for a catalogue of other, possible examples of top-down cascade. There are multiple-
step trophic cascades in marine ecosystems. We just have a very slim file of
convincing demonstrations. Despite that, understanding of stock-size interactions
among predators and prey has come to hold a prominent place in the ecological
understanding and lately the management of ocean ecosystems, both oceanic and
coastal. Food-web models now hold a prominent place in codifying this understanding
and making it useful. We will examine just one such approach, Ecopath models.
Marine Food-Web Modeling: Ecopath and
Ecosim
(^) An important issue in modeling food webs is assignment of correct trophic
efficiencies. The ratio between tissue formed and food eaten is an animal’s gross
growth efficiency (GGE). Values of GGE must be greater than ecological efficiencies,
which are defined in terms of populations (or trophic levels) and incorporate more
losses than metabolism and defecation. Specifically, any non-predatory mortality (due
to genetic diseases, parasites, and infections) are additional transfers to the
decomposer trophic compartment of the ecosystem. Thus, ecological efficiency refers
to transfers “up” the food web. However, keep in mind that there are returns from the
decomposer level to the “main” prey-to-predator trophic path. For example, copepods
feeding at depth on falling fecal pellets can be eaten by myctophid fish that are later
eaten by squid. Gross growth efficiencies set an upper limit on ecological efficiency,
and are somewhat helpful with problems like approximating how much fish might be
harvested from a region based on its primary production (see Chapter 17). The GGE
of an animal is not fixed, but changes with size, age, and habitat conditions. This is
obvious for people, who grow very rapidly immediately after birth, their weight
doubling in a few months, but normally (obesity problems excepted) stop growing
altogether before 20 years of age. We do not stop eating, but our growth efficiency
drops to zero. Many fish continue to grow throughout life, but growth slows strongly.
Given a replete diet, growth efficiency of larvae and early juveniles is typically
∼30%, then drops off. For example, tank-reared, juvenile bluefish, Pomatomus
saltatrix, initially weighing 1–2 g, grow at GGE = 29%. Over 90 days they increase to
86 g, with GGE dropping to 15% (Buckel et al. 1995). At ages of several years, GGE
drops to a few percent at most. Both growth rates and GGE depend upon temperature,
generally higher in the warmer half of the habitable range, and food abundance and
quality. Restricted rations require larger proportions expended on metabolism, with a
lower limit on GGE of zero, although some increases in assimilation efficiency are
possible.
(^) Growth efficiencies of zooplankton, both protists and metazoans, are more uniform