When predators aren’t so predaceous

Posted on March 28, 2013

Some sixty years ago it was proposed that productive ecosystems, where sources of energy are plentiful, should be able to support longer food chains.  An associated idea is that when system productivity is low, predators are more likely to broaden their forage base to obtain enough energy, thus becoming more herbivorous.  Given its general ecological relevance, it’s perhaps surprising that tests of this “energy hypothesis” have been few and far between.  In a recent North American study, chlorophyll, zooplankton and  zooplankton-feeding invertebrates (chaoborid midges and mysid shrimp) were collected from 14 lakes that varied widely in terms of productivity.  In each case, the trophic (food chain) position of the sampled animals was measured using stable isotope techniques.  The results showed that the extent of omnivory shown by the midges and mysids did indeed depend on lake productivity.   In lakes with low levels of nutrients, algae and primary production, the difference between the food chain positions of zooplankton on the one hand, and mysids or midges on the other, was relatively low – less than 1%  in terms of the stable nitrogen isotope ratio (δ15N).  This showed that under low nutrient conditions, mysids, midges and zooplankton all shared a largely herbivorous lifestyle.  In contrast, in the most productive lakes, the difference in the stable isotope ratio was much higher (> 4%), which indicated that mysids and midges were feeding further up the food chain, on zooplankton.  In lakes with intermediate productivity, differences in the isotope ratio were also intermediate (1-4%) and here mysids and midges appeared to be omnivorous.  This finding helped to explain the results of a parallel study that revealed that the strength of top-down control (i.e., the ability of mysids and midges to regulate herbivorous zooplankton) increased with the productivity of the system.  Another message from this work is that because food webs are highly dynamic, traditional neat classifications of aquatic species as herbivores, omnivores or carnivores are too rigid and simplistic.

Reference:  France, R. 2012.   Omnivory, vertical food-web structure and system productivity: stable isotope analysis of freshwater planktonic food webs.  Freshwater Biology 57, 787–794.   http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2427.2012.02744.x/pdf