9 Ecotrophic efficiency

Ecotrophic efficiency (EE) was defined by Ricker^[1] as the proportion of a prey’s production that is consumed by predators. Polovina used the term for the original Ecopath model of the unexploited French Frigate Shoals.^[2]

Subsequently, we have for EwE modified the term to include exports from the system, (which notably are due to fisheries). Based on the second Ecopath Master Equation (See The energy balance of a box chapter) we have,

[latex]EE_i=\frac {M2_i \cdot B_i+C_i+BA_i+E_i}{P_i}\tag{1}[/latex]

so, EE can be estimated as the ratio between the summed predation M2, catch C, biomass accumulation BA and net migration E relative to the production P for any group i.

If your model is descriptive (as Polovina’s model was), your aim likely is to describe the energy flow in the entire ecosystem. If that’s the case, the EE indeed is an “ecotrophic efficiency” that describes the proportion of the energy produced by a group that it passed on through the trophic web or exported (e.g., through fisheries). So, if EE is 0.95 then 95% of the production of the group is passed on to predators or fisheries.

But what about predictive (or MICE) models? Such models tend to be focused on the specific policy/research questions they are built to address, and as such may not give a complete picture of the food web interactions in the given ecosystem. That boils down to there being a considerable amount of unexplained mortality (M1) in the model. In such cases the EE isn’t really an “ecotrophic efficiency”. Hence, the following may be a better way to grasp EE.

It is difficult to estimate EE independently, and few, if any, direct estimates appear to exist. Recognizing this, an EE of 0.95, based on Ricker (1968) was used for many groups in Polovina’s original model^[3] and in a number of later models.

Evaluating EE

Intuitively one would expect EE to be very close to 1 for small prey organisms, diseases and starvation probably being, for such groups, much less frequent than predation. For some groups, EE, may however, be low.

It is often seen that phytoplankton simply die off (as “snow”) in systems where blooms occur (EE of 0.5 or less). Also, kelps and seagrasses are hardly consumed when alive (EE of 0.1 or so), and apex predators have very low EEs when fishing intensity is low. There are indeed many incidences of tunas or cetaceans simply dying and sinking reported from open oceans, with abyssal organisms (e.g., ratfishes) specialized in feeding on such carcasses.

If EEs are estimated it is often because of lack of biomasses. It should not be because of lack of P/B or Q/B values – it is better to guess those than to let them be estimated from the Ecopath mass balance.  When biomasses are estimated, one needs to carefully examine how realistic those biomasses are. We’ve seen examples where there were biomasses entered for only unexploited high trophic level groups, and everything else being estimated from guessed EEs. So, if you have to used EEs as input, check out PreBal^[4] and compare the estimated biomasses to estimates from similar ecosystems.