Body size and food chain position

Posted on March 28, 2013

In studies of aquatic food webs, stable isotope techniques are finding wide application because the concentration of the stable nitrogen isotope δ15N in an organism’s tissues varies with its position on the food chain.  Also, the δ15N signature of an animal’s body reflects its total diet over a period of time.  However, because the stable isotope approach is quite specialized there’s interest in knowing how well simpler measures, such as body size, relate to food chain position.  There’s evidence that body size is a good guide to feeding level in marine fish communities, but until recently it wasn’t known whether this was also true for freshwater lake ecosystems.  To examine the relationship between body size and food chain position estimated using the δ15N technique, zooplankton and fish were collected from a range of lakes in Ontario, Canada.  For Daphnia (water flea) species,  some of the most common types of zooplankton, the strength of the relationship was high, with 50% of the variation in trophic position being explained by body size.  However, for another common species of water flea (Holopedium gibberum), the correlation wasn’t significant, possibly because the size range of food particles taken by that species was relatively narrow.  In the case of fish, almost all relationships were positive, but not all were significant.  Although the relationship was strong at the community level, the rate at which trophic level increased with body size varied significantly between species and between lakes.  This variability presumably reflected differences in feeding conditions and diet choice: for example, fish species with non-significant feeding-size relationships tended to be opportunistic omnivores that feed at several trophic levels at the same time.  In summary, food chain position does show a general increase with body size in both zooplankton and lake fish, but single-species relationships often have low predictive power and should be applied with caution.

Reference:  Persaud, A.D., Dillon, P.J., Molot, L.A. & Hargan, K.E.  2012.  Relationships between body size and trophic position of consumers in temperate freshwater lakes.  Aquatic Sciences 74, 203–212.