through life, in some cases with egg production of adults sustaining output
efficiencies nearly equivalent to early growth. Straile (1997) reviewed data to that
date and stated that “all taxa (i.e. nano/microflagellates, dinoflagellates, ciliates,
rotifers, cladocerans, and copepods) were found to have mean and median GGE of
∼20–30%”. The most important factor influencing variability was food availability,
with lesser but positive effects of warmer temperatures within the usual habitat limits.
The comparison of zooplankton with fish, much of the food consumption of which is
by older, slow-growing individuals, suggests there is a major reduction of ecological
efficiency as nutrient ascends the food web, simply due to progressive reduction of
GGE. Estimating the overall ecological efficiency between any given trophic levels,
even of a species population, remains largely a matter of guessing well.
(^) Ecopath models are complex and can only be sketched here in a general fashion.
They were initially developed by Jeffery Polovina in the 1980s, and then extended by
Daniel Pauly, Villy Christensen, Carl Walters, and many others (Christensen &
Walters 2004). They are steady-state models with populations represented by their
biomass (or energy) and connected to each other by trophic relations. The basic
assumption is a conservation of biomass (energy) such that input of food equals a sum
of outputs for each population in a food web:
(^) Tissue formed (production) is then partitioned according its ultimate fate:
(^) In the models, each population (stock) in a food web is assigned one of a linked set
of equations based on those two. Generally, it is necessary to define at least some
food-web “populations” in a general way, say as ecological guilds: “phytoplankton”
and “zooplankton”, for example, or “small squid” and “small mesopelagic fish”.
Other populations, those of central interest, can be particular species, perhaps chinook
salmon or yellowfin tuna. A given species can be both prey and predator. As prey
species i, it is subject to predation by a series of predators given subscripts j. The
input–output (or mass-balance) equation (Heymans et al. 2007) becomes:
(^) The product on the left is: prey stock biomass (B
i) times prey production to biomass
ratio ([P/B]i) times “ecotrophic efficiency” (EEi = fraction of prey production
consumed by predators or emigrating from the area of interest). This is the total of all
growth by the prey stock over some interval (implied by the production rate P). It may
seem odd to write Bi•(P/B)i instead of just P. The reason is that Bi can be