk and escape costs are both affected by water velocity.  As velocities increase, predator pressure tends to drop off because predators need larger amounts of energy to forage in strong currents.  At the same time, in fast currents prey species should be less willing to move from favourable positions because of the dangers of increased exposure and displacement.  This prediction was tested by comparing the responses of three mayflies found in different habitats (slow pools, fast riffles and very fast cascades).  Mayfly larvae were exposed to a range of flows in an experimental channel.  A simulated predator (a black plastic disc with a cross-section similar to that of a stonefly predator) was used to generate an escape response and the displacement distance was measured.  The mayflies from the fastest streams (Epeorus) reduced their average displacement dramatically – from over 60 mm to less than 20 mm – as the current speed was increased from 5 to 15 cm/sec.  In other words, when Epeorus larvae were threatened in their preferred, high-velocity conditions, they substantially reduced their potential costs of flight, in line with the tradeoff prediction.  In contrast, for mayflies from low-velocity streams the tradeoff was different because the costs of moving from favoured, low-velocity locations varied little across velocities.   These findings show that water velocity can affect predator-prey interactions in hitherto unexplored ways. 

Reference:  Hoover, T.M. & Richardson, J.S.  2010.  Does water velocity influence optimal escape behaviors in stream insects?  Behavioral Ecology 21, 242–249.