Top-down control and patterns of land use

Posted on March 26, 2010

Top-down control by consumer species  is known to be important in structuring freshwater communities, but how are natural top-down processes affected by human disturbance?  While streams in undeveloped, forested environments have relatively clean substrates, those in suburban and urban settings usually have higher sediment loads and less tree cover, with increased light penetration and more algae.  These differences  mean that as environments become more modified, the dominant stream consumers  tend to shift from specialists feeding on bottom-dwelling invertebrates to generalists that depend more on midwater invertebrates and algae.  ANorth Carolinastudy compared top-down effects in streams in forested, agricultural, suburban and urban settings.  At each site, electric fields  were used to exclude dominant consumers  (fish and crayfish) from some substrates but not others.  Although species of top consumers, insects and algae varied from site to site, at all locations there were significant increases in chlorophyll and insect biomass when the top consumers  were excluded, showing that they normally restrict the size of populations  lower in the food chain.  The consistency of this trend showed that top-down control can remain important even though community structure may change as a result of human disturbance.

Reference:  Schofield, K.A., Pringle, C.M., Meyer, J.L. & Rosi-Marshall, E.J.  2008.  Functional redundancy of stream macroconsumers despite differences in catchment land use.   Freshwater Biology 53, 2587–2599.