Don’t waste energy – use this equation

Posted on September 23, 2013

Ecological and behavioural  investigations often use energy as a common currency.  For example, ecosystem processes can be understood in terms of energy transfers through the food web, and optimal foraging studies use net energy intake as an indicator of fitness.  Bioenergetic models predict predator growth rates by calculating how much surplus energy is available after the body’s metabolic needs have been satisfied.  Energy inputs from ingested food are traditionally estimated by using a bomb calorimeter to measure the energy released from samples of prey.  This can be a time-consuming and costly process and is sometimes plagued by sample size limitations.  Alternatively, “secondhand” energy density estimates can be imported from the literature, but this approach can lead to significant errors in model outputs because variations in energy content across space and time are ignored.  A new, and very simple, method of estimating energy density values uses the fact that energy content depends on  lipid levels, which are inversely related to the moisture content.  This means that it’s possible to use the proportional dry weight (dry weight divided by wet weight) of animal tissue to predict its energy density.   Researchers found that the energy – dry weight relation is very consistent across a wide range of freshwater and marine invertebrates, from jellyfish to tunicates.  Proportional dry mass (pDM) explained 96% of the variation in energy density (ED; J/g wet mass), the predictive equation being ED = 22,960pDM – 174.2.  In other words, for every 10% increase in dry weight, energy density rises by about 2200 J/g.

Reference:  James, D.A. et al.   2012.  A generalized model for estimating the energy density of invertebrates.  Freshwater Science 31(1):69–77.