Turnover rates and the inverted food pyramid - Effects of fishes

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).