Turnover
rates and the inverted food pyramid
The biomass
(mass per unit area) of fishes or other animals in a habitat indicates
something about the nature of the community. However, biomass is a static
depiction, basically a snapshot, of a very dynamic situation for which a moving
picture would tell us more. Turnover, the ratio of production to
standing crop biomass (P : B), provides the added information. Turnover,
expressed in units of mass per unit area per unit time (e.g., g/m2/year) is a measure of how productive a population is over time
and takes into account life table schedules of birth and death, population
density, individual growth rate, and development time (Benke 1993; Production). For example, a seeming paradox
occurs in many freshwater habitats when prey consumption rates of fishes are
investigated. Trout in the Horokiwi Stream of New Zealand consume about 20
times the standing crop biomass of invertebrates annually; trout and stonefly
consumption of prey in a Colorado stream is about 10 times greater than
standing crop biomass of prey. In some streams, the biomass of predators
exceeds that of prey, which would seem to violate laws of ecology and
thermodynamics.
Obviously,
just looking at biomass tells us little about ecosystem dynamics in such a
situation. The paradox of how fish can consume more prey than exist, and how
more predators than prey can be maintained in a habitat, is solved when one
looks at turnover rates, namely how quickly animals reach maturity, how
many times they reproduce, and how many young they produce. If benthic
invertebrates go through several generations per year (which they do), then
their annual production can greatly exceed biomass at any one moment and the
invertebrate community can support a much larger fish assemblage than if the
fishes were solely dependent on standing crop biomass. Production values three
to 10 times greater than biomass are not unusual (Benke 1976, 1993; Allan
1983).
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