Short-term studies may mislead

Posted on September 21, 2011


In freshwater streams, consumers such as fish and crustaceans are known to have strong influences on ecosystem structure and function through their top-down effects on primary production, nutrient cycling and energy flow.  However, it can be difficult to assess these impacts when background environmental conditions are continually changing.  In many streams in central and eastern North America, crayfish and stoneroller minnows are the dominant benthic consumers.  Experiments in Little Mulberry Creek, Arkansas have shed light on the ways that the top-down effects of crayfish and stonerollers are affected by the environmental changes that occur as streams dry up in summer. Consumer impacts on organisms at lower levels of the food chain were measured by comparing the occurrence of algae, midge larvae and organic sediment on ceramic tiles in control quadrats with their occurrence in quadrats from which consumers had been  excluded by electric fields.  Early in the summer of 2006 the water level dropped rapidly and stayed low.  Crayfish and stonerollers significantly reduced the occurrence of midge prey, algae, and organic matter, and this trend strengthened as the drought continued.  Changes in organic sediment were positively related to crayfish density in June but to stoneroller density in August, which suggested that fish replaced crayfish as dominant consumers.  In summer 2007 a similar exclusion experiment was carried out in the same creek.  In that year drying reduced the stream to a series of pools, but in contrast to 2006 the dry period was broken by a series of spates that scoured the stream, and the presence or absence of consumers had no significant impact on algal densities, invertebrates or sediment dry matter.  The combined findings from the two studies showed that seasonal and year-to-year changes in environmental conditions can have a dramatic impact on the ways that consumers such as fish and crustaceans affect stream ecology.  As a result, long-term studies of consumer effects are required and short-term findings should be interpreted with caution.

References:

Ludlam, J. P. & Magoulick, D.D.  2009.  Spatial and temporal variation in the effects of fish and crayfish on benthic communities during stream drying. Journal of the North American Benthological Society 28, 371–382.

Ludlam, J. P. & Magoulick, D.D.  2010.  Environmental conditions and biotic interactions influence ecosystem structure and function in a drying stream.  Hydrobiologia 644, 127–137.

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